Advance instruments DMM3 - Page 1
Forum section: electronics / repair

author: Slh
author_rank: Regular Contributor
timestamp: August 15, 2025, 07:07:40 am
content: Just picked this up for the princely sum of £3. I was assured that it was working by the seller who was waxing lyrically about the Nixie tubes while haggling himself down from £5 to £4. It's going to cost me more to get power cables for it!Anyway, it displays 000 (unless volts is chosen in which case the + lights up) which suggests that the power supply is probably fine. I might clean the switch contacts but I have a feeling that there's an issue with the ADC somewhere and I'm not sure if the leading 1/0 should light up. Does anyone have the full manual for it? I found a partial manual with a schematic but I have no idea what the actual specs are.This is going to be a long term restoration as I don't have much time for it but it would be nice to have it lurking in the background for SiC and GaN power supply development

author: factory
author_rank: Super Contributor
timestamp: August 15, 2025, 09:17:26 am
content: You robbed that seller, they cost a lot more on feebay.I've got a couple of these (& the older version), most of them have had issues with the 4mm diameter capacitors going short circuit, preventing various things from working. I remember one that would only work in one polarity, then stopped completely when a cap shorted.David

author: Slh
author_rank: Regular Contributor
timestamp: August 15, 2025, 10:47:04 am
content: I did all my research after I got home from the flea market and was impressed at the online prices! I guess it's just because of the Nixie tubes? That's the only reason I got it. Guess the seller was tired of carting it around.Thanks for the tip. I'll have a look at those caps.Cheers,Stephen

author: Slh
author_rank: Regular Contributor
timestamp: August 16, 2025, 06:23:13 am
content: So far, I've opened it up and confirmed that all the power supplies are good apart from the 5V which measured 2.5V. i guess I'll look around there.

author: factory
author_rank: Super Contributor
timestamp: August 16, 2025, 09:20:00 pm
content: Is this scan any better than the one you probably found already?David

author: Slh
author_rank: Regular Contributor
timestamp: August 18, 2025, 11:56:37 am
content: Hi David,Yes that's clearer thank you!Cheers,Stephen

author: Slh
author_rank: Regular Contributor
timestamp: August 19, 2025, 12:53:23 pm
content: Replacing C46 and C47 with 22uF fixed the 5V. They were both measuring <1uF.No change to overall function though so the hunt continues (this will be a long slow hunt ad I have to squeeze it around a toddler).

author: Slh
author_rank: Regular Contributor
timestamp: August 24, 2025, 07:20:21 pm
content: C34 was also knackered. It should have been 1u6 so I put a 1uF in and that started the GT (gate?) signal.Unfortunately, a small probing issue established that the designers ran 190V right through all the low voltage stuff as I slipped while repositioning a probe on the CA (carry) signal. D37 is on the 190V and stays high voltage for more than a few seconds after turning the unit off. A brave transistor (TR23) sacrificed itself to protect IC3 so now I need to find a 2N2369 or equivalent rather the unique looking IC3.Annoyingly, the transistor is obsolete but I can at least put a 2n2904 in to make it kind of work. I assume that the faster switching of the 2n2369 is required for accuracy.

author: factory
author_rank: Super Contributor
timestamp: August 24, 2025, 10:02:41 pm
content: If you can't find a 2N2369, try a BSX20 instead (which is the part listed in the manual, they are interchangeable), unfortunately they seem to be on last time buy, a lot of through hole parts are going obsolete.I recommend changing the other 1.6uF capacitors, they are very unreliable, C34 was one that shorted during testing in mine, the other had the C60 fail short, which stopped the polarity indication working properly. C54 is the only other one, it's across a 12V rail. For these I selected from some new 2.2uF, choosing those that measured a bit low, I've got a few expensive 4mm diameter 1.8uF to try for the DMM2, where space is a bit limited.Be careful probing, there is a very dead Philips counter here which I had a probe mishap with.I've got a prototype replacement for the custom IC, just haven't got round to completing the design for a getting a PCB made, not helped after I figured it doesn't reset the counter part to 0000.David

author: TimFox
author_rank: Super Contributor
timestamp: August 24, 2025, 10:24:46 pm
content: Mouser (US) has classic 2N2369 and 2N2369A TO-18 (metal case) in stock, for roughly $4.00 ea in singles.https://www.mouser.com/c/?q=2N2369&instock=y