Help a noob? AVT150h dead after mismatch

[VomitTaxi]

Hi there,

Firstly thanks for reading this and giving me the time of day, having read thru this forum anon before im aware im waaayy out of my depth, so both sorry and thanks in advance!

My problem, i have a busted amp - a Marshall AVT150h - which stopped working after a prolonged accidental impedance mismatch (not my fault btw ). The emulated line and headphone outs are still working fine, it is only the speaker output itself which is dead, zilch coming out on all channels. As this is the case i asume the chips and valve are fine. There is no indication of burning or damage on the board and the fuse is fine also.

If any one could offer a suggestion about what component(s) are the problem and how to go about sorting it i will metaphoricly buy you a virtual beer. If the job is an easy one then id like to tackle it myself, if not, then id like to pass it onto someone else with all the parts ordered and necessary information they need.

Thanks!

[olddawg]

Check out this thread:
http://music-electronics-forum.com/showthread.php?t=813

[VomitTaxi]

Thanks, i did read thru that thread before but didnt think there was any connection. Its suggested those TDA7293V seem to go by themselves, wearas my issue was most definatly caused by the mismatch. But, if these things only affect the speaker output and not the rest of the unit, then they may well be the issue. Any thoughts?

[olddawg]

Thanks, i did read thru that thread before but didnt think there was any connection. Its suggested those TDA7293V seem to go by themselves, wearas my issue was most definatly caused by the mismatch. But, if these things only affect the speaker output and not the rest of the unit, then they may well be the issue. Any thoughts?

I would find a schematic and make sure all of your rail voltages are there. You may have simply have opened an on board fuse or a current limiting resistor. In my case I would check it out with a scope to see if there was an input signal at the output amps, you could also check it by signal injection. In the absence of a schematic get a pinout print of the generic replacement IC mentioned in the thread. I really doubt that your damage was caused by a speaker load unless the load was less than 2 ohms. Running it with no load would cause no problems. It is a solid state output amp.

[VomitTaxi]

Nice that seems like a good amount of information to start with, i will pass it onto someone who knows their stuff and hopefully can find the fault.
I should have mentioned before that it was the 8Ω out patched to a 4Ω in on the cab, it could be unrelated to the problem but seems strange it would happen after 7 years of good service. Will report back when the problem is solved for the record. Many thanks!

[olddawg]

Before you do anything make sure that the speaker cable that you used and the speaker cabinet are not shorted. That is what usually kills SS output amplifiers.

[52 Bill]

If the headphone jack is still working, check to see if the signal for the headphone jack is derived from the output amplifier.

Many amps have headphone jacks that cut off the signal to the speakers. If the switching contact on the headphone jack doesn't make solid contact it will give the symptoms that you describe.

With the amp off try cleaning the headphone jack by spraying a little contact cleaner into the jack and then plug a set of headphones in and out of the jack a few times to work the switch contact clean. I have seen these jacks fail so badly that replacement was the only real cure.

[Enzo]

You say you used the amp for an extended time with the impedance mismatch, yet you are sure that is the cause of the TDA7293 failure. I would be more inclined to think the impedance mismatch was irrelevant. In any case, I would be betting my lunch money on a bad TDA7293 REGARDLESS of why it failed. Seems real unlikely that it would take 7 years for something to cause damage. Things fail, everything works until it doesn;t.

Without looking, I am pretty certain the phones have their own driver circuit.

But of course, test the speaker and speaker wiring first.