[R-390] R390A AGC troubles

[Jim Miller]

My old S&W had AGC problems which seemed to be centered around Z503.  I 
remember having to work with it to eliminate a short to ground between one 
of its pins and the chassis.  Seemed the chassis cut out for the transformer 
needed enlarging a bit.  Another oddity that happened in the same radio was 
a breakdown in the center insulation of the small shielded cable carrying 
the diode load signal...would cause popping and loss of AGC..this doesn;t 
seem to be your case however.
----- Original Message ----- 
From: "Bill Coleman N2BC" <n2bc at stny.rr.com>
To: <R-390 at mailman.qth.net>
Sent: Tuesday, July 26, 2005 3:46 PM
Subject: [R-390] R390A AGC troubles


> Maybe my earlier posts were lost in the fan / audiophile / Depot Dawg 
> discussions.
>
> Observations:
>
> 1) Low AGC voltage (zero signal = -0.5V)
> 2) Suppressor grid bus (V504 & V508) is +3.4V, should be -2.5V)
> 3) Z503 will not peak before it's slug "tops out" in the core
>
> I have isolated the decks, measured everything that is measurable, 
> replaced caps.  I have isolated individual IF stages and swapped tubes. 
> All without making progress.
>
> The more I poke, measure and isolate, the more I am convinced that the 
> problem is in the AGC Amp stage(s) rather than out on the AGC bus.
>
> All resistance measurements along the AGC bus are OK, voltage measurements 
> when fed with a separate source (9V battery) are OK.
>
> Along the way I managed to destroy Z503 so I pulled one out of a 'cadaver' 
> IF strip.  I noted that it's slug is fairly well centered in the core. 
> Upon replacing Z503, the 'new' one behaved the same way.... increasing 
> stage output as the slug was cranked toward the top but it runs out of 
> adjustment before peaking.
>
> I fed the AGC Amp stage directly with my siggen at 450kc and Z503 will 
> peak with the slug slightly further down in the core.
>
> I have snugged all the ground points in the entire IF chassis & replaced 
> everything in the V508 stage except the tube socket.
>
> I think I'll disassemble the gear train, polish the parts and reassemble 
> blindfolded, that will be less frustrating!
>
> Ideas greatly appreciated
>
> 73, Bill   N2BC
>
> PS:  I have no fan installed, use a plain highly oxygenated line cord, and 
> the receiver's mixed Stewart Warner / Collins / EAC heritage is usually 
> just a lovable as my Lab/?/Shepard best friend.
>
> _____________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/faq.htm
> Post: mailto:R-390 at mailman.qth.net
> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>