Marshall AVT275 Dead Headphone

[steves4891]

My headphone jack has a dead channel, don't recall if it's right or left. I'm sure it was caused by me using a mono jack on it when I was younger. I'm hoping someone could tell me what components are likely fried. Attached is the wiring diagrammarshall_avt150_avt275.pdf. Page 6 has the headphone jack. I was guessing the IC's but I don't know that much. Any direction would be appreciated.

Thanks, Jacob.

[52 Bill]

Welcome to the place.

If the problem is a bad chip, then the two chips that drive the headphones are IC101 and IC102, but the problem may not be a bad chip.

What have you done so far to diagnose the problem?

[steves4891]

I've only looked at the boards to see if there was anything visually fried. Have not tested anything. I assume it replacing those doesn't work I should keep replacing up the chain to 103 and 104?

[Enzo]

No. If you want a can of corn from your pantry, do you start opening cans until you find corn? or do you determine which can is corn and THEN open it?

Who says your problem is a bad part? You need to isolate the problem. See the jacks above and below the phones jack? They look like line out jacks to me, for the two channels. Do they both work, or is one of them out too?

Have you tested the headphones to make sure they work?

You could have a phones jack not making contact with the phones plug - a sprung contact blade in other words. Or cracked solder under the jack.

The signal might not be there for those ICs to handle. But they are simple to test. Plug in and put on your head phones. Use a pointy metal something: a small screwdriver, a paper clip bent straight, even just a resistor lead. Now touch pin 5 of each IC101 with it held in your fingers, does that produce a hum in one side? Now the other side IC102, same? If both do, then 101 and 102 is working. Now move back to pin 3 of each and touch the pin, same questions, both hum or one not? Now touch pin 3 of IC103 then 104, same questions?

No need to replace them to see if they work, just test them.

[steves4891]

Thanks Enzo. I did a quick search and didn't find a way to test the IC's but I'll give that a shot.

[Jazz P Bass]

The IC's are 'tested' in a number of ways.
First: are the supply pins at the correct levels?
You can run a signal through the circuit and looking at the output.
It's either there or it's not (ignoring for the moment whether or not there are any distortion issues)

A simple volt meter set to read volts ac (for the signal) ang volts dc (for the supply) is all you need.

On dual supply circuits the outputs typically sit at zero Vdc.