TSL100 Hum

[drewl]

I have a Marshall TSL100 that develops a hum when it warms up.
Starts out nice and quiet, and gets louder and louder until it peaks in about 10 minutes.
It's worst on the clean channel, and if you run a jumper between the fx loop jacks it goes away.
Something in the switching, the jacks and connections on that board all look good.
Anyone encounter this before?
Thanks.

[drewl]

The hum is between V3 and V4, when RLY1 is engaged the hum goes away. RLY is engaged when you switch channels or use the main fx loop.

[AmpFix]

Do you have a scope to have a look at the amount of noise and determine exactly where it starts in the circuit?

[R.G.]

Have you cleaned the shunt contacts in the Fx loop jacks very, very carefully?

[AmpFix]

That wouldn't be the cause of hum, just noise/loss of volume (the Fx Loop jacks being dirty)

[drewl]

Cleaned the contacts of course, but the noise seems to be coming from the bottom board where the loop switching takes place.
When the signal is routed through the fx loop the clean channels volume pot is now after the hum, when the loop jacks are not switched in, the volume pot has no effect.

The amp is quiet when first powered up, and then the hum just gets louder and louder.
When the signal is routed through the fx loop jacks, the portion that goes through the loop is quiet, the portion that doesn't has the hum.
The FX mix pot can dial between the hum side and the quiet side.
I need to pull that bottom board and check everything, but it seems like one of the optos or something in the switching circuit is crapping out when warm.
The power supplies for all the switching circuits are nice and clean.