Crate GT1200H won't switch to Solo channel.

[Enzo]

You have control signals A,B,C etc they come from page 2. Ther is a chart on that page called Switching Logic. That tells you the voltages on the control lines for each state. Start there, are ALL those A, B, etc control voltages toggling as you flip switches?

yes, ther is an LED for Solo.

If all the control voltages are being made on page 2, then see if they are reaching the JFETs like Q29, Q2, etc.


On all the op amp ICs, pin 8 has +15, pin 4 has -15.

JFETs are normally ON, with a lowish resistance between source and drain. I expect something like 100-200 ohms. A voltage on the gate turns it OFF, meaning a very high resistance between source and drain. Thus us acts as a switch controlled by the gate. If you get gate voltage going off and on with the channel selecting, then measure resistance from source to drain. Do this while the amp is RUNNING, otherwise there is no gate voltage. Your ohm mater may make the amp pop. Does the resistance source to drain =go high and low with the gate voltage switching on and off?

[Duncandod]

Thanks for the road map. I'll post my results.

[Duncandod]

You have control signals A,B,C etc they come from page 2. Ther is a chart on that page called Switching Logic. That tells you the voltages on the control lines for each state. Start there, are ALL those A, B, etc control voltages toggling as you flip switches?

yes, ther is an LED for Solo.

If all the control voltages are being made on page 2, then see if they are reaching the JFETs like Q29, Q2, etc.


On all the op amp ICs, pin 8 has +15, pin 4 has -15.

JFETs are normally ON, with a lowish resistance between source and drain. I expect something like 100-200 ohms. A voltage on the gate turns it OFF, meaning a very high resistance between source and drain. Thus us acts as a switch controlled by the gate. If you get gate voltage going off and on with the channel selecting, then measure resistance from source to drain. Do this while the amp is RUNNING, otherwise there is no gate voltage. Your ohm mater may make the amp pop. Does the resistance source to drain =go high and low with the gate voltage switching on and off?

.

Where do I check the control voltages? I see hexagonals on the drawing that are labeled a, b, c, etc.

On another note, I touched the negative probe of the tester on the heatsink mount that the tip142s mount to and it sparked. Now it's not stitching from OD. The heatsink is grounded to chassis and the positive probe was far from the unit...?? Did I just screw the pooch? Nevermind. Unplugged it and now it's back to its old self... wierd, though.

[Enzo]

Wherever you see them on the schematic. The hex box letters are them. They are created on page 2 by the switching circuits, and a control voltage like A or B is used in more than one place. So A might be found at Q4 and Q21. It needs to get to both of those, but the schematic is telling you all those points are connected together. So voltage A can be checked at Q4, Q21, or over at Q17 where it comes from on page 2.

[Jazz P Bass]

To add: the middle leg of each JFet is the grid.
That is the control pin.
Drain, Grid, Source is the pinout if you are facing the flat part of the transistor.

[Enzo]

You know, I am never confident which lead is the gate, it can be any one of the three on any particular JFETs. I mean type, clearly all J174 will be the same, but PV for example has a whole chart of JFET pinouts. I always verify with a meter.

[Duncandod]

Thankfully, the board is labeled D, G and S at the pins, and no, they were not all in the center, but most were Anywhoo, the switching logic on the schematic is far from What I'm getting.

S'posed to be:
A B C
Solo -10 / +10 / 0
OD -10 / 0 / +10
Clean 0 / +10 / 0 or +10

I Got:
A B C
Solo -14 / 11.7 / .6
OD -14 / 11.7 / .6
Clean 0 / 11.7 / 13.7
This was on all test points labeled A, B or C, including those labeled C2 and B2. Curiously, (to me, anyway) D16 measured 11.7v on the + end and 12.95v on the end with the black line and R113 measured 13.7 on one side and 14.7 on the other...

[Jazz P Bass]

Are the LEDs functioning as they should?

You are missing the negative voltage.

On TP Clean/ A, that voltage should come through the Green LED.

[Duncandod]

Yes. On clean channel the led is on. Just verified A tests on Q4, Q21, R71, C41 and Q21. . Black probe to ground, red to g leg of ic's and power side of other components.

[Duncandod]

Solo led is not working. Other two are fine. Od light stays on when solo switch is engaged. Solo led tests fine.

[Duncandod]

Is this the right schematic? U can't find another one on the net. Mine is the halfstack model. It looks like the schematic may be the combo?

[Duncandod]

Can't find a different one on the net...

[g1]

The schematic should be correct.
When the green Led is on, what is the voltage on each side of R50 ?

[Duncandod]

-15.26 and -.01

[g1]

And on each side of R50 when green Led is off?