Marshall AVT150 - No output, fans, foot pedal

[ccwicks]

Troubleshooting my son's Marshall AVT150.

Background: Problem occured when trying to plug into the FX loop in the back of the amp where accidentially, the plugs may have been plugged into the line out or headphone out and into the FX loop in or out. Puff of smoke, and no more sign of amp life other than the beautiful glow of the Marshall switch.

Symptoms: As mentioned above, the power switch lights but no power to the footswitch (leds don't light), no power to the dual fans, no amplification, and the preamp tube does not power up.

Troublshooting so far: I have checked the one fuse (which would have to be good to have the switch light up), and the transformer which is providing the appropriate voltage. Looks like the power from the transformer gets to the main board but then no longer powers other components (AKA rookie diagnosis by me...)

Would appreciate a next step to look at or suggestions from the forum. In the mean time, from research within the forum, I will try looking at the 511-TDA7293V if I can figure it out.

P.S. Enzo thank you for the pdf's up front - much, much appreciated and resptful of your knowledge.

[Mark Black]

Sounds very much like an AVT-100 I am currently working on.

I don't know for sure that somebody plugged something bad into the FX return, but the trail of carnage would seem to suggest it.

On this one I have found (so far):

4 shorted 1N4148 diodes and shorted opamp in the FX return immediate vicinity, exploded cap in the negative 15V supply near the regulators, bad regulators (both), and burned resistors on the AC side of the +/- 15V bridge rectifier.

Additionally there is a shorted DG212 switch IC on the front PCB which I now have to order. After I get that and install it I hope to have reached the end of the destruction, but we'll see...

Hope that's a little help & good luck!

[ccwicks]

Mark - thank you. I'll follow your lead on this and see if it's the same path.

[Mark Black]

A little update to the AVT-100 I am working on in case the similarities help you:

Turns out pretty much ALL the IC's on the front panel PCB are toast, and many died quietly without smoking or discoloration - actually failed open instead of shorting. I ran out my supply of chips so now it's back to waiting for parts again.

How is your AVT-150 project coming along?

[ccwicks]

I looked in the mirror and realized that I'm not as good/smart/brave as you guys are on the forum, so I punted and had a local repair guy take the amp in.

So far:
* He has traced the damage along the +/- 15V lines of the circuit board, following the path of destruction from the FX loop back to the power supply entry point - that's where my kid connected the line out or headphone out into the FX loop.
* He has replaced a rectifier, resistors and diodes along that path so far and seems like that will be it - have not picked it up yet or tested.
* Separately and proactively, I asked him to apply the RFC (request for change) from Marshall to the amp where 2 zener diodes are applied to the input side of the amp to prevent any future static electricty zap to the amp. This is a simple one that I could have done.

To date, I have to thank you Mark for the ideas and support as well as Enzo.