The third bay of my recently acquired TM-503 mainframe ate two plug-ins, releasing their magic smoke.  The it didn’t do this to the module that came originally installed in the 3rd bay, and here’s why:

The mainframe has two power transistors for each bay that are wired to the edge connectors.  They’re meant to be used by the plugin’s power supply.  The DM-502 that was originally in the 3rd bay didn’t actually use these transistors, but the DC504 and FG501 did.  The photo below shows 3 of these transistors.  The one on the far right is for the center bay, and it has a mica disc insulating it from the chassis.  The two on the left did not have those mica insulators.  Some knucklehead that replaced the left-most transistor (center one removed by me), forgot to re-install the mica disc, and I found one floating around the inside when I opened it up.  This meant that one of the pins was shorting to ground, and he was only lucky not to get bit because of the plug-in he was using.

You’ll also see a 5v regulator bodged onto the unit, it’s output has been snipped.

Here’s the damage caused by the error:

A blown trace on the DC504

And one of two blown resistors on the FG501.  I’ll have to order replacements, they’re .27Ω, of some reasonable wattage. 

The trace repaired:

And the mica replaced:

DC504 is back to life. It’s not the greatest counter, but it claims it’ll go up to 80Mhz.