[HAM] 147 "helicopter" noise at low volume

[Scott Hawthorn]

At 08:48 AM 7/12/2007, Hugh wrote:
>I'm working on a 147 with unknown prior history that works reasonably
>well but certain frequency pairs, especially low frequencies like say
>G2 and B2 together from an analog synth creates a "helicopter" noise
>about as loud as the tones. I've swapped out the amp with a 125 amp
>and the problem disappears. I've also swapped out the 12AU7 which is
>the only spare tube I have.
>
>Any suggestions on what to check first?  I've already recapped all
>the electrolytics and measured the resistors for obvious
>discrepancies and found none. I'm guessing it's an untamed
>oscillation somewhere, but where?

Check all operating voltages against the schematic. If you don't know how 
to do this safely, call a technician. I suspect one of the other, 
non-electrolytic caps. This isn't a common problem with a stock solution, 
AFAIK.