Crate GLX212 preamp screeching sound

[robertro]

My Crate GLX212 makes a high pitched sound when turned on. The only control that impacts the sound is the DSP mode control, when I turn it fully clockwise the sound is a bit diminished. No guitar sound comes through at any setting.

I am able to use the amp directly through the Effects Return input on the rear, with perfect sound, so I assume it's something on the preamp section or the DSP board.

I see no physical signs of damage/melting/breaks, and I've cleaned all the many knobs and switches.

Any help is welcomed - thanks in advance!

[Jazz P Bass]

The perfect sound through the FX Return jack pretty much narrows the problem down to the preamp or DSP board.
But first, run a jumper cord from the FX Send to the FX Return jack.
If that fixes it (which I doubt) then the FX Return jack needs attention.

Do you have the setup & equipment to signal trace the unit?

Insert a 100 mv/ 1K signal at the input jack.
Put all controls at 12:00

I would go directly to the DSP header.

Is the +10Vdc there?
Is the ground connection good?
Is signal going in?
Is signal coming out?

GLX 212 DSP.pdf

[robertro]

Thanks - I tested a jumper cable and no change unfortunately.

I don't have a setup for signal tracing.

I will go check the DSP header and report back.

One thing that I didn't explicitly mention - none of the switches seem to impact the sound; i.e. whether or not DSP is on or off and no matter which channel I switch to, the sound remains the same. Only the dsp pot makes any difference, and that is only a change in the frequency of the noise.

[Jazz P Bass]

Do you have access to a Digital Volt Meter.

If so, that can be used to trace out the signal.

Note: There could be a multitude of problems , I am just shooting for a faulty DSP card.

Check that the DSP board has +10Vdc & ground.
Set the meter to read Volts ac.
Ground the Black lead to the chassis & use the Red lead to probe the DSP Signal in pin.
With the amp on & controls set at 12:00, bang a chord on your guitar (set to full on).
If you see any sort of Vac signal, then that narrows it down considerably.

Same set up, but with the DMM Red lead on the DSP Signal Out pin.
Bang a chord & see if there is a signal (you may have to adjust the DSP knobs for gain).

If you have no signal, then the DSP board needs attention.

[robertro]

So I did both tests above and no AC voltage registered in my multimeter. My digital mm has a lowest range of 200vac, so I repeated with an old analog multimeter that has a 10V range with same results, ie no voltage reading. When I touched the +ve pin - 4th pin, the sound frequency dropped.

I then unplugged the J10 connector as well as the one that goes to the pots, assuming that would remove the dsp board - no change. It was not obvious how to remove the third connector - so I let it be.

When I turn off the amp, it takes a few seconds for the sound to die out - capacitors? Because none of the control knobs or switches make any difference, I had assumed preamp problem. For example, whether or nit dsp is selected by pushbutton, I get the same sound.

[robertro]

I also noticed that R96 gets very hot after even 30 seconds of operation.

[Jazz P Bass]

Crate GLX212

And R95 does not?

Usually they both do.
It is a pretty poor way of dropping from 40 volts down to 16.

GLX212.pdf


Anyway.
As I said, I jumped to the DSP.
(if the DSP card dos not work, it will not pas the signal back out.)

I would start the testing at the input jack.
Just so you have some idea of what the signal voltage will look like as you work your way down through the circuit.

I usually apply the 'Divide to Conquer' method.
Start at the input jack. Have signal?
Now we know that the DSP card is not getting signal, so divide the input jack to the DSP in half.

The idea is to make huge jumps to find where you have signal & where you do not.
By dividing like this you can narrow it down real quick.

[robertro]

Yes, R96 is uncomfortable to touch after even 15 seconds while R95 is not even lukewarm.

If the dsp card is the culprit, I can replace it, otherwise I'm going to have to find a circuit testing 101 tutorial :-)

[g1]

Schematic in post #7 dead.

[Jazz P Bass]

What a PITA.

Forget to check it.

Here is the zip file.

GLX212.zip

[Jazz P Bass]

Yes, R96 is uncomfortable to touch after even 15 seconds while R95 is not even lukewarm.

If the dsp card is the culprit, I can replace it, otherwise I'm going to have to find a circuit testing 101 tutorial :-)

The DSP card can be disconnected, can it not.

I don't think that is your problem.

If you can, measure the Volts dc as noted on the schematic.
Specifically the voltages on either side of R96 & R95. (TP 14 & 15)

If you can get to the 16 volt zeners, I think you will find that the +16 is loaded down.
If so, that is why the preamp is shutdown.

[robertro]

Thanks, that sounds like a good plan.

When I the look at the schematic, I see +40v at TP14 /R96 and -40v at TP15 / R95, so I'm assuming that after the resistors I should see +16VAC and -16VAC respectively.

The kids are sleeping now, so I'll have to do my tests tomorrow!

[g1]

+/- 16 VDC, not VAC.
And I'll gamble that the -16 is not there, hence the cool resistor. There should be 2W worth of heat there.

[robertro]

Yes, DCV ....late night typo.

Checked the resistors:
R96: +36.6v, 0.06v this is the hot one
R95: -37.2v, -16.25v this is the cool one
I did not run the amp for more than a couple of minutes, so perhaps R95 never got to usual temperature?

[Jazz P Bass]

Something is shorting out the output of R96.

Check the 16 volt zeners for a short.