The IC-751A Saga
I'm getting closer to getting this thing figured out...
I was half right - the antenna switching unit (also called the connector board) had a bad transistor and a resistor that was so burnt I couldn't even see it until I turned the light WAY up. The numerous times I've visually inspected that board and come up with nothing because the light wasn't bright enough to see that it wasn't a ferrite choke on a wire, but a burnt resistor is staggering.
The radio has gone from outputting less than a watt to just over five watts. It's supposed to output 100 watts (or thereabouts) CW.
Next step is searching for cascade failed components (from that one failure) on both the connector board and the PA itself.
I've found one on the pre-driver already. a 2.2 Ohm (yes Ohm, not KOhm.. Ohm. Weird, huh?) resistor is reading 3.8 MOhm on the emitter of the pre-driver amplifier. so either it got hit hard when the other components on the connector board failed and popped nearly open VERY quickly, or I've got other issues I'm going to need to find.
Luckily, I managed to find the rear plate from an IC-751 (not EXACTLY the same as the 751A, but close enough to scavange components off of) so I've got a plethora of parts to test and use.
I was, while I had the back plate off, able to test the thermal switches that run the fan and protect the PA from overheat, and they're both working as they should. this is, indeed, a lot of fun, but I wish I had the real tools to do this right, like a signal generator and a working oscilloscope.
Enough typing for now. I want to get back into this rig and get it talking again, and I'm going to need to leave in a couple hours to pick my lovely wife up from work.
73 for now.