Yamaha EMX860ST with bad output channel

[rf7]

The schematics for this beast are on the last 2 pages: manual download

This PA head/mixer blew up it's Right channel pretty thoroughly. I've replaced 20 bad components and fixed some burnt traces, and I've got it to where it works, but the bottom half of the output waveform folds up under a load. All the voltages in the Right channel power amp section measure pretty closely to the Left channel (which is working fine) and I just replaced all of the output devices in that channel this last go around.

The only anomaly that I've noticed is that the input line PAR is 0.04Vdc while input lines PAL and PAM are sitting at 0.023Vdc, which is not a big difference.

Any ideas? Are Transistors Q208, 210 & 214 the most probable culprits even though they seem to measure fine?

[Enzo]

Everything isn't bad transistors.

Of course it can be any number of things, but when I see an amp that puts out a clean signal and at most of its range WITHOUT a load, but it collapses under a load, I usually first suspect the output stage. The drivers alone can move the output signal around, but it takes the output transistors to supply current to the load.

The transistors can be just fine, but an open ballast resistor will be the same as an open transistor to the circuit. Burnt cracked or damaged traces can do th same. The driver may be producing its output, but it has to reach the output transistor base to do anything. SO check all resistors.


When I have a mega blown amp, I find it helpful to make a Xerox of the schematic, at least the blown section, and as I find bad parts, I highlight them on my copy with a yellow marker. Pretty soon the failure current path through the amp emerges to view. Now we already are pretty well aqware of the transistors we replaced and such,but when you see a totem pole of burnt parts, and in the middle of it is a non-highlighted component, well, we might want to swap that opart out anyway. After all, the same current that toasted all the other stuff ran through it too, even if it tests OK on a meter.

Please don;t scribble on your actual schematics, just the working copies. Nothng I hate more than to get out a schematic and find a big circle around something and BAD written there.


Since the amp works without a load, I'd bet the DC voltages are mostly OK. Getting a voltage signal to the output is not your problem. Getting current there is.

[HDSarge]

Some Yamaha EMX mixers rely on back panel screws to make channel ground connections. So, check ground integrity.

[rf7]

Thanks for the responses!

I'll have to try replacing the components in the current path, that's a great idea! I had already done the highlighting of the replaced parts, just to keep track of what had happened. Yeah, and I hate when people mark on the originals too.