Wow, what fun we’re having!
This rig has been one problem after another. After I got all of the bad resistors replaced, a new problem occurred. No or close to no RF output. I can coax a watt or so out of it on 80 meters, maybe 5 W out on 10 meters. My troubleshooting efforts were hampered by the fact that while I could probe around all I wanted with an O-scope or meter, nothing in the manual gave any indication (at least none that I could find) of the expected RF signal levels in the path – the carrier oscillator, VFO, etc. So I could see the carrier oscillator was producing about small fraction of a volt peak to peak, but I had no idea if that was right or not (but it seemed awfully low to me).
Here’s where the folks in the Heathkit group at groups.io really were helpful. It turns out that the schematics printed in the manual for the Heathkit HW-101, which is nearly identical to the SB-101, do indeed show expected RF signal levels at numerous points in the circuit. Armed with that information, I confirmed that at least part of the problem was a very low signal from the carrier oscillator. In an effort to double-check my work, I went back and re-checked every part I had replaced – after which the oscillator signal was fine (more or less). That told me there was a physical problem. It could be bad solder joints, bad ground, a cracked board, or a cracked part. Just poking around with the meter probes had changed things.
After much trial, tribulation and scope work, I spent some time de-soldering and re-soldering most of the modulator circuit board. That seemed to stabilize the carrier oscillator in a working state. I had to do the same to the audio board to solve a newly developed scratchy intermittent noise issue that appeared. Along the way I discovered a bad connection to the LMO output RCA plug. Then the LMO started randomly jumping 1, 2, or maybe 10 kHz whenever it was tapped or touched. I ended up peening the two pop rivets holding the phono jack in the back of the LMO to fix that one.
Now at least I’m back to having a relatively stable, quiet rig with a fixed symptom I can chase. That symptom is a good signal from the carrier oscillator reaching the balanced modulator, a relatively normal signal at the input to the modulator transformer, and not much of anything coming out of that transformer. Right now I’m feeling a little foolish because I had that transformer out of the radio this morning, opened it up, re-glued the coil form to the base, checked that the wires were still attached to the pins… and did not check the windings for continuity. What a rookie move. I’ll check that in the morning.