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Crate SS amp problem...?

6/22/2004 5:31 PM
Crate SS amp problem...?
Crate GX212...I'm wondering if it's worth fixing.  
Very low output. Low and distorted, even in the clean channel. Red LED indicating OD channel on doesn't light, but the switch works. Reverb doesn't work, either. Speakers check out ok.  
At first glance, all wiring is correct and board components all look ok. Any suggestions on where to start testing and/or looking closer...?  
Thanks for your help.  
6/22/2004 11:29 PM

Isolate the problem. It is either in the preamp, the power amp, or the power supply, or a combination. First check the power supply. The high voltage for the power amp is maybe 40V plus and minus - maybe 38, maybe 45, but something in that area. Are both supplies present and about the same voltage? Then there is the low voltage supplies for the op amps in the preamp. This will be +/- 15V. Are both these supplies present and about the right voltage?  
Once the voltage is verified, we move on to the audio path. Plug a guitar into the power amp in jack, or if it has none, the effects return jack. Does it still sound bad? If so the power amp is suspect. If it sounds OK, then the power amp is OK.  
Connect the preamp out or the effects send to another amp so you can hear what the preamp itself sounds like. Is it clear or troubled?  
These tests will at least help narrow down what part of the thing needs service.  
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6/23/2004 2:31 AM

Enzo...I'm from Southern California, never been to Michigan. Now you know 2 of us... :)  
Anyway, I haven't checked the voltages yet, but the insert/effects loop jack test tells me the power amp is fine (not too loud, but definitely clear), and the preamp is the problem (still very distorted running through another amp).  
Ideas? Thanks again.
6/23/2004 5:16 AM

OK, it is the preamp. Now make sure both 15V rails are there. On the 8 leg dual op amps, the supplies are on pins 4 and 8. Pick a few chips scattered around the board and check. A line could have opened between sections of the board, but it is more likely that if there is power trouble, it is one of the supplies missing.  
Touch each chip with your finger. If any are hot they are bad. A bad chip can load down a power rail, so check for hot ICs first if one or both supplies are low. That your LED doesn't work makes me think missing supply.  
If the supplies are OK, then check the ICs. On the op amps, pins 1 and 7 are the output pins. Mostly there should not be any DC offset on those pins. If there is a couple volts or more, the IC is suspect. SOme ICs drive LEDs however, so in those cases we do expect DC. Making the lights go off and on can verify if that is what an IC is doing.  
After that, get the schematic, apply a sine wave to the input, and trace along until you find the cancer.
7/7/2004 8:09 PM

I'm not really sure where to start checking for voltages in the pre-amp. Nothing feels hot, not even warm. Around the board, there are 5 8-legged chips, 20 or so small half-cylindrical 3-legged chips and 4 big flat 3-legged guys .  
I am suspicious of the red LED that should light up when the overdrive channel is does not light up. The switch has one of the small 3-legged half-cylinders up against it.  
Please point me in some direction...thanks!  
7/7/2004 11:05 PM

The amp may well be worth fixing, the problem is probably basic. I don't mean this unkindly, but if you can't identify the transistors, this may be more than I can lead you through easily.  
The 8 leg things are op amps, and as I said there should be +/-15v - or maybe only 12 - on pins 4 and 8. Hold the IC so the little notch in the end is up, or if it has a round dot in the corner, that is upper left. Upper left corner is pin 1, and count down one side and back up the other, so pin 8 is upper right and pin 4 is lower left.  
Measure voltage with respect to chassis. I would bet one or both those voltages are missing.  
The large three leg thingies are the output transistors. You will probably find the center leg will have +/-30-40 volts. SOmewhere in the amp there will be a pair of cement resistors with the 30-40 volts on one end and ought to be 15 on the other. Check them for open. If they get hot, the zener on the other end is shorted, if they are cold they ,may be open.  
Without looking it up, I assumed zener regulators for the 15, but it is possible you have three terminal regulators, but probably not from your description.  
The small half cylinder things are signal transistors of some sort.  
No sound and no LED screams no power supply to me.
7/7/2004 11:57 PM

Op amps:  
All measure -16.7vdc pin 4 and 1.5vdc pin 8.  
Output transistors center leg:  
2 of these measure 39vdc, and the other 2 measure -39vdc  
There are 3 pairs of 5 watt cement resistors. All of them check out for correct resistance. Only one pair have any measureable voltage...the 270 ohm pair nearest the large filter caps. And they are VERY hot:  
#1: -40vdc on one end, -17vdc on the other  
#2: 40vdc on one end, 1.5vdc on the other  
One of the zener diodes (N535) at one end of 270 ohm resistor #2 appears to be shorted.  
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