NEED HELP REPAIRING YAMAHA EMX860ST

I hope someone here can suggest what is wrong with a EMX860ST PA Head. Basically the input signal inserted in almost any front panel input jack ends up at other outputs when it should not. i.e. 1Khz sine wave through 1/4 phono or XLR input, any channel 1-6, nominal gain, panned left- produces a signal on both L&R & Monitor Led's and speaker outs, not equally, seems to favor the R side. Sweeping the channel pan knob causes some cancellation near mid point. Same thing happens raising the master gain. Raised a little further and the wave becomes non-symmetrically distorted shown on the O-scope. The only input that doesn't do this is the R ch (non-mono) inputs of ch 7&8, where it doesn't go to the L output even when panned L.

BTW: This unit had quite a few bad components starting with visually cracked power trans, both EQ chips had one leg burned off, the EFX processor chip, 15V regulator, and others. At least now it doesn't draw high current through the Variac when powering up!
 
Well, start at the start. Verify all the power supplies as up to voltage and free of ripple.

If you had as bunch of burnt stuff, look closely at the copper tracery for burnt open spots, usually at a narrow spot.

Apply a signal to one of those inputs and scope the power rails to see if the signal is riding them.
 
Hi ...

This is not my area of expertise but when reading what you write my thought was that it could be feasible to isolate matters as much as possible and then look at minor parts of the circuitry at a time. I.e. look at the input circuitry and see how it performs isolated from the other parts of the circuitry etc.

This way you might be able to find out where what doesn't work as it is supposed to. My guess is that your oscilloscope could guide you in this.

Regards,

Jesper
 
First thing I would do is go through and check every single ground node has continuity to the main ground. A floating node can sometimes cause very strange signal routing faults.

Actually, the first thing I would check is that all ribbon and pcb interconnect cables are making good contact and are plugged in the right way around. ;)
 
I was suggesting putting a signal through the system and set up so the unwanted crosstalk is occurring. The check your power supplies with a scope to see if your signal can be seen on the power supply. In other words is your +15 (or -15) sitting there at about 15 volts, but also wiggling to the music. A lost ground could cause a loss of decoupling between stages.