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

[Larry H]

Hi Mack,  This is usually caused by feedback due to weak bypass caps
(screen, plate, or cathode).  This is an easy check (grab a .1 - .5 mfd 400
v cap and ground the neg end and touch the + end to all those appropriate
points).  It can also be caused by a weak agc cap - try the slow position
and see.  And it could be a weak audio out B+ filter cap.

Have fun.

Regards, Larry

On Sat, Feb 15, 2020 at 1:31 PM Mack McCormick <w4ax.mack at gmail.com> wrote:

> My R-391 has developed motor boating (audio oscillation) when the RF gain
> control is advanced beyond 7.
>
> If you're run across this issue I'd appreciate knowing how you fixed it.
>
> I have rebuilt about a dozen R-390a and have a full test equipment bench.
> I'm just hoping for a head start before I begin troubleshooting the IF
> stage. Yes, I tried the usual searches but didn't find anything.
>
> Thanks in advance.
>
> 73,
>
> Mack
> W4AX
> https://www.qrz.com/db/W4AX
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