[Collins] KWM-2A Meter Problem

[Dr. Gerald N. Johnson]

On Thu, 2007-02-08 at 22:56 -0500, EOLEEJR at aol.com wrote:
> Hello, My KWM-2A meter is not working in the receive mode. The needle is  all 
> the way to the right and the RF gain has no effect on it. The radio receives  
> ok, although when the RF gain is fully clockwise it does not seem to have the 
>  volume that it normally has. The meter works fine in the transmit mode. When 
> the  radio is turned off the meter goes back to 0. Any suggestions on what to 
> check?  I am thinking it might be a tube problem.
>  
> Thanks,
> Gene
> WB4NUM

The meter is switched to different circuits on transmit from receive
using the main vox relay. On receive its in the screen circuit of the
1st IF amp, V1B. On transmit its in the screen circuit of V4A. So V1B is
suspect along with the S-meter zero pot and the receiver AVC line.

If the AVC line is being driven way negative that cuts the screen
current of the tube and pushes the S meter to peg at full scale. Or if
the tube has low emission that can happen and that would contribute to
low gain. The receiver maximum gain is also set by a pot in the cathode
of that stage which surely will affect the S-meter setting, especially
the s-meter zero.

The RF gain control also applies a negative bias to the AVC line.

Leaky time constant capacitors like C93 will apply more bias than
planned. The black beauties have already been replaced? They make for
some odd performance as they are leaky.

The bias supply is up to snuff in the power supply? A bad bias supply
makes for some odd results too and the old selenium rectifier is a few
decades beyond its expected end of life.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer