[R-390] Varying gain [WAS: Filters]

[Charles P. Steinmetz]

>I've got all replacement caps coming for C551 as well as all the 
>bypass and coupling caps.  The Orange Drops I put in the other 
>module were so large, I opted to buy smaller radial Panasonic ones 
>for this module.  The ODs make it nearly impossible to work on.

As has been pointed out before, the film capacitors in tube equipment 
should really be "film-and-foil" types rather than the smaller 
metallized film caps.  Most especially, C551.  (For the reasons, 
consult the archive -- the search terms "self-healing" or "self-heal" 
may be helpful, as well as "film-and-foil".)

>I've found Z503 on the module with bad AGC is open.  I've tried 
>tracing the fine wires back to where it goes in the glue, but nada. 
>That will kill the AGC for sure. Fair Radio has a used one for 
>$16.  Anyone else have any?

Look in the archive for a thread from a couple of years ago regarding 
replacements for Z503.  Search "Z503".  There are many ways to 
improvise a suitable replacement.  One suggestion: the inductor 
should be reasonably well shielded so it doesn't radiate 455 kHz into 
the IF grid circuits.

Best regards,

Charles