HP-3440 - Nixie multimeter - Repair or not repair? - Page 1
Forum section: electronics / repair

author: Paulo Peres
author_rank: Contributor
timestamp: December 24, 2023, 06:14:54 pm
content: Hi All!Merry Christmas!Today I was checking if my old HP-3440 was working and discovered that an old issue with one of the displays is back.I rescued this equipment from a scrap dump almost 20 years ago with same problem.This equipment is notorious because it is a Digital Multimeter that does not use integrated circuits.Internally, it is a frequency counter that uses the read voltage values to change an oscillator frequency. Basically, it's a voltage-frequency converter.At the time of its conception, the suitable display for the application was the nixie tube, so all circuitry gravitates around it.Turn out that the counter are made with two transistor flip-flop cells, that produces a BCD code as result of the counting. Each BCD digit is produced thru its own board with a nixie display. There is 4 identical boards here. And each digit must be converted to the decimal code used in the nixies.As I said, it doesn't use ICs. So its creators conceived a solution that had the touch of a genius of the HP designers of the time: a baquelite box with 8 neon bulbs at one side and a conveniently designed PCB on opposite side. And under the bulbs light beam, a small layer of photo-conductive material is painted on PCB (see photos). The BCD digit shown by the neon bulbs is decoded by the electrical path formed when the photoresistive paint is illuminated.Wonderful, isn't?But let's back to the unit on my bench.Its a equipment manufactured in 1971, full of carbon mass resistors (yes, that cursed ones). The repair I did years ago was done with silver ink, because there was interrupted tracks in the PCB with the photoresists and was effective.Today I tested and found no broken tracks, so it could be something else. I haven't yet devised a proper test for the entire circuit, but I'm wondering: should I really try to fix this?I mean, I tested some of that resistors and found up to 7% of resistance drift on some 5% units. If the photoresistive PCB is defective, I have no replacement and would have to redesign it with some kind of modern equivalent component.So, I'm asking to the colleagues that uses to repair this things: Do you think it worth the effort? The general accuracy of the equipment is fine (the measurement circuits have more stable components), but I'm not sure if I shoud change something in the original design just to make it work again. It is a wonder of its time just because that circuitry.What do you think?

author: factory
author_rank: Super Contributor
timestamp: December 24, 2023, 07:03:26 pm
content: Repair it, early DVMs are getting much harder to find, far too many people destroy our history, many more frequency counters were made than this DVM.A few of the boards are shared with HP counters of the era, they did replace the optical decoder section with a CMOS BCD IC & 10 transistors, in the 1980s for the 5245L, not found any of these later boards, but I believe they won't fade from one digit to another in the same way.The decoder neons are a known problem as they age, they can fail to light in darkness. The decoder plate corrosion is a less common problem, I've had this too, I bought a selection of used boards for parts.The carbon resistors on the counter boards should be a problem, other brands used 10% resistors and 20% ceramic caps instead of the silver micas, for the same type of circuit, more critical areas will use film resistors.David

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 25, 2023, 12:56:39 am
content: ++ for the repair and please publish a short summary under this thread.The multi function unit makes this device useful even in today's workbench or just as a kind of 'living fossil'.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 25, 2023, 01:24:30 am
content: If you have the time, resources, and skill.... then repair it.If not, then give it to someone who does.Most definitely do NOT throw it out in e-waste!!!

author: Paulo Peres
author_rank: Contributor
timestamp: December 25, 2023, 02:17:35 am
content: QuoteWell... I didn't mean to scrap it.My main question is how far should I go if I can't find original replacements. The Russian version of the 7442 with some extra circuitry could be the basis for a functional equivalence of the board, but this is an instrument designed *without* ICs.I don't know. It's like swapping a Spitfire's Merlin for a PT6 turbine.It could work, but the thing loses its soul.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 25, 2023, 02:35:58 am
content: Don't be so pessimistic.You have four identical counter/nixie driver modules, so You can swap them, compare the waveforms etc. to narrow the culprit.From my experience the most common issue with that modules are the germanium transistors, not the custom LDR arrays.

author: Stray Electron
author_rank: Super Contributor
timestamp: December 25, 2023, 03:05:27 am
content: If you do decide to not repair it, I could use three of the Nixies to replace the three that I'm missing in my HP 5321B Freq Counters.IIRC my counter (built in 1969) uses 7442s and one of them is also missing. It looks like someone tried to work on it and made a pretty good mess of it and also left some of the parts out!

author: Arts
author_rank: Regular Contributor
timestamp: December 25, 2023, 04:36:12 am
content: Repair it; defects in these units are rarely terminal and are generally easily fixed. They can take some time to track down though.And even if you do have to make some modifications, meh. Whatever you need to do to keep her running today can always be reversed tomorrow when a better solution becomes available

author: factory
author_rank: Super Contributor
timestamp: December 25, 2023, 11:25:29 am
content: Quote from: doktor pyta on December 25, 2023, 02:35:58 amThis might vary depending where in the world you are & how badly TE is treated, here in the UK they get abandoned in cold damp storage conditions (lofts, sheds, garages), corrosion of those traces on those ceramic phototransistor plates is definitely a problem here. Along with rusty iron wires to the various components (neon lamps included), they break off.Not seen much problems with the Ge transistors on the low speed boards, from the HP TE in my collection, but the higher speed boards with early Si transistors, I've had a few failures on those.Quote from: Stray Electron on December 25, 2023, 03:05:27 amThis has almost nothing in common with this DVM, it's a whole generation newer, also the counter ICs are all custom parts designed by HP, there is a HP Journal covering those. Only a few HP counters of this age used standard 74xx series logic in the display counter and the 5321 isn't one of them.David

author: factory
author_rank: Super Contributor
timestamp: December 25, 2023, 12:09:35 pm
content: Quote from: Paulo Peres on December 25, 2023, 02:17:35 amThis is the evolution of the 5212L-4A latching display counter board (+1224 logic variant);The original was 5212L-4A, Ge diodes & transistors for counter part, neon optical decoder for the display & storage.This became 05212-6016 after HP renumbered the boards, older boards used Ge parts, later ones (around mid 1970s) replaced the Ge parts in the counter part with Si parts, Ge 2N404 got replaced by Si MPS404A (still made in surface mount form), a few resistors & capacitor also changed with the Ge replacement.The last versions 05212-6018 (1979) & 05212-06027 (1980) replaced the optical decoder part with a CMOS 4514 or 14514 decoder/latch IC, 10 transistors, the IC power supply was a Zener based circuit.David

author: jonpaul
author_rank: Super Contributor
timestamp: December 25, 2023, 12:40:05 pm
content: MUSEUM PIECE! Valuable to save.easy to check the Nixies with a 300V PSU and resistor.Most switches and PCB edge connectors may have corrosion/oxidation, through cleaning first.j

author: Paulo Peres
author_rank: Contributor
timestamp: December 25, 2023, 08:46:36 pm
content: Thank you guys!I'm convinced. I will repair the DVM.I'll post the pics here...

author: factory
author_rank: Super Contributor
timestamp: December 27, 2023, 12:09:32 pm
content: This was hard to find a picture of, it's not suitable for the 3440A, as this one is for an option the 3440A never had, but to give an idea of the short lived 1979 design, with a extra board added to the older design board, before they redesigned it to get everything onto one board.David

author: Paulo Peres
author_rank: Contributor
timestamp: December 27, 2023, 05:21:40 pm
content: Wow...Very interesting. Is not a direct replacement, but is something to think about.I found the schematics. Thank you!

author: factory
author_rank: Super Contributor
timestamp: December 27, 2023, 07:52:51 pm
content: That diagram iswrongfor your boards, there are no pictures of the +1224 BCD logic version on the web.The diagram in reply #9 is thecorrect+1224 BCD logic version, see the different outputs used from the decoder IC. The photoresistor plate was made in two versions for the same reasons.Of course you should figure what parts are the problem first.David

author: Paulo Peres
author_rank: Contributor
timestamp: December 27, 2023, 08:28:24 pm
content: QuoteYou are right.I liked the idea and didn't pay attention to the details.***Later today I will do some tests in the instrument. I'll share the results.

author: Paulo Peres
author_rank: Contributor
timestamp: December 28, 2023, 02:26:09 pm
content: And here we go...I did some tests and found some things with wrong values (voltages, mainly).These are some things I saw:This is how my Fluke 8050A reads the test voltage (11.83V)And this the HP-3840/3844 versionThis is how my Fluke 8050A reads the test voltage (38V)And this the HP-3840/3844 versionThe semiconductors are apparently ok, and when I change the module position the defect follows it. I did some comparative tests too, setting the two more significative displays to 3 (33V) and checking the flip-flops Q outputs, them comparing the results. No significant differences observed.D flip flip in both boards. The result for C is the same. These waveforms was acquired in cathodes of CR5&CR6 and CR7&CR8. All in 20V/div scale (2V/div signaling is because the last 2 probes don't have the attenuator signaling).I'll check the rest of circuitry from begin before continue.

author: factory
author_rank: Super Contributor
timestamp: December 28, 2023, 07:26:35 pm
content: That haze with no single digit lit, is how mine with faulty photoresistor plates were, I proved this by swapping the plates from one board to another. Be careful if doing this. I avoid removing the tubes unless absolutely necessary as they can crack, they are often in poor condition from pin corrosion (look up oxide jacking) in the UK.The picture in the first post shows possible corrosion in the circled areas, also the repair could be bridging traces (silver migration?).David

author: coromonadalix
author_rank: Super Contributor
timestamp: December 29, 2023, 10:51:37 am
content: it is damaged on many places ... not just the red circles

author: Paulo Peres
author_rank: Contributor
timestamp: December 30, 2023, 12:33:46 am
content: Well...Actually I tested the red circles and these points are ok. The blurred track at lower right is the old silver ink. The other track is just scratched, not cut.I also swaped the photoresist boards between two modules and the problem really is in the pictured board.Tomorrow I'll check it again with a microscope.I was playing to the schematics and the logic to narrow the search. I believe I can fix a broken track, even if tiny. But if is the photoresist, I'll need another board.

author: Paulo Peres
author_rank: Contributor
timestamp: January 01, 2024, 06:23:41 pm
content: Yep!I checked the board with the microscope (unfortunately its a old model and I have no reliable means to take good pics).The tracks are a bit ugly, with several scratches but without interruptions.I have a problem with the photoresist.Happy new year, btw.

author: Paulo Peres
author_rank: Contributor
timestamp: January 03, 2024, 03:00:23 am
content: While I don't have a solution for the photoresist, I still troubleshooting the DVM.I found a bad Electrolytic Cap and was able to get a fair calibration, but now I found this:

author: factory
author_rank: Super Contributor
timestamp: January 03, 2024, 08:25:14 pm
content: Try a wanted post for the photoresistor, or a another board.The thumbnail picture showing the broken wire, I can't seem to make it bigger, it worked fine this morning.I've got a 3444A here somewhere, I could have a look to see where it connects to.David

author: The Doktor
author_rank: Regular Contributor
timestamp: January 03, 2024, 08:34:41 pm
content: I do have one possible suggestion for replacing the photoresist section. I am not at all familiar with this meter, maybe somebody who is can say whether my idea is workable or just complete nonsense, but either way here goes. Is there any chance of using a bunch of CDS cells to replace the photoresist strips? You can buy a bag of 20 of them for about three dollars off of aliexpress. It would probably not be a very pretty repair, but as I doubt the resistance values are supercritical, it may very well work.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2024, 09:03:38 pm
content: There are some 17 LDRs in the chip and they see quite some voltage (150 V). The available space for the LDRs circuit is limited and especially the smaller LDRs may not like the high voltage.Also the neons have a limited size and thus a size limite from that too.The easier replcement would be with a decoder chip like in later versions.

author: The Doktor
author_rank: Regular Contributor
timestamp: January 03, 2024, 09:17:34 pm
content: I see. It sounds like the LDRs drive the nixie tubes directly. I do suspect a CDS cell would not appreciate 150 V, especially if it was a small one which would have a chance of fitting in that space. It seemed like a good idea at the time

author: Paulo Peres
author_rank: Contributor
timestamp: January 03, 2024, 10:16:11 pm
content: QuoteThank you David!Here's the photo again.I found this wire wrapped around the potentiometer shaft. Apparently it got loose and tangled the wire.At some point I broke it trying to calibrate the plugin.The wire is connected in the 1.6164M resistor.

author: duak
author_rank: Super Contributor
timestamp: January 04, 2024, 01:40:57 am
content: A 7441 driver output is rated for about 70 VDC, tops. There's a little trick in these circuits. In a gas discharge tube, there's essentially no current flow if the tube is not glowing, ie. no conduction through the gas until the gas atoms are ionized. When the output transistor turns on, its collector goes close to 0 V and about 180 V is applied across the gas in the tube that causes ionization and conduction. When the transistor shuts off, its Vce rises, the current falls and at some point there isn't enough voltage across the tube to maintain ionization, the tube goes open circuit and the current stops. I'd bet it's a close run thing. If the transistor BVce was say only 30 V, the circuit wouldn't work as the transistor would break down and continue to conduct. It would be interesting to see a 7441 die to analyze the output transistor structure and drive circuit to see what the designers did.In the HP driver, the CdS cell only has to have a high enough resistance to reduce the current enough to extinguish the ionization and cause the tube to go open circuit. I think a megohm might be enough, but I don't know nor do I have a Nixie to try out.

author: factory
author_rank: Super Contributor
timestamp: January 04, 2024, 08:02:46 pm
content: Quote from: Paulo Peres on January 03, 2024, 10:16:11 pmOK, the broken wire is white orange yellow, this is wire 934. To read these, the first number is the base color, large stripe is second number and thin stripe is third number.Looking at the barely readable diagram it connects to 3 switch wafers and the 1.6164M resistor, will have a closer look at the module once it is retrieved, hopefully at the weekend.David

author: factory
author_rank: Super Contributor
timestamp: January 04, 2024, 08:31:45 pm
content: Quote from: duak on January 04, 2024, 01:40:57 am7441 or 74141 would be no use here, the standard display counter/storage/decoder board in the 3440 is +1224 BCD code. The neons are used as display code storage too.There are no die pictures for any driver IC on the forum AFAIK, but the HP custom driver die mask is shown in this journal.https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1967-08.pdfGoing back to my suggestion of looking for a spare board, it's the most common type used in varous counters, these boards turn up from time to time, I got some from Sphere (minus the tubes of course) a few years ago, of both 1224 & 1248 BCD logic, as I have many devices here that use these and I've have several go bad from corrosion.David

author: Paulo Peres
author_rank: Contributor
timestamp: January 05, 2024, 01:30:21 am
content: Thank you again David!I'm managing to buy another board.While I was debugging it I found something that cam be useful for others. I'll share what I did and the results.First, I placed a known good board at a known distance of a light source." alt="" class="bbc_img" />(Hmm... I need clean that floor...)So, I numbered the pins according to the resistor 0845-0001:" alt="" class="bbc_img" />I also had desoldered the two terminals at right and referenced the multimeter at the lower terminal" alt="" class="bbc_img" />When I measured with the lid closed, all numbered terminals have a impedance greater than 2M ohms (actually it can reach tens of mega ohms), in both boards.When I did the measurements with the board exposed and under the light, I got the values below:Good board:1 - 4k2 - 4k3 - 4k4 - 4k5 - 4k6 - 3k7 - 4k8 - 4k9 - 5k10 - 5kBad board:1 - 4k2 - 4k3 - 15k4 - 15k5 - 15k6 - 3k7 - 3k8 - 3k9 - 4k10 - 16kThe distance of the light source is not so critical. It just need to be the same for all the boards.About the values, I considered only the stable digits. I mean, values oscillating from 3.4 to 3.7 (for instance; I know that its the wrong way to round a number) was registered simply as 3. I'm interested basically in the orders of magnitude, not absolute values.So, I think is reasonable assume that uniform values should be evidence of a good board.

author: factory
author_rank: Super Contributor
timestamp: January 05, 2024, 06:58:36 pm
content: Some pictures of the 3444A wiring, showing the 934 wire.And the switch diagram from the change sheet.David

author: Paulo Peres
author_rank: Contributor
timestamp: January 06, 2024, 02:34:31 am
content: And I'm glad to say that the ohms scales are back!Thank you again David!