Hi folks,
I am repairing a radio that was previously 'fixed up'; the final was a
real mess, and no surprise, one of the MRF475s is broken. That is, the leg
of one of the transistors is broken off. Wow. So much blobby soldering and
flux, could hardly tell what was going on.
As I was working on cleaning this mess up, I noticed that the module's
design is a bit different than the available schematics. On this amp board,
instead of a single bias adjustment, there are two - one for the AB driver
(as expected), and the other for the final pair. The circuit for the final
pair bias is a copy of that used for the driver, and it is applied to the
bases of the MRF475s through the low-turn-count base windings of the driver
to final stage transformer.
At some point I'll have to adjust these pots after the repair. I'm going
to figure that the 'set for 6mA bias' from the manuals is not valid here.
I have some guesses on how to adjust this circuit - but what do you all
think? Or, is there somebody with an older revision of the manual that
describes how this is done?
Thanks,
--
David Goncalves
W1EUJ
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