Yamaha G100B-212 help with troubleshooting and repair.

[Helmholtz]

With the switch open, the gate voltage will be the same as drain and source voltage as there's no gate current.
And there's no drain-source DC current either.

[JAelec]

"It is not necessary to resolder the transistor.
It is enough to short the pluses of capacitors 1/25 and 1/35 (7.6V points).
Then check the "distortion" on the output."

So do you have output distortion with this arrangement?

I soldered a jumper between those caps, with the transistor still out... No distortion... could it be the diodes? What to try next?

I have access to a signal generator and an oscilloscope... perhaps I could trace the signal in the distortion circuit? Never done this before though...

Would it be necessary to have a dummy load when signal tracing? I know you have to have it on a tube amp, but what about solid state?

[Helmholtz]

Is the distortion pot itself good?. Sometimes the wiper of a pot loses contact.

If yes, verify TR3 and TR4 pin voltages (schematic voltages are given for a reason).

If good acc. to schematic, remove and test the coupling caps in the distortion path.

[JAelec]

Is the distortion pot itself good?. Sometimes the wiper of a pot loses contact.

If yes, verify TR3 and TR4 pin voltages (schematic voltages are given for a reason).

If good acc. to schematic, remove and test the coupling caps in the distortion path.

When I tested the pot it was OK. However the resistive material had worn off at the far ends of the trace, so the total resistance was bit lower, 44k instead of 50k.

I'll test your suggestions tomorrow... but how do I know which caps are coupling caps?

I've numbered the caps in the distortion circuit from 1-7 in the picture below... which ones are coupling caps? Which numbers should I test?

https://imgur.com/a/DI7VNAG

[Helmholtz]

Did you really test if the wiper (middle lug) makes contact to the track?
Measure resistance from middle lug to both outer lugs.

Except No.1, 3 and 6 all circled caps in your pic are coupling caps in the distortion path.
If any is open, no signal can pass.

[g1]

I think you should have enough posts to be able to attach pictures now, can you try? Imgur does not work for my browser.

[JAelec]

I think you should have enough posts to be able to attach pictures now, can you try? Imgur does not work for my browser.

Here's the latest pic... seems to work now!

[JAelec]

Did you really test if the wiper (middle lug) makes contact to the track?
Measure resistance from middle lug to both outer lugs.

Except No.1, 3 and 6 all circled caps in your pic are coupling caps in the distortion path.
If any is open, no signal can pass.

Pot/Switch
I remeasured the pot/switch again...It goes from 0 Ohms (turned fully clock wise) to 23.4 kOhms (fully anti-clock wise). However this is in circuit. I'm pretty sure I measured 0-44 kOhms out of circuit. Either way it has a smooth action when turning the wiper...no drop outs. As said before only problem is that some of the resistive material is worn off at the beginning and end of the resistive strip. So first degrees of turning makes no change in resistance, 0 ohms even while turning... then when it meets the resistive material the measurement goes up then it stops again for the last degrees at 23.4 kOhms at the end, because the worn out part doesn't add any resistance.

Pin voltages of TR3

T7B c1681	Schematic	Measurement
Base	1.2	1.33
Collector	1.9	2.0
Emitter	0.7	0.75

Pin voltages of TR4

T7A c1681	Schematic	Measurement
Base	1.9	2.0
Collector	23.4	25.2
Emitter	1.3	1.38

Cap measurements (out of circuit)

#	Spec	Capacitance	V loss	ESR
1	1uF	-	-	-
2	1uF/35V	0.988uF	0.2%	3.3 Ohms
3	100pF	-	-	-
4	1uF/50V	1.289uF	0.5%	2.7 Ohms
5	1uF/50V	1.289uF	0.5%	4.0 Ohms
6	220uF	-	-	-
7	1uF/35V	0.955uF	0.1%	2.9 Ohms

Results:

Pot/switch, TR3, TR4, and caps 2, 3, 5 7 seem to measure OK am I right?

What to check next?

[x-pro]

I suspect insufficient gain of the first two transistors and/or the tone controls are not in their maximum positions. Connect a generator to TR1 input 50-100 mV, frequency 500Hz. Set DT control to maximum signal.
Connect the oscilloscope to the minus of capacitor "5".
View the view.

[Helmholtz]

Looks ok so far.
Now lift one end of each of the 2 clipping diodes.
The distortion pot should have some effect now, though without diode clipping.
This also allows you the check the diodes with your tester.
One of them might be shorted.

[JAelec]

I suspect insufficient gain of the first two transistors and/or the tone controls are not in their maximum positions. Connect a generator to TR1 input 50-100 mV, frequency 500Hz. Set DT control to maximum signal.
Connect the oscilloscope to the minus of capacitor "5".
View the view.

I used a 620 hz signal 100mV peak to peak.

I made measurements at several points in the circuit. 50mV per division



Cap 5, minus side:
Signal is distorted. While turning the distortion knob the signal goes from sine to more and more distorted smoothly.

So diodes are probably not broken.

PPVoltage is lower than when measuring the output signal from the generator. Is this normal?

Cap 5, plus side:
Signal is quite a bit weaker than minus side.

Cap 7, plus side:
This is the same point as cap 5 plus side so same result as cap 5 plus side.

Cap 7, minus side:
No signal! This point is connected to ground!

A100K Volume pot:
Outer pins:
No signal! Both outer pins are connected to ground!
Middle pin:
No signal! Connected to ground when the dial is at the start and at the end of the resistive element. Turning the knob past the very start and back from the very end, the pot has resistance.
There is no signal even when measuring the middle pin and turning the pot through its full scale.

The volume pot has been replaced by someone before me it is not working correctly... but still the normal signal gets through but not the distortion. I would expect a signal at the middle pin since the amp gets super loud when turning the volume knob up... so the pot works just like it should for the normal signal even though no signal can be read on non of the pot pins... why is that?

[x-pro]

Connect the V and BR controls according to the diagram.
A clean signal is likely to pass through BR. Used as a V controller.

[Helmholtz]

There's no sign of diode clipping in the scope pics. Probably due to shorted C7 output, pulling down the signal.
Also a scope is not only for viewing the waveshape. When calibrated it is also a measuring instrument.
But that requires to know the scales.

[JAelec]

Connect the V and BR controls according to the diagram.
A clean signal is likely to pass through BR. Used as a V controller.

I'm not sure I understand... are you saying the V and BR controls aren't wired correctly? And that I should check the diagram (schematic?) to see if everything is connected properly?

Don't understand the part about the clean signal passing through BR used as a V controller.

How can the BR be used as a V controller... turning the BR control only effects the brightness... turning the volume controller effects the volume...

[JAelec]

There's no sign of diode clipping in the scope pics. Probably due to shorted C7 output, pulling down the signal.
Also a scope is not only for viewing the waveshape. When calibrated it is also a measuring instrument.
But that requires to know the scales.

Do you think the C7 is shorted? But I have tested that one and it's OK according to my ESR meter. Or do you mean that something else on the output side is shorted?

Did I misread the scale of my scope, if so please tell me how?