Marshall AVT-150 Head Sounds Bad

[misterc57]

Amp sounds like crap on all channels. The 6 FX knobs are all set to zero. Signal fluctuates, sounds scratchy.

At first I suspected dirty pots and jacks. I especially thought the master volume pot was suspect.

Things I have done and tried...

I have sprayed all pots and jacks. No change.
I have jumped the effects loop. No change.

I have run the effects send to another amp, all 4 channels sound great in the other amp.
I have run another amp into the effects return, it sounds normal.

Currently I have the pre-amp board floating off the chassis for cleaning and diagnostics. It was tough prying that board off its pins.

I am attaching the schematic. I need to take a good look and see what is between the pre-amp and amp, since the above two steps would indicate (to me) that both are good.

I have a scope (but not much scope experience). I will run a sine-wave and see what the output looks like on the clean channel. I would like to figure out where else to probe the signal in the path to try and isolate the issue.

Any guidance and other ideas much appreciated.

Thank you, MarkO

marshall_avt150_avt275.pdf

[g1]

I have run the effects send to another amp, all 4 channels sound great in the other amp.
I have run another amp into the effects return, it sounds normal.

I would retry with the FX loop mix pot turned up. It's a parallel loop so the normal troubleshooting steps may not show the issues the same way.

[misterc57]

Hello G1.

When I ran another amp's pre-amp into the effect return of the AVT-150, I had to dial up the FX Loop knob to hear anything. Is this what you meant? Or are you saying to try the AVT-150 on its own with the FX Loop dialed up?

Thank you

[g1]

Actually I was wondering about when you use the FX send jack to another amp. Just in case the mix set to 10 adds any noise.
I've not figured out yet the loop as far as the schematic goes, or operation for that matter. Can you use the loop and the DFX at the same time? Does the footswitch have a shut off for either?

[misterc57]

Actually I was wondering about when you use the FX send jack to another amp. Just in case the mix set to 10 adds any noise.
I've not figured out yet the loop as far as the schematic goes, or operation for that matter. Can you use the loop and the DFX at the same time? Does the footswitch have a shut off for either?

FX Mix and DFX do nothing when sending signal to another amp.

I have the 5 button footswitch but I do not have the 15 pin d sub cable.

[misterc57]

I was trying to see the signal on the scope, when I finally dialed it in it looked good on the final output. Hmmm? I plugged a guitar in and the amp appears to be working fine now. Will give it a few more tries before I am convinced all problems are gone.

[misterc57]

Anyone know where to get a cable for the pedal? 15 pin, two rows. Was hoping there is a data cable source instead of a Marshall designated cable which are very pricey for just the cable.

[g1]

That's like a VGA cable for a computer monitor (D-sub) but with female on both ends. Maybe you can get a male to female D-sub turnaround and use a VGA cable.

[misterc57]

I think the VGA 15 pin are three rows. The Marshall uses a 15 pin two row...

[g1]

Ok, not VGA, how about serial, DB15 ?

[misterc57]

Yes that is it, DB15 serial cable!

[g1]

Probably boxes of them for 50 cents each at my local thrift store. Not sure if they make a Female to Female but I would think you can get a M to F turnaround.