Hi Michael,
Last night my curiosity got the better of me and I searched for and found the DM501 schematics.
BTW, the Tek Wiki's DM501 (not DM501A) manual is incomplete, and I found one with complete, if unmerged, schematic pages. Good enough for my needs. But I should get this to the Tek Wiki admin to update. That's on me.
With that I brought in the plug-in which sacrificed itself for the cause. Along with its stink - that was a mistake, but it's cold out in the garage. I studied it against the schematics and board layout. Here's the dope: The trace that fried is not only connected to 33v common, but also traces back into the plug-in where it is grounded to the chassis. More careful examination of the failed trace does look like current was trying to travel down both branches to both "grounds". The connection to pin9B happened to loose first. But there's a distinct source from R421 that travels down both branches, one to 9B, the other to the chassis ground. So R421 seems to be the source of the current.
I did initially think: "the 33v common is ungrounded!" I ran back down to the garage, pulled out the TM504 and verified that indeed the 33v common has continuity to ground. A couple/few ohms due to connector grime? So it's not like the 33v common has something untoward on it. I did also verify the source fuses are correct capacity and type. I powered up the unit and verified the 33v common is at 0v relative to chassis ground.
Somewhat humorously, R421 is driven by a pot, R420 from pin 5 of U420, and R419 from pin 6 of U420. This is the 5v adjust. That circuit is driven by 12v derived from the +33v supply and a Zener, VR410, 12v. If VR410 opens, does the 12v rise and create over-voltage on the +12v supply in the plug-in? I would test, but I'm not plugging that thing back into a working PS unit quite yet.
The second plug-in that had started to burn had a singular trace from pin 7 (gnd) of a 7400 14 pin DIP. No apparent trace to chassis ground, nor any other component. I didn't dig too much deeper into that plug-in: DC504 (not A). The 7400 is one of many IC components driven of +5v derived from +33v by U405. Again, not powering that up any time soon to further debug.
So at this point the issue seems to be bad plug-ins, not something generally wrong with the power supply units. I do need better test equipment (working on that), and should find a better current detection setup before I go testing more plug-ins.
I should note these plug-ins are ancient. Many/most have calibration seals on them with due dates in the 90s - if not earlier. The seals on these two plug-ins were unbroken suggesting they haven't seen power in almost 30 years. As other recent experience has revealed, things do go bad just sitting in dry warm climates. Time to bust out a lightbulb setup? I also do have a vaiac that is due to arrive tomorrow. Just part of the learning process. The TM5xx series units are _not_ robust supply units. Rather quick-n-dirty cheap products. A little disappointing, but caveat emptor.
Dave
Dave Peterson