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:
And one of two blown resistors on the FG501. I’ll have to order replacements, they’re .27Ω, of some reasonable wattage.
DC504 is back to life. It’s not the greatest counter, but it claims it’ll go up to 80Mhz.