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Need help with a Weston 1242 Nixie Tube Multimeter that is acting weird

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Just picked up a Weston 1242 Multimeter, and it's exhibiting some weird behavior.  It displays garbage, and acts like it's locked in an over-range state.  The mA input fuse was blown when I got it, so I'm thinking input circuitry may be the culprit.  Perhaps the damage is deeper into the conversion/logic sections? Looks like the range switch is working, as can be seen by the decimal point changing, but from the schematic it looks like there is a dedicated deck switch for the decimal point, so maybe that is ALL that is working.  There is a manual/schematic on BAMA if you'd like to look at that and see if someone can point me in the right direction.  All I have done so far is measure the PS rails. All seem to be within spec.

+12V = 11.84V
-12V = -12.19V
+6.2V = 6.24V
-6.2V = -6.27V
5V = 5.00V
HV = 96.9V


What a nice meter, please do try and repair it.

The first thing I'd check is if the 1MHz oscillator is running. Do you have a scope to do that?

Another thing is the Nixie tubes showing two numbers at once. That should not be happening at all, could possibly point to defective U201...U204. In the manual it says these are '9315' nixie tube decoders/drivers. I'm not 100% sure but IIRC they should be compatible with standard 7441 or 74141 TTL drivers. That said, even the standard TTL parts might be hard to find today  ::) .

Its not a driver problem as the digits react....
I suspect a logic problem.
On the logic board : replace all ttl ics (74xx) . Most likely that will fix it.
The two big memories are ok.

Had a few minutes to probe the crystal today.  I could only let the scope & meter warm up for about 20 minutes, but this is what I got.  Looks pretty decent to me.  Not a "perfect" wave, but I think it's enough to say the oscillator is ok?  What say you all, I'm just a noob... ::)

Oscillator looks fine.  Maybe the 99990 detector would be a good place to start?


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