Marshall AVT150H Problems

[Force3fan]

I am new here so I would like to start by saying Hello to everyone.
I found this site by searching for help with Marshall AVT amps so I hope I am in the right place. Sorry but this is going to b a long post.

I have a Marshall AVT150H serial number M-201-38-1305B about 6 years old.
A problem started one day with bad sound on the OD channels then the next day no sound from OD channels Clean and Ac Sim ok. Next couple of days a bad hum started and the leds on control panel not working properly. They glow dim and all on at the same time also leds on foot switch don't work.I tried disconnecting foot switch no change.I tried replacing the tube but did not fix the problem. Also the Effects don't work now.

After reading some threads here I looked at the output drivers with the TDA7293 chips. I thought the one on the right (with control panel towards you) may have had a burnt spot so I unpluged both connector (all connections have been done with Power off first) turned Pwr on no change still hum and leds funny. Unpluged other output driver connectors. The hum stopped probably normal. Pluged the right driver only in turned Pwr on got a loud squeel. Turned Pwr off quick. When reconnecting right driver Found it was a little loose on heat sink, took it off for better insp. looked ok what I thought was a burn was onlt a piece of trash maybe plastic. Reinstalled it also tightened the driver on the left. It really was not loose. Turned PWR on got loud squeel and pop and Saw a flash the left TDA7293 burnt.

Have any of you had this problem? Would the TDA7293 chips cause trouble with the effects and leds on control panel and foot switch along with output problems? I know I need both TDA7293 chips now. Before I replace them where else do I need to check for trouble?
I also need a fan. It is noisy for a few secounds on Power up then Sounds ok but can feel a sligth vibration in it the other is ok. Where can I buy them.

Again sorry for the long post but I want to give all the information I can.
Please help if you can
Thanks Force3fan

[Enzo]

You should not have been disconnecting and connecting the 7293 modules. You can power the amp up with them both disconnected, but don;t be messing with them otherwise. Especially while it is powered up.

I can't think of a way the power amp ICs would be involved in the low voltage supplpy issues you seem to have. The LEDs and op amps run off the low voltage rails, which are +/-15v I think. Make sure both those power rails are right. Nothing else will work right if they are not right.

Then to the original problem - I would suspect you had an op amp failing. I would check the output pins of each op amp on the preamp looking for a DC offset. Not worried about a few millivolts, we are looking for a DC offset of several volts.

[Force3fan]

Thank You for your fast reply. Education is expensive. I need to replace the now bad 7293's (one has a hole burnt in it the other is probably bad now also).
I will need a schematic to locate the low voltage rails. Could you please send me one?
Also what would your recomendation be? Replace 7293's, leave both connectors off both modules and make voltage tests on low volt rails and op amps to find other problems.

[Force3fan]

I wanted to clarify that when I first started looking at the problem I never disconnected or reconnected anything with the Power on.

The 7293 that I saw burn up was with all connectors connected before turning Power on.

After replacing them what should I do before turning the Power on so I don't burn out the new ones.

Also I still have to find the oriainal problem of no sound on the OD channels,all led's on control on dim at the same time' no effects working and no led's on foot switch working.

Another question I have does it matter where I get the replacement TDA7293's from. Would original Marshall sound diffrent than other manufactures?

Thank you for all your help !!!
Force3fan

[Enzo]

No difference in parts. Marshall or elsewhere.


I bring up a circuit like that on a variac while I monitor mains current. Lacking a variac, search this board for "light bulb limiter." and make one of those.

I suggested something o\for the original problem in post #2.

[satriani09]

Originally Posted by Enzo

No difference in parts. Marshall or elsewhere.


I bring up a circuit like that on a variac while I monitor mains current. Lacking a variac, search this board for "light bulb limiter." and make one of those.

I suggested something o\for the original problem in post #2.

I have a similar problem with mine, avt150h
it blows the 4amp fuse every time I turn it on.
Any suggestions?

Thanks

[Phostenix]

What's your level of experience troubleshooting electronics? Do you have any test tools?

[Enzo]

And I would bet my lunch money that you have blown power modules.

[satriani09]

And about how much is that to repair?

Thanks

[Enzo]

Depending upon whether you are in Antarctuca, SOuth Africa, Ukraine, Scotland, or the USA, it is hard to say.

In my shop here - in Michigan, I expect that repair to cost you something like $90-100.