PA Power Aplifier

[Robin]

Hi Guys

The left channel of my power amplifier doesn’t work properly anymore. Sometimes it works and sometimes it doesn’t. It’s not scratchy at all. I’m sure the pots ( volume control) aren’t the problem. The right channel is fine so I guess there is nothing wrong with the power supply.

The sound on the left channel doesn’t fade out, it just stops. Turning the volume control won’t help. If I power the amp off and then on again, It works fine again. (most of the time)

I don’t know if it is the protect circuit or maybe a bad capacitor or transistor which could be the problem.
Does anyone have any suggestions for something that I could measure?

I do know quite a bit about electronics, but with this problem, I don’t know where to start.

Hope someone has an idea?

[dom0]

Protection circuits in  consumer amplifiers almost always affect the entire amplifier and not single channels (less cost).

- Check/replace speaker relay for sticky / oxidized contacts
- Check/replace mechanical source select switch, if any

[David_AVD]

I've seen it before where a 2 pole protection relay on a stereo amplifier goes intermittent on one pole, causing one channel to drop in and out.

Also make sure the relay coil is getting enough voltage (compare to the marked rating) as it may not be pulling all that well.

[DimitriP]

You might just have to figure it out the old fashioned way:
Trace the signal from the input to the output and find out where it stops.

[Alex Eisenhut]

Does it have an A-B speaker switch? Try switching that back and forth a few times.

[JoeB83]

Can you tell us more about the amp? Make, model, manufacture date, etc?

[Robin]

Thanks for the advice, I'm going to check the speaker relay like you guys suggested.
After that, I'll trace the signal from the input to the output.

The amplifier isn't a well known brand in the USA, it's a DAP Audio Club Range 500. And it could be 10 years old. So it's likely to be the relay, but that's only my guess.

I'll let you guys know when I have measured the relay!

[Robin]

Does it have an A-B speaker switch? Try switching that back and forth a few times.

No it doesn't 

[Alex Eisenhut]

Does it have an A-B speaker switch? Try switching that back and forth a few times.

No it doesn't 

Ah, I see, you really mean a PA like "sound reinforcement", I thought you meant like a "receiver" or home stereo.

I don't know, check the connections on the amp and speakers? Re-seat the cables? I'd try that before taking it apart. It costs nothing to try that first.

[Robin]

Just powered it on, left channel obviously didn't work. But after tapping the speaker protection relay, it did work!
I also checked the coil voltage of the relay and it's even a little over the recommended 12volts. So exept for the relay everything is fine.
I could just try to clean it's contacts, but if I want a permanent fix, I could probably be better off just buying a new replacement relay.

Do I have to match the resistance of the coil? A higher resistance wouldn't hurt I guess...

Thanks for all the suggestions!

[David_AVD]

Just replace the relay.  It will be a common type.