240Hz hum in a power amp???

With no signal going in, upon warming up a couple minutes after power on, this amplifier starts outputting a low level hum at exactly 240 hertz.

60Hz or 120Hz would make sense, but any theories what could cause this at 4x AC line freq?

http://www.theatreorgans.com/hammond/faq/a-100/schem3.jpg

Thanks
-Bob

Power supply filter capacitors.
Replace all of them to prevent a similar problem in the future.
Any multiple of 60hz hum can almost always be traced to failing filter capacitors.

Mike Cleaver said:

Power supply filter capacitors.
Replace all of them to prevent a similar problem in the future.
Any multiple of 60hz hum can almost always be traced to failing filter capacitors.

Click to expand...

That's what I thought. I'll see what a recapping does for it. 

Funny thin is thatit doesn't happen immediately after warm-up.  It sounds fine for a minute or so, then starts the hum, which sounds like a pure sine wave.  I thought the B below middle C was stuck coming through, but the hum beats slightly flat when I play a B.  I'm thinking what I hear is an oscillation that is being phase-locked to the power supply ripple. 

There are several moving parts in power amps that change with warm up...  If class A/B the class A current will change during warm up until stabilized probably affecting the ripple voltage on PS.

May be something other than caps, but clearly mains frequency related, so PS is suspect.

JR