Yamaha G100II head

[leonpiper]

I have done alot of googling on this amp head and I found out alot, but I still can't find some stuff.

I'm trying to find the schematic and user manual for it. I found on yamaha's website the manual for the III but no info for the II.

I picked it up a few yrs back and most of it works. I picked it up cheap because the B channel does not work at all. I had used a pedal with it but since PCSed a few yrs ago I haven't even played it.

The clean channel sounds great. But I decided maybe it's time to try figure out what's wrong with the B channel.

While I'm not an electronics guru, I did go to college for HVAC so I am familiar with wiring diagrams and working on things. I also build guns and cars and do everything from welding and machining to finish work. I have also been into car audio since before I could drive and built my own crossovers. Basically I like working with my hands and don't mind getting dirty and I think I can handle this project.

Does anyone have a source for a schematic? and/or any tips on troubleshooting this beauty? I'm hoping it's a simple fix like the A/B switch but I need to start tearing into it.

thanks for any help
Leon

[DrGonz78]

Hopefully file is not too big... lets check...

[phatt]

Thanks DrGonz the file loaded.
the amp has both a front panel switch and a foot switch to activate the other channel.

I'd check that foot switch socket and make sure it's not stuffed, if not  then the relay circuit might be stuffed which will be harder to fix.
(Tr103--Tr106).

Teemu might know more about these Amps.
Teemu are you out there? 8|
Meantime this might be helpful for general info;
http://www.ssguitar.com/index.php?topic=2179.0

Oh worth a note;
A lot of SS amps from this era had horribull distortion channels and with so many pedals around these days,,, time and money might be better spent finding a pedal that works well with this amplifier. (just my view)

Phil.

[Roly]

When you operate the panel switch and/or footswitch, do the channel indicator lights change?

Have you tried an actual footswitch? (channel B has to be preselected on the panel switch)

[leonpiper]

I do not have a foot switch to try so I have never tried that. I believe the light changes when a/b is switched but I will have to double check.

The baby will go down for a nap in a bit and I can mess with it then.

And as far as pedal go vs. repair.... I picked the head up so cheap that was my thought at first but after having the head for a few yrs its just bugging me so i thought why not at least take a look.

[leonpiper]

Hopefully file is not too big... lets check...

Thank you for the file, this should help a bunch.

[leonpiper]

had a few minutes while the boy napped and I was able to check a couple things. the light does switch to the B channel.

Also I noticed (don't know why I never checked before) that A and B channels both have a light hiss with no signal plugged in. So that should mean the amp section in the B channel is working and that it's somewhere in the preamp signal path.

[phatt]

Refer to Roly's Q as it's important for us to help you.  8|
You need to Check with a pedal while also flipping the front panel switch to a/b. The pedal switch will only work with the front panel switch in the correct position.

The pedal socket maybe worn or broken but as the led changes then it might be the circuit and as we have a schematic, there is a very good chance it is fixable. :tu:

Yes highly likely the switching circuit is in need of repair, a common issue.
If I had a dollar for every Amp where the audio path is in perfect order while the switching system is stuffed.  Hence I have a pet hate for multi channel amps. Rant over

Phil.

[leonpiper]

I get to leave the house today to run a couple errands so i will swing by GC an see if they have any cheap footswitches.

and this may sound dumb, but what does "stuffed" mean? do you mean there might be something actually jammed in the socket?

[Roly]

"stuffed" is a polite Australianism for F$&#ed - unserviceable, broken.

If the channel lights are following the front panel switch then you don't need to do the footswitch test; the question was because the footswitch socket also has a switching contact and these are often problematic, make poor connection, however if the LED's are changing then that isn't the problem. {you can also fake a footswitch by plugging in a normal guitar lead and shorting the end with something metallic.}

Next test, while operating the channel switch (and observing that the LED's are changing reliably) turn all the volume controls right down and see if you can hear a faint click coming from the amp innards each time the channel is (gently) changed.  It's possible you may hear a sharp little click with each channel change or only when channel A is selected.

There are two relays controlled by this switching circuit.  We can now be fairly sure that the switching drive circuit is working thanks to the LED's, but now we need to know if both relays are operating.  If so the next suspect would be the contacts on the B channel relay.  If not it could be the B relay coil has gone open, or the transistor that drives it has failed (etc).

It's also possible that these relays are so small you won't be able to hear them operate, but give it a go.

[leonpiper]

 well, I was out and picked up a footswitch anyway. It switched from a to b with the foot switch although at first it flickered between the two so it looks like I may need to clean up some solder joints as well.

but, still no sound out of the b channel. but, I expected that.

On a side note I was wrong about one thing. I thought I could hear static on the b channel but the house was quieter today and with the a cranked full you could hear the static (white noise) nice and clear and the B channel .......nothing. So it could be in the amp section... I was hoping not.

now I have to get the meter out an start tracing