This amp. is not working!!!

[covik]

Hello!
It is an car amplifier 50W+50W but is not working.
When i connect both channels everything is ok untill i turn high the volume speakers start buzzing. There is a small "protect" led and turn on when this happens.
Some kind of protection.
When i connect only "left channel" it works well at high volumes so i think the dc-dc convert stage is working perfectly, i have also measured dc-dc stage voltage and is +-30V.
Also i have connected another amplifier and works without problems.
I think the right channel is oscillating but i do not why.
I have replaced all output transistors MJE2955/3055 but the phenomenom is always there.

[ocaukrell]

You say that if you only use the left it works ok.
What happens if you only use the right ?

When the fault appears, if you turn the volume back down does it start working normally again ?

[covik]

I must turn off completely the amplifier because it stay blocked when this faulthy thing happens.
It is not working the right channel, only with small but very small amount of volume is working but then vhen i turn volume up it stops.

What can be wrong?

[ocaukrell]

Being a bipolar design I would doubt it's oscilation.

Check all large resistors, and also look at the transistors feeding the output transistors.

Cheap way to test/confirm is to swap parts from one channel to the other.

once the protection system triggers it obviously won't reset until power has been removed.

[covik]

Thank you.
Ill try repair it or ill cut the power DC-DC stage, remove the amplifier and preamplifier, then put a brand new amplifier in.

[ocaukrell]

Looking at the photo, I wouldn't bother doing that.
If you can't fix it cheaply and easily I'd just toss it.
The power supply just doesn't inspire confidence to me.

Could also be a dry solder joint.
I've repaired an amp with a very similar looking board but for the life of me I can't remeber what brand it was.

Was some so called american amp that claimed big numbers.

[Original Burnedfingers]

Cheap way to test/confirm is to swap parts from one channel to the other.

Are you actually talking about taking apart a working channel and swapping parts with a non working channel? If so I don't agree at all. Never and I repeat never swap parts from a good channel of ANY amp. The good channel should be used as a reference to measure voltages. If you start swapping parts you can end up with two non working channels and twice the work.

Check your load, connections and things of that nature. Make sure your load on the side that is having problems doesn't end up to be a short. I would suggest borrowing a impedance meter and checking both channels and this will eliminate load as a problem. Don't swap channels with loads before checking for shorts.

[Jexx]

Originally posted by burnedfingers


Are you actually talking about taking apart a working channel and swapping parts with a non working channel? If so I don't agree at all. Never and I repeat never swap parts from a good channel of ANY amp. The good channel should be used as a reference to measure voltages. If you start swapping parts you can end up with two non working channels and twice the work.

Check your load, connections and things of that nature. Make sure your load on the side that is having problems doesn't end up to be a short. I would suggest borrowing a impedance meter and checking both channels and this will eliminate load as a problem. Don't swap channels with loads before checking for shorts.

I was thinking the same thing myself.