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Fixing a Yamaha G100 112 ii

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Hello all!  New to the forum.  So before I lay out the problem, know that I have some experience with electronics.  I've been building effects pedals for a few years now and I can debug a pedal alright.  If I had a schematic this may be a little easier, but I don't and I can't find one free online.  If anyone has one to share I would be very grateful! 

Okay-  so I got this amp in a trade.  I've heard awesome things about them for years.  It powers on, everything lights up, but there is no sound.  No signal, nothing.  I probed it and it sounds like the input jacks are working, but the signal dies once it gets past the input jack solder joints on the PCB.

The guy I got it from says he doesn't know anything about it, and that he never did anything to it.  In the chassis I found two old power transistors, so it looks like they were changed at some point.  There is a spot on the preamp pcb where it looks like someone tried to fix a busted trace and wired in a jumper to reconnect it. See picture.

I took out the power board the transistors are mounted to, and cleaned up the contacts leading to the second power board (at least thats what I think it is).  They seem to be making good contact.  I've checked the fuses and they're all good. 

Power Board with Trannies:

Power Board up top:

Besides the jumpered trace the only thing I can find that is suspicious is a blue solid core wire that looks like its supposed to be a ground-wire since it is bolted to the chassis (see 4th picture).  It's just hanging out and I'm not absolutely sure where it goes, but I do have a guess:  On the 2nd power pcb there is a blue stranded wire running from the preamp board.  On the underside of the pcb the solder joint where that stranded wire meets the power pcb has a little brown dot in the middle- almost looks to me like something had broken off at some point (it's right on the top edge of the PCB marked with a "-" above it).  I want to alligator clip that solid wire to that joint to see if its just a ground point- but since I don't have a schematic I'm not all that confident.   

Since the signal is dying at the preamp pcb, my guess is that the preamp pcb isn't getting power somehow, or the signal is grounding out somewhere.  My primary request is just for a schematic, but if you have any bright ideas I would love to hear those too.  Thanks everyone! 

Here is a Yamaha G100 service manual.

The blue wire screwed to the chassis is pretty stiff wire, right?  Notice all the various colored wires running right past it?  You wrap that piece of wire around the ones going somewhere, and it holds them in place.  Essentially it is a wire tie.  it does not connect to anything.

PLEASE.  Do not just conect a clip wire to see if grounding something works.  That is not how to do it, you will likely damage something more often than guessing right.

The guy may be honest about never doing anything to it.  An amp this old has had plenty of chances for other people to have repaired it.

The heat sink module with the six pins that plug into the main board - it is your power amp.  I rarely see anything wrong with them other than blown output transistors or cracked solder on teh female pins on the big board.  That doesn't mean something else can't be a problem, but on average not.

Start at the start.  No sound?  Does the speaker work?  Pull the wires off it and touch a 9v battery briefly to its terminals.  A working speaker will pop or thump when you do that.

Galaxiex posted the series III manual, but the series II manual is on the same web site, and the first file in the zip is the schematics.

Then look for power supply.  The preamp runs on +/-20v supplies, and there are 15v supplies for the preamp as well.  Are they all four present?  The power amp runs on a single supply, I don't know the voltage, but it has its own fuse.  Even if that voltage was way wrong, the amp should still make sound, so what I would look for is that power supply is either present or not present.  If it is there, it is probably OK.  Try switching channels, do the channel A and B LEDs go on and off?

The output of the power amp goes through a cap to the speaker, because it is single supply.  So make sure that cap is not disconnected somehow.

With power on, turn the reverb up some and rock the amp to make the reverb springs crash.  Does that sound out the speaker?  APply a signal to the input, and plug a cord from the REVERB FOOT SWITCH jack to some other amp.  And turn the reverb up some.  We are using that jack as a preamp out for testing.  DO you hear sound over in the other amp now?

Just wanted to post another great site to find tons of free Yamaha schematics. This site is excellent and a great resource.

Thanks for the schematics, and thanks for the feedback Enzo!  I'll work on it this week and see if I can't figure it out and report back.  Thanks a lot for all your help!


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