Yamaha G100B-212 help with troubleshooting and repair.

[Helmholtz]

Do you think the C7 is shorted?

No, if C7 was shorted, signal at both ends would be the same.
But as you reported both outer lugs of the pot are connected to ground.
Means the negative end of C7 is shorted to ground killing the signal there.
Either the pot is defective or there's a wiring error.
In circuit the upper end of the 100k pot should measure around 50k to ground

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

Sorry, I missed that you reported scope sensitivity as 50mV/div.
So signal level is too low for diode clipping.
Below a signal of around 600mVpeak the diodes have no effect.

[g1]

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!
...

The volume pot has been replaced by someone before me it is not working correctly...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?

I think there was an error made by whoever replaced the Volume pot.
I believe what x-pro meant in post #27 was that in 'clean' mode, you can still get signal leaking through the BR pot even if the Vol. pot is mis-wired. And the Vol. pot could still be somewhat functional for clean. Although I would expect the Vol. pot in 'clean' mode to start dropping off again past the half-way point.

[x-pro]

Draw a wiring diagram of the V-regulator yourself and present it to us. It is not difficult.

[JAelec]

I have found the problem (with the help of you guys!) and the amp now works including distortion.

I desoldered the V-pot and it tested perfectly out of circuit... and with the pot out there was no short between the pads on the circuit board

I noticed that there was a lot of flux residue around the V-pot pads on the circuit board. I had noticed it before but did not think it would be able to short out the pot and distortion! Scraped it off and cleaned with IPA, put the pot back in and now the amp+distortion works!!!

The pot measured from 0-24k at the first 70% then the last 30% it went from 24k-0 while in circuit. So not entirely at the half way point, perhaps the flux residue tipped the scale somehow.

It was a good idea to make a drawing... I now think I understand why the pot goes from 0 up to 24k in the middle then down to 0 again.





Please correct me if I'm wrong.

Since both outer legs of the V- pot are connected to ground when the wiper is at the outer edges of the resistive material it's just a dead short to ground then moving towards the middle more resistance is added to the path the ohms go up until the middle of the resistive track, beyond that the current from the multimeter takes the other way around through the shorted outer legs since this is now the path with least resistance.




And I also now think I understand how the volume pot works. Again, correct me if I'm wrong.



In pic #1, above, the pot is turned all the way to the left (or right) and the all the clean signal is shorted to ground. No sound!




In pic #2 the volume pot is turned up to 25k so less of the signal gets shorted to ground. Signal moves on in circuit and there is sound!

[Helmholtz]

Look at the schematic, only one of the outer lugs connects to ground.
Connecting both outer lugs makes no sense. No one would do that.
The other outer lug should measure around 50k to ground in circuit, as said earlier.

Out of circuit you should read 100k between the outer lugs.
Between center lug (wiper) and either outer lug resistance should vary between 0 and 100k.
If not, the pot is bad.

[g1]

Look at the schematic, only one of the outer lugs connects to ground.

I think he was describing what had happened under fault conditions, when hot lug was shorted to ground.

[Helmholtz]

I think he was describing what had happened under fault conditions, when hot lug was shorted to ground.

It's all written in present tense, so I have to assume there's still something wrong.

[g1]

He said

I have found the problem (with the help of you guys!) and the amp now works including distortion.

[Helmholtz]

He said...

Yes, I saw but wasn't sure the amp and pot work as intended now.

[Helmholtz]

To answer the question:

Yes, when the ends of a pot are shorted the maximum resistance between wiper and either end is always total value divided by 4.
This is independent of the taper.
Furthermore the position of maximum resistance is the 50% point.
With a linear taper pot this should be in the middle of the track, but with lots of use the taper can change thus shifting the 50% point.

[JAelec]

Sorry for the confusion,

The penciled schematic is just to describe and understand what was happening in the faulty circuit (with a short between outer legs).

To answer the question:

Furthermore the position of maximum resistance is the 50% point.
With a linear taper pot this should be in the middle of the track, but with lots of use the taper can change thus shifting the 50% point.

OK so that explains, since the V-pot is an audio taper the max resistance would not be at 50% but at 70% (which actually was the case with this V-pot)

To answer the question:

Yes, when the ends of a pot are shorted the maximum resistance between wiper and either end is always total value divided by 4.
This is independent of the taper.

Why is this? I'm not sure I understand. I would expect the max resistance for a pot with shorted outer legs to be total value divided by 2?

[Helmholtz]

With outer lugs open, the resistance from wiper to each end is 50%.
With outer lugs shorted, the 2 resistances to each end are paralleled, so result is 25%.

[JAelec]

With outer lugs open, the resistance from wiper to each end is 50%.
With outer lugs shorted, the 2 resistances to each end are paralleled, so result is 25%.

I see, thanks for explaining, Helmholtz. That makes sense.

[JAelec]

Sorry, I missed that you reported scope sensitivity as 50mV/div.
So signal level is too low for diode clipping.
Below a signal of around 600mVpeak the diodes have no effect.

Sorry I have another question for you Helmholtz about the diode clipping.

You talked about earlier that the diodes only clip signal above 600mVpeak and that the sine of 50mVpeak I had when testing the amp was too low for diode clipping.

After the amp was repaired I tried injecting the same 50mVpeak sine and now the scope looks like this:

(Scope setting: 50mV/division)



Pic #1




This is the 50mV signal output from the sig generator



Pic #2





This is what signal looks like on the minus side of cap#7 with distortion turned off


Pic#3




This is what signal looks like on the minus side of cap#7 with distortion turned on




So after the repair it seems the diodes are clipping even though the signal is the same low 50mV as before...

Where did you find out about 600mVpeak limit of the diodes? Could you please explain?

[Helmholtz]

A silicon diode doesn't noticeably conduct below a voltage of around 0.45 to 0.6V depending on diode type.

There's a 11:1 voltage divider before C7, so voltage across the diodes should be around 0.55Vpeak judging from your scope pic.
This is assumig that the inner knob of the channel sensitivity selector switch is in its calibrated (arrested) position.