Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A) - Page 1
Forum section: electronics / testgear

author: beanflying
author_rank: Super Contributor
timestamp: February 07, 2018, 03:53:42 am
content: Lots to love about this precision boat anchor. 1000V dc precision DC voltage standard and NIXIE tubes combined with proper clicky rotary knobsManual is herehttp://www.hparchive.com/Manuals/HP-740B-Manual.pdfAnd some early references to it's predecessor the 740A (without the Nixies) to it herehttp://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1965-05.pdfApplication Notes on the use of 740B and 741B (** Worth a read for everyone)http://hpmemoryproject.org/an/pdf/an_70.pdfSo why buy it - because I stumbled on it on evilbay and it looked very neat and tidy. Price was reasonable so punch the button and wait.First photo below reveals an unhappy surprise on opening the packageSecond Photo after a few hours warm up helped me smile as the 50 year old + Tracked my 34401a and month old 121GW to the last digit on the 10V range. The 100V and 1000V ranges are out of Cal but not horribly (under 10mV at 10V)The tougher part of the purchase was to try and find the unobtanium input and output boxes and leads. CUBDRIVER has been looking for the input Box for a few years for his but seems I tin potted one 3 days after buying my 740B and snagged that too. Sorry again PatSo anyone knows of a source of these Pat needs an input one and I need an output one.Plenty more photos and full teardown to go including some metalwork fixups, calibration and possibly modding the high voltage inlet and outlet plugs to handle the 1000 VDC. There is a Mercury 1.35V cell to arrange a stable replacement for as well.

author: beanflying
author_rank: Super Contributor
timestamp: February 07, 2018, 07:15:08 am
content: Due to the weight of the beast the broken handle isn't likely to be needed to throw it around much (most likely will rack mount it). Went and had a chat to a local Aluminium welder today about TIG'ing it back together. His feeling and a few of the others working there was don't. But either brazing or soldering would be as good or better and lower risk.So started the process this afternoon with some careful localised heating after first giving it a general warm up. Like Cast Iron old cast aluminium is brittle as you might be able to pickup on the sheared surfaces so anything you do should minimise thermal shock so NO quenching and post work heating is generally required to relieve stresses.Some Aluminium fluxes to be chased up now as they aren't really that common anymore. If you are looking at this some are Nitric acid based and some of the solders used to be Cadmium based, the first is dangerous and the second can kill you.Pre and post bending/heating got the handle sitting back fairly square again and the bent Rack Ear got put under the press earlier today

author: tautech
author_rank: Super Contributor
timestamp: February 07, 2018, 07:32:06 am
content: Quote from: beanflying on February 07, 2018, 07:15:08 amThe 'right' guy can certainly weld this but there'll be some rework (filing) needed.Instead of welding as advised (wisely if skill levels aren't there) there are indeed high strength solders available.Magna peddled their products in this part of the word as perfect for this kind of work.http://www.magnagroup.com/products.php?id=3&lang=en#info_30

author: beanflying
author_rank: Super Contributor
timestamp: February 08, 2018, 01:42:53 am
content: Thanks for the link. Interesting NZ has a distributor but no one in Oz.While I wait for the solder gear time to lift the covers.The likely origional owner I guess. Seems mine is exactly one year older than me so based on this the 740A may have only been sold for 2 or 3 years. No cooling fans so very clean inside. Dont think these Caps need changing either.

author: beanflying
author_rank: Super Contributor
timestamp: February 08, 2018, 01:56:43 am
content: Couldn't figure out what the strange tool clipped inside was for? Easy when you crack the covers. Most of the boards are double sided with single sided load. The oven looks like the cover may come off but as it is likely crusty foam inside and the stability is great unlikely I will lift the lid.

author: beanflying
author_rank: Super Contributor
timestamp: February 08, 2018, 01:15:31 pm
content: Main range select switchTested over 4 weeks prior to shippingNixie BackendNixie is gang number 1 none of this micro controller rubbishResistors are a mix of HP 'matched', Draven or precision hand wound on the voltage set rotaries

author: beanflying
author_rank: Super Contributor
timestamp: February 08, 2018, 01:20:57 pm
content: Nixie pre IC control then off to the plug in cards for high voltage transistor logic.Bodge 240V cable because I don't have a vintage Belden one.Inductors various. Sealed and oil filled of some sort but most are now weeping. Can't see that will be an issue.Do I now need a recorder to go with the 740B

author: kj7e
author_rank: Frequent Contributor
timestamp: February 08, 2018, 02:56:38 pm
content: You got to love classic 1960's HP constriction.

author: beanflying
author_rank: Super Contributor
timestamp: February 14, 2018, 11:50:26 pm
content: Nice cool day here and the Aluminium soldering bits have arrived. Felder Flux and Bernzomatic Rods. Plenty of cheaper rods on evilbay but you wouls have no idea of what was in them.Plenty of videos on the soldering or brazing of Aluminium on YouTube most not using flux but I highly recommend it!Scrub clean and wire brush immediately before doing the job aluminium develops an oxide layer in minutes.

author: tautech
author_rank: Super Contributor
timestamp: February 15, 2018, 12:04:06 am
content: Quote from: beanflying on February 14, 2018, 11:50:26 pmOne of the things I learnt welding alloys was to finish prep with a file as to not leave carborundum contaminates behind that might spoil the weld. Apply same process to cast steels/irons or do the prep with a gouging rod.

author: Tony_G
author_rank: Super Contributor
timestamp: February 15, 2018, 02:49:44 am
content: Looking forward to see the results - BYU is a Uni in Idaho (next state over from me) - Did you get the gear from the US or was it already in Aus?TonyG

author: BradC
author_rank: Super Contributor
timestamp: February 15, 2018, 03:02:07 am
content: I prep with a set of stainless brushes that I keep in a box (to prevent my wife using them to clean the BBQ). They are exclusively for aluminium use. Between a standard wooden handle quadruple row brush and a set of different shaped dremel brushes and a couple of small single cut carbide burrs it's mostly covered. I generally do a post prep wash with acetone to make sure it's really grease free before welding. Aluminium soldering and I haven't really got on well in the past.I'll be super interested to see how this works out for you. Are you using an oxy or a gas/air torch?

author: beanflying
author_rank: Super Contributor
timestamp: February 15, 2018, 04:04:04 am
content: Jigged and reasonably straight.Sourced on evilbay from Utah from aseller of electronics.Straight butane is fine for aluminium. Flux made into a paste and scrubbed with stainless brushes.

author: beanflying
author_rank: Super Contributor
timestamp: February 15, 2018, 04:53:31 am
content: Cough cough smokey from the base board charring.Could do with a bigger torch but now it is tacked I should be able to remove it off the board.Working better without the flux with these rods?

author: tautech
author_rank: Super Contributor
timestamp: February 15, 2018, 05:43:56 am
content: Quote from: beanflying on February 15, 2018, 04:53:31 amYea it's easy to underestimate how much heat Ali sucks....but not as much as copper !QuoteIf the work is clean and uncontaminated by oxides or oils flux usage can be minimal although the 'correct for product' types are useful as an indicator of correct temp. Good and clean welding glasses let you see the minuscule amounts wet the whole joint.

author: beanflying
author_rank: Super Contributor
timestamp: February 15, 2018, 08:44:46 am
content: Still some more to do but fairly straight and it still fits where is should. Maybe wouldn't like to send it bungy jumping attached by the handle but it seems fine for lifting.

author: Tony_G
author_rank: Super Contributor
timestamp: February 15, 2018, 04:00:52 pm
content: Did you get the input and output attachments with it?Also it looks like it has a couple of photochoppers as modulators (A16 &A17) - Are they similar to the 419A construction? If so then they apparently benefit from being left on all the time (interesting discussion on the HP Yahoo Group regarding replacement neons as well). Annecdotally some choppers will come back to life if you leave them on for a few weeks (see discussions about the cadmium photocells) - Your mileage may vary on this though (my 419A never came back to life).

author: beanflying
author_rank: Super Contributor
timestamp: February 15, 2018, 11:24:51 pm
content: Similar photo chopper technology I gather and a Mercury reference cell somewhere inside I haven't come across yet. Recalibration and going over it in more detail when I get back together mechanically.As above I scored an Input connector box but I don't have the output one and another member has the reverse. The connectors seem to have only been used on the 740A and B and the input one has had the extra pins pulled I am assuming to prevent connecting to the wrong side. Most likely this will mean changing out this connector unless I find the other.edit: Just about finished cleaning up one of the breaks very happy with it. Last time I did any of this was nearly 30 years ago on aluminium irrigation couplings with oxy/acetylene. More heat would be a help here not more temperature.

author: beanflying
author_rank: Super Contributor
timestamp: February 16, 2018, 05:18:26 am
content: Sanded both side frames with 180 grit finish to take out a few scrapes from the the years and to blend the repair a little more.Vinyl and front panel cleaning time. But first a well earned beer

author: kj7e
author_rank: Frequent Contributor
timestamp: February 16, 2018, 05:54:37 am
content: Nice job

author: BradC
author_rank: Super Contributor
timestamp: February 16, 2018, 05:56:32 am
content: Quote from: beanflying on February 15, 2018, 11:24:51 pmI've had a little oxy/propane rig for years using incredibly expensive disposable oxygen bottles that last not much longer than a fart at $45 each. I used it when I really needed it, but have an array of air/fuel torches that got loads of use. Now Bunnings do no-rental swapsies of decent sized oxygen bottles (by comparison to the 100L disposables) I don't even get the gas/air torch out anymore. More heat, easier to control and the potential for the same heat in a much smaller area.I've also done some soldering with the TIG. The argon shield helps keep the oxides under control and because aluminium conducts heat so well you don't have to get the solder near the arc. Sometimes silver solder copper with it too.Nice job by the way!

author: beanflying
author_rank: Super Contributor
timestamp: February 16, 2018, 06:29:35 am
content: I have been looking seriously at a small Tig/Inverter again after avoiding buying one for years. I would put my combined welders bills at $1k'ish in the last 10 years so generally paying someone else has been the better option. Like all things keeping the practice up with welding is one of the keys.When I got turned down on this one by a 'professional' business and used my $15 evilbay torch and disposable Butane cans to get a result makes you go Mmmmm about current training and the narrow range of skills of some current tradies.I was taught by a 50+ year old at my first real job because taking the then 3 phase Tig out to the paddocks wasn't really practical and it was what was taught at Tech Schools before TIG'sOMG Starting to sound like an old man 'when I was a boy' ......

author: tautech
author_rank: Super Contributor
timestamp: February 16, 2018, 06:55:41 am
content: I've found LPG torches that you can use on your BBQ bottle quite effective heat sources and cheap to run.The 'old school' vinyl layers used to use quite large units with 'soft' flames for softening the vinyl when cove'ing under cabinetware toe kicks.I've got one of each but the smaller one sees a lot more use. Better with a 4kg cylinder than having to hump around a 9kg one.Something like this one:https://www.trademe.co.nz/Browse/Listing.aspx?id=1544249384&mkwid=sEIcRBR8m_dc&pcrid=225543738224&pkw=&pmt=&plid=&gclid=Cj0KCQiA_JTUBRD4ARIsAL7_VeXBL3kc2SVVvj_tDa6tHL9VmdrwC2upoOEt5lb5ayv_cAyU0L8RPrgaAqmMEALw_wcB

author: Cubdriver
author_rank: Unknown
timestamp: February 16, 2018, 07:44:30 am
content: You did a beautiful job fixing that broken casting! Bravo!The mercury standard cell in the 740B should be on the right front corner of the A10 board. Here's a screen cap (sorry for the poor quality) from the quickie video I did (and never uploaded until just now) of mine when I first got it a few years ago.The battery is visible briefly around the three minute mark in the video, in all its corroded glory:-Pat

author: beanflying
author_rank: Super Contributor
timestamp: February 16, 2018, 08:27:43 am
content: You mean like this crusty little bugga. 0.5mV aint doing much I suspect

author: Cubdriver
author_rank: Unknown
timestamp: February 16, 2018, 08:30:48 am
content: That would be the critter. I didn't bother to try to measure mine, though half a millivolt or less wouldn't surprise me...-Pat

author: beanflying
author_rank: Super Contributor
timestamp: February 16, 2018, 09:10:18 am
content: I think the sensible solution is to abandon a battery all together based on the replacement options. A small Daughter board in that space with an additional cal pot hole drilled in the lid and a modern reference fitted with appropriate dividers if needed. Temp Co shouldn't be a major issue as that bay will be heated by the reference and it is at least 30 minutes to get near being stable. So give the new reference an hour or two before tweaking the cal pot and it will be nice and toasty too and well above any ambient variations on temperature.

author: Cubdriver
author_rank: Unknown
timestamp: February 16, 2018, 09:21:11 am
content: Quote from: beanflying on February 16, 2018, 09:10:18 amI was thinking along those lines too, and did some searching back then and seem to recall finding a couple of methods people had used. Think I was initially looking for mercury battery substitutes - they're used in old school light meters too, so there's a market for such a thing. I recently got a pair of Chinese voltage reference boards (must have seen them here), adapting one of those into the circuit may be a good option. The instrument has been sitting on its side in the hall for must be three years now waiting for me to work on it...-Pat

author: beanflying
author_rank: Super Contributor
timestamp: February 16, 2018, 10:18:11 am
content: Rough idea but a 4.096V reference is easy to get a stable one that will drive a few mA without a buffer. Drop the output into 1/3's with the best precision resistors (4-5k total but not important) you can sensibly afford gets you 1.36V with no trimming. Any Pot you add to tweak the last bit can be any sort as it's influence on Tempco will be tiny.Most of these References will need 10-15V so finding a 12V rail should be simple.

author: beanflying
author_rank: Super Contributor
timestamp: February 16, 2018, 11:19:01 pm
content: One of the other things I was going to chase up was the old Belden/HP lead. Evilbay pricing shipped to Australia would be $50+ Based on another recent acquisition a 735A from the same era having a factory fitted IEC plug I think a change is sensible.Maxim reference IC ordered with the few supporting bits for the 1.35V replacement.

author: tautech
author_rank: Super Contributor
timestamp: February 16, 2018, 11:27:16 pm
content: Quote from: beanflying on February 16, 2018, 11:19:01 pmAlways.......when it can be done neatly !Sometimes an IEC socket with an inbuilt fuse or switch can be better to fully hide the OEM chassis punching.

author: beanflying
author_rank: Super Contributor
timestamp: February 17, 2018, 01:18:52 am
content: The switch/fuse ones are good for a cover upI made a template a while ago for my label printer helps keep it neat and also reduces scratches from doing the job. Catch bucket under it keeps the mess contained. Normally the nibbler it easy but the rear lip meant mainly filing to size and the dremel is to messy.

author: Cubdriver
author_rank: Unknown
timestamp: February 17, 2018, 01:31:52 am
content: Nice job fitting that C14 connector!! Those PH-163 power cords are enough of a pain to find here in the US; i don't even want to think about trying to get them where you are.-Pat

author: beanflying
author_rank: Super Contributor
timestamp: February 17, 2018, 01:45:38 am
content: Part of that reason for none locally is this one I removed is rated at 125V 15Amore like 5-8A for safety so perhaps for gear being shipped to the sensible part of the mains voltage worldmay have been fitted differently?

author: SeanB
author_rank: Super Contributor
timestamp: February 17, 2018, 04:44:40 am
content: The caps you will be replacing are the wet tantalum types with rubber seals, as they will be very dry by now. Sad, as otherwise wet tantalum is a very good cap, just not the lower cost versions with a rubber bung seal. Good thing is that in most cases you can replace with a regular modern Nichicon or panasonic ultra low ESR capacitor of the same value and same voltage, and in almost all cases this will work.

author: beanflying
author_rank: Super Contributor
timestamp: February 17, 2018, 05:51:34 am
content: Thanks, board by board pull and sort out coming up over the next few months so caps etc.The Battery replacement is next on the list now the physical repairs are sorted that way I can hack a temporary output test box and run through the cal cycle as I go.

author: beanflying
author_rank: Super Contributor
timestamp: February 18, 2018, 05:26:23 am
content: Just a few photos of Input cable unobtainium. If anyone has a couple of plugs in their junk bins Pat and I would love to hear from youDeutsch MDR07-7P-090 is the number. HP for safety given the 1kV DC potential has pulled some of the pins on the input side.

author: beanflying
author_rank: Super Contributor
timestamp: February 27, 2018, 09:43:35 am
content: Mercury cell replacement lashed up on the bread board tonight. 1.35xxx from 10-12v input. Resistors are low tempco Vishay's. Maxim 6341 csa for the 4.096 reference.Will pop it on a board in the next day or two and add it to the 740b providing I am not struck down for adding an IC to a classic HP

author: beanflying
author_rank: Super Contributor
timestamp: February 28, 2018, 12:24:30 am
content: Good enough for AustraliaJust need to figure out where to mount the Mercury Battery replacement now. Temp is fairly stable here today and it hasn't moved a digit in over an hour.edit yes I have now soldered the cap legRunning well on LiPo power at this stage. I need to sort out either a floating supply for the reference or figure out if the resistor divider network can be changed to allow a voltage compared to a circuit ground ..... The circuit diagram is a little interesting

author: beanflying
author_rank: Super Contributor
timestamp: February 28, 2018, 05:41:05 am
content: Must remember RTFM more carefullySeems there is actually two Mercury reference cells in the 740B.I did order two references so another to be made and crammed in here. Underneath the unit on board A11and under a guard cover hidden by wire and another board is the loop gain check circuitEven with desoldering a coax that goes through the aluminium this is as far as the board moves.

author: Cubdriver
author_rank: Unknown
timestamp: February 28, 2018, 07:00:07 am
content: Oh, snap! Good find! I'm watching this closely as mine will need a similar treatment at some point.-Pat

author: beanflying
author_rank: Super Contributor
timestamp: February 28, 2018, 07:11:02 am
content: Quote from: Cubdriver on February 28, 2018, 07:00:07 amGot it out, pre wired the reference wires in place and put it back together again. I need to print out the circuit diagrams and try and figure out a voltage reference point. The simple option will be drop a couple of isolated 12V supplies underneath near the main inputs. Plenty of room but adding more stuff isn't ideal.When you get a chance could you crack the top on your output box and take some similar snaps to my input one? Based on the snowballs chance in hell of either of us getting the ones we need I now have the Tellurium Posts to make a counterfeit box. I would prefer to make it fairly close to the real deal if possible.

author: Cubdriver
author_rank: Unknown
timestamp: February 28, 2018, 09:50:46 am
content: Quote from: beanflying on February 28, 2018, 07:11:02 amWill do. I need to figure out where I stashed it first, though...-Pat

author: factory
author_rank: Super Contributor
timestamp: December 21, 2018, 08:26:33 pm
content: Quote from: beanflying on February 28, 2018, 07:11:02 amHere are pictures of the Output box & connector (part no. MDR07-7PB-090), note the polarization slot in the connector is in a different place to the Input one.David

author: beanflying
author_rank: Super Contributor
timestamp: December 30, 2018, 08:20:16 am
content: Getting back to this after leaving it on the rack for a while. Ran across some gorgeous Renderings of the 740B and some other HP gear. More HP renders herehttps://www.flickr.com/photos/144543740@N05/43372501065/in/photostream/I think I am making an output box but may casually keep looking for the real deal.

author: beanflying
author_rank: Super Contributor
timestamp: March 21, 2019, 04:09:46 am
content: Just a first pass Model of the Output Box (I will also tweak it so a matching input box can be made). The Plan is to 3D Print it after finalising dimensions of components to be used in making a clone box for the unobtanium.The base will be a drop in fit for the genuine box and I have modelled in the slight taper on the side wall that would have been to clear the part from the injection mold. Generally it is size for size of the real thing.

author: Dave Wise
author_rank: Regular Contributor
timestamp: February 18, 2020, 11:26:34 pm
content: Good job so far. I should send you my notes, I have fixed several problems, some of them basic to the design.The batteries are not critical, you can adapt for various voltages. Your precision reference module is overkill but since it's in place, fine.A11BT1 will crap all over the board when it leaks. Really it has no business being there. The effluent will contaminate the board and allow conduction between the main output and the gain check circuit. I used a standard button cell with tabs spot-welded, and relocated it onto S10 GAIN CHECK where it should have been all along. If the new cell is different from mercury 1.35V, just calculate the correct meter displacements for minimum and maximum. This battery is never used in normal operation, so shelf life is the most important characteristic.When you press GAIN CHECK, the battery goes in series with the signal, which perturbs the output to a degree depending on the amplifier open-loop gain. This is used to monitor the condition of the modulator and demodulator photocells in A17. I replaced mine with H11F1 photofet optoisolators, so my gain will be stable for generations to come and I only needed to check it while troubleshooting the new assembly, which has much higher efficiency. I adjusted a resistor value on A3 (not the one mentioned in the manual, that's bogus) to bring the gain back down to normal.A10BT1 powers the ZERO ADJUST control, and its voltage is not critical although it's nice if it is stable over temperature. I used a 3V lithium button cell and added a series resistor to drop back to normal voltage to maintain the same adjustment range as before. Current drain is a constant 200 nanoamps which means the battery will last for its entire shelf life.One of my two instruments lacks cables, and I have given up on finding original connectors. I will construct cables, and for connectors use whatever I can find that meets spec for insulation resistance and voltage withstand, or maybe just hardwire them. Since I have triax cable but not the custom-made stuff HP used, I will run one cable for input, arranged +, -, guard from inside to outside, and run a separate wire for chassis. For output, I will use two triax. Number one will be + out, - out, guard, and number two will be + sense, - sense, guard, with again a separate wire for chassis. Have to make do with what I can get.Dave Wise

author: beanflying
author_rank: Super Contributor
timestamp: February 19, 2020, 12:21:25 am
content: Thanks for the Info.Progress has stopped at present along with the supply of Output/Input Boxes everywhereSo when I have a test print and design I am happy with I will put the STL files here so anyone can either print them or have them printed for theirs.Hoping to clear a dedicated space so I can leave a few bits of test gear refurb and repair out without blocking the day to day in a few months. In chicken and the egg style I first have to make new trusses and replace a second Garage Roof before that happens

author: Dave Wise
author_rank: Regular Contributor
timestamp: February 05, 2021, 11:06:50 pm
content: I built my boxes and they worked out great. I used a generic type of hobby box from Amazon, high-voltage triax cable with spiral wrap to hold it together, and Amphenol CPC Series 2 connectors. Shell size 11 just fits, with flange behind the panel andsecured with countersunk flat-head 6-32 screws. To keep the WARNING HV neon lamp bright at moderate voltage, I put it in series with half resistor, half current limiter made of an LND150 depletion-mode MOSFET.I used cheap binding posts as proof of concept. If warranted, I'll replace them with good ones later.I'm pleased with the way the cables and connectors work. They're not push-on pull-off like the old Deutsch ones, but they only need a quarter-turn to mate and unmate. I used some of the input-jack contacts to guard the input-plus line. I have verified that I'm still at least 10 gigohms at 1V and up.The two plugs mate either jack, but I haven't got them backwards yet, partly because the cables are clearly different. (Output is two triax while input is one.) Anyway I wired them so it's mostly harmless. The only thing I couldn't fix is if you put the output box on the input jack, the plus-sense terminal goes live with plus-out voltage even if the output jack is turned off. It's a side effect of the guarding. I could paint the input and output with complementary colors if I was more worried.

author: beanflying
author_rank: Super Contributor
timestamp: February 06, 2021, 07:16:58 am
content: Sounds good Dave butOther than that can you link or provide a bit more detail on the Triax bits. I have all but given up waiting for the second unobtanium box for mine.

author: Dave Wise
author_rank: Regular Contributor
timestamp: February 07, 2021, 12:54:52 am
content: Here are pictures of my DIY input and output cables. Hope this works, preview seems broken.

author: Dave Wise
author_rank: Regular Contributor
timestamp: February 07, 2021, 12:55:35 am
content: Here's 740B #2 with original jacks for comparison.And here is my cable pinout.Oh that didn't edit right, what am I doing?

author: Dave Wise
author_rank: Regular Contributor
timestamp: February 07, 2021, 12:58:11 am
content: And a text version of my notes.000LN was omitted from the CODE LIST OF MANUFACTURERS, but it isDeutschMDR07-7P-090 (input), NSN 5935-00-169-9672MDR07-7PB-090 (output), NSN 5935-00-410-7256Mounting hole is 0.685" .AMP CPC Series 2 shell 11 just fits, flange behind panel,secured with countersunk flat-head 6-32 screws.Plug P/N is 206434-1.TODO: Get one Series 2 Keying Plug to prevent input/output cross-mating.NEW CABLE PINOUTSHELL CONTACT NUMBERING looking at contact side of plug1 23 4 56 7 8INPUT1 2 3 4 5 6 7 8G I- C O O X O I+G = GuardI = InputC = ChassisO = Signal Guard. Continuous version of O+, i.e. 5 wire betweenA11 and K1A.1 and C4. Plug has these contacts populated butnot connected to anything. This shields pin 8 from the rest.X = No contactCable is triax.Center = I+Inner shield = I-Outer shield = GChassis is a separate wire, bundled to the triax with spiral wrap.OUTPUT1 2 3 4 5 6 7 8G S- C J S+ O- J O+G = GuardS = SenseC = ChassisJ = Jumper between pins 4 and 7 in plug, energizes output relay in jack.O = OutputCables are triax.CABLE 1Center = S+Inner shield = S-Outer shield = GCABLE 2Center = O+Inner shield = O-Outer shield = GChassis is a separate wire, bundled to the triax with spiral wrap.Cables are strain-relieved at boxes by the common plastic strain reliefused on zip cord, with the projection ground down to accommodate thelarger size of our cable without kinking it. Smaller size relief oninput (1-triax) cable, larger size relief on output (2-triax) cable.WARNING: Unless you fit the INPUT jack pin 6 with a keying plugto prevent cross-mating, if the OUTPUT cable is inadvertentlyplugged to the INPUT jack, amplifier output voltage will appearat the OUTPUT box's SENSE+ terminal even when output is disabled.

author: Dave Wise
author_rank: Regular Contributor
timestamp: February 07, 2021, 01:06:26 am
content: I can't remember where I got the triax. Some eBay seller I suppose. The outside jacket is smooth and slippery-feeling like teflon. The center conductor insulation is thin but I'm sure it was designed to take high voltage because it is covered by a conductive coating. At the plug and box it's fiddly but essential to scrape it off without nicking the insulation. Using my Fluke 895A as a high-R ohmmeter, I can't detect any leakage at all. Virtually all of the instrument's 10+ gigohm input resistance is the connectors or inside the instrument. The latter is partly the A9 input/range board and the S1 and S2 MODE and RANGE switches, partly the amplifier input current.

author: beanflying
author_rank: Super Contributor
timestamp: February 07, 2021, 01:59:26 am
content: Thanks Dave

author: Dave Wise
author_rank: Regular Contributor
timestamp: February 07, 2021, 05:01:47 pm
content: I have detailed notes any time you want.Having accidentally blown up the Germanium Q1 and Q2 years ago, 740B #1 is retrofitted with Silicon, plus mods* to accommodate its different characteristics. (Currently KSA1010 driving MJ2955 but I want to try TIP42C driving BD912. I also found some 2N6331's. I'll drive that one with an MJ2955, similar to HP's own choice.)* A 1nF/330R in series from collector to base, or remove A7C8; and an RCD "tracking voltage clamp" across T4 primary to tame the leakage-inductance spike. That's 10uF, 1K, UF4003.It's a good idea to put a heat sink on A7Q3 if main switch Q1 is Germanium. A7Q3 has its work cut out for it discharging that big heavy base, and it's the hottest transistor in the instrument. (With Silicon Q1, A7Q3 barely conducts at all.)Make sure A7Q2 is working; if it goes open, A7Q3/R7/R9 will fry.In late production, HP added a freewheel diode across Q1 to prevent transformer ringing from driving the collector positive. They seem to have used a garden variety rectifier but I discovered that a fast-recovery part like the MUR115 or UF4003 is way better.740B #1 also has a DIY HV transformer wound on ferrite instead of iron because the original overheated when operated full-throttle overnight. Its relatively low saturation ceiling forced me to develop a simple mod to the A7 board current limiter which is compatible with the original parts.I refitted both 740B #1 and #2 with H11F-based choppers, and I recently realized I had matched the modulator parts for the wrong characteristic. They need to be matched for temperature coefficient. If this results in a large net offset, it's easy to trim out, by simply inserting a resistor at one or the other end of the front panel ZERO control. I matched the parts in a toaster oven, 20C-60C, using a Keithley 147 Nanovolt Null Detector to measure the offset. Out of nine same-lot H11F2's, I found two good matches with less than 2uV drift over 20C-60C. The mod-demod efficiency increases and it's desirable to reduce AC amplifier gain to avoid overload on the millivolt ranges. Adjust A3R19 not A3R9, I don't know what HP were thinking there.While I was in there, I reduced A4 noise by replacing transistors. I used KSC900.Beware of the 2N3391 data sheets. Central Semi, National Semi, Fairchild, and New Jersey Semi call the 2N3391 "general-purpose", but GE (HP's vendor for 1854-0033) is "Ultra High Beta, Low Noise".The original neon-and-photocell input protection circuit is trouble - the HV cell HP used to disconnect the input generates 10-100uV of offset. (This is how I discovered the oven was oscillating - offset ramped up and down as oven load affected the light level on the photocell.) Even if it didn't, the circuit is not usable with H11F's with their 30V absolute max. I engineered a new circuit based on the Supertex LND150 high-voltage depletion-mode MOSFET which is optimized for current limiting, and a TVS. There's also an SSR to keep input current out of the KVD in STD mode. This circuit works better than the original did when it was new.Years ago I discovered that 740B #1's oven had run away and cooked the PCB inside so I rebuilt it with the two separately-floating circuits on separate boards to avoid leakage. It's important to keep the reference zener thermally connected to the inner can. The thermistor was cooked too so I replaced it with a modern part, discovering in the process that control stability can be improved by attaching the thermistor to oven wires instead of the can, to introduce some phase lead. I still had to reduce control loop gain to keep it from oscillating. I discovered that 740B #2's oven was oscillating too and reduced its loop gain until it stopped. The frequency is in the milliHertz; I graphed it with a storage scope while perturbing it with an HP 203A oscillator.Speaking of leakage, I discovered that T2, the transformer powering the in-guard system, has a lot of leakage between the +34V and -12V secondaries, which messes up low-level readings. A simple mod reduces the voltage between them and hence the leakage.When I was doing hi-pot testing, 500V between MINUS and GUARD in VM mode leaked into A2C17 and punctured it. This proves HP never did that particular test. Symptom is no reading in DVM mode.I added a TVS across the new cap. SMAJ30CA since I was already using that to protect the H11F's. It can be almost anything, normally the cap has less than a volt across it.(I've said this one before, I'm repeating it here to put it all in one place.) Battery leakage messes up circuit operation and must be cleaned. The battery on A10 can be anything you like. I used a lithium coin cell with tabs. It will last for its shelf life. If you use some kind of power supply instead, it must float 1000V with no leakage. The battery on A11 should be relocated onto the GAIN CHECK switch. It can't be 3V lithium as that amount of offset will overload the chopper amp. I used a silver oxide button with tabs. It will last for its shelf life. If it's not 1.35V you need to adjust the GAIN CHECK bogie value. If you do away with the photocell choppers, GAIN CHECK becomes a pointless function anyhow since the H11F's will last forever.PROCEDURAL NOTE: When doing INTERNAL CAL, wait several minutes after you switch between 1-9 and 10-12 to allow dielectric absorption in the switches to relax.

author: Cubdriver
author_rank: Unknown
timestamp: February 07, 2021, 07:59:40 pm
content: Dave, thanks for the very thorough writeup! I will definitely be going through that again when I eventually get the beast onto the bench.Much appreciated!-Pat

author: Dave Wise
author_rank: Regular Contributor
timestamp: February 07, 2021, 09:36:29 pm
content: Don't rush into removing all the chopper photocells even if your gain is below spec. Try modernizing just the demodulator side first and see if that gives you enough boost in efficiency. I think the main challenge will be syncing the demodulator drive to the modulator, unless you punt and drive both from a new circuit. (BTW I have PIC source code for that.) Don't try for a final design, just breadboard something to see if it's enough. (I'm speaking from the hassle I went through matching pairs of H11F.) If you do essay to install H11F's, the main amp has to be H11F1 or H11F2 for the 30V breakdown. (The main amp - even when gain = 1 - has to generate 30V to get the expected slew rate out of A4.) The meter amp can use any H11F including the 15V H11F3.Be careful with the mod-demod phasing - Figure 3 in the manual is wrong. On the demod section of A17, V4 is actually lit by DS1 and V3 by DS2. The chopper amp overall (2 wire to 926 wire) should invert.Do be suspicious of A9V2. Mine was generating on the order of 100uV photovoltaic offset. I've replaced it with an SSR; Ixys CPC1981Y works fine and leakage is negligible. Standex SMP-1A40 would also be great. You can wire in the LED in place of A9DS2 if you add series resistance. I'm sure HP would have used one of these SSRs if they existed in 1965.

author: Dave Wise
author_rank: Regular Contributor
timestamp: June 06, 2022, 02:44:37 am
content: I'd like to add one thing. The H11F1 characteristic that needs to be matched is not offset - by manipulating resistors A10R6/R7/R13, you can null out at least 1000uV. The important thing is drift. You want a pair of H11F1's that drift in the same direction, the same amount. This is even more tedious than offset matching, but I was able to get several satisfactory pairs out of about 20 candidates using my toaster oven as a cheap environmental chamber.

author: Dave Wise
author_rank: Regular Contributor
timestamp: July 07, 2022, 07:36:19 pm
content: NOTE: Everything in this thread is applicable to the 740A, 741A, and 741B, as well as the 740B. It may be useful for the 419A Null Detector as long as power consumption stays low.NOTE: With respect to chopper phasing, the 740B schematics are a confusedmuddle, including my well-intentioned annotations.For now, my definitions: Both Main and Meter. Physically, cells arenumbered top looking down V1 thru V4 counterclockwise. V1 is mod series,V2 mod shunt, V3 demod shunt, V4 demod series. V2/V3 lit by neon DS2 orLEDs DS2/DS3, V1/V4 lit by neon DS1 or LEDs DS1/DS4. DS3 and DS4 areinternal to the H11F1 demodulator switches.I have new findings. I am grading my photocells by scoping them against a current source, adjusting illumination so R-lit is 20K, and recording the time it takes the cell to recover to 100K.Recovery is a straight line at normal illumination, which means that T10 (HP's figure of merit, see AN-919) is meaningless unless you also adjust illumination so all cells have the same R-lit.Of my sixteen cells (two 740B's), eight look very promising. I think I can use photocell modulators along with H11F1 demodulators. The latter's near-ideal behavior lets us do a very important trick.FINDINGS1. Asymmetric Demodulation TimingWe must turn on series switch V4 at V1 on and keep it on throughout V2, to route the entire half-cycle waveform to the A4 integrator. This boosts efficiency by around 30%.Use a microcontroller.Four states. V1+V4, V4, V2+V3, none.2. Customized Modulator DriveWe MUST tune the light intensity on each cell, which rules out neon. I suggest poking LEDs down the empty V3/V4 wells.You should insulate and guard.With custom drive currents, modulator pulse width is not critical, just set 50% dark ratio (beta) - on both channels - and forget it.Doing this gives me another big boost in efficiency. My results are comparable to my estimate of original new-instrument performance. Don't go overboard, remember HP put lower limits on V1-V2 average series resistance. (160K on A17 - main, 350K on A16 - meter.)3. Asymmetric Modulation Timing (partially true)Bright short pulse V2, dim wide pulse V1.E.g. A17#1V1 treated as V2. 100uA 5ms is about 10K valley andit hits 100K about 1.8ms after turnoff. 300uA 500us is also10K valley, but it hits 100K about 1.6ms after turnoff.dR/dt is almost constant for a given Rlit, slowing down as the cell is driven harder.So if you maintain a constant Rlit, you will have essentially constant dR/dtregardless of pulse width.On the other hand, a shorter pulse makes room for more dead time,so it is still useful: brighten V2 and shorten V2/V3, stopping short ofdegrading cap charge, and lengthen V1/V4, stopping shortof degrading Rin.

author: Dave Wise
author_rank: Regular Contributor
timestamp: July 21, 2022, 03:14:55 pm
content: UPDATE.My fastest photocell is also nearly blind; using a 24V Luxeon 3535HV LED, 3mA is required to get the cell down to 20K. Using a 3V Cree JB2835, I expected 24mA, but it actually requires 60mA, a factor of 20 instead of 8.A less-blind cell that needs 1mA on 24V, needs 10mA at 3V, a factor of 10 instead of 8. This points to a steep drop in LED efficiency as current goes up.This means I cannot run 3V LEDs on 5V, I must use 24V LEDs driven by transistor switches. The extra space taken precludes through-hole hand-assembly of a prototype, plus it's not usable in a 419 Null Detector with its 24V supply. You can probably manage with 9V LEDs, which should find themselves on the left side of the efficiency knee. (12V-18V parts are expensive for some reason. The cheap ones are 3/6/9/24/48.)I'm working on a circuit board layout with surface-mount parts including in-circuit-programmed uC. There's enough space for everything. The board will mount on the demodulator side of the old chopper housing, sticking LEDs down the V3/V4 wells. (Remove the old photocells first.) The back side of the board is copper fill connected to Guard. Cover it with a few layers of Kapton or polypropylene tape to insulate from the grounded chopper housing. The H11F1's mount on edge, with the LED pins in through-hole pads above the V3/V4 wells. Air-wire the signal pins to the old terminals.I'm a total noob with KiCad and OSHPark. Maybe someone can look over my shoulder and stop me from making dumb mistakes I can't back out of.HP-Chopper-Photocell-Driver-first-draft.pdf(17.29 kB - downloaded 89 times.)

author: Dave Wise
author_rank: Regular Contributor
timestamp: July 25, 2022, 04:25:36 am
content: I think we have a winner. Wonder what OSHPark will say.Now I get to lay out the LED discs that go into the photocell wells.I tried to make it friendly to hand-solder, with 0805 and 1206 parts.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 25, 2022, 06:00:30 am
content: For the driver circuit it would help to also show the schematics. For the layout the design rule checks and PCB tools usually do a good job ckecking for major errors.From what it looks like the optocouplers ouput side is not connected, not even to some connectors, which is odd.The origianl neon bulb circuit has one hidden function, that may not be obvious: it alternates between the 2 neons and has a little pause in between.It is kind of normal that the fast LDRs are less sensitive, but there is also a point when the sensitivity gets too low. With very high power from the LEDs one gets increasing levels of thermal effects that can cause thermal EMF. This would not be real drift, but cause a rather slow settling for the meter to warm up.

author: Dave Wise
author_rank: Regular Contributor
timestamp: July 26, 2022, 02:56:00 am
content: Thanks, Kleinstein. I added the schematic to post number 61.Everything on the board is at or near Guard potential, which may be up to 500V away from the signal pins. I will solder in only the LED side of the optocouplers. I'll connect the signal pins to the module terminals with point-to-point wire, to avoid leakage. I wasn't willing to put in the work of creating a custom footprint for half of an H11F1, so in KiCad I just pretended they were hanging off the edge, with the signal pins set as no-connect. (The only DRC warnings were "Silkscreen outside board outline".)The microcontroller program will include dead time. The original 740B driver has dead time on the Main chopper but not the Meter chopper. Testing showed that it's of critical benefit to both.I listened to my blind, brightly-lit photocell with my General Radio 1232-A Tuned Amplifier, and didn't hear a thing.I worried about those four clearance holes. The teflon insulators the holes are meant to clear extend right to the edge. A drill bit would want to bend. I made them recesses in the outline, so OSHPark can mill them instead of drilling. I figured out how to make KiCad do it. It's tedious but I did it. String together lines and arcs. It's finicky about the outline making a closed shape, you have to drag line ends together at extreme zoom.Back copper is a fill zone, connected to Guard. Have to insulate from the chopper housing. Kapton tape, polypropylene tape (Scotch "Magic" tape)... how about a phone screen protector?See any show-stoppers?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 26, 2022, 07:29:56 am
content: Partially mounting of the OK makes absolute sense. The drive level is quite high and this can cause some thermal offsets, that may increse the warm up drift (would not make is slower but more amplitude).I would assume the LEDs to drive LDRs are for the input modulator and the H11F1 is for the demodulator. The demodulator part is after amplification and thus way less critical.I don't hink the H11F1 would need that much current for the demodulator - the signal current is still relatively low.From the datasheet there is rather little gain from using more than some 5 mA and even 2 mA should be enough.Anyway the drive current could come from the 5 V supply - no need to start from 24 or 40 V to drive a 1.2 V IR LED. Of cause this does not help with the 5 V derived from the 40 V with a linear regulator.some 10 mA for the H11F1 would be quite some power from the 40 V rail !For the 5 V regulation it may with adding more filter capacitance - for just the µC and low power the simple zener may be enough.Similar the 40 V supply for the LEDs should have a filter capacitor (e.g. , so the extrernal supply would not see much of the modulated current.For the LEDs the focussing of the LEDs can also have quite some effect. The LEDs for illumination use tend to be wide angle. The viewing angle can also be different for the 3 V LED.The Cree LED is spedied for 150 mA, so at 60 mA the efficiency should still be good. Another point could be the color: the white LEDs have quite some red and blue. The red light part may not have any effect and the blue part could be already less effective. The color temperature make quite some difference to the spectrum. The question would be if a high efficiency green or even orange LEDs could work better.

author: Dave Wise
author_rank: Regular Contributor
timestamp: July 26, 2022, 03:46:08 pm
content: Thank you!You have it exactly right, I'm using a pair of H11F1's on the demodulator side - where offset can be ignored - while keeping the modulator photocells and driving them with LEDs.With a 3V LED, my observation did not agree with the datasheet; Cree JB2835 needed 60mA to excite the photocell the same as my 24V LED at 3mA.Or maybe the spectrum changes when you drive it hard, to something the photocell doesn't like.Anyway, I abandoned that design path.The LEDs I tested are supposed to emit 120-150 degrees which covers the whole photocell.During "cruise", the demodulator only carries a few uA, but when slewing, the shunt switch has to charge 1uF to 20V in a couple milliseconds. The meter loop series switch peaks around 200uA.Are we looking at the same datasheet? OnSemi Rev 2 (2021) Figure 2 Output Characteristics shows almost equal-size increases in saturated current for drive at 2-6-10-14-18mA, with 16mA yielding about 600uA. I gravitated to 16mA because it's the test current for several specs.Using the -42V rail, current transients are not an issue since the instrument's PWM output pulls amps not milliamps.That said, if 9V LEDs work, I want to ensure the board can be used on the 419A, so I am looking at the lower voltage supplies, like +16.5 or -16.5 . And since the board has moved from the main chassis to the (grounded) chopper housing, there's an exciting possibility of running it off one of the in-guard rails like +22 or -22 which would simplify the insulation. Like -42V, these power high-level signals that don't need to be isolated from transients. The 419A expects a quiet battery so I will see if I can squeeze an RC filter onto the board.UPDATE.9V LEDs are usable!My blind photocell that needs 3mA on Lumileds 24V and 60mA on 3V Cree JB2835BWT-G-H30GA0000-N0000001, needs* 12mA with 9V 2700K 80CRI Lumileds L128-2780EC35000B1* 20mA with 9V 3000K 80CRI Cree JK2835BWT-W-H40EC0000-N0000001

author: Kleinstein
author_rank: Super Contributor
timestamp: July 26, 2022, 05:54:35 pm
content: For the datasheet i have lloked at the 2002 Fairchild version. The main difference is whether looking at the on resistance or the the maximum current (e.g. some 2 V). For the current a higher drive seems to help, for the on resistance mot very much beyond some 6 mA. I don't expect the slew rate to be that relevant. The analog meter and input filter are slow anyway. I don't see a need to be faster than some 100 ms to come out of saturation.If the H11F1 really needs some 10 mA, I would looks for a different solution that does not get as hot. Photomos may be option, though a litte on the slow side (e.g. 0.2 to 2 ms).Even it the circuit may tolerate some ripple in the supply, it is still worth having supply filtering. The LED current part (especially with unequal current) is in sync with the chopper signal and can thus effect the result. Mains ripple would be far less critical. Filtering is easy (just a resistor (e.g. some 100 Ohms) and a electrolytic cap (e.g. 470 µF). There is anyway pelnty of voltage to loose on the series resistor.The spectrum can change a little with intensity, but not very much. The color temperature of the white LEDs can make a much biger difference. From looking at LDR datasheets it looks like they are most sensitive for green to orange light, just the part of the spectrum where most white LED are rather week. So both a green and an orange LED could be an alternative even though the brightness to the human eye may be lower. The extra red and blue has mainly a negative effect in heating up the LDR and possibly causing thermal EMF problems (more warm up drift). The other point with high power is heating up the circuit and this way indirectly causing possible drift from the LED intensity changing with temperature.

author: Dave Wise
author_rank: Regular Contributor
timestamp: July 27, 2022, 03:41:44 am
content: Looks like I could fit in a 6.3mm through-hole cap; that's either 47uF/50V or 100uF/35V. There's not enough room for 470uF. I don't think 100uF is big enough to make a difference.In the 740B, +/-22V don't have enough filter cap; we're stuck with out-guard supplies. Insulate the board to 500V. Use -42V or string it across +/-16.5V; these tolerate noise. Either connection can use 24V or 9V LEDs.The 741B doesn't have Guard. Use -34V (24V LED) or +25V (9V LED). These tolerate noise.In the 419A, disable the neon driver (disconnect XA2 pins 9 and 11) to save on battery. Use 9V LEDs, and string the new board across +/-13V behind an external RC filter. Insulate the board to 500V.In all cases, select the zener series resistor and the LED dropping resistors to get the appropriate current with your supply voltage. In the 419A, pick a supply voltage, tune pulse widths on an external power supply, then adjust the R in the RC filter to get that voltage.I tested two eBay H11F3's, one marked GE, the other Harris. Both far exceeded H11F1 datasheet Figure 2 Output Characteristics current, and U3 would not need 16mA drive. Put them on a curve tracer, with LED on Base and FET on Collector, and see how much you need to make the switch conduct 0.5mA .I'll put all this in an Installation Guide, if there are enough people in the world interested enough to use it.

author: Dave Wise
author_rank: Regular Contributor
timestamp: August 02, 2022, 02:50:03 am
content: Boards ordered from OSHPark.

author: Dave Wise
author_rank: Regular Contributor
timestamp: August 18, 2022, 06:58:43 pm
content: Assembling one to see if I need to fine-tune any mechanical interference issues. The A16 meter-loop chopper assembly is near some HV points on the A11 board.

author: Dave Wise
author_rank: Regular Contributor
timestamp: August 22, 2022, 05:09:31 pm
content: Board v1.0 has some trouble. It's fine in the spacious and quiet A17 position, but in A16 you have to bend the connector pins 45 degrees to avoid nearby board A11. New v1.2 layout ordered. This one has connectors up high to avoid mechanical interference. It's also EMI-hardened because A11 emits high-voltage spikes.

author: Dave Wise
author_rank: Regular Contributor
timestamp: August 29, 2022, 08:08:36 pm
content: Pictures of v1.0 board and how it's installed in the chopper module. I'm using a scrap of 0.01-inch clear PET film as the insulator. It was the packaging for some random trinket at the store. My Fluke 895A reads > 2e12 ohms at 1000V. If Henkel doesn't warn me off it, I will secure the film to the chopper housing with Loctite "Stik-N-Seal" Outdoor Adhesive, then secure the board to the film with dabs of the same filling the voids around the PTFE turret insulators.

author: Dave Wise
author_rank: Regular Contributor
timestamp: September 13, 2022, 06:44:43 pm
content: Henkel says, they don't test Stik-N-Seal for electrical, therefore, don't use it for electrical.I tested it and it's fine. It's a synthetic rubber in a volatile solvent. With dielectric absorption settled and about 50% relative humidity, I read about 1e14 ohms at 1000V sustained for an hour. Understand, I'm using it because I have it, and it's cheap and abundant. Normally, Guard and In-minus are at the same potential, but the spec allows them to be 500V apart and I don't want to be the weak link.A scope probe bypassed by 10K in the A16 space picks up about 200mV peak-to-peak from A11 (home of HV rectifier A11CR1) when the instrument is idling. This will not interfere with calibration. (I see about 1V at 1000V out.) Anyway, you can calibrate the module outside the instrument - unless your photocells are marginal (in which case why are you doing this?), just select and trim the frequency and leave the pulse timing at default.I made neon-less mounting plates out of polycarbonate sheet. You could also cut up a CD/DVD/BD. I'll include a dimensioned sketch in the writeup.Since 740B#2 contains my input protection re-design, I've replaced A9DS1 with 30V TVS diodes across A17V1 and V2. This will protect them from surges in case the operator accidentally switches range or mode with high voltage present. As soon as I've verified all the program's calibration functions, I'll build a V1.2 board, set its LED drive for A16's photocells, pull out the KVD module, and install A16 and A17.The attached picture shows the A17 powered on the bench. V2 is brighter because that photocell is blind. It doesn't show in the picture but I had to replace the old capacitor because (a) it's too bulky and (b) one lead is too short. I threw together a cal harness, it reuses the 5-pin ICSP jack.

author: Dave Wise
author_rank: Regular Contributor
timestamp: November 16, 2022, 08:05:24 pm
content: I have installed my hybrid modems in 740B number two. It's looking good. They easily exceed efficiency and resistance requirements. There's less drift, and less input current. (Compared to previous quad-photofet modems.) After I've recorded some observations, I'll modify 740B number one. I'm continuing to test as time and energy permits, and I'm working on a writeup.

author: Dave Wise
author_rank: Regular Contributor
timestamp: December 12, 2022, 10:41:57 pm
content: Here it is. For 740, 741, 3420, and 419, with possibility of repurpose elsewhere.Writeup, MPASM source code and hex, and KiCad schematic and board layout. Please look it over and comment.

author: Dave Wise
author_rank: Regular Contributor
timestamp: December 13, 2022, 05:00:48 pm
content: Fix a couple of typos in the writeup, and upload its LibreOffice Writer source.Meanwhile, I'm still goofing around with individual photocells... probably reproducing bits of 1950s-era research. The time it takes for a cell to recover to ten times its lit resistance, T10 (and by extension T5 which I was using) turns out to have an essentially linear relationship with R0, the lit resistance itself. Dim -> long. But the slope is much less than 1, so going dim is good (dR/dt gets faster and faster) until you run into cell/load voltage divider losses. Intercepts are all over the place but always positive.Most cells have a slight upward curve on their R/t graph, but one cell has a distinct curve the other way which suggests a parasitic conductance alongside the light-sensitive one.

author: Dave Wise
author_rank: Regular Contributor
timestamp: January 11, 2023, 06:34:08 pm
content: Both 740B's are looking good with my hybrid choppers.I updated the writeup to version 1.1 . Most of the change is a simpler board insulator, LDPE film instead of semirigid PET. The LDPE is easy to cut with a craft knife and you can use the board itself as a template.Just for fun I measured the generic CdS photocells in my junkbox. Their T5 runs the gamut from five times the HP cells, to 25 times. Chopper cells are super fast. According to a Journal article (I think), their active material is a mix of CdS and CdSe, the latter being fast but unstable. A couple of niche scientific companies make PbS and PbSe cells but they are crazy-expensive.I also used a tight beam to probe some HP cells looking for fast and slow spots. Nope, each cell is uniform across the whole active area. The slow cells were coated with a bad batch, not an insufficiently-mixed batch.I put the fastest eight cells in my two 740B's. Here are my notes for the other cells.PHOTOCELL DATACurrent for 20K, T(20K-100K)1 3uA 8ms NOT USABLE2 20uA 3.7ms USABLE BESIDE A FAST CELL3 50uA 13ms NOT USABLE4 20uA 3.2ms USABLE BESIDE A FAST CELL5 45uA 3.2ms MAYBE USABLE BESIDE A FAST CELL (1)6 15uA 2.8ms USABLE7 65uA 2.4ms GOOD8 20uA 2.1ms GOOD205 350uA 1.8ms GOOD (2)PC5191 7uA 4.4ms USABLE BESIDE A FAST CELL (3)(1) Cell resistance rises to a relatively low value asymptotically.(2) Was A9V1, marked PC5191-205(3) Was A9V1, marked PC5191Most cells R vs t curves upward a bit, so T(20K-100K) < 0.5 * T(10K-100K).T10 10K-100K 20K-200K 30K-300K 40K-400K2 5.3ms 7.8ms >10ms >10ms4 4.6ms 5.5ms TBD 9.3ms5 5.4ms 8.6ms >10ms >10ms6 3.8ms 5.1ms 6.0ms 7.0ms7 3.2ms 3.9ms 4.5ms 5.3ms8 2.6ms 3.5ms 4.2ms 4.9ms205 2.3ms 3.5ms 4.8ms 6.2msPC5191 5.3ms 6.8ms 8.0ms 9.4ms

author: Kleinstein
author_rank: Super Contributor
timestamp: January 11, 2023, 06:56:20 pm
content: The speed of the LDRs also effects the sensitivity. So an LDR that is faster is naturally also less sensitive. In the LDRs the photogenrated carriers or one sort are caught by trapping centers and as long as they are trapped the counterpart is free to contribute to the conductivity. Long life trapping centers give more conductivity (the same effect for a longer time), but of cause also a slow decay.The correlation can also be seen in the data for the old cells.The chopper cells are specially fast, but for this reason also not super sensitive. As such they arather special product.The PbS / PbSe cells are used because of the IR sensitity (AFAIR up to some 3500 nm) - the high price is likely because of low numbers and more precision ones for a measurement as opposed to the CdS cells made for low cost uses for a simple trigger level. Because of the IR sensitity I would expect them to be rather temperature sensitive.AFAIK there are a few rather special, fast photoresistors made from direct bandgap materials (e.g. InP, GaAs or such) - here fast means really fast with response in the ns range. It could be tricky to find them and they may be to low in resistance to get good high frequency performance.AFAIK the CdS/CdSe mix was also used in cells for exposure meters. The composition effects the response to different wavelengths a little.

author: Dave Wise
author_rank: Regular Contributor
timestamp: November 19, 2023, 05:07:15 pm
content: I'm still happily using my rehabilitated 740B's. I just noticed that I forgot to make my pcb design shareable at oshpark. I'm "davetheresurrector" there.In post #66 (July 26, 2022, 05:54:35 pm), Kleinstein wonders if photomos i.e. Solid-State Relay is a viable demodulator. I suppose so, but their timing delays are a nontrivial fraction of the sampling period so you'd have to drive them before the corresponding modulator edge. And I worry that the delays drift with temperature or time. I could add this photomos advance timing to my microcontroller program but I won't, because I'm completely satisfied with the H11F1's.