Mesa Simul 2:90

[DELAMBG]

Hi
I have this simul 290 that only hums when the deep and modern functions are engaged. It hums a little when either the deep or the modern is switched on, but when both are on together it is a LOUD hum. It is only in channel A and it goes away when the volume on the 290 is turned all the way up. I already changed the filters as they were a little weak but the hum still hasn't gone away.
Cap in the feedback loop? I dunno

[Don Moose]

I don't see anything in the schematic that would do that - just a little tuning in the FB loop.

... unless the relay supply filter is hosed and AC on the coils is coupling into the signal contacts.

[DELAMBG]

But......
It only does it on the one channel not both, and the 10,000uF caps in the switching circuits are common to both channels.

[Don Moose]

[grasping desperately at a straw] maybe the relay contacts for channel A are closer to the coil or the coil wiring, and channel B is just far enough away?

What we gotta do here is look at what those relays change when engaged.
I see that the NO contacts are open. Looking at the switching transistors, I'm going to assume that the inactive state for DEEP and MODERN are with the relays relaxed (C connected to NC), which would have resistors in parallel with a cap (DEEP) and an RC series set (MODERN). Engaging those relays would pull those parallel elements out of the circuit, but not leave anything floating.

Unless there's a problem with the non-switched paths in that section of the feedback network, and from your description it would have to be both paths, I'm not seeing a way for this to add hum.

You might look at the C4-to-first-cap and PRESpot-to-RC wiring - especially of the PRES pot works better in inactive mode (compared with channel B).

Hope this helps!