Solved! -SB-101 AVC Hang/SLow Recovery

[Dick KF4NS]

John,
I Had this plague me for years in my HW100 and found out an even
simpler approach to eliminating the problem. All I did was put a diode
in the screen supply lead immediately after the tap to the driver and
before the finals. Worked great and no trouble since then.
73, Dick KF4NS
St Petersburg, FL 33714 USA
Keep The Glow!

----- Original
Message -------------------------------------------------------------------------
> SNIP>
> Date:    Mon, 21 Jul 2008 08:46:22 -0400
> From:    John A Diefenbach <jadief at GMAIL.COM>
> Subject: Re: Solved! -SB-101 AVC Hang/SLow Recovery
>
> Like Sherlock Holmes, you, as well as Len, are great detectives.
> Good
> job...and thanks.
>
> On Sun, Jul 20, 2008 at 4:49 PM, John P. <jpetrocxme at yahoo.com>
> wrote:
>
>> Hi ALL,
>>
>>     After many, many hours of sleuthing found the problem with the
>> SB-101
>> AVC hang problem.  When keying the transmitter, with no output
>> (mike gain @
>> zero) for about 2 to 3 minutes, upon returning to receive the
>> S-Meter would
>> kick up to S-9 and slowly decay back to normal over 40 to 60
>> seconds.  I
>> applied ALL of the factory and non-factory fixes to no avail.  THis
>> includes
>> checking voltages and resistances.
>>
>>    Finally Len (SM5DFF) eMailed me via the Yahoo group about hisr
>> fix which
>> was to ground the screen grids of the 6146's during receive.  I
>> found this
>> curious and investigated.
>>
>>    What I found was...  That during transmit the screens of the
>> 6146's were
>> at 300 volts, but during transmit the were "floated" in the
>> circuit - at
>> about 0 volts (per the book).
>>
>>    Well, I found that for some reason that the screen grid circuit
>> was
>> holding at about 150 volts and SLOWLY decaying.  This circuit
>> HAPPENS to be
>> supplying the screens of the isolation amplifier which is just
>> after the
>> balanced modulator.  This isolation amplifier (6AU6) should be cut
>> off too
>> during receive but alas, it was seeing the decaying positive
>> voltage too as
>> I watched it and its affect on the AVC - as the residual screen
>> grid voltage
>> on this tube decayed the AVC would slowly recover.
>>
>>     ANY residual signal from the balanced modulator (very very
>> small) was
>> "leaking" thru to the "IF" stage and causing the stage to be
>> desensitized by
>> the AVC.
>>
>>   I tested this theory that it was the 6146's by removing the 800V
>> plate
>> voltage and just keying the transmitter and there was NO PROBLEM
>> with the
>> positive voltage or AVC.
>>
>>   The solution is rather simple now. Add a relay to ground the
>> screens of
>> the 6146's during receive.  I did CAREFULLY wire in a switch to
>> manually
>> simulate this grounding action and the problem was eliminated.

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