Crate GX140C schematic needed

[Jazz P Bass]

Whew.
I can't make heads or tails out of that info.
Sorry.

How about injecting the signal Into the input jack?
Then measure the TP's with a meter.

[Tone Meister]

See post #6

Whew.
I can't make heads or tails out of that info.
Sorry.

How about injecting the signal Into the input jack?
Then measure the TP's with a meter.

[g1]

Power amps work. You have nothing when injecting signal at TP8 of preamp.
That leaves page 2 of the preamp schematic.

[Tone Meister]

Power amps work. You have nothing when injecting signal at TP8 of preamp.
That leaves page 2 of the preamp schematic.

If I'm reading the schematic correctly, the issue should be on the first page because I can put a signal into that J2 TRS INSERT jack and can get throughput to both amps. The problem should be upstream of that, right?

[g1]

TP8 is the end of the first page. So the problem must be after it.
Look at the second page. The point called OT is TP8.
There is a J2 insert shown on page 2 of preamp, and also a J2 on power amp schematic.

[Jazz P Bass]

O/k, I read Post #6.

I don't know if you followed the prescribed control settings.

If you did, the test readings that you have do not agree with the chart.
Tp1: A & B are good. C fails low.
Tp2: A & B fail low. C is good.
Tp3: A & B fail low. C fails high.

I would get the front end straightened out.

[Tone Meister]

If you mean the TEST POINT CONDITIONS from the schematic, then yes I worked using those control parameters.

[Jazz P Bass]

Well then, It appears that it is failing at the beginning of the circuit.

Do you have any spare opamps?

[Tone Meister]

Nope, but I'm ordering some in the morning. Thanks JPB!

[Enzo]

Your injection has to be appropriate. if you inject 60mv of signal where the amp is expecting several volts, then of course you will hear nothing.


If you cannot inject a signal at TP8, then the rest of the first page is irrelevant, TP8 has no idea where the signal comes from it just passes it along - or should.

And remember that mute at TP8? And how it was controlled by the input jack shunt contact? REgardless of where you inject a signal, you still MUST have something plugged into the input jack to unmute the TP8 stage.

But assuming we have no action after TP8, it was said the problem must be on page 2, but you said no because the insert jack works. OK, but there are two components on page 2 before the signal path gets to the jack. It may be just two small parts, but they ARE on page 2. But I am betting you had an empty input jack for those tests.

[Tone Meister]

...But I am betting you had an empty input jack for those tests.

And you are correct.

With a jack in the input, I am still unable to get throughput from TP8 (pin 7), but I can hear the tone when I inject the signal to pin 6 of IC4. Nothing at any other pin of IC4 and nothing at TP7 with the injected signal.

IC4: With the multimeter, I measure ~14.5 VDC at pin 8 and ~-14.0 VDC at pin 7. At pin 4 there is -16.1 VDC.

I can measure ~14 VDC across both R37 and R38.

[Enzo]

Then you have trouble at IC4. either the IC is bad or Q25 is shorted source to gate.

[g1]

I am still unable to get throughput from TP8 (pin 7), but I can hear the tone when I inject the signal to pin 6 of IC4.

Could someone please explain how this is possible? Signal can be heard if signal is injected to the input of IC4 (pin6), but is not heard if signal is injected at the output of IC4 (pin7) ?

[Enzo]

I wondered too, but ultimately he has DC on the op amp out, so it has to go anyway.


My rationalization might be that the chip is slammed over, but still has gain to smash itself against the stuck output leg. And that becomes audible... or something.

[Tone Meister]

Yeah, also remember that I have the reverb send and return cables connected together with a barrel connector. Could the signal be seeping back into the reverb mix because of that?