From: Garey Barrell 
Subject: Re: [drakelist] R4C/TX4C Sidetone Problem


Paul -

Quite likely the electrolytic capacitor at C107.  Easy to check by 
bridging with a good 60-100 uF cap.  When the ESR of the cap goes up, 
the tone leaks through the B+ line to the grid of V7, which is the 
sidetone amp, bypassing the diode switch.

Also possibly a leaky diode at CR15, but less likely than C107.

The mic amp V9A is connected as an audio oscillator through the phase 
shift network, and is keyed whenever the PTT (or semi-breakin) is 
keyed.  The audio tone is shorted to ground by the keying bias applied 
to CR15 when the key is up.

My bet it the cap.

73, Garey - K4OAH
Atlanta

Paulprato{at}aol.com wrote:

> While sending cw a steady, but weaker tone, is heard in the background.  
> It continues until the relay switches back to receive.  Any suggestions 
> as to what part or parts have deteriorated?  Thanks
> Paul
> K7CKZ
> 

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