Valhalla 2720GS - Page 1
Forum section: electronics / general

author: jfphp
author_rank: Regular Contributor
timestamp: March 23, 2014, 11:08:53 am
content: Hello,I was very happy with a Valhalla DC standard 2720GS but unfortunately it died in two steps : I lost the differential voltmeter ("DVM repair" as on the one offered now on Ebay) and now "Reference repair". After a lot of tests including the various stacked power supplies, I suspect the Pal deconding chip on the CPU, an EP900. Did anybody read it ? I can read it on a chip programmer but, of course, I down know if it is a good or wrong JEDEC file.Another question : does anybody know the default password to reset the 2720GS to the factory calibration default state ?Thanks.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 24, 2014, 12:02:33 am
content: I don't have any information on the 2720GS.But I'd love to see some pictures of it! (And so would other people that might be able to help more than I can)

author: TiN
author_rank: Super Contributor
timestamp: March 24, 2014, 04:41:46 am
content: Same here, one of few units using statistic voltage references.That pal is like simple version of CPLD, so it's just couple logic gates in there probably.In theory should be possible to replace it with some bodged logic circuit, if you can find out what are input signals, what are outputs and whatfunction that PAL was supposed to do. These may have security bit, so even if somebody have working 2720GS, they may not be able to read it.Tear-down hi-res photos would be gold.

author: Vgkid
author_rank: Super Contributor
timestamp: March 24, 2014, 06:35:08 am
content: I would also require pictures.

author: jfphp
author_rank: Regular Contributor
timestamp: March 25, 2014, 06:14:51 pm
content: http://www.adret-electronique.fr/valhalla_2720/adret_valhalla_2720.htmlSome pictures and documentation before the crash...

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 26, 2014, 04:30:10 am
content: " This system's internal 30 bits A to D converter continually monitors each reference zener. Each reference module contains its own non-volatile memory wich stores calibration and drift rate data."This is something I've wondered about quite a bit. What does the A/D compare the A to in this configuration?I mean, at its core an A/D only determines ratios.... "so much more" or "so much less" than V_ref.But if you're measuring something that's *the most accurate* already... what's more accurate than that?My guess is the A/D compares the zeners to one another -- in a bit of a kilogram divergence problem:So can this make the reference more accurate? Is there some statistical magic or model for zener drift that helps divine the true value of the volt? (e.g. if they all drifted the same amount, how would you detect this?)

author: jfphp
author_rank: Regular Contributor
timestamp: March 26, 2014, 10:47:24 am
content: It is exactly the same problem with the Fluke 732A : you need 4 to be sure of the respective drift of one to another (therefore 4 references in the Datron 4910) : you compare each day or week ... 1 v 2, 1 v 3, 1 v4 , 2 v 3... In comparing the various drifts you can isolate the (eventually) faulty one (drift exceeding the mean value) and you can take as reference the mean of the 4 or 3 if one faulty and minimize noise (reduction of noise of root square 6 or. Superiority of the Valhalla is the computerized mesurement of the respective drifts in comparison with the drifts stored in EEPROMs of 6 zeners or 8 as an option.In the high stability option the 6 or 8 LM399AH (selected !) are replaced by 3 or 4 LTZ1000A (1 zener per card instead of 2).

author: jfphp
author_rank: Regular Contributor
timestamp: June 30, 2014, 10:28:04 am
content: Thank you very much to all people who neither helped nor answered : you have forced Bruno and me to work. We have now reconstruct all the logical equations of the PAL, burned new ones and find the factory reset password (one week work for this very carefully hidden password). Various failures (nullmeter, output board...) were easy to fix. Two 2720GS are now living again...

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: July 05, 2014, 04:31:53 pm
content: To jfphp,I am curious, are any of the precision wire wound resistors in your 2720Gs yellow?Sorry no one could help but Valhalla keeps any details they view as proprietary out of the manuals and only a few people at the factory, the engineers who designed this unit, have knowledge of such things. They also like to charge a lot for their manuals if they are available, which would not have been much help either. It is a bit strange that Valhalla claims to not have the microcode though, that should have been in the engineering documentation and that should have been kept on file.

author: Mickle T.
author_rank: Frequent Contributor
timestamp: July 05, 2014, 08:10:00 pm
content: QuoteVery strange. AFAIK the first of the Valhalla 2720GS calibrators don't have any of it, but only selected 1N827 zeners in the ovens. In the mid-1980s, LTZ1000 did not exist.P.S. And how about factory reset password?

author: Kosmic
author_rank: Super Contributor
timestamp: May 15, 2018, 05:02:08 pm
content: Quote from: Mickle T. on July 05, 2014, 08:10:00 pmI recently acquired a unit built in 1988. It's equipped with the HSR option (High Stability) and has 8 x LM399 voltage ref.@jfphp: I would be really interested if you had any information to share on the reset password and how to reflash the PAL.Thanks!

author: TheSteve
author_rank: Unknown
timestamp: May 16, 2018, 04:14:03 pm
content: Quote from: Kosmic on May 15, 2018, 05:02:08 pmI don't have any info to share but am glad it stayed in Canada. I was local to it and I was really considering it.If it does need repair please consider starting a thread in the repair section to share the problems and fixes.

author: Kosmic
author_rank: Super Contributor
timestamp: May 16, 2018, 05:36:52 pm
content: Quote from: TheSteve on May 16, 2018, 04:14:03 pmYup it made the trip from BC in one piece (I'm on the east coast)I will definitely start a thread since there is a couple of issues. For one, the fan is not working and I'm pretty sure it's not normalAfter a hour it get really hot. Nevertheless after running the internal check all the voltage ref were back online. So that's a good start

author: branadic
author_rank: Super Contributor
timestamp: May 06, 2023, 06:34:01 pm
content: Quite some old thread, but I thought to revive it.QuoteIsn't it more like the 2720GS contains three reference boards with a total of 6x LMx99, while the HSR option has four board with a total of 8x LMx99?Was someone else successful in replacing the Xircor X2212 chips? A replacement was mentionedhere, but without further details.Same goes for all the rest mentioned like factory reset password or firmware file for the PAL. Already contacted jfphp via his website, but yet without any feedback.Quote-branadic-

author: branadic
author_rank: Super Contributor
timestamp: May 15, 2023, 05:48:58 pm
content: Hello everyone,I received such a unit today, so I will put everything I find out about it here so everyone can benefit from it.When the unit was about to be sold to me the seller tested the unit, which tripped his circuit breaker. He also noticed some smell of burning components inside the unit. Okay, so it was eventually sold as defect, which is fine, right? Let's see what needs to be fixed...

author: branadic
author_rank: Super Contributor
timestamp: May 15, 2023, 05:54:38 pm
content: First thing I've noticed was the front panel being pretty yellow. A good looking meter is good motivation to get things fixed. So the first step I've done afterThe Arrivalwas using pure ethanol to remove whatever this yellow film was. Could have been either a transparrent but yellowed protection coating or human residue from the last decades? Who knows, but in any case it was removable and is almost gone.

author: branadic
author_rank: Super Contributor
timestamp: May 15, 2023, 06:01:54 pm
content: Next I pulled some cards that obviously contained physical large axial capacitors and what do you know? It's almost always a bad electrolytic capacitor, also here. Since the guts of the caps was released only a few days ago vinegar helped for the most part. So next step is a fully recapping of the unit. I also noticed that some of the fuses are blown, maybe a good sign and the rest was well protected? Fingers crossed.However, after removing the caps I've noticed at least two burn marks creating short circuits between traces, which needs fixing as well. I scraped away the carbon and filled the remaining grooves with some UHU endfest. First fix done.

author: branadic
author_rank: Super Contributor
timestamp: May 15, 2023, 07:01:37 pm
content: On the front panel board is some AA 3.6V lithium primary cell, measuring it showed not a single bit of live. So it needs replacement too.-branadic-

author: Kosmic
author_rank: Super Contributor
timestamp: May 15, 2023, 07:21:16 pm
content: Make sure you are really gentle with the high voltage transformer. Mine shorted after being removed / reinstalled. It's not like I dropped it or something. I guess the cables were bent a bit too muchanyway I still need to source a new one.

author: branadic
author_rank: Super Contributor
timestamp: May 16, 2023, 04:43:11 pm
content: Thanks for the hint, I will keep that in mind.The caps on the power supply board all measure good, nevertheless they will be replaced in any case. The caps on the high voltage power supply seem to be the main issue and were already removed.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: May 19, 2023, 08:12:20 am
content: Ordered most of the caps, fuses and battery. I guess they will arrive next week. I already replace the 100 µF/25V with 100 µF/50V electrolytics on low voltage and high voltage power supply boards and noticed, that some of them started leaking. So it's good advice to replace all caps in the unit.Slow but at least some progress, that will hopefully bring back the unit to life. I will take more photos of all boards and link them here.Someone by accident has the CAL key? It's missing on my unit, but before I order a replacement key switch, maybe someone can help?-branadic-

author: Kosmic
author_rank: Super Contributor
timestamp: May 19, 2023, 01:15:44 pm
content: Quote from: branadic on May 19, 2023, 08:12:20 amI have one. I'll check if I can make a copy.

author: Kosmic
author_rank: Super Contributor
timestamp: May 19, 2023, 06:27:46 pm
content: Quote from: Kosmic on May 19, 2023, 01:15:44 pmDone! I had to go see a locksmith but that was no problems.

author: branadic
author_rank: Super Contributor
timestamp: May 20, 2023, 06:03:05 pm
content: QuoteHouston we have a problem. Seems like the dying caps took the HV transformer with them. Anyone has any detailed info about that xformer?I better check low voltage transformer too.-branadic-

author: coromonadalix
author_rank: Super Contributor
timestamp: May 20, 2023, 06:16:07 pm
content: sv manual for thoses i havent seen it ...https://xdevs.com/doc/Valhalla/2720GS/https://xdevs.com/doc/Valhalla/2720GS/Valhalla_Scientific_2720GS_Vol.%202_sm.pdfMan thats not funny, and you need at least some specs for sure, and or find an defective unit hoping the xformer is still good ??page 6 show the connector pinout of the xformers ...searching for the voltagesp34 p35 shows some of them not sure if we all of them2x16v ac ? with an ct2x 115v ac ?2x 1100v ac ??that give us 7 wires ?? possibly 8 because we faintly see 115vac CT going to the 16vac CT ??

author: Kosmic
author_rank: Super Contributor
timestamp: May 21, 2023, 01:21:59 am
content: Quote from: branadic on May 20, 2023, 06:03:05 pmLook like I might had the same issue. In my case, the transformer failed just after I fixed the highvoltage psu where a cap shorted. The transfo survived the operation but failed shortly after.coromonadalix is right, the transformer has 3 windings on the secondary side. 16V, 115V, and 1100V. Not sure of the VA requirement for each windings though.I was thinking to ask a quote fromhttps://www.heyboertransformers.com/. Let me know if you are interested, we might have a better deal if we ask for 2 units.The other option is to rewind the transfo manually but I would definitely prefer not to.

author: branadic
author_rank: Super Contributor
timestamp: May 21, 2023, 06:21:48 pm
content: QuoteAny possible option is welcome. Let's see how far we can get.In parallel I'm also trying to find out who is able to replicate the transformer here in Germany. In best case and if required to get prices down we could have made even a few more, I'm sure we are not the only guys with that issue?Let me ask, is your unit working even without the HV power supply board and the HV transformer or does it throw out an error if both are missing?QuoteMe neither.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: May 21, 2023, 08:12:15 pm
content: Attached the EEPROM (AT27C256) content of "Main Microprocessor Board" in case someone needs it in the future.Anyone has an idea on how to read X2210 and X2212 with TL866II? In theory it should be possible, as FM16W08 and FM18W08 could be read/written using corresponding Dallas chips, but there is nothing similar for these chips.On the other hand these chips could be replaced with FM16W08, leaving out four of the address lines.https://www.eevblog.com/forum/projects/x2210-and-x2212-nvram/msg4861997/#msg4861997Any guess or suggestion?-branadic-

author: Kosmic
author_rank: Super Contributor
timestamp: May 22, 2023, 01:54:53 pm
content: Quote from: branadic on May 21, 2023, 06:21:48 pmI only tried with the HV transformer unplug and the unit start but refuse to output anything, even low voltage.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 22, 2023, 05:36:58 pm
content: must be the 16v lines i think .....

author: branadic
author_rank: Super Contributor
timestamp: May 22, 2023, 06:04:15 pm
content: QuoteYes, 7 wires, 2x red, 2x green, 2x purple, 1x yellow. But that is only one connector, there is one more including two wires that are directly soldered to the main board and one earth wire? screwed to the base of the xformer.Already tried contacting some companies today, but couldn't reach some person with the proper knowledge yet. I give it another try tomorrow.-branadic-

author: coromonadalix
author_rank: Super Contributor
timestamp: May 22, 2023, 08:13:05 pm
content: sorry wasn't clear enough, i think it must have some voltage detection(s) on theses line(s)if one is dead it doesn't work

author: Kosmic
author_rank: Super Contributor
timestamp: May 23, 2023, 01:39:27 am
content: My shorted transfo has been sitting in paint thinner for quite some time now (to soften the varnish and allow disassembly), maybe it's time to start unwinding to understand how it was constructed exactly.

author: TheDefpom
author_rank: Frequent Contributor
timestamp: May 23, 2023, 03:52:13 am
content: Quote from: Kosmic on May 19, 2023, 06:27:46 pmExcellent, my 2703 is missing its cal key and I am tired of using lock picks on it LOL... maybe that picture will help me sort out a replacement, the Datron key is similar but won't turn :-(

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: May 23, 2023, 02:18:54 pm
content: Guess where the recent 2720GS E-bay sale end up. So far it seems to work reasonably well. Passes all tests and calibrations except for the DVM, which fails checks and calibrations and gives somewhat flaky readings. The initial drift upon turn-on is not too bad.The nice aspect of this system is very high voltage resolution, 0.002 ppm below 6V. However, the output is noisier than I would have liked. There are also periodic spikes every 10 min which I believe are related to internal comparison between the voltage references. Has anyone else seen those or is it a fault in this device?In the long term it maybe interesting to replace LM399 with ADR1399.

author: Kosmic
author_rank: Super Contributor
timestamp: May 23, 2023, 02:47:52 pm
content: Quick, check all the caps before it blows up

author: Kosmic
author_rank: Super Contributor
timestamp: May 23, 2023, 02:52:05 pm
content: Quote from: maxwell3e10 on May 23, 2023, 02:18:54 pmI remember monitoring my unit for 24h on 10V with a 34401a and did not noticed any spikes. But, it was just a really quick test and I might had missed something. On my side self tests were passing with success.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: May 23, 2023, 04:15:00 pm
content: I was pleased that it had a calibration sticker from 2020, so hopefully mostly kept on, which should help with the capacitor health. They look fine, but should be replaced, of course. The parts list should help with BOM.It is supposed to check each of the references against the average every 10 min. I am not sure if that requires disconnecting each one in turn from the average, which would of course shift the mean around.Here are a few more picture. It seems to have a battery style different from other pictures I saw.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 23, 2023, 05:55:02 pm
content: i would check if the hv xformer seems ok ... non burned epoxy enamel etc ... some good visual look ...

author: branadic
author_rank: Super Contributor
timestamp: May 24, 2023, 05:29:47 pm
content: Congrats to your new toy. Hopefully, you can contribute with photos of all your boards and measurements on your HV transformer? That would be great for a few people.I highly recommend to quickly replace all the electrolytics before they go bang. I received a first package full of capacitors yesterday, but haven't worked through all the boards yet to spot all needed values, so a second order is likely, before I can share my capacitor replacement list.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: May 28, 2023, 07:10:22 am
content: I've pulled the transformer and took a few photos, please find them attached. The dimensions of the EI core are 96 mm x 80 mm x 33 mm, with a single chamber.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: May 29, 2023, 10:34:18 am
content: Attached is my recapping list with links to mouser.I also attached the ROM file from the TMS27C256 that is sitting on the front panel board.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: June 01, 2023, 07:38:19 pm
content: QuoteHave you started your disassembly? I'm currently off without access to the device, but received a view quotations already, spreading from 500 to more than 1400€. I also asked for a ring core transformer, which should be possible, but that requires better specification of the required output power for the several output voltages.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: June 01, 2023, 07:48:48 pm
content: maxwell3e10, could you help us to measure resistance and impedance of the HV transformer, (both primary windings as well as all secondary windings) as long as yours is still intact? That would help us a lot in asking manufacturers to replicate it or to wind a custom ringcore transformer.-branadic-

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: June 01, 2023, 08:00:33 pm
content: Yes, I can measure the inductance and resistance of the windings. But I wonder if it would be possible to just use a transformer with a 16V output and live without 100V and 1000V ranges (I would almost prefer that, less chance of things blowing up).I think the self-test system is relatively forgiving. On mine it first says that all ranges have a fault, then it says the DVM has a fault (the DVM is a V-to-F meter of the output voltage) and after that the output works fine. So I think it just gives up on using the DVM to check the output voltage.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: June 03, 2023, 11:40:55 am
content: Here are the transformer measurements. One thing it note is that nomenclature is a little off, 16VAC is actually 32V CT and 115VAC is 230V CT. The center taps for the two windings are common. The two extra wires coming from the transformer to the board are just connected to each other on the board.

author: Kosmic
author_rank: Super Contributor
timestamp: June 03, 2023, 02:44:51 pm
content: Quote from: branadic on June 01, 2023, 07:38:19 pmSorry, no update on my side. Got busy with some other stuff.

author: branadic
author_rank: Super Contributor
timestamp: June 08, 2023, 07:38:30 pm
content: Attached is what I found on my transformer. What is hard to spot in the schematic page 34 due to poor quality is the common center tap for both, the 32 Vac and the 230 Vac. Looking at the connector it already indicates this scheme by how the wires are arranged.The 1100 Vac winding is as expected shorted and shows some 0.6 Ω on my transformer.Green/green (230 Vac with CT) winding reads 71.3 Ω (52 Ω measured by maxwell3e10), violet/violet (32 Vac with CT) reads 6.9 Ω (8 Ω measured by maxwell3e10) or if you want to put it in a different way: green/violet/yellow/violet/green = 31.7 Ω/35 Ω/38.3 Ω/71.3 Ωmaxwell3e10, could you please measure resistance on the low power transformer too? I'd like to have a sanity check to be sure it is not defect.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: June 10, 2023, 02:24:18 pm
content: Maybe the mods are gentle enough to move that thread into the metrology section? It's better located over there.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: June 10, 2023, 07:49:38 pm
content: After recapping the unit, replacing the carbon composite resistors inside the oven and socketing the X2212 on the reference boards I took some quick pictures.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: June 10, 2023, 07:50:49 pm
content: And a few more...

author: tautech
author_rank: Super Contributor
timestamp: June 11, 2023, 10:38:35 am
content: Quote from: branadic on June 10, 2023, 02:24:18 pmNot without endorsement from the OP or they can do it themself with the Move Topic feature only they can see at the foot of each page.

author: branadic
author_rank: Super Contributor
timestamp: June 19, 2023, 08:33:08 pm
content: Thanks to the mods for moving the threadI still hadn't the time to take high-resolution images of all boards, but will do before I assemble everything.Any updates to the HV-transformer analysis, Kosmic?I'm still in contact with several companies, but without definitve result so far. I'm also considering having a ring-core transformer being made, but they asked for power consumption of all rails. I've sent them the image with the resistor values of each winding and still hope to get some answer.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: June 22, 2023, 10:34:30 am
content: Good news:I have a company at hand, that is able to analyze and replicate the transformer at reasonable costs. Only drawback is, the transformer will be 15mm longer, as these core sheet length is no longer available, which results in slightly more power being available. A rewinding would be way more expensive, since the coating has to be removed first, which takes a lot of time.Analyzing the transformer is about 150€ and a copy costs 135€, so 285€ maximum, that's what I was told on the phone.So if multiple people come together, we could share the costs for analysis and get working copies. Who is in?-branadic-

author: branadic
author_rank: Super Contributor
timestamp: June 26, 2023, 06:10:57 pm
content: So I have placed an order for 3 HV transformers today. Final dimensions are 95 x 8 0x 70 mm³. I will now package the defect one and send it to the company for analysis.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: June 30, 2023, 08:59:53 pm
content: The transformer is at the company, they will start with their work next week.Question to the German followers, is someone here with a programmer for Xicor X2210, X2212 and willing to lend it to me? The unit is full of those NOVRAMs and I would like to save their content.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 06, 2023, 11:02:22 am
content: I today received a BeeProg+ from a friendly forum member (spent for a few days) and read the 8x NVRAMs distributed in the unit, with the files in the attachement together with an image of the content of reference 1 vs. 2 vs. 3 (from left to right). First two rows are identical.Meanwhile, a first transformer copy was fabricated and will make its way to me tomorrow for testing whether or not it is working.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 11, 2023, 09:29:31 am
content: In a former post the following image was drawn:What is missing here are the two wires connected to E9 and E10 on theMother Board Assemblyand the additional wire that is directly connected to the transformer core with a ring lug. maxwell3e10 could you try to find out how they are connected and if they are connected together? That would explain at least something.Today, a first sample of the HV-transforer arrived, that only has one shield wire, so it is missing the two additional wires for E9 and E10. Need to talk with the manufacturer why. Other than that the transformer looks nicely build.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 11, 2023, 10:42:33 am
content: I had a phone call with the manufacturer... the additional two wires were screens (copper foil) between the secondary windings (32 Vac with CT and 115 Vac as well as 115 Vac and 1100 Vac). In the sample I currently have only the screen between primary and secondary is in place.In the next samples they will add the screens between the secondaries too, so that the transformer is as original as possible. However, their technican was the opinion, that such additional screens were not necessary, as they are only good for high frequencies.Another important thing to note is, that they found a palette with the absolut identical core sheets, so the outer dimensions of this replacement are identical to the original transformer, which is great news.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 12, 2023, 11:53:28 am
content: Have checked the transformer yesterday by resistance measurements, but also by voltage measurements and found, that something was odd and different to the schematic I've drawn in my previous post. User maxwell3e10 already confirmed, that all secondary windings should be symmetrical to the center tap, except for the 1100 Vac.Unfortunately, the 230 Vac CT (2 115 Vac) are not as they supposed to be on the sample they have sent. I already wrote the manufacturer yesterday and had a call with him this morning. They will now implement it hopefully in the correct manner including the screens and will send a new sample by the end of this week, that I hopefully receive by the beginning of next week.So I'm kind of lucky to have made the sanity check first, before installing the transformer in the instrument --> better safe than sorry-branadic-

author: Kosmic
author_rank: Super Contributor
timestamp: July 13, 2023, 01:06:05 pm
content: It's nice of them to send prototype like that. Looking good so far

author: branadic
author_rank: Super Contributor
timestamp: July 14, 2023, 05:42:43 am
content: Yes and the bill that came with it is for the first working sample. By the way, a new sample is ready for shipment and will arrive by the beginning of next week. This mornings call revealed, that in theory everything should be as drawn, continuous winding with center tap as well as screens. Fingers crossed.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 15, 2023, 06:30:59 am
content: From what we know we can now update the transformer scheme.There is a fist screen (white/brown) between primary and secondary 1100 Vac, a second screen (white/green) between 1100 Vac and 32 Vac CT and a third screen (white/yellow) between 32 Vac CT and 230 Vac CT. The first screen is tied to the core of the transformer, while the other two screens are connected to E9 and E10, which themselves are interconnected on the Mother Board Assembly.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 17, 2023, 06:45:31 pm
content: Today a new sample arrived and connection-wise it looks about right. Tomorrow I will power it up and measure the voltages.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 18, 2023, 07:30:52 pm
content: I today performed the voltage measurements and it all looks fine. I'm already in the process of reassembling the unit, but had to stop at some point today, but will move on in the next days. Fingers crossed we have a working 2720GS soon.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 19, 2023, 11:57:02 am
content: She's alive-branadic-

author: Kosmic
author_rank: Super Contributor
timestamp: July 19, 2023, 01:12:45 pm
content: Quote from: branadic on July 19, 2023, 11:57:02 amHo Nice!Good Job

author: TheSteve
author_rank: Unknown
timestamp: July 19, 2023, 07:57:33 pm
content: Show us the volts!

author: branadic
author_rank: Super Contributor
timestamp: July 20, 2023, 08:07:11 pm
content: Quote from: TheSteve on July 19, 2023, 07:57:33 pmJust for you Steve.-branadic-

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: July 20, 2023, 10:11:31 pm
content: Would be useful to see a recording as a function of time, to see if there are any spikes associated with cross-comparison, as I was seeing, and general level of drift and noise.

author: branadic
author_rank: Super Contributor
timestamp: July 23, 2023, 07:41:42 am
content: I performed some measurements yesterday and compared it to our 3458A, for which I had to estimate new correction coefficients, based on the history I have so far. Unfortunately, I don't have a valid baseline point for this year by now. A deaper expaination about how I did that and the results I've found so far coming soon in a full repair article on xdevs. However, I found that both instruments agree within <2 µV/V for 10 V, which would be pretty good.A fun project would be to replicate the reference card and equip it with ADR1399KHZ. There is a free slot 4 on the GS and I have copied the content of the NOVRAM to some spare parts I've bought, so I could populate it with the original drift history of another reference. I guess at the beginning this 4th reference would be rejected by the instrument, due to the fact that, compared to its drift history, the instrument would notice a sudden jump for thiis card. But I guess at some point, assuming constant calibration to an external trustworthy reference, it could accept the new reference board. What is the opinion on that?-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 23, 2023, 12:40:54 pm
content: 0.1 - 10 Hz noise measurement @ 10V with all 6x LM399 being active, so about 8 - 9 µVpp.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 30, 2023, 11:13:58 am
content: I made some progressReverse-engineering complete, schematic needs some clean-up though before I can post it. So the way of having a 4th reference board is created, but even ADR1399 upgrade is now possible and other sorts of things that come into mind.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 01, 2023, 07:48:07 pm
content: This is the best I could arrange with a limited amount of time.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 05, 2023, 09:34:27 am
content: A first reference board copy is almost good to go.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2023, 12:29:54 pm
content: The plan looks a bit odd in some places. Tr6 and TR7 may have gate and source swapped. Just the gate conneded to easch other makes little sense.

author: branadic
author_rank: Super Contributor
timestamp: August 05, 2023, 05:09:50 pm
content: You are right, there was an error in the lib, which resulted in swapped gate and source. Already updated the schematic.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 06, 2023, 08:06:03 am
content: Updated the schematic again, now everthing should be complete and correct, but schematic needs some re-arranging to make it more readable. It is consistent with the layout in the post before.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2023, 08:53:11 am
content: I think there is likely a link missing from the source of TR6 / TR7 to some output / connector. These 2 should be switches to choose between the 2 references.

author: branadic
author_rank: Super Contributor
timestamp: August 07, 2023, 09:41:06 pm
content: You are right, the path got lost when I updated the pinout of the transistors in the schematic the other day. Schematic and board image have been updated. I'm happy that the path was there when I've ordered the board, so it won't need a bodge wire-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 12, 2023, 07:08:42 pm
content: Schematic was cleaned up and updated. It is more readable now.Aluminum casing was hand over to me, I'm almost there to power up the board. The right oven setting is still unknown.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 13, 2023, 02:04:47 pm
content: It's done, the board is ready. It only took 4 bodge wires to fix some reverse-engineering errors I made. Below is the board and a comparison to an original reference board of the unit.The reference is working, but ...It has the NOVRAM content of reference 1. The memory content seems to contain some serial number, which causes an issue during boot sequence, once reference 1 and my copy cat are installed at the same time.If one of them is pulled, the instruments powers up without issues. So I need a NOVRAM with the content of a totally different reference board to get that board working with the rest of the references or need to find out which part of the NOVRAM content causes the conflict. Other than that it works as it should. I also tested with only my copy cat installed and get reasonable readings.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 14, 2023, 08:59:00 pm
content: Updated schematic with the information on how to adjust the oven settings:https://www.eevblog.com/forum/metrology/valhalla-2720gs/msg4991971/#msg4991971From the schematic we can now extract what to do for an ADR1399 upgrade, we need to replace R15 = 4.32k and add the output filter as well.-branadic-

author: martinr33
author_rank: Frequent Contributor
timestamp: August 17, 2023, 03:14:01 am
content: branadic, have you dumped the EPROM for the main 6809 processor? That might contain some clues.I have attached a zip file with converted data files and a short file containing the contents of each memory (without labels).BTW, congrats on a very nice piece of engineering on the reference board.Back in 6809 days, floating-point hardware did not really exist, and there were not many applications for it because MPUs were generally slow and simple. I've found some useful references to a 6809 floating point library thatmighthave been used. If not floating point, the system could have used 16 or 24 bit arithmetic (16 bit with a 65mV offset, which seems narrow, so 24 bit is more likely.)https://gtoal.com/src/asm-expand/https://jefftranter.blogspot.com/2019/04/a-6809-single-board-computer-mc6839.html

author: branadic
author_rank: Super Contributor
timestamp: August 17, 2023, 07:18:01 am
content: Hello martin,you missed the attachement, however, you find the EPROM content here:https://www.eevblog.com/forum/metrology/valhalla-2720gs/msg4873862/#msg4873862Front panel EPROM here:https://www.eevblog.com/forum/metrology/valhalla-2720gs/msg4884920/#msg4884920and all the rest here:https://www.eevblog.com/forum/metrology/valhalla-2720gs/msg4945894/#msg4945894-branadic-

author: martinr33
author_rank: Frequent Contributor
timestamp: August 20, 2023, 10:50:05 pm
content: Thanks, got it!I started the disassembly. The ROM is "only" half full, but the code is very dense with no obvious text blocks. So nothing much yet.Also, the data sheet states that the system stores 30-bit drift and calibration data in te local memory of each reference, so we are looking for patterns in 4-byte blocks.The data sheet also indicates that the system could update the data, which means that you might be able to track which bytes change, and from there eliminate some more memory locations. I think that the least significant bit in such a word might represent 10 nV (based on the 1e9 address space of 30 bits) , so we might have a way in there by looking for values that represent the 7+ volts or so that the reference produces.

author: PrecisionFreak
author_rank: Contributor
timestamp: August 21, 2023, 12:59:49 pm
content: After several emails with branadic, I registered here, finally.I started disassembling the AT27C256.BIN, also, and found some 48-bit constants at addresses 8264, 8BBE.And a list of GPIB-Commands, I think, at B14A:FDB comUD_A398FDB comPO_823EFDB comDR_9CB3FDB comRD_ADF4FDB comRP_AD33FDB comER_9C49FDB comFC_9AE1FDB comFD_9B19FDB comCS_9488FDB comTP_9C1FFDB comCT_84C4FDB comSW_9B63FDB comCM_9C56FDB comCL_9560FDB comCN_9591FDB comCA_9766FDB comCE_97CAFDB comIL_9CC0FDB comIE_9D31FDB comSC_9D7AFDB comIC_9D7FFDB comSZ_9C8FFDB comIL_9CC0My aim is to find the password / passphrase and to check if it is possible to read / write the nvrams in situ, in the 2720GS without opening it.Attached you find the files to generate a disassembly with BEEBDIS.I tried to work with IDA, but I think to disassemble 6809 I need the costy pro-version.Can we use Ghidra or Radare?martinr33, which disassembler did you use?maxwell3e10, do you have a better (printed?) copy of the manual? The photo 2720Transformer.jpg shows good resolution.It would be very helpful to understand the circuits, e.g. the signal names and IC-pins.Did anyone see hidden keys in the front panel? The manual shows a matrix with 8*7=56 keys, but only 45 are visible.See attached JPG for the missing 11 / 12. (Enter are 2 separate keys)There is a photo with all the displays / LED designators, too.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 22, 2023, 05:02:56 am
content: I did get a printed manual, but it does not seem to be much better in resolution than the online pdf version. If there is a particular place in the circuit you want me to test, I can scan a page or two.

author: branadic
author_rank: Super Contributor
timestamp: August 22, 2023, 07:13:29 pm
content: I started comparing the memory content of the NOVRAMs (removed the leading zero first) of the 3 references to find similarities between them but also similarities within each memory. I color coded the corresponding sections.-branadic-

author: PrecisionFreak
author_rank: Contributor
timestamp: August 22, 2023, 08:12:28 pm
content: Hi all, branadic, I did the same thing. But as long as it is difficult and time consuming to change RAM contents, what would you do?Does anyone have the 2720GS connected to a working GPIB-bus?I checked the commands, if you send a request SW, it should answer with the firmware version, here 61.Additionally to the beforementioned commands, there are also some System / Service / Internal Calls (adress table in 9D3C / 9D6A):comSC_9D7ALDX #tblSC_9D6ABRA L9D82comIC_9D7FLDX #L9D3CL9D82LDA <L005FCLR <L005Dwhich I am looking at.I hope to find commands to modify NVram contents in the device.maxwell3e10: Thank you for your time, pages 15 to 21 of 99 would be interesting, as they are poor in the pdf.There must be a manual with around 150 instead of 99 pages (including volume 1?), does someone have this?I updated the valhal2.zip-file with disassembly-listing.One thing to note: Valhalla connected the address-decoders 74138 and 74154 in an unusual way:the outputs are not addressed in linear order, because for example A15 is wired to IC4/1 (input A), not IC4/3 (input C), as one would expect.Update: Branadic, the oven adjustment must be -0.5V (ca. 53°C) for RV1, not -0.8V (71°C). RV2 is for overtemp, where 71°C are correct.Attached you find a simulation. I put the zener D7 to constant 69°C to get fixed 6.3V. The AD590 generates a current in µA proportional to absolute temperature (PTAT), at 27°C 300µA, which results in 5.8V at R7+R8=19320 OhmsI would run the reference at a lower temperature, e.g. 45°C, depending on ambient and internal device temperature.

author: branadic
author_rank: Super Contributor
timestamp: August 23, 2023, 05:43:02 pm
content: QuoteI simply don't know. My best chance is that someone else with a 2720GS shares his memory contents and I then have some NOVRAM chips programmed.Some MBM2212-20 arrived today, but there is also CAT22C10 available. I'm not sure if it makes sense to put an empty memory chip into the copy cat reference board and see how the instrument deals with thatQuoteYou are right, that was a copy&paste error. Corrected the schematic.-branadic-

author: Mickle T.
author_rank: Frequent Contributor
timestamp: August 23, 2023, 06:00:01 pm
content: Can I hope that someday we will see graphs of non-linearityand short-term instabilityof the 2720GS?

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 23, 2023, 07:46:23 pm
content: Here is for example page 19 of the service manual. One can't really see board markings.I have posted some initial stability plots shortly after turn-on.

author: dietert1
author_rank: Super Contributor
timestamp: August 24, 2023, 07:17:42 am
content: The memory dumps shown in "Ref1-Ref2-Ref3.jpg" and the more recent "MemoryComparison.png" are different, e.g last byte in second row. Why is that?Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: August 24, 2023, 01:09:12 pm
content: QuoteSeems like a typo with "0" instead of "9". Fixed.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 24, 2023, 03:09:31 pm
content: QuoteOne step after the otherI quickly tried fitting a reference board with an empty NOVRAM, but the instrument doesn't like that either and stops during boot phase.-branadic-

author: PrecisionFreak
author_rank: Contributor
timestamp: August 24, 2023, 05:37:19 pm
content: I think I found the default password (by chance after 1 hour lurking in the code), it was not that difficult. The sum of the digits is 31.What is it good for?I think, one can set a password to the cal-key additionaly. If you forget this password, you need the "universal key" master default pwd.Branadic, did the 2720 show any message with the empty NVRAM? I think it should show "REF 0 NO CALDATA" or "REF 0 LOST DATA " (replace 0 with the number of the board). At page 2 of the manual we find:---- snip ----REFERENCE MODULE OPTIONSOPTION # REF BDS LOCATIONSHSR 4 BDS. A, B, C, DGS 3 BDS. A, B, C54-4T 2 BDS. A, BALL REFERENCE BOARDS ARE TO BE COMPLETELY SOAKED ANDTAKEN FROM 2710 REFERENCE SYSTEM. ANY GRADE BOARDMAY BE USED IN OPTION 54-4T. GS OR HSR GRADE BOARDSMAY BE USED IN OPTION GS. ONLY HSR GRADE BOARDS MAYBE USED IN OPTION HSR. BOARDS MUST BE COMPLETELYINSERTED INTO CARD GUIDES.---- snip ----AFAIUnderstand, the 2710 reference system is their station to burn in and test the ref boards. And probably write to the NOVRAM.We have to look if and how they marked the grade on the boards.We could find HSR grade boards in lower grade units.COMPLETELY SOAKED could mean: washed with isopropanol thoroughly.Here is a listing of all 169 display messages (there are some more short strings):1 1 string $A470 16 ; "ILIM 13V 000mA "2 2 string $A481 16 ; "ILIM 130V 000mA "3 3 string $A492 16 ; "ILIM1200V 000mA "4 4 string $A4A3 16 ; "-- 100mA MAX ---"5 5 string $A4B4 16 ; "--- 30mA MAX ---"6 6 string $A4C5 16 ; "UPPER VOLT LIMIT"7 7 string $A4D6 16 ; "LOWER VOLT LIMIT"8 8 string $A4E7 16 ; "SETTING DEFAULTS"9 9 string $A4F8 16 ; "INT REF 0000DAYS"10 A string $A509 16 ; "DIVIDER 0000DAYS"11 B string $A51A 16 ; "INT CAL 0000DAYS"12 C string $A52B 16 ; "NULLMTR 0000DAYS"13 D string $A53C 16 ; "SETZERO 0000DAYS"14 E string $A54D 16 ; "EXT REF 0000DAYS"15 F string $A55E 16 ; "- VERSION 0000 -"16 10 string $A56F 16 ; "I/O ROM FAILURE "17 11 string $A580 16 ; " CLOCK FAILURE "18 12 string $A591 16 ; " I/O uP FAILURE "19 13 string $A5A2 16 ; "I/O RAM FAILURE "20 14 string $A5B3 16 ; "- IEEE FAILURE -"21 15 string $A5C4 16 ; " INPUT VOLTAGE? "22 16 string $A5D5 16 ; "DATA ENTRY ERROR"23 17 string $A5E6 16 ; "NOT WARM 00 MINS"24 18 string $A5F7 16 ; " DIVISION RATIO?"25 19 string $A608 16 ; "- CHANGE DATE? -"26 1A string $A619 16 ; "TEMPERATURE 00C"27 1B string $A62A 16 ; " TEMPERATURE ? "28 1C string $A63B 16 ; "- CAL ABORTED -"29 1D string $A64C 16 ; " CONNECTIONS? "30 1E string $A65D 16 ; "- AMMETER CAL -"31 1F string $A66E 16 ; " AMMETER CAL? "32 20 string $A67F 16 ; "PROVIDE 100ohm ?"33 21 string $A690 16 ; "- CAL FAILURE -"34 22 string $A6A1 16 ; " DRIFT ANALYSIS "35 23 string $A6B2 32 ; "-- USE DATA FOR DRIFT ANALYSIS? "36 24 string $A6D3 16 ; "TURN OFF CAL KEY"37 25 string $A6E4 16 ; "CALIBRATION END "38 26 string $A6F5 16 ; " ENTER 10V STD "39 27 string $A706 16 ; " ENTER 1V STD "40 28 string $A717 16 ; " ENTER 100V STD "41 29 string $A728 16 ; " ENTER 1000V STD"42 2A string $A739 16 ; "IEEE ADDRESS 00 "43 2B string $A74A 16 ; "NO A-D CONVERTOR"44 2C string $A75B 16 ; "- INTERNAL CAL -"45 2D string $A76C 16 ; " OUTPUT GAIN ? "46 2E string $A77D 16 ; " OUTPUT ZEROES? "47 2F string $A78E 16 ; "INT CAL COMPLETE"48 30 string $A79F 16 ; "----- PASS -----"49 31 string $A7B0 16 ; "----- FAIL -----"50 32 string $A7C1 16 ; "- CAL - KREFAV -"51 33 string $A7D2 16 ; "-- CAL -- KB2 --"52 34 string $A7E3 16 ; "-- CAL -- KA2 --"53 35 string $A7F4 16 ; "-- CAL - KA10 --"54 36 string $A805 16 ; "-- CAL KA100 --"55 37 string $A816 16 ; " CAL - DVM ZERO "56 38 string $A827 16 ; " CAL DVM SCALING"57 39 string $A838 16 ; " CAL - DAM ZERO "58 3A string $A849 16 ; "CAL - NULL ZERO "59 3B string $A85A 16 ; "CAL - NULLMETER "60 3C string $A86B 16 ; " CAL ¶5V ZERO "61 3D string $A87C 16 ; " CAL -MDAC ZERO "62 3E string $A88D 16 ; " CAL 650mV ZERO "63 3F string $A89E 16 ; "CAL 1300mV ZERO "64 40 string $A8AF 16 ; " CAL °65V ZERO "65 41 string $A8C0 16 ; " CAL ±3V ZERO "66 42 string $A8D1 16 ; " CAL 13V ZERO "67 43 string $A8E2 16 ; " CAL 26V ZERO "68 44 string $A8F3 16 ; " CAL 65V ZERO "69 45 string $A904 16 ; " CAL 130V ZERO "70 46 string $A915 16 ; " CAL 600V ZERO "71 47 string $A926 16 ; " CAL 1200V ZERO "72 48 string $A937 16 ; " CAL - EXT REF "73 49 string $A948 16 ; " NO SUCH MEMORY "74 4A string $A959 16 ; "--MEMORY--000---"75 4B string $A96A 16 ; "- ERROR LIMIT? -"76 4C string $A97B 16 ; "--- STANDBY? ---"77 4D string $A98C 16 ; "--- HI THERE ---"78 4E string $A99D 16 ; "--- HI THERE ---"79 4F string $A9AE 16 ; " RESET REQUIRED "80 50 string $A9BF 16 ; "REF 0 NOT FITTED"81 51 string $A9D0 16 ; "REF 0 NO CALDATA"82 52 string $A9E1 16 ; "REF 0 FAULTY "83 53 string $A9F2 16 ; "REF 0 NEEDS CAL "84 54 string $AA03 16 ; "REF 0 NEEDS DATA"85 55 string $AA14 16 ; "REF 0 SMALL ERR "86 56 string $AA25 16 ; "REF 0 OVEN HOT "87 57 string $AA36 16 ; "REF 0 LOST DATA "88 58 string $AA47 16 ; "REF 0 BEING USED"89 59 string $AA58 16 ; "EXREF NOT FITTED"90 5A string $AA69 16 ; "- EXREF FAULTY -"91 5B string $AA7A 16 ; "--- EXREF OK ---"92 5C string $AA8B 16 ; "- OPTION EXR1 --"93 5D string $AA9C 16 ; "- OPTION EXR7 --"94 5E string $AAAD 16 ; "- OPTION EXR10 -"95 5F string $AABE 16 ; "---- DVM OK ----"96 60 string $AACF 16 ; "-- DVM FAULTY --"97 61 string $AAE0 16 ; "-- AMMETER OK --"98 62 string $AAF1 16 ; " AMMETER FAULTY "99 63 string $AB02 16 ; "--- AtoD OK ----"100 64 string $AB13 16 ; "-- AtoD FAULTY -"101 65 string $AB24 16 ; "NO OPTn BAT/EBU "102 66 string $AB35 16 ; "LOW VOLT BAT/EBU"103 67 string $AB46 16 ; " BAT/EBU FITTED "104 68 string $AB57 16 ; "- NO NULLMETER -"105 69 string $AB68 16 ; "FAULTY NULLMETER"106 6A string $AB79 16 ; "- NULLMETER OK -"107 6B string $AB8A 16 ; " 13V AMP OK "108 6C string $AB9B 16 ; " 13V AMP FAULT "109 6D string $ABAC 16 ; " 130V AMP OK "110 6E string $ABBD 16 ; " 130V AMP FAULT "111 6F string $ABCE 16 ; " 1200V AMP OK "112 70 string $ABDF 16 ; "1200V AMP FAULT "113 71 string $ABF0 16 ; "- SYSTEM CHECK -"114 72 string $AC01 16 ; "--- DIGITAL? ---"115 73 string $AC12 16 ; "CHECKING DIGITAL"116 74 string $AC23 16 ; "---- AtoD's? ---"117 75 string $AC34 16 ; " CHECKING DVM "118 76 string $AC45 16 ; "CHECKING AMMETER"119 77 string $AC56 16 ; " CHECKING AtoD "120 78 string $AC67 16 ; "--- OUTPUT ? ---"121 79 string $AC78 16 ; "CHECKING 13V AMP"122 7A string $AC89 16 ; "CHECKING 130VAMP"123 7B string $AC9A 16 ; "CHECKING1200VAMP"124 7C string $ACAB 16 ; "-- NULLMETER? --"125 7D string $ACBC 16 ; "CHECKING REF SYS"126 7E string $ACCD 16 ; " NOVRAM FAULTY "127 7F string $ACDE 16 ; " OUTSIDE LIMITS "128 80 string $ACEF 16 ; "--- EXT REF? ---"129 81 string $AD00 16 ; "CHECKING EXT REF"130 82 string $AD11 16 ; "CHECKS COMPLETED"131 83 string $AD22 36 ; "-- REMOVE CONNECTIONS (NO TO ABORT) "132 84 string $AD47 16 ; "NO FAULTY PARTS "133 85 string $AD58 16 ; "REQUIRES EXT CAL"134 86 string $AD69 16 ; "DIVIDER OVEN HOT"135 87 string $AD7A 16 ; "-- DIVIDER OK --"136 88 string $AD8B 16 ; "REQUIRES INT CAL"137 89 string $AD9C 16 ; "INT CAL DATA OK "138 8A string $ADAD 16 ; "WARMUP IS 00MINS"139 8B string $ADBE 16 ; "SYSTEM ERR- MATH"140 8C string $ADCF 16 ; " CANNOT OUTPUT "141 8D string $ADE0 16 ; "- OUTPUT SLOW -"142 8E string $ADF1 16 ; " NO REFERENCES "143 8F string $AE02 16 ; "-- OVER TEMP --"144 90 string $AE13 16 ; " CANNOT CAL YET "145 91 string $AE24 25 ; "-- UNDEFINED SYSTEM ERROR"146 92 string $AE3E 16 ; " ENTER PASSWORD "147 93 string $AE4F 16 ; " WRONG PASSWORD "148 94 string $AE60 21 ; "-- NO DIVIDER NOVRAM "149 95 string $AE76 21 ; "-- NO INT CAL NOVRAM "150 96 string $AE8C 16 ; "-- UNDER TEMP --"151 97 string $AE9D 16 ; "--- CHANGES ----"152 98 string $AEAE 16 ; "-- SELF TEST --"153 99 string $AEBF 16 ; " SERIAL NUMBER ?"154 9A string $AED0 16 ; " CHECK REF SYS? "155 9B string $AEE1 22 ; "-- INCORRECT SOFTWARE "156 9C string $AEF8 16 ; " REPAIR DATA ? "157 9D string $AF09 16 ; "REFERENCE REPAIR"158 9E string $AF1A 16 ; " DIVIDER REPAIR "159 9F string $AF2B 16 ; " DVM REPAIR "160 A0 string $AF3C 16 ; " DAM REPAIR "161 A1 string $AF4D 16 ; " KB2 REPAIR "162 A2 string $AF5E 16 ; " KA2 REPAIR "163 A3 string $AF6F 16 ; " KA10 REPAIR "164 A4 string $AF80 16 ; " Ka100 REPAIR "165 A5 string $AF91 16 ; " ZERO REPAIR "166 A6 string $AFA2 16 ; " EXR REPAIR "167 A7 string $AFB3 16 ; "NULLMETER REPAIR"168 A8 string $AFC4 16 ; "----------------"169 A9 string $AFD4 36 ; "JANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDEC"

author: branadic
author_rank: Super Contributor
timestamp: August 24, 2023, 05:52:50 pm
content: QuoteNo, the system shows "Hi There" on both screens, then the lower display shows the firmware version "61.61" and then beeps. That happens with the copy cat reference having an empty memory, but also if the memory content is one of the other three references.That's why I believe that at least some content has to be present, like some serial number or the like.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 24, 2023, 06:12:11 pm
content: I wonder if it would help to start the instrument with the key in CAL position? Will give that a try tomorrow.QuoteQuoteQuoteQuoteQuote-branadic-

author: PrecisionFreak
author_rank: Contributor
timestamp: August 24, 2023, 07:32:57 pm
content: I just wanted to post this, but the session timed out, now it it too late ;-)I wonder what the following messages mean:156 9C string $AEF8 16 ; " REPAIR DATA ? "157 9D string $AF09 16 ; "REFERENCE REPAIR"158 9E string $AF1A 16 ; " DIVIDER REPAIR "159 9F string $AF2B 16 ; " DVM REPAIR "160 A0 string $AF3C 16 ; " DAM REPAIR "161 A1 string $AF4D 16 ; " KB2 REPAIR "162 A2 string $AF5E 16 ; " KA2 REPAIR "163 A3 string $AF6F 16 ; " KA10 REPAIR "164 A4 string $AF80 16 ; " Ka100 REPAIR "165 A5 string $AF91 16 ; " ZERO REPAIR "166 A6 string $AFA2 16 ; " EXR REPAIR "167 A7 string $AFB3 16 ; "NULLMETER REPAIR"What to do to come into repair-mode?By the way: Does't the 6803G have a serial interface (IC10/10, 11, 12)?Maybe the 2-letter commands I posted aren't GPIB, but serial commands.The two processors communicate serially via optocouplers (68A09 via IC10 ACIA 6850)Branadic, do you still have means to program the NOVRAMs?We could do a binary mix-search with two NOVRAMs:Board NOVRAM1 12 23 1/2 of 2 and 1/2 of 3In the case that a serial number is in the first half of the contents, it should reject board 3. Otherwise the serial number is in the second half.Then go on looking in a quarter of the contents and so on.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 24, 2023, 07:44:17 pm
content: One can also do a calibration, that would change some string of numbers. From what I recall, it saves the last several calibrations together with dates. I am not sure if there is a way to perform a calibration on only a single reference and then do it several times to replace all numbers except presumably the serial number.

author: branadic
author_rank: Super Contributor
timestamp: August 24, 2023, 08:11:36 pm
content: QuoteNo I don't, unless I get the TL866-II programmer running it.https://www.mikrocontroller.net/topic/556243#7448143QuoteI will try different things this weekend.-branadic-

author: PrecisionFreak
author_rank: Contributor
timestamp: August 24, 2023, 08:57:52 pm
content: Another possibility would be a permanently connected arduino (Mega256 with more i/o), that resets the processor 68A09 and takes over the bus. So you could change NVRAM contents very quickly and then release the bus to give back control to the µP. But take care of different Gnd-Levels, a battery-operated laptop would be best.

author: dietert1
author_rank: Super Contributor
timestamp: August 25, 2023, 05:56:43 pm
content: There is a service manual online and it has a schematic "Reference Module" (page 40 out of 95) that shows the permutation of the X22C12 address bits. PrecisionFreak already mentioned unusual addresses. After simulating the permutation, branadics three memory dumps looked less random.Apart from the fixed header there are subrecords (probably calibration numbers) tagged with 0014, 00E0, 00E8 and FFFF (tail). The subrecords are at the same position and have the same length in all three dumps. The serial number should be in the last subrecord tagged FFFF:FFFF 0000 FFFF 0000 FFFF,FFFF 0000 F7FF 0C00 FFFB andFFFF 040C F7FF 0C00 F7F7.An educated guess may give you a fourth valid example, e.g.:FFFF 080C F7FF 0C00 F7FBRegards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: August 25, 2023, 06:01:31 pm
content: Poor quality leads to pure misunderstanding. While it seems like the over temperature detection should be set to -0.8 V from the schematics in the service manual:https://xdevs.com/doc/Valhalla/2720GS/Valhalla_Scientific_2720GS_Vol.%202_sm.pdfthe operation and maintenance manual says set it 0.1 V higher than the oven setting (page 9-9ff)https://xdevs.com/doc/Valhalla/2720GS/Valhalla_Scientific_2720GS_Vol.%201_um.pdfQuoteThat means for 23 °C ambient temperature the voltages should be set to 0.44V and 0.54V respectively.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 26, 2023, 06:18:56 am
content: QuoteI assume you are refering to that part of the schematic? Would you mind sharing the rearrangend Hex code?As far as I understand the memory content was read in the address sequence 6,5,4,3,2,16,17,1 but 3,4,5,6,2,1,16,17 is needed to read the real content.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: August 26, 2023, 09:00:24 am
content: With the correct unmapping of address permutation i got:5A C5 C5 C1 C1 00 00 0000 00 00 08 00 EF 70 7900 E8 30 43 EA FB 00 E0F2 3D C7 BD 00 14 AD 8ED0 00 E8 30 CF 46 36 00E0 F2 70 55 E1 00 14 A38E A3 00 E8 30 CD 30 7B00 E0 F2 B6 DB 99 00 14AB 8E 22 00 E8 30 C3 AFC3 00 E0 F2 BB 68 B3 0014 AB 8E 48 00 E8 30 C289 A2 00 E0 F2 36 12 3400 14 AB 8E BB 00 E8 3041 67 A9 00 E0 F2 3A 3A77 00 14 AB 8E 9E FF FF04 0C F7 FF 0C 00 F7 F75A C5 C5 C1 C1 00 00 0000 00 00 08 00 EF 70 7900 E8 9C 7D BC 1F 00 E86B D8 27 DC 00 14 AD 8EE5 00 E8 9C FD 27 FA 00E8 6B 35 82 A1 00 14 A38E F0 00 E8 9C FE B9 DA00 E8 6B 3A 08 DA 00 14AB 8E EB 00 E8 9C F9 F5D5 00 E8 6B 3C F5 04 0014 AB 8E 79 00 E8 9C FA77 B5 00 E8 6B D3 61 7200 14 AB 8E 24 00 E8 9C77 F4 EA 00 E8 6B D6 F3E9 00 14 AB 8E FB FF FF00 00 FF FF 00 00 FF FF5A C5 C5 C1 C1 00 00 0000 00 00 08 00 EF 70 7900 E8 41 F4 DB 59 00 E866 3A D7 5A 00 14 AD 8E19 00 E8 49 09 B4 87 00E8 66 B6 DC CA 00 14 A38E 34 00 E8 49 0E 3E 0F00 E8 66 33 7F C7 00 14AB 8E 74 00 E8 41 F7 B3B2 00 E8 66 B9 F8 B3 0014 AB 8E AF 00 E8 41 F9DA B4 00 E8 66 3F 4F 4F00 14 AB 8E D1 00 E8 4172 53 65 00 E8 66 39 3487 00 14 AB 8E 57 FF FF00 00 F7 FF 0C 00 FF FBThe first example parses as follows:Fixed Header00e8 3 0 4 3 e a f b00e0 f 2 3 d c 7 b d0014 a d 8 e d 000e8 3 0 c f 4 6 3 600e0 f 2 7 0 5 5 e 10014 a 3 8 e a 300e8 3 0 c d 3 0 7 b00e0 f 2 b 6 d b 9 90014 a b 8 e 2 200e8 3 0 c 3 a f c 300e0 f 2 b b 6 8 b 30014 a b 8 e 4 800e8 3 0 c 2 8 9 a 200e0 f 2 3 6 1 2 3 40014 a b 8 e b b00e8 3 0 4 1 6 7 a 900e0 f 2 3 a 3 a 7 70014 a b 8 e 9 effff 0 4 0 c f 7 f f 0 c 0 0 f 7 f 7So there are 6 groups of 3 records plus the tail. The tail should have the different serial numbers.Regards, DieterPS: I corrected a minor confusion between address bits 6 and 7.

author: PrecisionFreak
author_rank: Contributor
timestamp: August 26, 2023, 10:38:06 am
content: In the 156 page manual all GPIB commands are listed on pages 75ff, there are some to read the calibration values: GCAL0 .. GCAL87get calibration constant0 .. 7 Reference 1 through 8 voltage at the time of the latest calibration (in volts).8 Reference Divider ratio (divider used for 0.65V, 1.3V, 650mV and 1300mV ranges). The data in nominally 100% and is the data at the time of the latest 1V external calibration.16 Reference averaging coefficient (100% nominal)33,35,37,39,41 Voltages of reference 1 at the times of previous calibrations (earliest to latest but one resp.)34 .. 72 ..... for all 8 references73 .. 77 Reference Divider rations at the time of previous calibrations79 .. 86 presently measured reference voltages (as obtained by the OBRMS system) in voltsAlso GSTA (get overall status) gives some data of the 8 references in the first 8 characters of the reply.I repeat my question: who has the possibilty to connect the device to GPIB?Branadic: could you please post the OCR-version of the Vol. 1_UM.pdf?

author: branadic
author_rank: Super Contributor
timestamp: August 26, 2023, 12:49:50 pm
content: Okay, here are some news. The unit is running with all 4 reference boards. It was "just" a matter of hitting the right buttons at the right time, no independant NOVRAM content required1. Started the instrument with the fourth board fitted with an empty NOVRAM. The unit stops during boot sequence showing firmware version.2. Pushed the SYS RST button, unit notice bad NOVRAM content3. with the CAL key in CAL position and pushing the hidden key (below Last Cal) the instrument is able to repair the "broken" memory content, once the secret password was enteredIMPORTANT: Whenever the unit asks "CONNECTION ?" it expects a connention between SENSE terminals and NULLMETER, otherwhise an error new to me appears that tells you "CANNOT OUTPUT". Afterwards the voltage output doesn't work anymore. Ask me how I knowIt took quite some time to figure out how to get away with that error, as it is not clearly described in the manual (missing image too).Meanwhile, I also adjusted all oven settings, which seems to have affected at least one of the references, but consequent Internal Cal should get away with it at some point once everything has stabilized again. That's at least what the manual tells you.QuoteNot here, not at the moment, need to prepare something first, as all our original Agilent GPIB-USB adapters failed. I wonder whyQuoteThe file is too large to post it here-branadic-

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 26, 2023, 01:26:38 pm
content: Quote from: branadic on August 26, 2023, 12:49:50 pmCan you share the password?QuoteThanks for figuring this out! I also had this problem and would go in circles doing self-tests until it concluded the Nullmeter is faulty and then work OK

author: PrecisionFreak
author_rank: Contributor
timestamp: August 26, 2023, 03:26:13 pm
content: As the secret was found by me, I will not release it directly, but will give hints every few days how to find it.One tip was given already: the cross sum (sum of all single digits) ist 31.Today's tip: it is in the front-board EPROMDieter, your tipps are valuable!But as I said, they often store 6-byte (48-bit)-values.Wasn't there a 6 byte floating point representation? in Turbo Pascal?My suggestion is: (my lines 5-8 are swapped with your lines 9-12, as A7 is swapped with A5, but Dieter's representation fits better)Freak 8 byte wise5A C5 C5 C1 C1 00 00 0000 00 00 08 00 EF 70 7900 E8 30 43 EA FB 00 E0F2 3D C7 BD 00 14 AD 8EAB 8E 22 00 E8 30 C3 AFC3 00 E0 F2 BB 68 B3 0014 AB 8E 48 00 E8 30 C289 A2 00 E0 F2 36 12 34D0 00 E8 30 CF 46 36 00E0 F2 70 55 E1 00 14 A38E A3 00 E8 30 CD 30 7B00 E0 F2 B6 DB 99 00 1400 14 AB 8E BB 00 E8 3041 67 A9 00 E0 F2 3A 3A77 00 14 AB 8E 9E FF FF04 0C F7 FF 0C 00 F7 F7END5A C5 C5 C1 C100 00 00 00 00 0008 00 EF 70 79 00E8 30 43 EA FB 00E0 F2 3D C7 BD 0014 AD 8E AB 8E 22 00E8 30 C3 AF C3 00E0 F2 BB 68 B3 0014 AB 8E 48 00E8 30 C2 89 A2 00E0 F2 36 12 34 D0 00E8 30 CF 46 36 00E0 F2 70 55 E1 0014 A3 8E A3 00E8 30 CD 30 7B 00E0 F2 B6 DB 99 0014 00 14 AB 8E BB 00E8 30 41 67 A9 00E0 F2 3A 3A 77 0014 AB 8E 9E FF FF04 0C F7 FF0C 00 F7 F7Dieter 8-byte-wise5A C5 C5 C1 C1 00 00 0000 00 00 08 00 EF 70 7900 E8 30 43 EA FB 00 E0F2 3D C7 BD 00 14 AD 8ED0 00 E8 30 CF 46 36 00E0 F2 70 55 E1 00 14 A38E A3 00 E8 30 CD 30 7B00 E0 F2 B6 DB 99 00 14AB 8E 22 00 E8 30 C3 AFC3 00 E0 F2 BB 68 B3 0014 AB 8E 48 00 E8 30 C289 A2 00 E0 F2 36 12 3400 14 AB 8E BB 00 E8 3041 67 A9 00 E0 F2 3A 3A77 00 14 AB 8E 9E FF FF04 0C F7 FF 0C 00 F7 F7Dieter 5 or 6-byte-wise5A C5 C5 C1 C100 00 00 00 0000 08 00 EF 70 7900 E8 30 43 EA FB00 E0 F2 3D C7 BD00 14 AD 8E D000 E8 30 CF 46 3600 E0 F2 70 55 E100 14 A3 8E A300 E8 30 CD 30 7B00 E0 F2 B6 DB 9900 14 AB 8E 2200 E8 30 C3 AF C300 E0 F2 BB 68 B300 14 AB 8E 4800 E8 30 C2 89 A200 E0 F2 36 12 3400 14 AB 8E BB00 E8 30 41 67 A900 E0 F2 3A 3A 7700 14 AB 8E 9EFF FF 04 0C F7FF 0C 00 F7 F7Branadic could read the new NVRAM contents, but he cannot, missing a proper reader. Once again, I can give you my Elnec Jetprog for some months.www.elnec.com/en/products/device-programmers/jetprog/But I understand that now, after his hard-gained success, the motivation for this may be < than small. ;-)

author: branadic
author_rank: Super Contributor
timestamp: August 26, 2023, 07:36:28 pm
content: QuoteHe could, but lags a parallel interface on his modern computers-branadic-

author: branadic
author_rank: Super Contributor
timestamp: August 28, 2023, 05:56:06 am
content: It is worth mentioning, that with the fourth reference board in place the averaged reference voltage has changed, leading to a shift of the output voltages and dropping out reference 2 and 7 (one of the references of my copy cat), which means the instrument now needs external calibration to account for that. Lucky enough I can use the ADR1399KHZ converted AN3200 and a 3458A as a first sanity check to calibrate the 10 V and the 1 V and a Vishay 100R resistor.QuoteQuoteQuoteQuoteQuoteMeanwhile, a second board with ADR1399KHZ was almost completed, it's just missing R15, which I currently try to figure out the required value.-branadic-

author: Messtechniker
author_rank: Frequent Contributor
timestamp: August 28, 2023, 06:09:20 am
content: Quote from: branadic on August 26, 2023, 07:36:28 pmThere are things like this: DELOCK 89445

author: branadic
author_rank: Super Contributor
timestamp: August 29, 2023, 05:39:40 pm
content: More HV transformers arrived today. I can already spot some happy owners and more working 2720GS.I tried calibrating (10 V, 1V, ammeter) my instrument today, which worked to some extend, but just a few minutes after adjustment with all reference being active the unit dropped reference 2 and 7 again. So I need to read the manual carefully again to spot any hints.-branadic-

author: PrecisionFreak
author_rank: Contributor
timestamp: August 29, 2023, 09:05:32 pm
content: New HV-transformers:I hope they corrected the colors of the leads from the windings to the soldering posts, green and violet/blue were interchanged!green is 115V, blue is 16VIf not, please inform them about it. They could use the correct colors of the primaries too, at least at the printing.Appended you find my suggestion of the label (read it with monospaced (courier) font).If you see the colors and connect the mating wires, the 16V circuitry will see 115V and not be happy about it.Be careful and measure!At the very first transformer the printed on description differed from the actual winding / connection.Do you still have this transformer? Maybe only the printing was false.Or was the sequence 16 16 0 115 115? Even then it could be rewired (separate the enamel wires at the soldering posts).I suggest to use separate primary fuses (2 because of 115 parallel mode) for the HV-transformer.And / or fusible 1k 10W resistors R201, R202 at the 1100V secondary.Could you please measure the Ohms and Henries of the windings?I am not sure if the 26 / 52 ohm is the 115v to 16V or 115 to CT (center tap).It could be 22 /44 ohms.Where are the two shields E9 and E10 connected to?I think they are connected to each other, but see a wire on the solder side of the mainboard.Where does it lead to?General hint:Units that saw no voltage for longer periods of time should be powered up slowly, via a bulb or a variac with ammeter and sensitive fuse over several hours.About the new references: Probably they should burn in for some dozend hours before a new calibration.Sorry for the lost time.Tip of the day: digits are grouped by 2.

author: branadic
author_rank: Super Contributor
timestamp: August 30, 2023, 05:33:51 pm
content: I today repeated calibration, but this time denied the drift analysis facility. First, all references were in use, but after some time ref 2 and some time later ref7 were dropped again. I will continuously retry external calibration, hopefully to be able to get all of them in use at some point in time.Another solution would be to convert all of them to ADR1399 and start with empty memories.-branadic-Quote

author: branadic
author_rank: Super Contributor
timestamp: September 01, 2023, 06:47:28 am
content: I've updated the schematichttps://www.eevblog.com/forum/metrology/valhalla-2720gs/msg4991971/#msg4991971with the correct signal path designators (+HS that is +18V unreg. with 1N4001 diode, +RS that is +6V, -ZS, -HS that is 16 VCT), that were barely readable in the scans. I still can't make full sense out of the voltage controlled current source, supplying both LM399. Hence, I'm still stuck to what is needed to convert the reference boards into ADR1399.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: September 01, 2023, 07:47:05 am
content: The current source use D10 (1N827) as a voltage reference and than R16 and R18 to set the current to a relatively high value of some 6.2 V / 2.94 K , so a bit over 2 mA each. The circuit uses the JFETs as drivers - a more conventional circuit would use PNPs or P-channel MOSFETs at the same place.So the current should be high enough for the ADR1399 already and likely no change needed - just the added RC element for damping. C5 and C6 may already to that job though the ESR may not be that well defined.

author: branadic
author_rank: Super Contributor
timestamp: September 01, 2023, 07:57:06 am
content: I meanwhile measured the referene voltages, which to some extend could explain why the unit is dropping Ref2 and Ref7. Maybe Valhalla Scientific selected references being as close as possible together? Need to check, whether or not I have some LM399 left, that are way closer to the rest of the references.However, if that is the reason I'm very surprised. I would have expected that during external calibration the unit calculates the mean for each reference combination being in use, so that the absolute value of each reference is not of interest and could deviate largely from the others.RefVoltage Deviation from mean [µV/V]Comment16.9405-1223.226.892-8202.636.93-2734.246.921-4029.456.9045-6403.866.919-4317.277.05815685.7Copy Cat Reference87.02711224.6Copy Cat Referencemean 1-6: 6.918 Vmean 1-8: 6.949 V-branadic-

author: branadic
author_rank: Super Contributor
timestamp: September 01, 2023, 08:02:02 am
content: Thanks for that explanation, Kleinstein. LM399 only requires 1 mA, while ADR1399 requires 3 mA, that's why I was concerned.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: September 02, 2023, 12:03:51 pm
content: So I've checked all of my stock LM399, but couldn't find any that are close to the average of the existing ones inside the 2720GS. Hence, I decided to measured 12 ADR1399 that I have at hand:RefVoltage 16.93226.93936.915546.891556.90566.90976.93686.94396.9355106.9355116.933126.943replaced Ref7 and Ref8 with ADR1399-1 and ADR1399-11 and added the filter on RefBoard-4.I left the instrument on for quite a while, but by now the instrument didn't complain. Seems, like it feels way more comfortable with that choice.Next step is to replace the rest of the references with ADR1399 and add filter too. Since we know that it could deal with the reference values for Ref2 and Ref 5, which are on the lower side, I plan the following combinations:RefBoard-1: ADR1399-8 and ADR1399-12RefBoard-2: ADR1399-2 and ADR1399-7RefBoard-3: ADR1399-9 and ADR1399-10This way they are as close as possible together with the references I have at hand.-branadic-

author: PrecisionFreak
author_rank: Contributor
timestamp: September 02, 2023, 01:28:06 pm
content: > but this time denied the drift analysis facility.It's clear to me, all things must settle first (wait 90 days after calibration): page 94 of 156:8.1.5 Drift AnalysisAfter the 2720GS has been in use for some time (typically 3 to6 months) the references and divider resistor networks will havefully stabilized to the environment and any remaining drift ratewill be both small and also linear with time. Thus the calibratorof the 2720GS can select for the change in calibration from theprevious data to be used for "drift analysis" (only if more than90 days has elapsed since the previous calibration was performed).With this enabled the drift rate is calculated and the value updatedevery day to reflect the expected data at that time. If morethan one pair of data points are available for drift analysisthen a multi-order curve fit routine is used to forward extrapolatethe data in time. The resultant value of this is the value usedby OBRMS to compare each references' deviation from the averagethus even if a reference is consistently drifty then the 2720GSwill “learn” its' drift rate and automatically correct for it.And page 103 of 156:9.3.8 Request if to Use Drift AnalysisIf the previous calibration was performed over 90 days previouslyand the temperature was within 3C of the present temperature,then the 2720GS can, if desired, perform drift analysis betweenthese data, and forward extrapolate this drift rate into the future.The 2720GS will always request if the user wishes to perform thisactivity. The user is not advised to utilize this facility inany of the following circumstances :a) If the 2720GS is under a year old.b) If the previous calibration was obtained on different equipmentc) If either calibration is known to contain errors.d) If the previous calibration was obtained by a different procedure.e) If any component in the 2720GS has been changed or alteredsince the previous calibration.And I wondered how they would do the averaging, as all the active references are summed via the 10k00 0.1% resistors.So the average voltage must be calculated from the active reference's actual voltages, I think.What if you drive the ADR1399 with less current so they come in the LM399's range?Or adjust their voltage via their temperature? ;-)

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2023, 02:51:14 pm
content: For the references there is the analog average via the 10 K resistors. In addition there are individual paths via JFET swiching. So the instrument should be able to measure the difference to the average for all the references.Changing the current to the reference would not change much - they are made to have a very low R_out. Adjusting the temperature is also no really an option with the very low TC due to the local fixed heater. There is only a tiny range by changing the heater supply and this way indirectly the temperature.The reference voltage don't look that bad, so I would not expect the absolute value to be a problem. of cause adding different values to the average will also shift the difference for the old references. So one may have to disable the drift check for all the references, not just the new ones. For the initial phase there is also a chance that the reference are actually drifting and this might even be enough to be detected - though there should at least be room for the +-4 µV typical popcorn steps to be inside the accepted range.

author: branadic
author_rank: Super Contributor
timestamp: September 02, 2023, 04:19:10 pm
content: As I said, my assumption is that the two 7.xxV LM399 I'd fitted to reference board 4 were way too far off for the system (see image below). Once I've replaced them by selected ADR1399 the instrument was happy and put all of the references in use.Meanwhile, all reference boards were upgraded with the aforementioned ADR1399 references and again the instrument appears to be happy and put them all into "in use" status. So this is the first ever Valhalla Scientific 2720GS/HSR-All-ADR1399 in the worldThe instruments now needs to relax and the references to settle, before we can perform any reasonable calibration. But first noise measurements can be performed tomorrow, I guess. This way we can see were we are.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: September 02, 2023, 06:32:20 pm
content: Quote from: Kleinstein on September 02, 2023, 02:51:14 pmIn my experience the ADR1399 does not exhibit popcorn noise when used properly.When looking at the Valhalla schematic i found the 10 KOhm averaging resistors bad. In my LM399 arrays i have been using 274R resistors and i think one should replace the 10K resistors by 1K. I think they used 10K since it is a 2-wire circuit, but one doesn't need full precision for the averaging coefficients.Regards, Dieter

author: maat
author_rank: Regular Contributor
timestamp: September 02, 2023, 09:31:56 pm
content: Quote from: dietert1 on September 02, 2023, 06:32:20 pmI wouldn't go that far in saying, but they are considerably better than the LM399. So far out of the 10 diodes I am testing two show popcorn noise (see attachment, its channel 6 and 9). This is way better than the 2/3 quota of the LM399 though. I will post more details in the ADR1399 thread when I get to 1000 h (500 h so far).

author: dietert1
author_rank: Super Contributor
timestamp: September 03, 2023, 05:50:13 am
content: Apparently there is something wrong with your test. I would not wait another 500 hours before trying to find out what may be wrong.When i got the ADR1399KEZ (SMD package) evaluation kit, there were several things wrong. But at least i did not have to think about soldering temperature profiles and the like. And they had the board cutout that everybody thinks is for thermal reasons. Meanwhile i think it is important to avoid mechanical stress. With the KHZ device (TO-46-4 package) it's easy to avoid mechanical stress by bending the four wires in S shape. A reference soldered into a board with straight wires cut to 5 mm will deteriorate. Another one is fixing the thermal cap against movement. Then the RC filter.Regards, Dieter

author: ch_scr
author_rank: Frequent Contributor
timestamp: September 03, 2023, 11:26:28 am
content: Quote from: branadic on September 02, 2023, 04:19:10 pmTo automate such selection tasks in general, I've made up a python script. With the example at hand, it looks like this:Code:grouping_optimizer.py 8 6.932 6.939 6.9155 6.8915 6.905 6.909 6.936 6.943 6.9355 6.9355 6.933 6.943Average value 6.93712Error sum 3928.14Average error 491.02 ppmWith these elements and corresponding errors:(6.932, 6.939, 6.936, 6.943, 6.9355, 6.9355, 6.933, 6.943)['-738.78', '270.28', '-162.17', '846.89', '-234.25', '-234.25', '-594.63', '846.89']It can also be asked for a specific target value as well:Code:grouping_optimizer.py 8 6.932 6.939 6.9155 6.8915 6.905 6.909 6.936 6.943 6.9355 6.9355 6.933 6.943 --target 6.918Average value 6.92519Error sum 15394.62Average error 1924.33 ppmWith these elements and corresponding errors:(6.932, 6.9155, 6.905, 6.909, 6.936, 6.9355, 6.9355, 6.933)['2023.71', '-361.38', '-1879.16', '-1300.95', '2601.91', '2529.63', '2529.63', '2168.26']As you can see, this one works happily with multiple elements of the same value. It can also show the iterations and handle priority elements much like theresistor divider ratio optimizer.Hope someone finds it useful.

author: branadic
author_rank: Super Contributor
timestamp: September 03, 2023, 08:02:45 pm
content: I performed some noise measurements today. Strange, I can see some low frequency hum at 2.3 Hz and 12 Hz, the source of it is yet unknown. Maybe the ovens? The service manual indicates, that in some revision C7 was added across R10 = 10 M. None of my boards has this capacitor installed. Looking at page 27 of the service manual we find 1 µF across the 10 M of the output system oven controller. So maybe this is something I should add to my boards too. Also C8, most likely 100 nF, was added between pin 4 and 8 of IC2 later, which is missing on my revision C? boards.We can compare the noise to ADR1399 converted AN3200, which is way quieter.I found that the clock is running a bit too fast. The clock, displayed on the lower display, is ahead by a few minutes every day.QuoteSeems like this is some old explanation, as Y1 a 4 MHz crystal in the schematic, while Y101 is a 32.768 kHz crystal. The test points are nevertheless there, so I will check them next and adjust C105 (not C8) accordingly.I also did my first GPIB communication with the instrument for the "GCAL" - Get CALibration constantCode:0 through 7 - Reference 1 through 8 voltage at the time of the latest calibration (in volts).+6.94570030+6.94534459+6.93843745+6.94167089+6.93890978+6.93802343+6.93563261+6.934024638 - Reference Divider ratio (divider used for 0.65V, 1.3V, 650mV and 1300mV ranges). The data in nominally 100% and is the data at the time of the latest 1V external calibration.+099.9873819 - Nullmeter scaling coefficient (1.073% nominal)+000.00395110 - Nullmeter high range zero offset (1.19% nominal)+000.00449311 - External reference scaling coefficient (14.39/100/143.9% nominal for the EXR1/7/10 respectively).12, 13 - Internal DVM. scaling and zero coefficients respectively (0.119% and 0.085% nominal resp.)+000.000221+000.00015414, 15 - Internal ammeter zero and scaling resp. (0.119% and 0.098% nominal resp.)+000.000226+000.00018316 - Reference averaging coefficient (100% nominal)+100.00032017 - 2:1 sense buffer scaling (for 1300mV,1.3V,13V,26V,130V and 1200V ranges) (100% nominal)+099.99891818 - 2:1 sense attenuator scaling (for 26V range) (100% nominal)+100.00127419, 20 - 10:1 and 100:1 sense attenuator scaling resp. (100% nominal)+099.993863+100.00323121 - Negative polarity DtoA convertor offset (0% nominal)+000.00000022 through 32 - 650mV, 1300mV, 0.65V, 1.3V, 6.5V, 13V, 26V, 65V, 130V, 600V and 1200V range zero offsets in voltage units+0.00000000+0.00000000+0.00000000+0.00000000+0.00000000+0.00000000+0.00000000+0.00000000+0.00000000+0.00000000+0.0000000033, 35, 37, 39, 41 - Voltages of reference 1 at the times of previous calibrations (earliest to latest but one resp.)+0.00000000+0.00000000+0.00000000+0.00000000+6.9500000034, 36, 38, 40, 42 - As above for reference 2+0.00000000+0.00000000+0.00000000+0.00000000+6.9500000043, 45, 47 ,49, 51 - As above for reference 3+0.00000000+0.00000000+0.00000000+0.00000000+6.9500000044, 46, 48, 50, 52 - As above for reference 4+0.00000000+0.00000000+0.00000000+0.00000000+6.9500000053, 55, 57, 59, 61 - As above for reference 5+0.00000000+0.00000000+0.00000000+0.00000000+6.9500000054, 56, 58, 60, 62 - As above for reference 6+0.00000000+0.00000000+0.00000000+0.00000000+6.9500000063, 65, 67, 69, 71 - As above for reference 7+0.00000000+0.00000000+0.00000000+0.00000000+6.9500000064, 66, 68, 70, 72 - As above for reference 8+0.00000000+0.00000000+0.00000000+0.00000000+6.9500000073 through 77 - Reference Divider ratios at the time of previous calibrations (earliest through latest but one resp.)+000.000000+000.000000+000.000000+000.000000+100.00000078 - User set Nullmeter zero offset (by SET ZERO command) in volts.+0.0000000079 through 86 - Presently measured reference voltages (as obtained by the OBRMS system) in volts+6.94570657+6.94533921+6.93843947+6.94166848+6.93891909+6.93802837+6.93563030+6.9340194487 - Presently measured reference averaging system error (as obtained by the OBRMS system) in %+100.000307-branadic-

author: branadic
author_rank: Super Contributor
timestamp: September 03, 2023, 09:09:23 pm
content: By the way, does anyone have one of the options, such as EXR1/7/10? Would be nice to get some images and the NOVRAM content... yes, another one-branadic-

author: branadic
author_rank: Super Contributor
timestamp: September 04, 2023, 07:14:42 am
content: This morning I've added 1 µF X7R 0805 caps between pin 1 and 2 of the corresponding opamp on all of the boards and yes, the oscillation vanished. So definitively something to add, if not already populated.-branadic-

author: PrecisionFreak
author_rank: Contributor
timestamp: September 04, 2023, 11:58:50 am
content: Very very good, branadic! To adjust the RTC real time clock: variable capacitor C105 on page 9 of 99.Thank you for the GCAL values. BTW, I found three missing GPIB-commands not mentioned in the manual:string $9672 4 ; "SRNG"set range? set random generator?string $96FC 4 ; "GSEP"get separator?string $9703 4 ; "GTRM"get terminator?About storing voltages in NOVRAM: results fromhttps://www.omnicalculator.com/other/floating-point(the results are not that exact !?! 64bit should be 401BCCCCCCCCCCCD, not 401BCCCCCCCCCC00)the hex representation of e.g. 6.95 is:Single-precision (32-bit) floating point6.95 converted to 32-bit floating point is01000000110111100110011001100110 (40DE6666H) with:Sign: 0Exponent: 10000001Fraction: 10111100110011001100110The real value of this representation is 6.94999980926513672.The machine error resulting from this conversion is 1.90734863281250010587911840679e-7.Double-precision (64-bit) floating point6.95 converted to 64-bit floating point is0100000000011011110011001100110011001100110011001100110011001101 (401BCCCCCCCCCC00H) with:Sign: 0Exponent: 10000000001Fraction: 1011110011001100110011001100110011001100110011001101The real value of this representation is 6.95000000000000018.The machine error resulting from this conversion is 1.77635683940025046467781066895e-16.Still missing is the 6byte (48bit) representation as used in "real"-type in turbo pascal.https://moddingwiki.shikadi.net/wiki/Turbo_Pascal_Real(they count the 8 bits from 0 (MSB) to 7 (LSB), not from 7 to 0 as usual).

author: branadic
author_rank: Super Contributor
timestamp: September 06, 2023, 06:03:57 pm
content: I today adjusted the 32.768 kHz of the RTC using our 12digit, GPSDO driven frequency counter, but the clock still runs too fast.Even if I adjust the frequency to the lower adjustment range the clock runs too fast, which makes me belive that during the instrument being turned on, the clock is driven from the 4 MHz not from the 32.768 kHz. Could that be?-branadic-

author: PrecisionFreak
author_rank: Contributor
timestamp: September 06, 2023, 07:16:43 pm
content: You mean, after turning on, the µC reads the RTC and then the clock runs software-driven?The same happens in every? PC. You could be right. There are timer-driven interrupts.What happens if you read the clock via GPIB (GCLK get clock), which one is asked, hardware or software clock?Will the RTC be re-read after a Reset? 37 RESE 7.6.1.1 Perform Device Clear> The real time clock is provided with a 32.768KHz reference clock> by the crystal Y1 and is adjusted by means of C8 (the 32.768KHz> should be monitored between TP1 and TP2 (GND)).> The microprocessor obtains its clock from an internal oscillator> controlled by the 4MHz crystal Y1 and the capacitors C2 and C3.Change the values of C2 / C3?> 11.5.1.3 Interrupt Routines> There are two sources of interrupt to the I/O uP in the 2720Gs.> These are :> i) Real Time Clock Interrupt> This interrupt is generated by the real time clock and occurs> every 500uS (approx.). The uP uses this interrupt to multiplex> the displays and also for timing functions. The front panel> RESET key is actioned within this routine also.Ah, IC102/19 (MC146818) generates the IRQ of IC10/5?? (MPU MC6803G)This means the 32768 Hz Xtal is the source of RTPlease measure this frequency (2048 Hz?)Datasheet of MC146818 can be found at page 3-697 (741 of 876):http://www.bitsavers.org/components/motorola/_dataBooks/1983_Motorola_8-Bit_Microprocessor_and_Peripheral_Data.pdfGenerated interrupts can have various sources. See page 3-712 (756 of 876).

author: branadic
author_rank: Super Contributor
timestamp: September 09, 2023, 09:02:44 pm
content: From what I can observe the output noise is mostly dominated by varying low frequencies of the oven controls, not by the noise of the references, the DAC or the output stage, which is limiting the performany. Every now and then one of the controls kick in and ruin the output voltage with low frequency oscillations. Not sure how to improve on that. Thermal insulation?-branadic-

author: alm
author_rank: Super Contributor
timestamp: September 09, 2023, 10:06:51 pm
content: Quote from: branadic on September 09, 2023, 09:02:44 pmDo you know if the interference is thermal (temperature fluctuations) or electric (e.g. the oven controller modulating the voltage rails)?

author: dietert1
author_rank: Super Contributor
timestamp: September 09, 2023, 10:36:27 pm
content: As far as i understand the original circuit would work in on-off mode like a common refrigerator. Adding a capacitor into it makes it an I-controller. In general it would still not be stable until you also add a P-term for damping. Only then you get the desired PI-controller. I posted a schematic here that contains a simple recipe.https://www.eevblog.com/forum/metrology/thermal-fluctuations-on-pcb/?action=dlattach;attach=1719803To arrive at a good solution one can test and tune the dynamics injecting some test signal (external error signal). E.g. a rectangular wave with a period of one or two minutes. Then you can see whether the oven controller has critical damping. As an example the image shows such test results for a 50 Hz digital PLL that i implemented for an integrating multimeter. Here the lock phase changed every 20 seconds.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: September 10, 2023, 07:13:55 pm
content: QuoteI simply can't answer the question at the moment, but have attached measurements at different times, showing all sorts of situations. The instrument is sitting in a temperature controlled lab (23 ± 1°C).I always wondered, why the instrument is specified with 30 µVRMS wideband noise (10Hz to 10 kHz) in 13 V range, no matter what version and how many reference boards you have (2720GS/54-4T, 2720GS, 2720GS/HSR). First I thought the noise limit is coming from the amp, but the schematics doesn't lead to the assumption that the noise is that high. If on the other hand the limits are given by the temperature variation, causing way larger output voltage variation that could explain the specs.One thing I could do is adding some non-conductive foam to the ADR1399s inside the oven and see what happens. That is not too much of an effort, as I'm quite busy at the moment.QuoteAs you can see from the images below, we have all sorts of situations, so in case you are right about that I wouldn't expect situations with "low" noise in between, or would I?I only added, what was given as a modification on board revision B in the service manual, so the missing 1 µF as well as 100 nF at the supply rails.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: October 01, 2023, 11:36:58 am
content: First INL measurement wasn't necessarily successful as instrument switched between rangesNeed to figure out how I can force the instrument to stay within the very same range. Someone already suggested to test setting the output voltage and use INCR command instead. I will give that a try.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: October 04, 2023, 04:25:16 pm
content: Seems like even ADR1399 can exhibit popcorn noise. During noise measurements I found, that one of the references installed to one of the reference boards, is jumpy.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: October 09, 2023, 10:45:16 am
content: For a few days I was now monitoring the repaired, upgraded and converted 2720GS with 3458A and want to share first results. The 3458A is continuously powered and performs ACAL each morning auto-magically.We can clearly see, that it took the unit quite some time to stabilize after I performed some noise measurements with the lid of the instrument removed and the LNA directly connected to the reference boards (2x5 pin header at the edge of each reference board).-branadic-

author: alm
author_rank: Super Contributor
timestamp: October 09, 2023, 10:54:21 am
content: Thanks for showing the data. Looks like it's been stable within roughly 0.5 uV/V over the past days?Could you plot voltage against temperature using whichever temperature sensor works better, excluding the part where you did the noise measurements? That might give a decent measure of temperature coefficient (of the entire setup) if other factors like drift aren't too large. If drift is a factor, then do it over only the first days when there were more temperature fluctuations.

author: branadic
author_rank: Super Contributor
timestamp: October 12, 2023, 07:36:49 pm
content: Not at the moment, I would like to observe the reference a bit longer to separate drift before.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: October 30, 2023, 03:53:34 pm
content: Here is a first update with 670 h of stability data measuring the 2720GS/HSR-ADR outputting 10 V with a 3458A.At some point I was brave enough to hit the internal cal button of the 2720, which lead to the jump of the output after about 260 h. Before that a slight increase in output voltage was already visable. That's also about the time when we had some experiements running in our lab, which lead to increased lab temperature during the day, making it hard to barely see anything as the t.c. of the whole setup was hiding additional details. However, while these experiments are now over and the lab is getting quieter we can again see some slight increase of the output voltage.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: October 30, 2023, 04:19:57 pm
content: As far as i remember you put ADR1399 references into that calibrator. Then a drift of 1 or 2 ppm over the first 600 hours isn't that unusual. There seems to be some correlation with the temperature peaks though, more apparent after the calibration. Might be a problem of the meter. I mean one would better compare to another calibrator.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: October 30, 2023, 04:54:49 pm
content: Yes I did put ADR1399 into the unit. I'm just reporting results since these were LT marked ones. To be honest, I expected way more drift in the first 600 h, but also opposite direction.-branadic-

author: Kosmic
author_rank: Super Contributor
timestamp: November 05, 2023, 10:42:57 pm
content: I got a package from GermanyThe transformer crossed the Atlantic and survived. Still smelling fresh varnish!Thanks branadic for all your efforts, much appreciated!

author: Kosmic
author_rank: Super Contributor
timestamp: November 05, 2023, 10:48:42 pm
content: As for my 2720GS, before I install the transformer, I want to backup all the X2212 on the reference boards. I'm currently working on a programmer based around a cheap STM32F103. More info later.

author: PrecisionFreak
author_rank: Contributor
timestamp: November 08, 2023, 03:05:50 pm
content: Hi Kosmic, please be careful with the colors of the wires, the isolation hoses of 16V of the replacement are green (gr for german grün), but the 16V wires of the original trafo are blue and vicy versa. Seehttps://www.eevblog.com/forum/metrology/valhalla-2720gs/msg5034961/#msg5034961After lot of dicussion with Langer, they agreed to change this to the correct colors in future orders.Also the three shield windings have the same color, but the one which shields the primary winding should be connected to chassis (PE), but the other two go to board Pins. How to distinguish? Langer did not understand this.Also I promised to give tips for the secret password: The numbers are in eeprom adress range 07xx of the front board (TMS27C256-FrontPanel-2720_V61.1_8_18_88.BIN). Remember the other tips! The first person to post the secret will be the champ of the day.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: November 08, 2023, 04:49:48 pm
content: Well, this makes it easy: 25 60 02 12 58

author: Kleinstein
author_rank: Super Contributor
timestamp: November 08, 2023, 05:04:00 pm
content: A capacitance measurement should tell which shield is close to which winding: e.g. the shield to PE should have the largest capacitance to the primaries.

author: branadic
author_rank: Super Contributor
timestamp: November 08, 2023, 09:58:18 pm
content: The primary winding is the inner one, so you can tell which of the screens is the inner one too (the closest to the core), which has to be connected to earth. The other two screens are physically tied together on the mainboard, so it doesn't make a difference to which solder point you are connecting them.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: December 30, 2023, 07:16:39 pm
content: Here are results of my Valhalla2720GS-HSR-ADR1399 running for almost 2200 h.One might be suprised by the bad behavior, but that can be explained. The measurements suffered from EMI of what I called a scream cube (a 1 m³ fine particle filter that was running near one of our test benches) nearby. This scream cube was switched off 11 days ago and we can see how temperature in the lab stabilized and output noise went down to a reasonable level since then. We can also see some jumps that resulted after performing internal cals of the Valhalla.Since the scream cube will not be turned on again, the related project has ended, I guess it makes sense to repeat the noise measurements, but also to further observe how stability turns out from here.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: April 14, 2024, 08:21:13 am
content: An update of my Valhalla 2720GS/HSR-ADR1399. I'm approaching 5000 h of operation after my repair, modifications and upgrade. Since removing the "scream cube" from the lab the extreme spikes disappeared and never showed up again by now.At some point I had to readjust the oven settings of my copycat reference board 4, as the reference board was continiously turned off by the unit due to an "oven high" failure. The components and thus the voltages had drifted quite a bit. Since I made the adjustment the board was never turned off again.What I observe are some large intermittent deviations, which I think are due to the units internal intercomparison of the references, as they appear at least at around 12:00 pm. I can tell, as ACAL of 3458A happens every day at 8:00 am, hence the gap in the data.The location of the equipment near the AC in the middle of the room is far from ideal and the south side of the lab with its mirrored glass window front isn't perfect either, but it is how it is. There is clearly some temperature depence that needs to be ruled out.I also received data of an INL measurement of another 2720GS against a Datron 1271 from some fellow voltnut that I attach. Again, results are disappointing given the claims Valhalla made: "The 2720GS delivers the performance required to calibrate the world's most accurate 7.5 and 8.5 digit DVM's yet does even more."-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 13, 2024, 04:09:35 pm
content: As already stated, I replaced all 6x LM399 references in my unit and also equiped my copycat reference board with them too, so a total of 8x ADR1399. Unfortunately, noise of the instrument didn't improve as I have hoped for. So I looked into the schematicshttps://xdevs.com/doc/Valhalla/2720GS/Valhalla_Scientific_2720GS_Vol.%202_sm.pdfand noticed IC202, IC315 and IC4 in the chain, all three of them being ICL7652CPD. So I today performed the exercise of removing them one after the other, replaced them with a socket and fitted a single AD4523-1 in each position, with the following outcome:IC202: noise didn't change at allIC315: noise went up by a factor of ~2.5IC4: noise went up by a factor of ~10The instrument is now back on ICL7652 and noise is back where I started (8-9 µVpp at 10 V output, 0.1 - 10 Hz). The only thing that was successful today, while I was at it I tightened the Cal keyswitch.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: July 13, 2024, 09:30:55 pm
content: The ADA4523 is a rather extreme case: low voltage noise, but really high current noise and thus not suitable for more than some 10 K of source impedance. In addition the ADA4523 may be more picky about supply decoupling and a bootstrapped supply and has a slight chance to oscillate. The higher speed could also be an issue. A more moderate replacement would be a LTC2057 or AD8638.The ICL7652 are not very low noise by todays standard, but should still be way better than 8 µV_pp for a 0.1-10 Hz band. It is still not clear how the noise was measured and especially the upper BW limit.A point to check could be the noise with different voltage settings. E.g. with the output set to 0 V still is the upper range should not include ref. noise. One could also look at the DAC output directly, without the output amplifier/buffer.

author: dietert1
author_rank: Super Contributor
timestamp: July 13, 2024, 10:55:45 pm
content: You may have a look here once more:https://www.eevblog.com/forum/metrology/valhalla-2720gs/msg5040739/?topicseen#msg5040739.Regards, Dieter

author: branadic
author_rank: Super Contributor
timestamp: July 14, 2024, 05:45:05 am
content: QuoteOutput set to 10 V, measured with a 0.1 - 10 Hz LNA, 80 dB amplification by pipelie.QuoteThanks, that is something I could look at.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: July 14, 2024, 03:33:39 pm
content: QuoteDid that, but without a change in terms of noise at the 10 V output (13 V range that is). Haven't yet checked the 6.5 V range, but this sort of proofs that the noise is generated somewhere in the chain after the references have been averaged.-branadic-

author: chuckb
author_rank: Frequent Contributor
timestamp: July 15, 2024, 05:42:24 pm
content: The data sheet for the ICL7652 lists the current noise at 0.004 pa / rt Hz. This number is most likely not a measurement. It may be a shot noise calculation based on Bias current.The LTC2057 looks like a good replacement with 0.2 uVpp noise. I have not calulated the output noise from the opamp current noise of 0.12pa / rt Hz. But the LTC2057 has the lowest current noise, of the low voltage noise chopper opamps.The ouput noise could be from residual PWM current spike noise or another grounding issue. My latest project took several revisions to get the grounding correct. Just going from memory I believe the Datum 4910 PWM filters had more sections.Good luck!

author: Kleinstein
author_rank: Super Contributor
timestamp: July 15, 2024, 08:17:44 pm
content: There are OP-amps with less current noise than the LTC2057. It still has a good combination of current and voltage noise, especially among the higher voltage ones.Chances are the OPA186 should be lower current noise (at least that is what the data-sheet says), but is also higher voltage noise.The ICL7652 should be low current noise, maybe not all the way to 4 fA/sqrt(Hz), but it may not be that much worse.Current noise specs with AZ OP-amps are anyway not very reliable: quite some false ones, calculated from the bias current and even the real value can depend on the conditions of use. There is a good chance to have correlation in the current noise for the 2 inputs - so a single number is not enough to describe it fully and there are different ways (resistor at one side of both) to have the resistance to test.The reported noise still looks higher than expected for the ICL7652 - so there are likely other sources.There are different stages that could be followed to see if the higher than expected noise is alread there at the reference, the PWM DAC or only at the output stage. Ripple in addition to noise a possibilty.

author: branadic
author_rank: Super Contributor
timestamp: July 17, 2024, 06:28:11 am
content: The 0.1 - 10 Hz at the 6.5 V output appears to be similar to the noise of the 10 V output (13 V range). So one of the things to look at is the noise of the averaged reference voltage.Someone suggested to stop the DAC by pulling the jumper PL201 connected to the 16.777216 MHz clock at the reference generator.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: September 14, 2024, 04:23:34 pm
content: Here is an update:At some point I stopped my long-term observation (LongtermStability.png) and adjusted all oven temperatures down to 45 °C. From there, I started a new observation (LongtermStability2.png). The huge peaks are from me entering the unit and adjusting the "oven hot" alarm levels.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: January 20, 2025, 06:53:39 pm
content: Attached an update of the Valhalla 2720GS/HSR-ADR1399. After adjusting all the 5 internal ovens to 45 °C and over temperature alarms the unit is now rather stable and almost all of the intermittent peaks have now vanished too.-branadic-

author: faraday
author_rank: Regular Contributor
timestamp: February 13, 2025, 09:15:43 pm
content: There is 5x this units in France for sale. I would really like to have one of thishttps://www.ebay.co.uk/itm/115950380650?_skw=valhalla+scientific&itmmeta=01JKV8NA368CDP26CZHM4B0DD0&hash=item1aff2e666a:g:h9wAAOSwYUJlNXJ1&itmprp=enc%3AAQAJAAAA8HoV3kP08IDx%2BKZ9MfhVJKmhe1gBMu%2FHftxww9HDX14R4qJ3gJK1UazCwIXR3T%2BQYJiByCrL8Y1WVg9couy0tFVDxPbkRRDMZociZaS4jLRA2BsgJ03pb1w%2BN5X5deX0N%2Bye6FnryY%2Fszv8i4H1rSFWazpM%2F9roSrAyvCUV%2Faw%2Fc4DwfSicPrZgu16FIL7%2BbBleDBWyPZjGv9ISNBhxueGcn153N%2Fa0Zp2LVDDeyDJZtbnSlVY03tttUhRYbvlJAhFwU0xzbNnK4zbcMZYNSCcBFk%2Ba8mg4yZP6hoHTDiI6JPxT0EFzapFcW%2BMqcGx1scw%3D%3D%7Ctkp%3ABk9SR9yh1eieZQ

author: picburner
author_rank: Frequent Contributor
timestamp: February 14, 2025, 05:50:17 am
content: They have been for sale for quite some time and the seller has not yet been able to sell them.The problem is only two of them seem complete (from what I can see in the photos), the other three are missing pieces.Before the seller asked 1500 eur for shipping, now he ships for free...

author: branadic
author_rank: Super Contributor
timestamp: February 14, 2025, 08:11:08 am
content: The offer is totally overpriced and the seller does not accept lowball offers.In best case, you can make 2 working units with what is present. One HV power supply board is missing to increase the number to 3, but it contains a special toroidal transformer, so nothing you can replicate easily.It's sad, but unless the seller asks for a real market value I think they will stay with him.-branadic-

author: dietert1
author_rank: Super Contributor
timestamp: February 14, 2025, 08:39:24 pm
content: It's hard to tell from branadics last post, but the stability appears to be marginally sub-ppm (observed sigma maybe 3 uV in 10 V or so). Considering the amount of work he put into the device, that's somewhat disappointing. But i neither have that Valhalla calibrator nor a Fluke 57xx to get a serious comparison.Regards, Dieter