well I have a T301 on 220 that has some serious power drop off and
microphonics issues. I had thought the microphonics were generated in the
VCO, and while some of them are, i have traced most down to the final amp.
It looks like just a bad amp design as the transistor has the collector commen
to the case, putting RF on the Heatsink. the heat sink seams to form a
capacitor between it and the shield so any movement or heating modulates the
signal.
We also have seen the output power go from 4 watts down to 1.2 watts as it
heats up. If we put a fan in there the output is stable at about 2 watts,
but it modulates the signal with a hum from the moving air.
So I was thinking to replace the output transistor with a MRF237 which has the
case tied to ground(emitter) which would seam to be a better idea.
For the time being we removed the final and are using a small brick amp that
puts out 3 watts stable, with no microphonics.
So anyone tried this or have a better fix for the issue?
--
Bryan Fields, KB9MCI
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