Yamaha G100B-212 help with troubleshooting and repair.

[JAelec]

Hi there!

I'm trying to repair a Yamaha G100B-212 amp, and would appreciate some help!

I'm quite the beginner at this sort of stuff but I'd like to learn...


It has a couple of issues

1. Noisy pots (tried spraying them with lubricating contact spray but they still seem a bit noisy, although much better then before)

2. The distortion circuit in channel #1 is not working. The normal un-distorted guitar signal gets through. Distortion on/off is supposed to be activated by foot switch, but it is not. Using the footswitch to turn on/off the Reverb is working fine. Turning the distortion control up does nothing.

I'd like to troubleshoot the distortion circuit but I'm not sure where I should look first.


here is the schematic: https://music-electronics-forum.com/...etch?id=845046

and here's a pic of the board:

https://imgur.com/a/q1yKoca


Thanks for any help!

[The Dude]

First, check the switch itself. When you switch the switch on and off, does the the plug that goes to the footswitch jack go from short to open (tip to sleeve)? Check right at the plug. You could have a bad switch, bad cable to switch, broken connection in the plug, etc.

[JAelec]

Yes, the foot switch shorts/opens the tip and sleeve, so the foot switch itself is not the issue, I've checked it at the plug and it is working properly.

There are two foot switch input jacks on the amp, one for the Reverb and one for Distortion.
Reverb is on by default even if no plug is inserted. Shorting the tip+ring with the foot switch de-activates the Reverb.

I'm guessing the distortion is not on by default but I'm no sure.

Using the same foot switch to test the distortion circuit...distortion is not activated in any of three states: Plugged+Open, Plugged+Shorted or Unplugged.

If you look at the picture of the circuit board the pot for the distortion looks different then the other pots. It is thicker than the others, with a big black plastic portion at the back with two extra terminals. From the terminals go two yellow wires.

Why is this pot larger/different then the others, could it be a push pull pot?

[x-pro]

The DT controller has switch contacts. That is why there are two yellow wires. In the diagram the connection between the switch and the slider of this resistor is shown with a dotted line.
Check this switch, transistor TR5, and the capacitors on the left 1/25 (input) and right 1/35 (output) pins of this transistor

[g1]

As far as I remember from this series of amps, the Dist. pot has a switch that is engaged when you turn from 0 to 1, not a push/pull. You did not mention hearing any click, so perhaps it has been replaced with a non-switch type, or the pot's switch is not working.
Even so, the footswitch should still be able to engage the circuit.

[JAelec]

The switch has no click... so perhaps it is not functioning correctly.

Should shorting the terminals on the back of the DT control bypass the switch? I tried that but no difference.
Or could I bypass the whole DT-control somehow with a jumper?

Can I test the transistor in circuit or do I have to remove it?

[x-pro]

Temporarily short the upper (in the diagram) leads of the field effect transistor. DT slider to the maximum of the signal. Check the output for distortion.
Without replacement, it is difficult but possible to check the transistor qualitatively

[JAelec]

I have desoldered and tested the transistor, input and output caps with an ESR meter. Also tested the switch.

Switch:
This works at it should. Measures 45k instead of 50k though, possibly because the far ends of the resistive track is worn off on both sides.
I completely missed the off mode because it required quite a bit of force to turn it off. When OFF the switch shorts the two terminals at the back of the pot. This is the same as happens when activating the foot switch I think. Some part of the distortion circuit is then shorted to ground.


1uF 35V cap:
Vloss = 0.1%
995.1 nF = 1uF
ESR = 2.9 Ohms (should be below 9 Ohms says the table on the back of my meter)


1uF 25V cap: (this one was actually 50V not 25 like it says in the schematic)
Vloss = 0.5%
1289 nF = 1.3 uF
ESR = 4 Ohms (should be below 9 Ohms)

Transistor T7A K30A
The ESR meter says this is a N-JFET
I = 0.71mA@Vg = 0.49V


After desoldering the transistor I noticed that the circuitboard has the number "8" printed under the transistor which is supposedly TR5 according to the Board layout in the service manual.
This got me a bit worried that I had gotten the wrong transistor and caps. All the transistors spots are numbered on the circuit board in order from 1-9. The board layout however names the transistors TR1 - TR9 but are not in order from left to right, so the numbers on the board and the numbers in the layout don't match up.

Should I go by the layout or the numbers on the board?

https://imgur.com/a/RG6ei5N
Here are pictures of the board, layout and schematic with circles around the components removed.

When looking at the test results above, to me it seems these components are all good. Could you please verify?

Where do I go from here? I did not try shorting the transistor yet, as suggested by x-pro... should I solder the components back and try these shorting the legs of that transistor? What does shorting the transistor accomplish?

[x-pro]

It is not necessary to resolder the transistor. It is enough to short the pluses of capacitors 1/25 and 1/35 (points with voltage 7.6 V). Then check the "distortion" at the output.
If there are no distortions, check the operation of the stage on TR3, TR4.
The field-effect transistor TR5 here acts as an electronic key. The amplified signal after TR3, TR4 is cut off by the limiter on two diodes, which causes distortion. The distorted signal passes through these capacitors and the switched TR5, to the input of the resistor "V" and then to the amplifier TR6, TR7, TR8.

[JAelec]

Thanks a lot x-pro for helping me out!

Good to know about the switch functionality of the TR5 transistor. I'd like to understand its function inside the circuit a bit more...

I tried reading up a bit and this is what I have learned so far:

If this is a N-type JFET (like my transistor tester says) it conducts current between drain and source even without voltage on the gate. If a negative voltage is supplied to the gate it shuts off the distortion signal going between the drain and source.

So is TR5 used to shut on/off the distortion signal? If so, how does the negative voltage get to the gate to be able to do that? From looking at the schematic I can not see any negative voltage going to the gate. Does the switch in the distortion pot or foot switch do it somehow? From my understanding of the schematic the distortion switch-pot and foot switch don't supply any negative voltage to the gate. They just short out the point after the 1M resistor to ground = 0 V. Zero volts is not a negative voltage so how can the gate close?

Please explain how this works!

[g1]

how does the negative voltage get to the gate to be able to do that? From looking at the schematic I can not see any negative voltage going to the gate.

It does not have to be negative voltage, the gate just needs to be more negative than the Source. When the switch closes, the gate is pulled to ground via the 1M resistor. The Drain and Source are held at 7.6V so the gate is approx. 7.6V more negative.

[JAelec]

Thanks you for you answer g1....I understand now...the transistor should only care about the potential difference between source and gate and 0 V can be a a negative voltage from the source-voltages point of view, as you explained.

Looking at the schematic I the drain and source voltages are 7.6 V

The gate voltage according to the schematic seems to be 0.001V

- Is this the voltage after the foot switch is activated, then?

- what is the voltage on the gate before the foot switch grounds the gate through the 1M resistor?

Is it possible to calculate this voltage just looking at the schematic, knowing that there is +36V at the top and there are 3 resistors connected in series between the +36 V and the gate: 33K, 100K and 1M? Or are there other components that would affect the voltage through parallel connections?

[JAelec]

I noticed that the volume pot had continuity to ground on all there terminals. That doesn't seem right... but still the Volume pot seems to work... any idea what is going on here?

[x-pro]

"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?

[g1]

The gate voltage according to the schematic seems to be 0.001V

- Is this the voltage after the foot switch is activated, then?

- what is the voltage on the gate before the foot switch grounds the gate through the 1M resistor?

For the gate voltage to go to 0V, either the pot switch or the footswitch jack would have to be closed. So I would say the pot's switch on the schematic is shown wrong.
As closing the switch turns the Fet off and disables the Distortion circuit, I think you will find the pot switch is 'NC' (normally closed) and opens when you turn the pot.
When the switch is open, the gate voltage will rise up to something near that of the drain and source. I don't think you can calculate it from the resistor/capacitor network at the gate.