Mesa Boogie Caliber 50+; effects loop problems

[Bernardduur]

Hey all!

I've got an Mesa Caliber 50 in that works like a charm EXCEPT for the FX loop. OK, it does it's function BUT with some effects it'll squeel like a pig (mostly with digital effects like delays or reverbs)......

First I thought it was a loop between the in and the output (measured I get a resistance of approx 70k) but it turned out to be the pull down resistors collecting up.

Any ideas how to fix this?
Schematic

THANKS!

[patafix]

I have exactly the same issues & same measurements. I tried to disconnent the wire that goes from the return jack tip to board... i still can hear some sound from the speaker... definitely something going wrong inside the amp itself... maybe the EQ circuit, maybe triode/triode leaking in V3...

[Bernardduur]

I fixed it........ but I cannot really recall what I did

I thought:
- Cleaned the jacks
- got the grounding right on the jacks (forced a new ground on em)
- and I think I enlarged the bleeder resistor from 47k to 1M

[soundguruman]

I fixed it........ but I cannot really recall what I did

I thought:
- Cleaned the jacks
- got the grounding right on the jacks (forced a new ground on em)
- and I think I enlarged the bleeder resistor from 47k to 1M

hahahahahahahahhhhhhhhh!
If you want to fix that squealer, you will have to modify it. And good luck.
To start with, there is no control over the send or return levels...and if you put contols in, you might be able to tame the swine.
But woe to thee, lest ye enter the realm of said circuit board. It is pure hell on earth to work within...
actually controls could be added, without removing the board.

[patafix]

It actually doesn't sound or look like a level issue...

[Jazz P Bass]

Please ignore SGM.

[patafix]

OK I did some mods including the 1M grounding resistor... nothing really impressive, still squealing... at the moment I feel like adding one 12ax7 so that V3 "sends & buffers" and the added 12ax7 "buffers&returns"...

[soundguruman]

OK I did some mods including the 1M grounding resistor... nothing really impressive, still squealing... at the moment I feel like adding one 12ax7 so that V3 "sends & buffers" and the added 12ax7 "buffers&returns"...

I'm thinking the level IS a problem...
How much audio is at effects send, in volts?
If it's more than 1/2 volt, it's easy to see why the input of effects unit is overloading.
The effects unit is supposed to be driven by guitar level signal, 1/2 volt, not high line level.

Also, is the effects unit output in phase or out of phase with the input signal (at effects send)?
If it's out of phase, you have changed the output phase of the amp also...
and I am not surprised it squeals because of that either.

[patafix]

The squeal occurs also with a line level effects processor with adjustable input level !!!
I might also think of making a phase inverter stomp...

[soundguruman]

The squeal occurs also with a line level effects processor with adjustable input level !!!
I might also think of making a phase inverter stomp...

Reversing the phase might be the problem...