Need help with Yamaha EMX860ST - blown monitor amp of powered mixer

[flsurflover]

I think I previously posted this in the wrong section and just found this Music Electronics forum. Please forgive the double post.

Hi, I recently acquired a Yamaha EMX860ST powered mixer, which has 3 amps inside (1 for monitors, and 2 for mains Left and Right). The mains amps work perfectly, even in bridged mode. The monitor outputs in the back of the amp do not output any sound, although monitor section on the front panel appears to be working. When the level knobs are turned up and a mic is used, the LED meters light up according to the level of input received from the mic. Also, the monitor out on the FRONT of the amp that is meant for you to send to an external monitor amp (does not run through an internal power amp) does work, and the rest of the monitor controls correctly modify that unamplified signal. So it appears only the power amp for the monitor section is not working, and everything else appears to be fully functional, including the EQ's, level meters, reverb effects, etc.

I'm a novice at electronics repair. 20 years ago in a past life I graduated with at EET degree but haven't used electronics repair since working in the IT field. I paid $100 for this amp used site unseen and I can get my money back from the seller. It still has some use as a rehearsal amp so I may keep it. But if I could get the monitor power amp working it would be fully functional for small gigs. However I know if I take it to a repair place it will be relatively expensive to have them fix and I would be better off buying a functional Peavey XR600f or the like.

Any advice on if there are common things on these old EMX powered mixers that might be an easy fix that would account for just the monitor power amp not working? I need to decide if it's even worth opening up or if I should just go get my money back. I can look for visible burnt traces and components, and repair/replace them, but I don't have much experience tracing voltages through circuits. Am I over my head and should punt?

Any help is greatly appreciated.

[Enzo]

Actually, it was fine in the repair section. REally, the place for it would have been the Other Amps section. But please don;t repost.

Here is what I posted on the DIY forum:

The vast majority of failed parts look no different from good ones. Flames and soot are very much a minority of failure modes.

I don;t have the schematic with me, so I cannot look it up at the moment, but if you feel like it, open it up and see if there are separate fuses for the three amp circuits. Beyond that, there is not much that won;t require soldering parts on the circuit board and identifying where the failure lies.

Connect a speaker ONLY to the monitor amp outs. Is there any sort of turn-on thump or pop when power is turned on? You say no sound comes out, as in signal, but is there ANY sort of background sound? For example the amp might be working but the signal is not reaching it. It might still then make some little sign-of-life noise.

And I don't have a good close up photo, but if there are any POWER AMP IN jacks on the panel, if there is one for the monitor amp, plug a signal into it and see if it drives the amp to produce output. A dirty cutout contact in such a jack could inte3rrupt the signal path.

And if the power amp assign switch mentions the monitor function on any position, exercise that switch a few times and se if that helps. More times that I would think, someone has brought me a mixer with a "dead" channel that was really just the amp assign switch in the wrong position or dirty.

That's the simple stuff.

[g1]

Here's a link to the service manual: YAMAHA EMX860ST Service Manual free download, schematics, eeprom, repair info for electronics
Doesn't appear to have separate fusing for the amps, the only simple thing to check is the relay to the speaker jacks.