Peavey Stereo amp-1 speaker not working

[manalishi]

Hello
New to the forum

I have a Peavey Transfex 208s-(25 watts X2) with problem.No sound comes out of the left speaker.Speaker is good when I swapped the wires from the working one.
Now when I plug my headphones into the stereo phone jack, I get stereo sound.I verified its stereo by changing the left/right effects levels.
Question is, does  the headphone circuit normally tap off the preamp or tap off after the power amp?
The amp uses 3 lm1875t power amps.Just wondering if its just a matter of replacing the amp that feeds the left.But if the headphone circuit comes after the power section then it seems that the power amp for the left is fine and its some other dubious problem.
Since the speakers do cut out when using headphones, is it possible the phone jack is at fault? 

TIA
Doug

[teemuk]

The block diagram in the manual shows that the power amps feed the headphone circuit, which should have a stereo switching jack that disconnects the signal to the speakers. I would check the condition of that jack as well as the condition of the external speaker jacks, which are also switching and disconnect their corresponding internal speaker.

Those diagrams are sometimes a bit too vague, though.

If you need, Peavey should be very helpful in supplying schematics and I'm pretty sure you can also trace out the neccessary parts of the signal path.

I'm kind of wondering what the third LM1875 does...

[manalishi]

Okay, Its fixed.
Found a broken solder joint at the left speaker out jack.
I also found a bad solder at the input jack which caused alot of nasty pops when plugging in.
Huge difference with an extra speaker working.This amp's effects are very good as far as onboard goes.Big sound for 2 8's
Next job is to get rid of the speaker pops when turning on/off the amp.

Thanks T for pointing me to the speaker out jacks.

Doug

[Enzo]

Popping at turn-off is not generally considered a "problem," and often is not reasonably eliminated.  Sure one could engineer in some sort of speaker relay and get it to drop out before the power supply discharges.  But in reality, that pop cannot be any larger than the signal peaks - it is limited by the power supply voltage.  SO unless the speaker was underrated for the amp in the first place, it can;t hurt the speaker.

I'd have to wonder about that third LM1875 too.  Are you sure the third one doesn;t have only three legs and say 7805 on it?

[manalishi]

Well, I was a little hasty in my search to find the problem and there are ONLY 2 lm1875.The other is a transistor of some sort mounted on another heatsink.Its position makes it difficult to identify the labeling.

Noise  during turn on/off may not be a problem persay but I think there is something not right with it.I assume that when new,  the amp wouldn't have this trait.
When first turned on, lights come on and such.I guess after the presets are loaded, then the "pop" comes in.Since the preset has delay in it, I get pop,pop,pop......