TIU Fixed Output short?

Top suspect for the fixed channels is the TVS protection diode, they frequently fail shorted and cause this symptom.

Awesome, thanks!  If I open up the TIU would the failed diode be evident (like a blown capacitor)?  I’m not trusting myself with messing with the board, so it looks like a trip to my local MTH service center is in store!

No, just take your meter and measure directly across the TVS diode.  If you read a dead short or anything less than around 4-5 ohms, the TVS is shorted.  If that is the case, you can clip it out from the top of the board and see if that restores operation.  If you don't feel like soldering one in (you're working on a PCB), you can just put one across the output terminals of that channel for the same protection.

Here's a suitable TVS to replace the dead one.

1500W 33.3V TVS at Digikey

@spaceman1179 posted:

Awesome, thanks!  If I open up the TIU would the failed diode be evident (like a blown capacitor)?  I’m not trusting myself with messing with the board, so it looks like a trip to my local MTH service center is in store!

In rare cases the failed TVS is obvious:

@H1000 posted:

In rare cases the failed TVS is obvious:

True, but I've only seen one that looked like that, but I've seen at least a dozen or so that were shorted and cosmetically looked perfect.

Just checked the diodes and they all checked out good.  No shorts on either of the fixed diodes (checked the variables as well just for kicks).  Going to apply power to the inputs and check the individual outputs with the voltmeter to rule out the TIU.  Maybe it is the track somehow?

Solved! I think.  The individual TPCs hooked up to the TIU only had voltage coming out of their respective fixed outputs.  But when both were connected (no outputs to the track, just voltmeter), voltage showed on each.  The ONLY thing connecting the two TPCs besides the comm cable was a wire jumping the U outputs to carry the Legacy signal.  I removed the jumper from the TPC 300 and both tracks were isolated.  Not sure why that’s an issue now since I’ve ALWAYS had them wired like that.  Maybe the ground is also coming from my ZW that carries the TIU aux input?  My basically electrical theory knowledge doesn’t explain why that would be the case, but having the Base3 go to the TPC-400, then carrying through the common ground of the TIU, both tracks receive a strong command signal.  Fingers crossed everything works when I hook the other tracks up!