Crate GLX212 preamp screeching sound

[robertro]

Couldn't load a pic here, so I posted it on Google Albums at:

https://plus.google.com/photos/11288...CPORrtD5naKuLA

Yes I think the diodes are fine and we are just seeing variations of meters. I think what threw some of us off was we thought you were getting the low resistance reading with the DMM, which would be very uncommon. For the analog meter it's no big surprise.

Can you post a picture of the connector?
Some connectors for ribbon wires are spring loaded and you push down on one side while pulling up on the cable.

[robertro]

Yes, they are spring loaded!

Will test.

Couldn't load a pic here, so I posted it on Google Albums at:

https://plus.google.com/photos/11288...CPORrtD5naKuLA

[g1]

You should be able to push down on the visible portion which will release the cable.

[g1]

Sorry, I got you to lift the wrong cap, try C14. The fault is on the +16V rail, not the -16.
Your second statement here seems to have the diodes reversed, I think you meant there was zero resistance to ground from both pads of D29, not D28 :

Resistance between pads:
D29 location: 0
D28 location: 150/450 ohms given by my analog meter depending on which way I connect the leads (my digital meter seems to have problems getting a reading, going from infinity to about 160, than back to infinity)

Resistance to ground:
D29: 150/450 ohms for anode side (no ring) pad, 0 for cathode side pad
D28: 0 for both pads

[robertro]

Did that and removed two ribbon cables.

There is a third cable that appears to be soldered at both ends.

Measurements did not change.

Thread is getting longer, but to recap, I can use the amp normally if I bypass the preamp and input into the "effects return" jack, so I assume any problem is with preamp, dsp board, or control panel.

You should be able to push down on the visible portion which will release the cable.

[robertro]

Yes, my statement was reversed - sorry!

Will return C12 and lift C14 in minute or two.

Sorry, I got you to lift the wrong cap, try C14. The fault is on the +16V rail, not the -16.
Your second statement here seems to have the diodes reversed, I think you meant there was zero resistance to ground from both pads of D29, not D28 :

[robertro]

So, with C14 and D28 and D29 lifted, definite change:
D29 cathode/striped side - about 200 ohms to ground.
D28 anode - about 200 ohms to ground

Yes, my statement was reversed - sorry!

Will return C12 and lift C14 in minute or two.

[robertro]

When I left this last night
" So, with C14 and D28 and D29 lifted, definite change:
D29 cathode/striped side - about 200 ohms to ground.
D28 anode - about 200 ohms to ground "

This morning, I took C14 and C12 out of yhe circuit to compare:
C12 gives a low resistance which gradually increases.
C14 gives a low resistance which stays low.

I'm assuming C14 is shorted and should be replaced. I have 10uF and 33uF capacitors handy both 100v - I could connect them in parallel and install them for now (43 uF close enough to 47 uF ? ).

Any thoughts on this, and whether I should look further upstream before trying this replacement?

[J M Fahey]

(Not to piss off Juan) Resistance reading: black lead on cathode reads 2.3 million ohms.
Reversing the leads reads nothing. (O.L.)

No pissed at all, just sad to see that we have a definite problem with the metering, now confirmed by Robertro.

Given:

On Rx1 scale 30. 30 ohms
On Rx10 scale 20. 200ohms

Inconsistent,

You bet
That's why we can't even mention Ohms in this context.

And since we are not there , *you* are our collective eyes .
We can only suggest something based on what you post here.

Now onto the meters themselves:

It's not a high-end instrument - some obscure handheld unit "rapitest electrical tester" that I bought some time ago - no manual, no units, etc.... that's why I keep returning to my antique trust Radio Shack analog meter :-) but it doesn't have a diode setting.

That's partn of the problem.

The whitebeards among us were weaned on analog meters and although they are neither linear nor accurate we learned and got used to what a good diode would show, on our particular meter and on a particular scale ... which is fine for own use during servicing, and terrible for communicating with a fellow Tech.

But since there were neither Forums nor Internet way back then, not much confusion would arise.

But today what you measure on your meter (we are talking resistance measurements, voltage and current are fine and trustable) does not help us, sorry.

At least buy one of these $10 digital meters, in fact I also use one, got used to it because I work "outside" a lot, with live bands, typical situation is that I go see somebody act and the say : "hey Juan!! , welcome !!! thanks God you came today ... could you please give us an idea why the d*mn xxxxxxxx which is not working?"

When I say I brought no tools, they proudly announce: "oh !!! we have a soldering iron !!! (with the tip badly eroded), some no name, plumber's quality solder and .... a multimeter !!!!!!!!!!!! "

Which is *always* one of these:


That said, I can trust the diode scale number , go figure (not the continuity buzzer).

So go get one

Edit: some Techs might rise an eyebrow on my "giving out free servicing".
Not at all, actual servicing is always done at the shop, unless I was hired to install or troubleshoot a fixed system, say a PA at a Club, School or Church, which of course gets directly charged or is part of a PA equipment sale.

Whatever a band can get out of courtesy is : "this cable is bad, junk it and bring a good one / you inserted the batteries the wrong way / this wall wart is the wrong polarity or dead / these batteries are dead / this extension cord is bad / you forgot to flip Standby ON " .... anything "real" gets referred to the shop.

It works more as Public Relations / good Karma than anything else and keeps my name in the Musician's minds, cheaper and more effective than an ad at some Guitar Magazine, what's not to like about that?

[Jazz P Bass]

C14 gives a low resistance which stays low.

I'm assuming C14 is shorted and should be replaced. I have 10uF and 33uF capacitors handy both 100v - I could connect them in parallel and install them for now (43 uF close enough to 47 uF ? ).

Any thoughts on this, and whether I should look further upstream before trying this replacement?

For now, the 33uf cap will do.

You are simply trying to resolve the low voltage issue.

[robertro]

Thanks, done.

Both diodes pads now behave the same way.

Should I now put back the diodes and fire up the beast, or is there a next suggested measurement?

For now, the 33uf cap will do.

You are simply trying to resolve the low voltage issue.

[robertro]

Couldn't wait .... I reassembled and plugged it in......

....and.....

....plugged in an old Series A Jackson-clone guitar ....

....and.....

TWANG, TWANG, TWANG .... It's rocking !!!

Tested out all channels and functions - all OK !!! And after 10 minutes or so of waking the neighborhood, R95 and R96 are hot as promised!

I can't thank you guys enough - this is a very helpful and knowledgeable community; and you've been very tolerant of what my son would term "noob" questions - THANK YOU !!!

BTW, local music store said it was the DSP card which is very hard to source and pointed at a stack of shiny new amps. I was suspicious because none of the controls worked and figured it was a more general problem.

Hopefully last questions for a while: How long is it safe to use the 33uF cap? Can I leave it that way for a week of so until I get to an electronic parts store? Same if I add the 10uF cap in parallel?

Thanks, done.

Both diodes pads now behave the same way.

Should I now put back the diodes and fire up the beast, or is there a next suggested measurement?

[Jazz P Bass]

I would say it will not hurt to run it with the 33uf, for now.
Slightly more ripple on the opamps.

Get two 47ufs and replace the -16 one while you are at it.

In retrospect:
I guess that I shot you down the incorrect path from the get go.
The very first item to check is the health of the power supply.
That would have nailed it from the start.

The awfully baffling meter readings had a lot to do with the absolute confusion.
Diodes good, when lifted, cap shorted.
End of problem.

[robertro]

Super - thanks again!

Note to self - ask for decent meter for Fathers Day :-)

I would say it will not hurt to run it with the 33uf, for now.
Slightly more ripple on the opamps.

Get two 47ufs and replace the -16 one while you are at it.

In retrospect:
I guess that I shot you down the incorrect path from the get go.
The very first item to check is the health of the power supply.
That would have nailed it from the start.

The awfully baffling meter readings had a lot to do with the absolute confusion.
Diodes good, when lifted, cap shorted.
End of problem.