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Crate GX-212 on the bench (schematic?)

3/9/2006 12:41 AM
Erik Miller Crate GX-212 on the bench (schematic?)
Client says that it emitted an ozone smell; forever thereafter it has exhibited a constant crackling and the volume controls act as if they are always full on.  
Opened it up and first noticed that IC2's top was literally blown off. You could see the little chip inside and everything.  
Also the following resistors had crisped: R39 (4.7K?), R47 (10K), and R43 (2K2?). The question marks denote resistors that I measured, but were too roasted to read the color code.  
Q6 (J175) was physically cracked in two.  
Replaced all the resistors and IC2, to no effect. It's still cracklin'. Need to source a J175 or equivalent.  
Interrupting the preamp-power amp connection via the jack shows that the power amp section is fine; no crackle, and running my signal generator in gives a clean, strong output.  
So my gremlin is somewhere in the preamp.  
Problem is, no schematic, and it's a Crate. Little tiny PCB traces, and they'll probably want me to pay them 10% of replacement cost of the piece for a schematic.  
Anyone got a schematic for this?
3/9/2006 1:31 AM

Those bad parts don't make sense. Unless water got on it or some hardware fell on the board. If IC2 blew up, fine, but the only connection it has to R39 is through the GAIN2 pot. Better check it.  
You got the values right. But the 10k R47 is in series with a film cap. how would it burn up? It iss just a rolloff network after the P6 level pot.  
If Q6 is bad, better check Q22, and any other JFET on line B - see drawings.  
Abd R43 is also in series with a cap on the way to GAIN1 pot.  
J175 (2SJ175) should be in stock cheap at Mouser.  
Forum etiquette suggests that if you ask for schematics you might put up an email address so we can send you one.  
Otherwise the $5-10 Crate might want to cover the expense if they won't email it for nothing is worth it.
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3/9/2006 3:44 AM
Erik Miller
>Those bad parts don't make sense.  
From the amount of tracing I could do, it seemed like the damage was all over the place. There's a blown trace in there somewhere, too, nowhere near the fried parts.  
I was thinking some kind of line surge? Lightning?  
>You got the values right.  
Thanks. As blackened as they were, I'm surprised they lived.  
>But the 10k R47 is in series with a film cap. how >would it burn up? It iss just a rolloff network >after the P6 level pot.  
That is weird. I'll let you know. In series with a cap?  
This kid must have been messing around somehow. But the amp showed no signs of clumsy entry, and the board had never been pulled. All screws perfectly torqued to the same tension....  
>If Q6 is bad, better check Q22, and any other JFET >on line B - see drawings.  
Will do.  
>And R43 is also in series with a cap on the way to >GAIN1 pot.  
He must have taken it in the tub and then tried to plug 120 into the in jack.  
>Forum etiquette suggests that if you ask for >schematics you might put up an email address so we >can send you one.  
(red face)  
Sorry for the etiquette violation. I forgot that we can't PM each other on Ampage.  
erik at erikmiller dot org should do it....
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