Interesting HW-12A problem

[Andrew R. Mitz]

Check for a gassy output tube.


At 08:36 PM 12/17/98 -0500, you wrote:
>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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*********************************************************************
Andy Mitz (WA3LTJ)                    arm at serge.gnode.org
a.k.a. LTJ Designs     www.somerset.net/arm/old_radios.html

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