[Drake] R4A AGC

[Rick Stealey]

It seems like I am always having AGC problems in my Drake rigs.  I just got out my old R4A and have been playing with it for awhile.  After it has been running for a couple hours I noticed the S meter reading unusually high, like 60 over 9 on broadcast signals on 40 meters, and SSB signals banging the S meter around like it was on fast AGC instead of slow.  Also the RF gain would not completely mute the receiver, and the S meter only went up to about 40 over when I turned off the RF gain.
To make a long story short, something was loading down the AGC line.  I found 2 of the 12BA6s were leaky.
The way you check for this is really easy.  Put a digital voltmeter on the AGC test probe on top of the chassis. It's labeled TP2 and located between the notch filter can and the passband tuner.  With the RF gain turned off note the voltage.  Then one by one unplug V9, V1, V4, V5 looking for a change.  If removing one of these causes the voltage to rise up (i.e. from -1.5 to -4.5) you have found a leaky tube.  In my case I had 2 bad tubes.
If this doesn't locate the source of the problem you may have some other component that is leaky.  A capacitor connected from the grid of one of the AGC controlled tubes to the plate of another could be leaking B+ and doing the same thing.

Rick  K2XT
 		 	   		  
_________________________________________________________________
Hotmail: Trusted email with powerful SPAM protection.
http://clk.atdmt.com/GBL/go/177141665/direct/01/