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I’m not sure that I’ve shared this on the blog, but I’ve long known that the reason that my “troublesome” 930 burns up PA drivers is a low-frequency oscillation.  This mechanism was also mentioned by Leeson in his comprehensive list of 930 upgrades and repairs.  At first, I thought that the 28-volt power supply’s filter capacitors might be the culprit—insufficient ripple-suppression.  After all, the oscillation sounds like it has a 60-Hz component (I’ve never tried to measure this because I’m concerned about blowing up more transistors).  I finally completed the capacitor replacement last night, realigned the power supply voltage and PA bias levels…and the oscillation is still there, just as I increase the drive past the point where I get 50 watts out.

There is a good Helge Granberg article in the September/October 1980 issue of RF Design titled “Good RF Construction Practices and Techniques.”  I would like to get a copy of the original article because Motorola, in their infinite wisdom, reprinted the article as AR164 but neglected to include the list of references.  Granberg devotes an entire section of the article to instabilities, a full of half of that section to low-frequency instabilities.  This is the area I will focus on first:

Causes for the low frequency instability are usually inadequate collector DC feed bypassing or an extremely poor ground in that area.  Two or three RF chokes together with various values of bypass capacitors from 1000 pF to several uF may be required in the DC line to stabilize the circuit.  (See examples in Reference 1.)

Ah, Reference 1, where (who) art thou?

If I ever get to the bottom of this problem, there should be a useful body of work and notes to prevent this from happening repeatedly to others.