[R-390] R-391 Motorboating with RF Gain beyond 7

[Charles Steinmetz]

> My R-391 has developed motor boating (audio oscillation) when the RF gain
> control is advanced beyond 7.

Most likely, unwanted feedback due to one or more defective bypass or B+ 
filter capacitors (don't assume it's just one bad cap).  Could be in the 
IF and/or AF section, and could be plate and/or screen caps (much less 
likely, cathode bypass caps).

Larry's suggestion to use a temporary parallel capacitor is a sound 
troubleshooting technique, and pretty quick.  I advise using spaghetti 
tubing or heat-shrink tubing on the "probe" lead, leaving just enough 
bare at the end to make contact with the target node.  This greatly 
reduces the risk of accidentally shorting the target node to somewhere 
unfortunate during the procedure.

Best regards,

Charles