Marconi 2022E RF signal generator repair and refurb - Page 1
Forum section: electronics / testgear

author: bd139
author_rank: Super Contributor
timestamp: July 22, 2022, 05:04:31 pm
content: Thought I should probably write this up in a dedicated thread rather than let it be lost in the noise of the TEA thread.Bought a Marconi 2022E from scotttrade12 on eBay for £100 delivered. This was advertised as dead, which it indeed was. Well packaged, accurately described. Good job.Has NO STD on some masking tape on the top. More to follow...

author: bd139
author_rank: Super Contributor
timestamp: July 22, 2022, 05:10:03 pm
content: After some dissasembling and testing, the root cause was found. Someone nicked the OCXO / TCXO module out of itThat's what NO STD means clearly.According to the service manual this drives the synthesizer and CPU.

author: bd139
author_rank: Super Contributor
timestamp: July 22, 2022, 05:19:11 pm
content: Some reverse engineering later, the missing OCXO pins were identified and a 10MHz TTL level signal was injected into the synthesizer boardTemporary bench set upIt boots and everything appears to work panel-wise. No idea about the RF situation yet.Next steps are to build a replacement OCXO module. There is a pot on the rear of the unit which gives an adjustment range on the ADJ pin noted on the board and there are +5V and +12V rail supplies on the OCXO footprint. I will ID which of these is supposed to supply the oven power and go from there. I've measured the footprint out and ordered 5x CTI OCXO modules from Aliexpress. I will produce an adapter PCB for it in Kicad and order from JLCPCB.Aliexpress link:https://www.aliexpress.com/item/1005003277492150.htmlStay tuned... in about a month and a half

author: Zenith
author_rank: Frequent Contributor
timestamp: July 22, 2022, 06:34:38 pm
content: STD, an unfortunate abbreviation which has another meaning.I thought you were being unduly pessimistic about the possible state of this. It has a nice sharp display.If this uses the standard Macaroni OCXO of that era, they are a dodgy number, especially at 30 years old. If it had come with one, it wouldn't have been as cheap and you would probably have had to change it anyway.A right result so far. I shall follow the thread with interest.

author: bd139
author_rank: Super Contributor
timestamp: July 22, 2022, 06:43:08 pm
content: STD may be appropriate for some hardware I’ve used (Telequipment I’m looking at you).Better to have disappointment elevated than happiness destroyedDesigning PCB now. I have a Kicad footprint for the OCXO module after building another one for an HP unit a couple of years back.Oh investigating the images suggests that the unit is 1989 vintage but the synthesiser board is 1994 vintage which is weird. Possibly factory replacement. The old synth board has a very naff crystal oscillator installed usually apparently. There aren’t even footprints for this on the board.For the money I’m very happy. I prefer to buy broken stuff cheap and fix it.

author: artag
author_rank: Super Contributor
timestamp: July 22, 2022, 08:18:44 pm
content: Isn't there an external std input on the back ? The INT EXT button switches to it. Should have made testing easy.While you've got it in bits, consider replacing the front panel buttons. They seem to die early and require heavy pushing, or bounce excessively. Replacements are available on ebay. I don't know whether it's worth replacing the lot, or just any that are faulty. You'll be kicking yourself if you get the osc working, put it back together, and then find the buttons are dodgy when you're using it in anger.I think you got a bargain - these can go for quite a lot of money, as do all RF generators. I've also had an item or two from that source. He seems willing to sell broken devices for properly low prices, which, as someone who likes to repair things both for the entertainment and the money-saving aspects, suits me very well.I will replace the buttons on mine (a 2022, no C, D or E) sometime and would also like to add an HPIB interface.You may findhttps://groups.io/g/Marconi-Test-Instrumentsuseful if you need information.

author: bd139
author_rank: Super Contributor
timestamp: July 22, 2022, 08:34:35 pm
content: I tried that originally. Unfortunately the CPU doesn't run if the internal oscillator is dead or missing so you can't select the external reference. And it doesn't actually use that for the computer side of things anyway, only the frequency synthesizer.Front panel buttons seem reliable. I will replace them in the future once I've checked it out.Thanks for the groups.io link - didn't know that existed. Already member of the HPAK and Tek onesDid some sleuthing and managed to construct a schematic for the original OCXO module that was fitted to the 2022 series. This had the same part number as their frequency counters and it appeared in the manual. Had to unfuck the Mauritron scan which I include here for reference:This suggests that the OCXO heater was run from the 12V rail and the oscillator from the 5V rail. I will look at options here as the OCXO replacement I have is 5V supplied.

author: artag
author_rank: Super Contributor
timestamp: July 22, 2022, 08:49:29 pm
content: Unfortunately you can't use the famous HP10811 as it wants a 20V oven supply. And is possibly too big anyway.Gerry Sweeny describes some comparable OCXO options here :https://gerrysweeney.com/diy-hpagilent-53131a-010-high-stability-timebase-optionApparently another point of failure are the small RF modules, OM345 and OM360. Seehttps://www.vintage-radio.net/forum/printthread.php?t=96747They can also be found on ebay - at present there seem to be some original OM360 and a pcb to substitute the OM345 with a MMIC. Mine haven't failed (yet).

author: bd139
author_rank: Super Contributor
timestamp: July 22, 2022, 09:20:12 pm
content: Yes definitely too large. Done the OCXO rodeo a few times now. Think I've got through about 6-7 of them now including 2x HP ones, couple of Blileys and a box of cheap CTI ones.But this is one different little box. The thing is phase locked to any external standard you plug in to avoid noise issues.The CTI 10MHz ones are 5x for £10.56 delivered from China and will drop in here. However they are a little hungry at 600mA heat and 250mA run currents. If I supply at 12V I've got to blow off 4.2 watts on a 7805 during heating and 1.75W steady state. If I supply at 5V I've got to pull 600mA off the 5V line while it's heating and 250mA when active. The 12V OCXO consumes max 400mA.Did the thermal calculations on sticking it on the 12V with a 7805 in there and it'd exceed junction temperature on the heat cycle with a heatsink that would fit in the space so that option is eliminated.VCTCXO is possibly an option phase locked to an external standard with the CTI in it. Those are cheaper to run...

author: xrunner
author_rank: Super Contributor
timestamp: July 22, 2022, 09:35:27 pm
content: Looks like you're on a good path forward. I found the thread on the one I had, dated back to 2015. I didn't remember what all I did to it, but I did find this -QuoteThe reason I wanted to know it was because the fan was making a lot of noise and I had to replace it. Be interested in how many hours your unit has (I don't remember how to check the register but it's in the manual).

author: bd139
author_rank: Super Contributor
timestamp: July 22, 2022, 09:39:01 pm
content: I will check it when I bring it up next time

author: bd139
author_rank: Super Contributor
timestamp: July 22, 2022, 11:09:35 pm
content: After some research and discussion elsewhere this may be a Frankenstein box. The synthesiser board may be from another 2022 series box.Thus another option is now appearing which is to build a replacement crystal oscillator module to restore the original 2022E oscillator and phase lock it to an external OCXO. This is a reasonable solution and allows me to build a GPSDO and distribution gubbins which goes nicely with the top floor balcony of my new apartment

author: bd139
author_rank: Super Contributor
timestamp: July 23, 2022, 08:20:49 am
content: Reverse engineered power supply. 5V limits at 2.7 amps. Investigating idle power draw next. Convenient 0.1 ohm sampling resistor in circuitPushing it. 5V option discarded. 12V option discarded. Final option is build internal crystal osc and discipline it externally.

author: AVGresponding
author_rank: Super Contributor
timestamp: July 23, 2022, 08:50:52 am
content: Is there space for something like an LM323K on a small heatsink, instead of a 7805?

author: bd139
author_rank: Super Contributor
timestamp: July 23, 2022, 08:56:35 am
content: It's not so much the space but the fact that it has to go somewhere or the junction temperature will be too high resulting in kapoof of the regulator. Also you have to drop 7 volts at 600mA which is about 5 additional watts during heating and then 1.75 additional watts when warm which is all being dumped into a sensitive box full of RF synthesizerAfter doing a KT of all the options, actually building a replacement crystal oscillator for it and phase locking that to an external GPSDO via the ref input seems to be the most sensible solution here with the least number of risks or surprises. I can do this for about £4 too.Plus as mentioned it give me the excuse to futz around and build a GPSDOI can't find any other differences between the AA1/1 and AA1/3 board doing a full inspection of both other than some additional decoupling and the missing crystal oscillator.

author: AVGresponding
author_rank: Super Contributor
timestamp: July 23, 2022, 09:08:52 am
content: Looks simple enough I guess. Is the pcb layout available?

author: bd139
author_rank: Super Contributor
timestamp: July 23, 2022, 09:15:05 am
content: It's in the 2022E SM but I will just dead bug one on some FR4. I may do a proper PCB if I can be bothered with it down the line but I want to prove the idea works first and check the entire unit over properly before I throw too much money and time into it.I need to order an N to BNC adapter anyway and Rapid seems to actually have all the bits I need for once. Will recycle some of the parts out of that HP create you pointed me at as wellOrdered - Rapid:10x 120pf NP0 ceramics10x 22pF ceramics2x ACT 10 MHz crystals (30pf loading to match the original)1x RF solutions N to BNC adapter100x 100nF X7R ceramics (for stock)100x 1N4148 (for stock)Ordered - Langrex:3x BB809 varactorsBang goes another 20 quid

author: AVGresponding
author_rank: Super Contributor
timestamp: July 23, 2022, 09:35:59 am
content: Glad you have some 150MHz transistors in stock, and didn't throw them out with the recycling!

author: bd139
author_rank: Super Contributor
timestamp: July 23, 2022, 09:39:32 am
content: Don't even go thereAny generic NPN will do this job so usual suspects 2n3904 / 2222 / BC547 etc are fine. I will reuse the Motorola 2N2222A's extracted from the 3776A for this only because I quite like metal can transistors

author: Fraser
author_rank: Super Contributor
timestamp: July 23, 2022, 10:14:46 am
content: I own a three versions of the venerable Marconi 2022 RF sig Gen.These Sig Gens are still sought after due to their compact size and decent performance. The most common fault on themis failure of one of the Philips OM series gain block hybrid amplifiers. Those hybrid amplifiers were common and relatively inexpensive in the 1980's. They were cheap enough to be used in TV aerial booster amplifiers as a gain building block component. Sadly their relative rarity in the modern world has made them somewhat more expensive. I have a 2022 that has suffered failure of many front panel button switches and that is a project that I will get around to doing when time permits.For information, the 2022E model was specified by the UK MOD who needed a tactical field use signal generator which did not have a long settling time before producing a stable output frequency. The OCXO was ditched and a TCXO employed to meet that fast set-up and use requirement. For the MOD application precise frequency accuracy was less important than the speed with which the unit became frequency stable. I personally like my 2022E as it is portable, fast to use and its accuracy is more than enough for most tasks that I have for it. If not, I connect a small Quartzlock 2A off-air frequency standard to it that uses the BBC Radio 4 198KHz carrier as its reference.From memory I own two 2022's, one 2022C and one 2022E. I was going to sell one or two but could not bring myself to do it as I have used these a lot in my career and I have a soft spot for them. I sold my bulky Marconi 2019 RF signal Generator instead.I believe that I may also have the service manual for the 2022E model, but sadly I am many thousands of miles away from home for a while so cannot check.Good luck with the restorationFraser

author: bd139
author_rank: Super Contributor
timestamp: July 24, 2022, 09:54:53 am
content: Thanks for the info - very interesting. I'm actually writing up all the notes people are providing and will publish a web page about it. There seem to be lots of little bits of information but nothing in one place so far.As for the original oscillator here I can see the design trade off that was made and it makes sense. I supposed the MoD were using it to work on typical FM stuff at the time so frequency precision isn't as important as say narrow bandwidth. And yes the OCXOs take a while to warm up. The previous set up I had was a 5334B with OCXO module and you had to leave that running for at least 30 minutes for it to be usable.So I got ahead of myself this morning and designed a PCB for the basic oscillator module.

author: Zenith
author_rank: Frequent Contributor
timestamp: July 24, 2022, 11:29:28 am
content: This is interesting because I have a Marconi 2019 which I use a lot. When it came the frequency it produced was way below what it indicated and the adjustment was at its upper limit.I found that someone on the WWW had opened the OCXO and traced the circuit. I recall there's was in a 2955 receiver test set. The circuit is similar to your OCXO, but with different transistors and varicap at least. I opened the OCXO and tweaked the preset cap quite a way, maybe 1/6th turn, and tried it. The adjustment multiturn preset could now bring it onto the correct frequency.It's accurate to less than 1ppm and drifts slightly over time. I don't know what the spec is off hand, but that seems somewhat disappointing for an OCXO. It takes a fair time to settle down properly. Realistically, it's more than accurate enough for what I use it for after it's been on for five minutes. I suppose we all have Frequency Counter and GPSDOitis these days.Since the 2019 works, I'm not inclined to start messing with it for no good reason. However it does occur that the OCXO is a dodgy number; it must have a lot of hours on it and it's likely been pulled well beyond its design limits. I wouldn't be surprised if it died or became unusable and I had to replace it. Finding one out of a scrapper wouldn't be easy and it would probably have the same problems, so making one up from new parts would make more sense.

author: xrunner
author_rank: Super Contributor
timestamp: July 24, 2022, 12:18:39 pm
content: Quote from: bd139 on July 24, 2022, 09:54:53 amThat looks nice. Come to think of it (I wish I still had mine but I sold it to get my Agilent 8648A) one of the problems I had at first was the ext. ref input - when I tried to use it nothing happened. It just wouldn't act like it had any stimulus from the ext. ref. As I said reflowing solder joints fixed that issue too. I followed the ext. ref. input and found where it stopped and I re-flowed solder connections in the area. That fixed it. I got off easier than you are doing.

author: bd139
author_rank: Super Contributor
timestamp: July 24, 2022, 12:44:09 pm
content: Quote from: Zenith on July 24, 2022, 11:29:28 amI had a 2019 for a while a few years back. Very nice bit of kit if a little large. Paid £5 for it from some ex EE who bought a sports car and was running off to the south of France with some floozie and just wanted rid of it! (slightly jealous). Bargain of the century that was. I'm not going to get into the demise of that unit here because that's a whole thread on its own but I will say that the previous owner was a bodger and it lead to more than 5V landing on the 5V supply which basically machine gunned everything in the box connected to the railAll OCXOs have a lifespan unfortunately. The crystals and/or other components will drift out of specification over time. I had that with a Racal standard a few years back with a 9444/12 in a 9915. I built a replacement for it and bypassed the frequency multiplier circuitry. Details here including photos of insides of the Racal unit:https://www.eevblog.com/forum/testgear/repairing-off-frequency-racal-9442-ocxo/The ex cell tower OCXOs on aliexpress etc have plenty of adjustment room yet. Plus also 5 for £10 it's probably worth just buying some to leave around as ornamentsActually I did a PCB for the Marconi as well and one of these OCXO's. That might be usable for your 2019 if it's the same standard. Not quite as elegant as the VCXO board and will require some pin trimming and spacers on the build but should work.It replaces the original standard which looks like this:

author: bd139
author_rank: Super Contributor
timestamp: July 24, 2022, 12:45:22 pm
content: Quote from: xrunner on July 24, 2022, 12:18:39 pmI'd rather have an 8648A but they don't turn up for £100 and some futzing hereI was expecting a fight with this one. It's not as bad as it could have been. At least it looks like it's recoverable

author: Zenith
author_rank: Frequent Contributor
timestamp: July 24, 2022, 12:59:50 pm
content: Quote from: bd139 on July 24, 2022, 12:44:09 pmThanks. I'll bag some of those. I've got enough stuff that uses OCXOs and I think it's only a matter of time before the one in the 2019 cashes its chips in.

author: bd139
author_rank: Super Contributor
timestamp: July 28, 2022, 06:02:53 pm
content: Non trivial update finally.At lunch today I threw together the prototype crystal oscillator replacement module based on the original schematic. This was tested with 0-5V which gave a pull range of 9.999800 to 10.000100 MHz. The pull range is +/-12VThis was crudely tacked into the 2022E. No prizes for mounting here.Booted straight up and the CPU runs fine. The phase lock is a bit dicky so might need some more debugging but we at least have a working frequency reference.When you poke EXT it complains about error 11 (bad reference) and occasionally error 12 (cant lock). Which is a lie because it's clearly working as shown here:The game is however not over. It does output a signal up to 250MHz but at 10dBm output it's about 15mV and on top of that it's well off frequency. That would possibly suggest that there's a problem with an OM345 somewhere in it as it's clearly not kicking out enough wumph to drive the output amplifier and/or synthesizer divider. Will wait until I've got the final oscillator board in for that.Found a nice debugging feature though. 2ND+3 then enter register number and you can shift binary values. I used this to test the attenuator drivers and relays to make sure that they were operating independently which they were.

author: bd139
author_rank: Super Contributor
timestamp: July 29, 2022, 03:07:23 pm
content: Going to need some OM345's for this so have designed a board. This uses the MiniCircuits MAR-7SM+. Unlike other boards there will be a different mounting method which will involve inserting a pre-trimmed 5-way header pin into the edge of the castellated board. The pins can then be soldered in while still on the header plastic which is then removed. This is then inserted into the board and soldered underneath. This avoids some of the mounting issues identified with other approaches. Disclaimer: might be worse. Also the module is the same size as the original one.

author: xrunner
author_rank: Super Contributor
timestamp: July 29, 2022, 05:02:34 pm
content: When I bought mine I researched that issue before it got to me, but luckily I didn't have to replace any.Does the board manufacturer populate your board for you? I noticed some of them will do that.

author: bd139
author_rank: Super Contributor
timestamp: July 29, 2022, 05:48:47 pm
content: I will order the PCBs from JLCPCB. They don’t sell the mini circuits part so will have to order those from mouser and hand solder. Will get the other two SM parts at the same time.I need to do a trial fit of the pins first which I will do when they turn up ….Did some debugging earlier and confirmed that OM345 IC10 is indeed dead. You can ID the dead ones because the input pin one goes to 0V and the output to 12V as the feedback network is broken.I’m actually going to order 30 boards as that’s the cheapest JLCPCB increment band and sell the rest on eBay

author: Fraser
author_rank: Super Contributor
timestamp: July 29, 2022, 05:49:23 pm
content: You may find this page helpful…https://ea4eoz.blogspot.com/2017/09/repairing-om345-hybrid-module.htmlDo remember that the OM345 is a 75 Ohm input and Output impedance and the MAR series MMIC’s are 50 Ohms.Fraser

author: bd139
author_rank: Super Contributor
timestamp: July 29, 2022, 05:50:46 pm
content: Yeah have been through that already. I did a calculation on 75 ohm loss and VSWR and it’s trivial at 50 ohms. It’s actually better than the original OM345 input VSWR quoted at 1:2.0The gain, noise figure are slightly better. Not sure about OIP3. I’ve seen that quoted but no data to back it up. Doing feedback amp calculations against transistor circuit with same topology I couldn’t get better than 20dB which is about as good as the MAR-7SM+.RF isn’t an exact science or magic

author: djsb
author_rank: Super Contributor
timestamp: July 29, 2022, 06:19:42 pm
content: I have one of these as well. Are you going to share your PCB files or the link to where you're selling any PCBs? I'd also be interested in any webpage/blog with additional info you may create. Thanks.

author: bd139
author_rank: Super Contributor
timestamp: July 29, 2022, 06:34:54 pm
content: I will write up and share everything once I'm sure it's all tested and workingEdit: just ordered enough parts to build 11x replacement OM345 modules. I'll replace my broken ones, keep some in reserve for to replace the rest of mine when they blow up and stuff the rest on ebay as kits. I can get 30x PCBs for the same as 5x so will over order and sell the remaining PCBs too. Will be sold at cost. Don't need the profit!Edit 2: perhaps I should lose the Ferengi avatar

author: bd139
author_rank: Super Contributor
timestamp: July 29, 2022, 08:07:15 pm
content: Meant to post this. RF board photo:

author: Fraser
author_rank: Super Contributor
timestamp: July 30, 2022, 07:58:32 am
content: bd139,Are you taking Gold Pressed Latinum in payment for your boardsFraser

author: bd139
author_rank: Super Contributor
timestamp: July 30, 2022, 08:21:20 am
content: As long as it’s not worthless gold

author: bd139
author_rank: Super Contributor
timestamp: July 30, 2022, 12:52:18 pm
content: Redoing the OM345 PCB without castellated edges as i didn't know they were an additional cost option

author: factory
author_rank: Super Contributor
timestamp: July 30, 2022, 03:02:44 pm
content: Would lead frame connections be better than the pin header connectors?Example:https://www.digikey.co.uk/en/products/detail/te-connectivity-amp-connectors/1544210-2/4731536David

author: bd139
author_rank: Super Contributor
timestamp: July 30, 2022, 03:35:54 pm
content: That’s a a great idea actually. Thanks for the suggestion. Those also won’t fall out when soldering!

author: George Edmonds
author_rank: Frequent Contributor
timestamp: July 30, 2022, 06:53:50 pm
content: HiJust a quick word.The ones used on the AA4 board in a 2955 are critical for overall dimensions due to the limited space available.PLEASE use soft pins for the legs, the last ones I used could not be cut with small side cutters, I had to use piano wire cutters to cut them, not a good idea.G Edmonds

author: G0HZU
author_rank: Super Contributor
timestamp: July 30, 2022, 07:14:44 pm
content: Note that the reverse isolation of a typical MMIC won't be quite as good as the original OM345. This might not matter in some locations in the 2022 sig gen but it probably does matter for the IC1 location at least.The OM345 reverse isolation is typically 22dB at about 100MHz and the MAR7 manages just 18dB here. The 22dB reverse isolation of the OM345 may already not be enough to prevent some FM modulation occurring on the AM for frequencies up to about 62.5MHz. IC1 acts as an isolation buffer between the AM modulator and the 160MHz VCO. I've checked several 2022 sig gens and they all suffer from a bit of unwanted FM when AM is selected below 62.5MHz. This includes the ones at work and these all have the original OM345 fitted. There's a good chance the MMIC replacement for IC1 will increase the amount of unwanted FM that occurs here.I designed a high reverse isolation alternative for IC1 but never got around to trying it out. This had about 50dB reverse isolation at 160MHz.

author: factory
author_rank: Super Contributor
timestamp: July 31, 2022, 12:47:51 pm
content: Quote from: bd139 on July 30, 2022, 03:35:54 pmThey were used in stuff at work, no-one knew what they were and some boards we had made (supposed to be an exact copy) they fitted right angle pin headers, they were a pain to remove, after we found the boards didn't work (eventually I found the problem).Many many years later, some of the leadframe connections turned up in the TEA thread and I asked to find out what they were. Seems there is lots of choice for, pitch, size & board thickness.David

author: bd139
author_rank: Super Contributor
timestamp: July 31, 2022, 12:50:43 pm
content: Quote from: George Edmonds on July 30, 2022, 06:53:50 pmThanks for the heads up. I located a picture of AA4 in the 2955 and it looks like my replacement module would fit in there.As for the soft pins, agreed. See my later reply from user factory which I haven't written yet as I post this

author: bd139
author_rank: Super Contributor
timestamp: July 31, 2022, 01:03:12 pm
content: Quote from: G0HZU on July 30, 2022, 07:14:44 pmInteresting. I will work out how to measure that when it comes to it. The 2022E is not a particularly wonderful generator specification-wise so I'm not overly bothered about some side effects as long as I am aware of them. I'm not going to be using this as a modulation source however so the modulation generators will be off all the time. I'm intending using it for very basic CW RF measurements such as MDS, return loss and occasionally VHF/UHF filter measurement. My AWG already makes a fairly decent RF generator with some exceptions.I saw your post on the designs on vintage radio forum as well.I actually looked at the W7ZOI termination insensitive amplifier as a design basis of another variant as they have excellent isolation and matching at both ends. This is a similar feedback amplifier topology. I shoved a BFR182W model into LTspice and got reasonable flatness up to 900MHz substituting all the transistors out for those. Obviously no idea on the noise figure. Would have to measure a prototype. I may persist with that design as well. Of course I need an RF generator and detector that is actually usable past 100MHz first. So I need to repair this with whatever I can.

author: bd139
author_rank: Super Contributor
timestamp: July 31, 2022, 01:05:18 pm
content: Quote from: factory on July 31, 2022, 12:47:51 pmInteresting bit of history. I have seen them before but didn't put two and two together on this one.I've actually ordered two sets of boards here to try them out (as the boards are so cheap!). I have ones with leadframe suitable pads on the edges and some that can just use 0.6mm wire (resistor legs) soldered in as that might be easier to fit for people. Will see what comes out of it!Starting to think I have a PCB problem at the moment

author: George Edmonds
author_rank: Frequent Contributor
timestamp: August 02, 2022, 01:17:34 am
content: HiIf it helps I have attached two photographs of a 2955 AA4 board showing my concerns. The last OM345 replacements on eBay require C37 to be moved to one side, not a good idea.G Edmonds

author: bd139
author_rank: Super Contributor
timestamp: August 02, 2022, 07:05:38 am
content: Excellent. Thanks for the photos. My design is considerably smaller than those ones from what I can see. They should fit in the same courtyard as the original OM345. The height is also close to the original.I redesigned the board slightly before they shipped from the above. The two sets of boards were ordered with 0.6mm thick substrate and the transmission lines approximated for that board thickness. The thin traces another poster mentioned earlier were a mistake on my part. I rechecked again and again before orderingThey’re still being manufactured so we’ll see in a few days.

author: bd139
author_rank: Super Contributor
timestamp: August 03, 2022, 02:00:12 pm
content: Mouser parts order for the final crystal oscillator, the OM345 replacements have arrived as have the CTI OCXO modules.All the CTI OCXOs are checked out and have been characterised for lowest current. I'll load up the 2022E power supply with an equivalent load and take some measurements when I get some time.Still waiting for PCBs which is the blocking issue at the moment.

author: wasedadoc
author_rank: Super Contributor
timestamp: August 03, 2022, 05:41:15 pm
content: My 5 CTI OSC5A2B02s came today. 4 appear to be OK. The other one is low in frequency and never takes more than 55 mA so its heater or heater control circuit is kaput. But 4 for £12.50 inc VAT and delivery is fine by me.

author: bd139
author_rank: Super Contributor
timestamp: August 03, 2022, 06:09:21 pm
content: Yeah that’s why I bought 5. I was expecting 1-2 not to work which was what happened when I bought the last batch a couple of years back.Cheapest commercial one is about £66 a go at the moment so it’s a winner either way

author: bd139
author_rank: Super Contributor
timestamp: August 10, 2022, 04:12:06 pm
content: PCBs finally arrived. Will populate and experiment further later...

author: xrunner
author_rank: Super Contributor
timestamp: August 10, 2022, 08:41:00 pm
content: Lookin' good, will be very interested in how they work.

author: bd139
author_rank: Super Contributor
timestamp: August 10, 2022, 08:52:39 pm
content: Ok here we go. Image heavy. Sorry. I am half way through moving house so the bench is an adhoc pile of kit on my work desk for the moment. Things will improve soonSo firstly I built the two replacement oscillator modules.The crystal oscillator module was tested quickly to make sure I hadn't futzed anything from the prototype build. I have an extensive list of things I screwed up before to check before I send a board off to China these days and there are decent DRC tooling in KiCad so it's hard to screw something up unless you made a boo boo.OCXO module was also tested. This is a backup module in case I can't get the external PLL phase locking to work down the line as it looks like there has been some rework on that and it wasn't behaving particularly well with the prototype module.The synthesizer board was fully removed from the chassis and the module pinout checked again so I don't blow anything up. It was reinstalled and wired up again. Am happy with this design so will distribute it (will upload to the thread later)The unit was powered up and booted as it should do with a permanent clock in it. Woohoo one problem solved. The EFC was twiddled to check the pull range and that was fine too. I will calibrate it later and look at setting up phase locking to an external standard.Anywhere here it is pretending to be an RF signal generator. I de-stickered and cleaned it when I was on a conference call last week. Multi tasking FTW.At this point I was considering buttoning it up and calling it a night but no I am a fool so I shut the brain box and flipped it over to the RF box to try and debug that. I'd already noted from another post on it that (a) there's probably at least one OM345 that is dead in it and (b) they're easy to find - just look for 12V across the input and output pins. After some poking with a DMM, IC10 was found to be borked.I tried to scratch off the coating but couldn't get any solder to hold reliably so shelved the "3.9k resistor hack over the capacitor" repair and went for the shonky "4.7k resistor across the input and output" repair. This sort of recovers some of the DC feedback network well enough to at least do a performance check and further assessment of how knackered it was.To my surprise this actually seems to have entirely fixed it. After checking the output ranges (mostly naively with my counter) all the RF bands are working up to around 800MHz at least which is where my counter gives up. Attached my Tek 465B, another recent repair, to it with a 50R termination, I checked the attenuators and ALC up to 200MHz. I'll dig the better scope up after I've moved but everything is working and the RF levels are absolutely spot on.100MHz:100% AM mod at 10MHz:Things are not finished of course. To do:1. Replace that bust OM345 with a custom replacement module (currently PCB on way from China) and re-test2. Try and get the internal clock phase locking to work with an external standard, failing that whack an OCXO in it and be done.3. Calibration and full check out.

author: bd139
author_rank: Super Contributor
timestamp: August 12, 2022, 12:20:22 pm
content: Have attached gerbers, BOM, schematics and sample picture to this post

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2022, 02:12:22 pm
content: Quote from: bd139 on August 10, 2022, 08:52:39 pmOld joke; "Why do women knit while they talk?" "To give their minds have something to do".I think the comparison is obvious

author: bd139
author_rank: Super Contributor
timestamp: August 12, 2022, 04:42:34 pm
content: Exactly that

author: artag
author_rank: Super Contributor
timestamp: October 23, 2022, 02:29:29 pm
content: I started reading this thread again and had an idea. Of course, it's all water under the bridge since you've got a good fix but I can't help wondering ..As I understand it, the problem you had with the supply was that the 5V wasn't high enough current and you didn't have anywhere to heatsink the regulator you'd need from the 12V rail. But .. many oven controllers use the dissipation of the series pass transistor controlling the heater to add to the heat sourcew (since they don't dissipate anything when the heater is inactive) - indeed, I've seen a design where just a transistor is used as a controllable heater.So, what if you heatsink the regulator to the OCXO can ? It will increase the thermal gain a bit, admittedly, but should keep itself at a low temperature.