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Old 6th Jun 2022, 9:05 pm   #1
factory
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Default Advance digital multimeter DMM2 repair

I've finally got round to looking at the Advance DMM2 multimeter, I acquired from Welsh Anorak earlier this year. OK I've already sourced some new Isostat button caps from Bulgaria, which arrived last week, note: they are an odd size.
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Yesterday I took it apart, there is a bit of corrosion to some of the electrolytic capacitors, these will probably all get replaced at some point, but I'm not one for doing a mass recap before some testing.
This DMM uses a regular mains transformer, followed by a step up converter, which prevents bringing it up slowly on the mains. It can also be run from a low voltage battery.
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First power up and there was no display, not surprizing as all the PSU voltages were reading low, they were very slowly creeping up. I printed out the relevant parts of the manual, for the PSU area & some of the parts list, I noted the voltages this time, again a bit higher than before, these were;
TP5 (+190V) = +120V
TP6 (+5V) = +3.6V
TP7 (-12V) = -8.7V
TP8 (+17V) = +11.8V
Voltage across C39 was 9V and the mains power draw was approx 8W.

After turning it off, I checked to see if anything was overheating and found C48 (8uF 350V) was warm, clearly not reforming properly. This was temporarily replaced with a 4.7uf 250V cap (need to order something more suitable). I also gave the low voltage caps a quick check with the ESR meter and found C45 (40uF 16V) to be high ESR, the lead promptly fell off on removal (corroded), a 47uf 40V cap was fitted in it's place.
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After powering it up again, the display was fully lit for the first time and voltages measured in at;
TP5 (+190V) = +180V
TP6 (+5V) = +5.2V
TP7 (-12V) = -11.8V
TP8 (+17V) = +16.4V

The next problem is I have is, more than one digit lit at the same time in each display. The input is functioning, as the digits change when switching the voltage range, the test source is a 3.7V back up battery (ex Tek scope).
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I will be investigating the decoder IC and associated switching transistors next weekend. I've had a similar fault with an AVO and several of the display driver transistors were shorted, it also uses the same decoder IC.

David
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Old 8th Jun 2022, 2:18 pm   #2
hamid_1
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Default Re: Advance digital multimeter DMM2 repair

Is the display driven by a 74141 chip or equivalent? I think it's IC4 on the circuit board.

I have had the same fault in a nixie tube clock, where 2 digits were lit together in the same tube. This is supposed to be impossible if the 74141 is working correctly. Replacing the 74141 cured the fault. It seems these chips are prone to developing internal leakage or short-circuits, causing more than one digit to be on at the same time.

In this multimeter it looks like one IC drives 3 nixie tubes which are multiplexed. A faulty 74141 IC would cause the same fault in all 3 tubes.

You can buy new old stock 74141 chips. There is also a Soviet (Russian?) 74141 equivalent called K155 or KM155ID1 . Those may have been manufactured more recently than the old 74141 ICs.

I suggest buying at least two 74141 or equivalent, and fit an IC socket in place of IC4. That way, if a NOS chip is faulty or it fails again you can easily swap it for another one.
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Old 8th Jun 2022, 7:48 pm   #3
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Default Re: Advance digital multimeter DMM2 repair

It's a 996079 (Fairchild/SGS) from the counter uLogic range, unfortunately not interchangeable with the 74141, but having said that I don't think I've had any problems with them in the past, I have several counters and an AVO DMM that use them too. I should probably obtain a few spare 996079 as I don't seem to have any, but I do have plenty spares of the 74141 & 7441.
Here is a better picture of the display decoder section.
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HP on the other hand used their own custom part*, completely different again and I've had a lot more trouble with those, the only source of spares is the boards that probably get thrown directly in the trash, when people rob the displays for n***e cl**ks.
*except for one item which used regular 74141, renumbered with a HP part code.

David
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Old 9th Jun 2022, 4:34 pm   #4
hamid_1
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Default Re: Advance digital multimeter DMM2 repair

The 996079 and 74141 display drivers are not directly interchangeable - the pinout is different. But they are functionally the same. It's possible to make an adaptor. See attached zip file. The PDF within it shows how one chip can be converted to the other - it's just wires. The other files are for making an adaptor PCB to make the job easier but you can simply wire it up by hand using the diagram. Maybe worth a try if you can't get an original 9960 or 996079 display driver. And the modification is reversible.

Since you already have some spare 74141s, this could help you to determine whether the fault is in the display driver or somewhere else.

P.S. No vintage test gear was destroyed when I built my nixie clock 17 years ago. I used NOS ITT GNP-17A tubes from Colomor Electronics. The Soviet KM155ID1 driver chips have been running the clock continuously for more than 16 years, so that makes them pretty reliable.
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Old 19th Jun 2022, 2:35 pm   #5
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Default Re: Advance digital multimeter DMM2 repair

Interesting adaptor, but it has a problem, the BCD decoder outputs of the 996079 are +V for 0 & 0V for logic 1, the 74141 has the opposite for the BCD output code. The adaptor board would need some NOT gates (or NAND wired for NOT) adding. There might also be a problem for devices that use lower supply voltages for the DTL/RTL logic too, the DMM2 uses +5V so not a problem here.

The BCD code should give a clue as to why I have double numbers showing, looking at the BCD outputs from the fancy custom counter IC, two are faulty with some weird negative output pulses (the counter IC does use -12V too). The two faulty output are the lowest, i.e. BCD 1 & 2. BCD 4 & 8 seem to be working OK.
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To find out if the problem was with the custom IC or the decoder IC, I disconnected the BCD lines between them and hooked up some test clips, to go through the BCD code & see what is displayed. This proved the 960 decoder IC was working fine & for the first time I've got the separate "1" neon to light. OL lights with with BCD code for 2.
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I'm now working on trying to invert the faulty outputs, with some 741 Op-Amp ICs, using the +5V & -12V rails as the supply, I've got something partially working, more in the next post.

David
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Old 4th Dec 2022, 10:56 pm   #6
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Default Re: Advance digital multimeter DMM2 repair

Very much a work in progress, but I've been working on a replacement for the custom display counter/multiplexer IC for the DMM2, if I can get it fully working then I will look at getting a small board made with surface mount parts.

This part is the display latch & multiplexer, using some BCD switches in place of the counter stage (to test if I needed to invert anything on the multiplexer part), needs further testing with the inverter IC bypassed, as I think that might be why the numbers are in the wrong order.

David
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Old 12th Dec 2022, 7:18 pm   #7
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Default Re: Advance digital multimeter DMM2 repair

I've spent a bit more time trying to figure out why the digits are appear in the wrong order, I think I've worked it out.

The sequence for the P & Q outputs (anodes) is;

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The sequence for the 74153 multiplexers is;

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This explains why the I'm getting the numbers in the wrong order, the sequence for the P,Q select outputs is out of sync with the 74153 multiplexers select inputs, all I need to do is correct the wiring, then I can move on to adding the counter stage.

David
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Old 12th Dec 2022, 11:02 pm   #8
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Default Re: Advance digital multimeter DMM2 repair

I have had the multi-digit effect and I think mine was caused by an open circuit.

Nixie tube drivers rely on one element being lit. They cannot turn them all off because then the voltage rises too high and exceeds the rating of the driver chip. So if one digit becomes open circuit then you get multiple elements lit.
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Old 13th Dec 2022, 3:30 pm   #9
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Default Re: Advance digital multimeter DMM2 repair

That can happen, but in my case the problem was the output from IC3 (the custom counter/latch/multiplexer IC), the outputs for BCD 1&2 had failed, the 996079 driver IC uses negative logic, the databook doesn't give information for BCD code above 9, the incorrect codes can give double numbers.

But in some other types of driver IC, they are used to provide blanking, HP used this in some of their counters, to blank digits that are not in use, some even had a switch to disable this and have the zeros displayed instead.

David
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Old 15th Dec 2022, 9:29 pm   #10
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Default Re: Advance digital multimeter DMM2 repair

Had to move the wiring to the BCD switches, to get the correct sequence.
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After that was resolved I added the 7490 ICs and was able to check the count was functioning correctly, using the logic pulser. Supply voltage was quite low with the 7490s, down to 4.1V.
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I managed to find three (out of four) 74LS90s and have now got 4.6V, still a bit low (forgot to order 74HCT390s in through hole version , 74HCT90 never made as far as I know).
Connected up count, load, reset & carry lines to the mainboard, it's sort of working, but reads about ten times the input voltage or resistance (shown on 20kΩ range in picture), I'm not getting the one thousand digit, or overload neon lighting at the moment.
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Will look at the inputs/outputs with the DSO at the weekend and compare with the logic analyzer results that were posted on eevblog by someone else with a faulty IC3. https://www.eevblog.com/forum/repair...594/#msg748594 I don't have a working logic analyzer.
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David
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Old 29th Dec 2022, 9:06 pm   #11
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Default Re: Advance digital multimeter DMM2 repair

A while ago I had a look at the inputs/outputs with the DSO, everything seemed OK apart from the number of count pulses before the carry output changes, this should be approx 1000 (or 999 from the logic analyser result taken by someone else), measuring ten count pulses and the width of the carry pulse I calculated I was getting approx 2400 pulses before it changed, no wonder things were very wrong, looking more closely with the zoom it was obvious the supply voltage was the problem, many pulses were quite low.

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The week before Christmas I put in a order with Mouser for some 74HCT390 and a few other bits, these arrived in four days.

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Today I built up a vero board with the 74HCT390s, I now have 5.1V on the supply rail, but seem to have another problem, need to check if it's a wiring error on the board I've made, or a dead tube.

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David
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Old 1st Jan 2023, 11:02 pm   #12
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Default Re: Advance digital multimeter DMM2 repair

Well it wasn't a wiring problem or faulty tube, turned out to be related to swapping out the 74LS74 for a lower power HCT version, it didn't have enough output power to drive the two multiplex ICs and the P, Q outputs. One of the P, Q lines was stuck at zero volts, now back working with the LS version.

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Just got one more problem to figure out now, the BCD counters switch the carry output at 1000 counts, but I need to figure out how to get it to display 0000 at this point, as at present I have sensible readings on volts & ohms if you minus 1000 from the displayed result, I thought this would happen.

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David

Last edited by factory; 1st Jan 2023 at 11:10 pm.
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Old 2nd Jan 2023, 6:17 pm   #13
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Default Re: Advance digital multimeter DMM2 repair

Could you replace your leading ONE with another nixie connected so that only ONE and ZERO can display. It sounds to me as though you could soon fix the digital part of problem.
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Old 2nd Jan 2023, 9:40 pm   #14
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Default Re: Advance digital multimeter DMM2 repair

They actually used three connections (all common connections due to the multiplexing), 0 is a hidden neon inside, 1 is the longer neon in the display and 2 is used as the overload indicator. There isn't really room to add another tube the same size as the others.

I've currently set up another counter stage using a 74HCT4040 (I was surprized the I had that one, draw was for the HC version), with an 8 input NAND, feeding a DG412 analog switch IC to toggle the count between the two different counter stages, it's currently set to switch at 999 counts.

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This has corrected the 99% of the digital problem, but still needs some minor adjustments, as the LSD is displaying 2 with no input, or shorted input. It was at 1 yesterday (as shown in the picture in my last post), the zero adjustment will not lower it.

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I now have more sensible readings, but can't get the full count of 1999, I suspect the problem is elsewhere. It tops out around 1500 ish, will try again with a mV source on the 200mV range, next weekend.

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I've also noticed the PSU is regulated on the primary side of the simple switching supply, it was low at around +9V (manual states to set it at +9.5V, before the zero adjustment), setting it to +9.5V gives +5.6V on the +5V rail, I'm not really sure how close the various secondary low voltage supply rails should be (they are listed as +17V, -12V, +5V), may have to put the original (defective) IC3 back to test, which will be tricky as pin 1 (+5V) fell off due to iron oxide issues.
Edit: the voltages were checked in post 1, C39 was at 9V then.

David

Last edited by factory; 2nd Jan 2023 at 10:07 pm.
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Old 2nd Jan 2023, 11:50 pm   #15
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Default Re: Advance digital multimeter DMM2 repair

One of these( or possibly a DMM1) came to me with a Nixie tube not lit. I was a little concerned, but a poke about with a scope showed waveforms reaching the tube, and the base of the controlling transistor The transistor associated with the tube had gone o/c c-e. ISTR it was a 2N5401( PNP?). I think I popped in a BCY72 just to see if it worked, it did, so I fitted the correct part recovered from a scrap board.
I remember jumpering across from the tube next to the duff one and had a digit on the duff one light, so I knew the tube was OK.
Also ISTR a lot of the blue electrolytic caps were going/ had gone low value.
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Old 3rd Jan 2023, 10:07 pm   #16
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Default Re: Advance digital multimeter DMM2 repair

That's very close, they used 2N5400 for switching the indicator tube anodes, not had any fail in this one yet, but have had similar problems with another DMM.

Most of the blue electrolytics are in the PSU section, had the Erie 350V one fail to reform and one of the Mullards the lead fell off due to corrosion, others checked OK, but will probably get replaced soon. Quite a few electrolytics on the AC measurement board look corroded, but I'm not too worried about them moment, need to get the DC side working properly first.

Probably need to check C50, part of the line frequency based reset circuit, it's listed as a 1.6uF 25V electrolytic, same as C24 (C23 recharge comparator circuit) & C27 (reverse polarity circuit), any of these could be gone low value.

David
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