[R-390] AGC voltage issue

[Charles Steinmetz]

Ryan wrote:

>So this C551 on my good EAC IF Deck:  When I removed it from the circuit it
>has 0.6V across it.  When you short it, the voltage goes away.  When you
>remove the short, the voltage is back.

It's bad, replace it.  You may have caught it just before it expired 
with finality, so it may not make much if any operational 
difference.  But if you don't replace it, you'll be back inside again 
very soon.

>I remember seeing on the messages sometime before about replacements for
>C551..  But I have to ask, is a 500V cap necessary?

If you switch the radio on from cold straight to Slow AGC, depending 
on how fast the power supply comes up and how long it takes V506A to 
warm up, C551 could see the full power supply voltage across it for a 
short time.  Because the radio uses a choke-input power supply 
(search the archives for "choke input" for discussions of the 
implications of this), it is possible for the B+ to be >400v for a 
short while at startup.  This is worse in radios in which the tube 
rectifier has been replaced with solid state diodes (and it is not 
helped by putting a resistor in series with the SS diodes).  So, I'd 
stick with a 500 or 600v cap.  As ever, I caution against using a 
metallized film capacitor -- use only a film-and-foil cap like a 
Series 225P Orange Drop or equivalent (for much more on this, search 
the archives for "film-and-foil" and "self-healing").

Best regards,

Charles