Electronics > Repair
Tektronix DM501A repair
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Rax:

--- Quote from: David Hess on June 01, 2024, 11:35:14 pm ---However later I decided that just dripping MG 801B into the switch and operating it was easier and worked better.

--- End quote ---

This far, the signs are very positive. I seem to have fixed two out of five units, and one other may be working by tomorrow, if conditioning of switches has the same distinctly positive result as I've seen. I think the 801B is worth the "(1) arm and (1) leg" bill. Though I guess long term performance will tell the whole story.

The rest of the units have bigger problems, but maybe their future is as parts mules.
Rax:

--- Quote from: David Hess on May 30, 2024, 05:19:04 pm ---High offset on ranges which rely on the decade divider is caused by excessive input bias current from the multiplexer, or damage to the input protection diodes.  The multiplexer is a known failure point in these designs, but it can be replaced with an external multiplexer and low input bias current precision operational amplifier.  I have the parts but have not quite gotten around to fixing mine.

--- End quote ---

At least one of the ones I have here seems to have this issue. No matter how much cleaning of the switches I'd perform, there's a large offset in the ranges over 20V, inclusively, in DCV (for instance, the 20V sees 1V as 9.994V...).

I did get a few DG419s, when I get a bit more time I think I'll try the mod.
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