Help! my AGC is taking over my K2!

[Merlarts]

Hi gang,
 
It's Merlin, W3ICT; you know, the guy who complains about the filter in the  
KSB2 all the time.  This time, it's actually something different.  
 
Recently I had to replace the finals in my K2 (2631).  In the process  of
measuring forward bias, I accidentally lunched the SC1969s with clumsy  metering
skills.  Got the finals put back no problem, but a strange thing  has
surfaced, and I wonder if anybody has seen it before.  
 
Unrelated to the finals, I turned the K2 on and the S meter pegged and the  
receiver shut down its 1350P IF amp.  I measured the AGC voltage.   It's way up
there at 5.5 volts, regardless of whether the AGC is disabled and  regardless
of the position of the RF gain control.  Sure enough, if I lift  the
collector on Q12, the receiver comes up to full sensitivity.  If I lift  d2 (these are
all components on the control board, by the way), thus depriving  the AGC amp
of a signal from the IF, it still makes no difference; the AGC  amplifier
still makes too much voltage.  Any thoughts?  Everything  else works, by the way.
 Does U2 just go bad sometimes?
 
By the way, in the matter of filters, I've come up with a simple shielding  
technique, using tin sheet from hobby shops, that actually improves the
ultimate  rejection and the shape factor of the seven pole device that's used in the  
KSB2.  
 
I bet somebody out there has experienced this weirdness.  I hope
 
Regards,
 
Merlin, W3ICT
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[Don Wilhelm-3]

Merlin,

First a correction - the IF signal path into the AGC does not flow through
D2, so removing it will not shut down the AGC, but rather will guarantee
that AGC will always be present - the IF signal comes into the mixer on pin
1 while C10 effectively grounds pin 2 at the IF frequency.

I have not seen or heard of a failure for Control Board U2, but I guess it
is possible - but before making any rash conclusions, can you provide the
voltage measurements at U2 pins 5, 6 and 7?  Also check the resistance
measured from U2 pin 7 to pin 6 - if it is much greater than 33k, check the
soldering at R5 and/or check for a bad R5.

PS - yes I would like to see your KSB2 shielding.  Can you put it on a
website or something like that? - I'm certain we will need pictures.

73,
Don W3FPR

> -----Original Message-----
>
> Unrelated to the finals, I turned the K2 on and the S meter
> pegged and the
> receiver shut down its 1350P IF amp.  I measured the AGC voltage.
>   It's way up
> there at 5.5 volts, regardless of whether the AGC is disabled and
>  regardless
> of the position of the RF gain control.  Sure enough, if I lift  the
> collector on Q12, the receiver comes up to full sensitivity.  If
> I lift  d2 (these are
> all components on the control board, by the way), thus depriving
> the AGC amp
> of a signal from the IF, it still makes no difference; the AGC  amplifier
> still makes too much voltage.  Any thoughts?  Everything  else
> works, by the way.
>  Does U2 just go bad sometimes?
>
> By the way, in the matter of filters, I've come up with a simple
> shielding
> technique, using tin sheet from hobby shops, that actually improves the
> ultimate  rejection and the shape factor of the seven pole device
> that's used in the
> KSB2.
>
> I bet somebody out there has experienced this weirdness.  I hope
>
>

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