Tek TM501 - help me fix it

[l0wside]

Hi everyone,

on my desk, I have two Tektronix TM501 (one 110V, one 220V version) and two AM503. The only working combination is the 110V TM501 and one of the AM503. Being in a 230V country, I would like to fix at least the 220V version of the TM501 to have a working current sensor unit.

On the (defective) 220V unit, I have already replaced all large caps (C12, C22, C35). I have checked the bleed times when turning off, these are roughly in the range of 2V/s (no precise measurement, just voltmeter and some counting). So I guess the caps are in place, otherwise bleed times would be much faster.
All voltages check out, but are slightly too high (25 VAC -> 27V, 33.5VDC -> 36.1V, 17.5VAC -> 20.3VAC, 25VAC -> 27.1VAC, 11.5VDC->13.1VDC). This can be partly explained by the 220V->230V conversion, which happened long after the unit was crafted. This does, however, not account for the full difference.

The working AM503 lights up when plugged into the 110V unit, but not when plugged into the 220V unit with its overvoltage.

I have also checked the two transistors (Q10 and Q12) by measuring their diodes. On the non-working TM501, all diodes give forward voltages in the 500...700mV range, which sounds plausible to me. Cross-check with the working unit shows that on the working TM501, Q12 does not seem to work. Strange. And no, I did not confuse the units.

Any suggestions? I am pretty much out of my wits.

Thank you from your TEA in the making 

[alm]

I don't expect the difference in (unregulated) voltages to be important. It would only increase dissipation slightly, something that is clearly not a problem if the plugin fails to power up with no load.

To me the most likely suspect are the external pass transistors. They may not be shorted, but could still have degraded performance. If you take off the covers of the TM501 and AM503, you may be able to measure the power rails on the AM503 while plugged into the TM501. For example, if one off the 16V rails is low, that would point at the transistors in the mainframe.

[texaspyro]

Be aware the the pass transistors in some of the TM50x mainframes have non-standard pinouts...