Marshall AVT 20 Loud Humming - diyAudio
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Old 30th December 2012, 10:25 PM   #1
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Default Marshall AVT 20 Loud Humming

Hi, I have a Marshall AVT 20 Valvestate that was given to me, which did not work. I got it so long ago I don't remember what was wrong with it, but I think it might have been doing the same thing as described below. I just now decided to start working on it.

A new TDA2050 was just installed to see if that fixed it, but obviously it didn't.
I also noticed that the trace side of ZD3 and ZD4 looked discolored like it got a bit over heated. I took them out of the circuit and they checked out like a diode should (using the diode setting on my meter).

I also noticed that R53 seemed like it might have got a bit heated, but I still get a reading of 8.7 ohms across it wile still in the board.

Seems to power up fine and is quiet, but when a guitar is plugged in, it immediately starts humming which increases when the gain and volume is plugged in.
When the OD is engaged, it increases even more and the two leds do not light up.

Any suggestions would be greatly appreciated.

Thanks!
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Old 31st December 2012, 07:48 PM   #2
Enzo is offline Enzo  United States
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Yes, you likely have an input jack problem, specifically, the ground connection is broken.

Pull the board and see if the solder is cracked or if the boards has been cracked by someone stepping on their cord. Of the jack itself could be broken.
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Old 31st December 2012, 07:55 PM   #3
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Quote:
Originally Posted by Enzo View Post
Yes, you likely have an input jack problem, specifically, the ground connection is broken.

Pull the board and see if the solder is cracked or if the boards has been cracked by someone stepping on their cord. Of the jack itself could be broken.
Enzo, thanks!

Would that have anything to do with the Red LED OD lights not lighting up, or might there be some other issues with it?
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Old 31st December 2012, 08:14 PM   #4
Enzo is offline Enzo  United States
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If the amp can't get any signal input because the input jack is broken, it would be hard for it to overdrive anything and light those lights.
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Old 31st December 2012, 08:30 PM   #5
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Quote:
Originally Posted by Enzo View Post
If the amp can't get any signal input because the input jack is broken, it would be hard for it to overdrive anything and light those lights.
Thanks again! I will inspect the jack.
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Old 31st December 2012, 09:55 PM   #6
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I swapped out the input jack with the External Speaker jack but the problem still remains.
Neither jack looked damaged in any way.

I will try and contact the original owner and see if he can remember what started the problem and what the original symptoms were.
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Old 1st January 2013, 08:17 PM   #7
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Ok, so I talked to my son-in-law who owned the amp. He said he was playing during a lightning storm and his guitar strings touched the on-off chain hanging from the ceiling fanlight and sent a charge into his amp via the input jack, which must have fried something. It also broke a few strings in the process.

He said it had the same symptoms Iím experiencing now, that is, the loud squealing and humming depending on the gain/volume level, yet being quiet when the instrument is not plugged in.

Any thoughts on troubleshooting?
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Old 2nd January 2013, 11:14 PM   #8
Enzo is offline Enzo  United States
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Sure. PLug a guitar cord into the input jack. Measure for continuity between the shell of the cord plug and the amp chassis. It should be there, if not, you have a burnt open ground trace most likely.

I still think the ground connection is missing from the input jack. Under the jack, the ground pin of the jack is the one near the front. Visually follow the copper traces on the board from that point. See if at some narrow point, the copper trace has evaporated - acted like a fuse.
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