TM500/5000 plug-in failure modes.

Dave Peterson

Hi all,

I'm going through a number of unknown TM500/5000 plug-ins and have encountered a couple that seem to fail in the same mode: a trace to pin 9B of the rear edge connector (33v common) overloads and begins to burn. I had one go completely smoky before I realized what was happening. After that I caught another one in time because I was monitoring the power supplies and saw a drop in the supply (not huge). It still caused noticeable trace damage.

The thing is, some plug-ins load this supply (and pin) and others don't. The one's that don't test just fine. Not that they work necessarily, they just don't go up in smoke. Now whenever I plug-in a unit that does include 33v sources I of course see a load on the power supplies. I don't have sophisticated load monitoring yet, but I do know about the issues with power supplies, pass transistors, and the TM500/5000s tester. For now I just blip the power on and immediately off if I see any load on 33v/17.5vac.

This has also happened on a couple of different power supply units: a TM503 and a TM504. These units test good for supply level, ac phase, and basic pass transistor continuity. I don't have a curve tracer at hand at the moment, but no shorts before and after testing. Also the failure mode does not appear to be on the pass transistor. Though I cannot be certain that the pass transistors aren't involved as I don't have good knowledge of the plug-in internals. But pin 9B isn't a pass transistor pin.

I'm pausing my testing for now until I have this sorted out so I can know the difference between a load on 33v and a plug-in on its way to death.

I'm thinking a current monitor on 33v common, but don't know how I'd implement that. Otherwise I'm left watching the 33v supply - 17.5vac also shows load when this happens. I can't tell the difference (yet), in a quantitative way, between a normal load and impending death. You can imagine, I'm not keen to sit and watch another plug-in load those supplies and wait to see if it's dying or just a normal load. Some of these are valuable.

So my question is, is this "33v common/pin 9B" overload a known thing? And how do I definitively monitor for it? Why the common pin and not the TM504 33v supply (or other) fuse?

I'm new to the TM500/5000 world and I'm trying to catch up. There's a lot out there to review, but I haven't seen anything yet on this failure mode.


Michael W. Lynch


What about "9A"? Any damage to that trace? Both 9A and 9B are tied to the 33.5 Common return. Are the fuses on the +33.5 Supply the correct "Fast Acting" fuses of the right current rating? I do not have answers to your other questions. I have never encountered or even heard of this phenomenon and I have several Power Modules as well as 2 dozen working plug ins.

The only way I have burned up a trace is when I plugged my flexible extender card to the module "upside down" (I know, DUMB Move!). I did have a few components go BOOM when i turned on the power. .

Michael Lynch
Dardanelle, AR

Dave Peterson

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.



My PS503 has a couple of OverVoltage SCRs on the +33 and -33 busses effective for S/Ns B022064 and below.
The +/-33V is also used to create regulated +/- 27V in a lot of other plugins, often by a pass transistor mounted in the plug-in, and +/- 5V via Zener but I would doubt it could draw that much current. I think the series resistor for that would limit the current sufficiently.