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Marshall AVT50 Guitar amp repair

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Gillieshill

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HI,

I have a Marshall AVT50 which has stopped working. The speaker has been blown but there are also other problems. I can get output from it by plugging a CD player into the CD in jack and unsing headphones in the headphone socket. Both of the clean and drive channel LED's light up but I only get a slight hum from the drive channel through the headphones. The main fuse is OK.

Clearly I need a new speaker, but there is obviously another issue. The guys at Marshall that I've spoken to (parts dept) suggested that the power stage of the amp could be blown. Does anyone have exoprience of this amp/problem?

Richard.
 
The amp will be a class a type, using possibly two or four output transistors, or could use ic modules or even VALVES!!!

See if you amp is listed here for schematics>>
 
Last edited:
Marshall AVT50

Guys,

Tha amp is a hybrid. It has 1 tube in the pre amp and a solid state power amp section. Other advice that I've had suggests that it's the power amp which has gone and taken the speaker with it. Any views or comments on this?
 
Maybe just a bad filter capacitor causing it to hum, or maybe a stressed out tube that has gone out of spec.

I don't have much expertise on tube amps though.
 
I am by no means an expert on these things, but a little googling turned up the manual: Marshall AVT50 Amp Handbook

Also, I found the following page which has schematics which claim to be for the AVT50: **broken link removed**


Good luck,

Torben
 
According to that circuit, the power amp is a TDA7293 IC - presumably that is blown, and took the speaker with it?.

However, it's easy to tell - just stick a voltmeter across the speaker output, there should be no DC on it (or certainly only a few 100mV). If there's no DC, it's probably just the speaker that's died - try hanging any old speaker on just to try it (get one out of an old TV or stereo).
 
I have the same head.I'm havin problems with it playing out of the cabinet. It turns on fine. If I plug headphones into the headphone jack it sounds perfect but nothing comes through the speaker output. I test the speaker cabinet with another amp an it worked fine. So given the evidence from troubleshooting, 1. It plays fine though the headphone jack. 2. The cabinet plays with other heads but not the Marshall. Therefore no signal or power is coming through the speaker jacks. Can anyone help?
 
I know this is an old thread, but here's something to be added.
I have an AVT50H that wouldn't power a cab. The TDA7293 was bad. The effects send worked fine, no headphone output.
I replaced it, checked components on the power board, put it back together and found that it would intermittently power a cab.
The headphone jack worked fine. I cleaned the contact surfaces on the
headphone jack and slightly tensioned (bent) the contacts with a jack installed in it and now it works great!
Remember, the headphone jack breaks the circuit going to the speaker
outputs and if it has bad connections/broken solder joints then no power will go to the speaker.

So if you can use headphones on these amps, then the TDA7293 is ok and the problem is in the
headphone jack or the circuit from the jack to the speaker output.
NOTE: IF REPLACING THE TDA7293, BE ABSOLUTELY SURE YOU DISCHARGE ALL CAPACITORS IN THE AMPLIFIER AND ON THE POWER BOARD BEFORE REPLACING OR THE CHIP WILL BE TOAST BEFORE YOU EVEN TURN THE AMP ON!
You can find the proper schematic at www.schematicsunlimited.com
 
I have the ATV 50H kid of the same problem but I tracked mine down to a resister but the color bands are unreadable it is the only resister that different in size , does anybody have a copy of the schematics that list all the resisters / parts ?
 
I know this is an old thread, but here's something to be added.
I have an AVT50H that wouldn't power a cab. The TDA7293 was bad. The effects send worked fine, no headphone output.
I replaced it, checked components on the power board, put it back together and found that it would intermittently power a cab.
The headphone jack worked fine. I cleaned the contact surfaces on the
headphone jack and slightly tensioned (bent) the contacts with a jack installed in it and now it works great!
Remember, the headphone jack breaks the circuit going to the speaker
outputs and if it has bad connections/broken solder joints then no power will go to the speaker.

So if you can use headphones on these amps, then the TDA7293 is ok and the problem is in the
headphone jack or the circuit from the jack to the speaker output.
NOTE: IF REPLACING THE TDA7293, BE ABSOLUTELY SURE YOU DISCHARGE ALL CAPACITORS IN THE AMPLIFIER AND ON THE POWER BOARD BEFORE REPLACING OR THE CHIP WILL BE TOAST BEFORE YOU EVEN TURN THE AMP ON!
You can find the proper schematic at www.schematicsunlimited.com
Hi Gearhead63, I had this EXACT same issue, and your technique resolved my problem. Thank you for your information. It was very helpful. Best regards, CentaurLD
 
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