Yamaha EM-150 mixer schematic, anyone?

[Neal]

Got together with some old bandmates last week....we're going to reform the band and do a few select dates. Over the course of many stories and very little practice, we got talking about old gear. The other guitar player walks in with an old Yamaha EM-150 powered mixer that we used to use for monitors back in the day. He said, "Take it home, or take it to the curb!" Naturally, I took it home. I thought it'd be great for the kids to practice with.
Problems.... when first powered up, it sounds fine. As it warms up, it makes crackling/popping sounds that increase in volume until I unplugged the speaker for fear of damage. Turning all controls to zero doesn't decrease the noise at all.
Secondary problem: no reverb on signal, although reverb crashes when the chassis is bumped. Gotta be in the reverb send, haven't traced it out yet.
If anyone has a schematic or tips on this old dinosaur, I'd appreciate it.
Neal

[Enzo]

Neal, I have the drawings, but they are very large fold out things, they are the size of a road map. Very hard to scan. Frankly I'd just order the service manual from Yamaha, they want probably $15, but it is a very thorough manual - schemos, layouts, parts lists, biasing procedures, disassembly, you name it. Yamaha makes great manuals.

SOunds like your problem is in the power amp. Open it up and see of there are any Molex connectors involved. Pull them apart and then remate them to freshen the contact surface. Also, get a can of freeze spray and chill sections of the board until you hit something that responds. That is what freeze spray is for.

You can leave the speaker load not connected, and watch the noise output on your scope. Even a meter set to AC volts would probably measure the noise level. After all, when we freeze things all we care about is does the noise disappear or not.

[Neal]

Thanks, Enzo!

Thanks for the reply, Enzo! When time permits, I'll start at the speaker jacks and work my way backwards. I haven't had time to properly troubleshoot yet, but I'll use the line outs to test out the mixer and EQ sections as well. I'm impressed by how much hand wiring is in this board, and also the design... take 6 screws out of the bottom, and the whole top mixer section hinges up from the bottom for easy access. Nice!
By the way, this board came from the same gent who gave me his old Sunn Beta Lead 4-10 amp. You gave me some great advice to fix it, and I took out the blown speakers and replaced them with aluminum-coned Hartke speakers I picked up cheap used. Our bass player loves the amp, and stacks it on a single 15" cab onstage. The original owner is highly amused that his old amp is still a road warrior. Thanks for helping resurrect it!
If circumstances ever take me to Michigan, I owe you a steak dinner....
Neal

[Enzo]

Yes, the old Yamahas are pretty accessible. I like to put them on their side on my table, and open them like a book - mixer on my right, power amp and bottom tray on the left. That way I can leave stuff plugged into the rear jacks without them getting crunched when I lift the mixer panel. And that way I can squirt cleaner into the pots easier than up and under the other way.