Crate GX-212

[stasisrant]

Hey guys im a nub here and to the solid state world. I have a Crate gx-212 and i like it, it perfectly fits into the trunk of my rx8 and is perfect for practice (120w) so I desperately want to fix it. Getting another practice amp would be a waste i think. Heres whats up:

-After playing for about 1/2 an hour at any setting above 5 (50%) the gain level drops to the point where the lead channel sounds like an overdriven clean channel, and then the clean channel can barely be heard(though it remains clean sounding.)
-This happens no matter which guitar or cable I use, AND it happens even when I use an extension cab.
-Nothing inside the amp is discolored or "burnt" looking.
-http://www.ssguitar.com/index.php?action=dlattach;topic=1075.0;attach=612-schematic

I found the schematic for this amp in the schematic section. I have built tube amplifiers before and i know how to use a dmm so just let me have it straight don't dull it down for me. I don't have an oscilloscope but I can get one. What should i check?

also if I fix it and don't fail I have plenty of other broken solid state crap that I will be able to fix, so I want to learn, not just fix and forget.

[teemuk]

The delay could be an indication that the problem is heat-induced. When heat increases enough the problem appears. Sounds like an intermittency issue. Most likely a failed solder joint somewhere.

These sorts of faults can be difficult to cure because you need to replicate the error mechanism and then reverse it. e.g. by spraying components with "cold spray" or by poking them with a stick hoping that the intermittent part will establish a proper contact. You may also need to heat some components to speed up the process of fault showing itself.

If you're lucky a visual inspection of the solder side may give some clues or point the problem instantly. Use a good lamp and a magnifying glass if neccessary.

Isolate the problematic section and narrow the isolated area until you find the part that's giving problems. Fortunately you already know its somewhere in the parts shared by both clean and OD channel and fortunately you have a schematic too so you have a map of whats what on the PC board. The first suspects would be parts under most stress: the potentiometers, switches and etc, as well as parts near heat sources such as power resistors. Of course it could be anything else as well.

Good luck.

[stasisrant]

On further inspection 2 of the transistors in the output are motorola (the TIP 147's) and two are a different brand maybe SL i think (the TIP-142's). Has someone been in this amp already?

-The Motorola transistors (Q15 and Q16) have -1volt DC on the base at all times even when playing. According to the schem they are only supposed to have -.06 volts. Every other DC measurement on the power amp side is ok +/- 5%.
-my speakers have ~40mv DC on them when nothing is being played.

Well, I guess im going to play the hell out of it on full volume right now for like 1/2 hour by myself and see what heats up. I have some cold spray to test what needs to be cooled to function.

[stasisrant]

Well I adjusted the "bias" to get 7 mv across R72, before the adjustment it was like 9mv. Incidentally all the other .47ohm resistors in series with the power transformers have 7mv across them now as well. I played for like 2 hours at full volume and nothing bad happened. Could I have fixed it with this adjustment? I find it hard to believe. I bet next band practice it will start its crapping out thing again. *sigh* Ill keep you guys posted. Any suggestions will be appreciated.

[phatt]

Try looking at the *Insert Jack*. (a common place for this kind of issue)

Bridge R41 to C26, thereby bypassing any possible intermitant connection issues.

If you have no need for EFX loops or insert jacks then consider disconnecting them from your circuit. IMO the dumbest thing to have on a guitar amp.
If the problem persists, then it maybe a switching issue. Check voltages at D25/D26.
Phil.

[teemuk]

I played for like 2 hours at full volume and nothing bad happened.

Did you have the chassis open while doing it? If yes, don't. If it's thermal-related you need to allow the heat to build up.

Could I have fixed it with this adjustment?

Unfortunately not. Excessive bias doesn't cause the kind of symptoms your amp has.

[Enzo]

I'm in the insert jack camp.

You have any effects loop jacks?  Next time this happens, plug a spare cord from FX send to FX return.  If that restores the sound, then the return jack needs service.  Likewise if there are a pair of jacks Preamp Out and Powr amp In, connect them together the same way.

In fact, if they are the problem, even when the amp is working, many times just probing into the return jack or powr amp jack with some plug will trigger the problem.

What I do inside the amp is measure the resistance across the tip cutout contacts on those jacks.  if it measures any more than half an ohm, the contacts need cleaning.


If it is not those, one thing I would watch for is a collapsing 15v power supply rail.  A failing zener most likely.