TS-930S PA/PS Postscript

August 11th, 2010 by k8gu Leave a reply »

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.