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.
Hi there, hope your well? Really enjoyed your blog, I cam across it after looking for an X Y DIsplay for some video synth work. ANyway, I wondered if I might ask you a question seeing as how you are clearly really brilliant at bringing life back into some of this equipment. I recently got a hold of a HP 1340A but it doesn’t have a power supply. It says 115v 400hz written on the back and seems to need some kind of 3 point plug. I live in Scotland and I wondered if you knew what kind of solution I might use for powering this unit? Many thanks for your time, kind regards, Matheu Watson
Hi matheu, thanks for your comment! Hmm… Sounds like that display may have been made for aircraft use. I bet you can find a 240v 50/60hz to 115v 400hz power supply out there, but if Scotland is anything like the states, you’ll probably have better luck finding another X/Y display for cheaper. A good source is old vectorscopes from TV stations, which if not already equipped with X/Y inputs, can be made to take X/y inputs with a little circuit exploration. Good luck, and let me know what you find!
Thank you for this excellent post. It is really thought provoking.
I recently bought a TM503B on eBay. The seller showed a variety of photos. I asked questions. I acknowledge that the price was low. I missed the ‘trick’ about the photos – they did not all match up. Sadly, now I have a mainframe that will not run my SG505 at rated performance levels. The TM504 I have been borrowing does so. Do you think I could fix this problem?
I’m sure it’s fixable. You’ll want to measure the voltages that the TM503 is delivering with the plug-in installed, which you can do from the top without an extender card (though they do make the job easier). Also remember that the mainframe has power transistors for optional use by the plug-in.