[R-390] C227 in R390A

[Charles Steinmetz]

Drew wrote:

>C227 is a cathode bypass for the RF amplifier stage. I've had this 
>cap fail once.  Symptom was loud popping noises in the receiver's 
>audio as the antenna trimmer control was operated, pointing to 
>oscillation of the RF amplifier stage. DC voltage at this circuit 
>point is relatively low, and so metalized film cap should work fine.

Also note that there is very little possibility of collateral damage 
if C227 fails, and no indication (IME) that it is prone to failure, 
so there is no reason to replace it unless it does fail.  In all but 
the earliest radios, C227 is paralleled by a 47 pF capacitor (C257) 
to improve the RF bypassing of V201's cathode.

Since C227 is an RF bypass capacitor, the best replacement is a 
ceramic (X7R or C0G/NP0).  For example, Mouser p/n 810-FK11C0G2A473J.

If someone is looking for caps to replace:  The cathode of V201 
(where C227 is located) goes through a 220 ohm cathode resistor to 
the "cathode bias line" (Terminal 1 of TB102, "RF Gain"), which is 
bypassed to ground with C103, a 50 uF electrolytic.  C103 is known to 
fail sometimes, so if you're on a mission to replace as many caps as 
possible, consider that one.  (But again, the chances of collateral 
damage are very low even if C103 does fail.)  I do not replace C103 
as a matter of course, but I do look it over for signs of distress 
and check it with a VTVM on the Rx1 range to make sure (i) it is not 
shorted and (ii) it appears to charge up at a rate consistent with a 50uF cap.

Best regards,

Charles