Long-time followers of the blog know that one of my TS-930S transceivers has been a money and time sink for about the past five years, fully 60% of the time I’ve owned it. So, if you haven’t been following the story over the past couple of months, I pretty much replaced (almost—get to this in a second) all of the electrolytic capacitors in the power supply and power amplifier, plus replaced the driver and final amplifier transistors. As I increased the drive past the point where I got 50 watts output, I started to get a lot of AC hum on the signal.
At first, I thought the hum might be associated with a low-frequency instability in the power amplifier. I read all the Helge Granberg articles I could find on the topic and tried all of the prescriptions he suggested. Last night, I even went so far as to tweak the feedback resistances in the PA stage to increase low-frequency stability. Still there. Finally, I measured the frequency of the AC hum—exactly 120 Hz—full-wave bridge rectifier leakage. Tonight, I pulled the power supply board out of the radio, which is a herculean task, by the way. There were still three small, insignificant-looking electrolytic capacitors that I hadn’t changed. I found two of them in my junk box and crossed my fingers on the third one (a 25 uF, 100-volt unit), leaving it in place. While I had it out, I also found and shunted a pair of dying PCB traces with pieces of wire.
After putting the board back into the radio, I disconnected the PA 28-volt line and powered it up. I checked the 28B voltage…right on 28.5 volts. So, I reconnected everything and it fired right up at 100 watts without the hum. Perseverance seems to have paid off. For now. I keep telling myself that the next time it breaks, I’m going to get a K3/100. But, I just can’t bear to buy a radio that’s worth more than my car.
