Pull-knob functions on Yamaha G50-112II not working

[lossfound]

I love the old Yamaha G-series amps and just added a G50-112II combo to my collection. It has three pull knobs. The clean and dirty sections both have a "pull fat" knob, and the treble knob in the EQ has a "pull bright" function (which is the one I'd really like to be able to engage-- the amp's a bit murky-sounding without it). None of these knobs seem to be working or doing anything at all on pull.

I can't imagine the knob switches themselves have all failed.

I've all but given up on my local techs-- our "most reputable" tech in the area charged me a $40 deposit to sit on one of my other amps for 13 months, finally returned this once-mint-condition vintage amp to me after ruining the tolex by using the cabinet as his personal drinkholder for that year or so, and didn't fix a thing (or offer to refund my deposit). That's allegedly the *best* guy we've got, and the others I've tried around here are little better. I have basic soldering skills (little else, although I can use a DMM if pointed at this or that) and figure this has to be something simple. What should I look for?

[Jazz P Bass]

Use your meter & check the switches. (power off)
Use the ohms scale.
You will have continuity in one position, and none in the other, or you won't.
That is a simplistic answer, I know.
I do not have that particular schematic, so there may circuit components that will confuse the reading.
It may be difficult to get at them, but that is how a switch is tested.
If the switches prove true, we take it from there.

[nashvillebill]

The schematic and owners manual are both in this thread http://music-electronics-forum.com/t16593/

It appears the footswitch must engage the proper channel for the fat or treble boost to work on that channel accordingly.

[Jazz P Bass]

Yamaha G50-112 II Schematic

I don't know.
Looks like the A/B (Panel) switch or the footswitch will select the respective pull switches.
Maybe I am reading it wrong

[lossfound]

There's a clean / dirty channel switch on the front. The footswitch overrides it. I've tested "pull fat" on the active channel (and the non-active one to be safe). The "pull bright" should work on both channels, as the EQ section is demonstrably "global." Will give the switches a test once I have the chassis out, but like I said, I would find it really strange if the switches had somehow failed in all three pots.

I took a look at that service manual / schem earlier but wasn't able to immediately see if there's some common point or part of the circuit where all three pull switches might have come simultaneously disconnected.

[nashvillebill]

I do see a common denominator, sort of. Both the fat switches as well as the bright switches are switched through relays, if they are not switching properly then that might be the issue. (I looked at the schematic hurriedly though)

The two Fat switches also share a common cap and resistor to ground, as well as the circuit traces to/from the switches. It's also possible there are two concurrent failures,here (one on the Fat circuit and one on the Bright section).

Fortunately the schematic also included the PCB layout. With a meter it should be fairly straightforward to find out what is and isn't being switched properly.

[lossfound]

Thanks, nashvillebill! For some reason I didn't spot the relays. Those do sounds like some very good places to start looking-- thanks again. I'll be back once I have it out of the box.

[MikeH]

The schematic and owners manual are both in this thread http://music-electronics-forum.com/t16593/

It appears the footswitch must engage the proper channel for the fat or treble boost to work on that channel accordingly.

I'm not certain about this; I have a g100-212 II and I have never had a footswitch for it, and the push-pulls work without it. Not the exact same amp, but it should function the same.

[Enzo]

I'd certainly check the switches first, it takes just a few seconds, after all. And while it may be hard to imagine they all failed, whatever conditions might have caused one to fial were also upon the others. The bottom line is...

Never think up reasons not to check something.

[Jazz P Bass]

Never think up reasons not to check something.

Footnote: This is Enzoism #4.

[Enzo]

The footswitch/panelswitch selects which relay is on, A or B. Those relays each have a contact in series with the A and B pull switches. SO the relays determine which pull switch will be ignored. At least that is how I read it.



#1. At home or at work... Get the right tool for the job.



Bonus points if you know where that advice comes from.

[Jazz P Bass]

#1. At home or at work... Get the right tool for the job.



Bonus points if you know where that advice comes from.

Mr Natural?

[Enzo]

THAT'S MY BOY!!!