Interesting HW-12A problem

[Paul K. Dean]

Hi, folks!

I'm looking for ideas!  Very strange problem with an HW-12A that I am
working on.  Set it to TUNE and the power creeps up slowly.  Takes 3 or 4
seconds to reach full output.  (as measured by RF out meter).  Hit the PTT
and the meter goes to about S9 then slowly drops to zero.  (Measurement in
that mode is made at the cathode of the xmtr IF amp).

I have traced the problem to a slow decay of cutoff bias applied through
the ALC circuit.  When you go into transmit, the bias drops quickly to a
few volts, then trails down to near zero very slowly.  Removing C74, the
ALC filter, stops the slow decay, but of course, this isn't the solution.
Checked C74 (using Heath cap checker), checks OK, R79, across it is OK.
Diodes D71 and D70 check OK on ohmmeter.  Cutoff bias, measured at the
relay drops quickly.  Removed C75 to see if bias was leaking through there.
 No change.

I checked the ALC on my HW-22A and there is a similar delay in dropping the
cutoff bias, but it is less than a second in duration.  Checked Joe Lutz's
fine service bulletins, but didn't see anything like this.

Any ideas?

Thanks, Happy Holidays and 73,
Paul, wb9hgz

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