Solved! -SB-101 AVC Hang/SLow Recovery

[John A Diefenbach]

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.
>
>   BTW, I also tried another pair of 6146's before trying the
> no-plate-voltage test. Also tried replacing MANY different tubes.
>
>   In many years of doing this it is a new one on me and something I would
> NEVER expect, but then again I shall never know everything
>
>   I suspect that there may be some space charge or residual and very small
> leakage (micro amps??) that is taking place from the hot plates of the
> 6146's that is causing this problem.
>
>   Maybe this is a design quirk/flaw in the SB-101.  I do not know.
>
>   As Sherlock Holmes would say - "After eliminating all the possibilities,
> whatever is left over, no matter how improbable or impossible, is the
> answer"  (with appologies to Sir Arthur Cannon Doyle).
>
>   Thanks for everyone's input and time !!!!
>
>
>
>
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for losses or misrepresentations of any kind.  Buyer beware!

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