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Hi everyone. I was hoping I could get some help/suggestions regarding a Crate GT1200H amp. With all the cables connected and the amp plugged into a bulb current limiter (or wall) as soon as I turn on the power switch there is a constant current draw (light on) and a buzz through the speaker. I unplugged the speaker cable (amp on) from the output jack and the current draw went away, I plugged the speaker back in and the amp works perfectly. The speaker is ohming out at 6.8, ok for a 8 ohm speaker. If I turn the amp off for one second and turn it back on the current draw is there again. I pulled the speaker cable out of the jack just far enough for the tip contact to close and there is a pop through the speaker, I then push the cable back in and the amp works. I would very much appreciate anybody's thoughts on this problem. Thank you.
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How much experience do you have repairing amps.
What equipment do you have?
I am not trying to rain on your parade but this may not be a simple fix.
The speaker out has a capacitor with one end on 40 Vdc.
This cap may be leaking.
The speaker ground goes through a 0.1 ohm resistor.
This may be a failure point.
The Output transistors themselves may be latching on for a multitude of reasons.
Please understand I am guestimating.
Without any firm data, derived from test measurements, that is my best.
Also, I would not pull the speaker plug out while the amp is under power! -
Hi Jazz P Bass. Thank you for your reply. I've repaired a few amps, but never came across a problem like this. The way you described the output transistors "latching" is perfect. Why would the amp work perfectly after the speaker output jack inner contact (tip) closes (when I pull the cable out) and push it right back in?? I did pull the two capacitors that are in that circuit, the only test I was able to do at that time was with an analog meter, charging and then looking for the "kick". They weren't shorted. My EICO cap tester will be back up and running when I get the new cap for it. I'll test them again. If both those caps and that resistor to ground is good, what would you please recommend next?? Thank you.Comment
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I would look for a measurement of any dc voltage, on the speaker out wires, when the amp is pulling current.Comment
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There is dc voltage at the speaker, I can't find my notes for the exact amount, but as soon as I pull the speaker plug and plug it back in it goes away and every function of the amp works. Could a cap be.............sticking????Comment
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I really wish you would stop doing that with the speaker jack
How much Vdc on the speaker.
Measure it.Comment
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Thank you for the input. When the amp is not working I measured 4.63 Vdc at the speaker (not good). When the amp is working there is .068 Vdc at the speaker.Comment
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The next step is to measure Test Point 11 & 12 on the power amp board.
It will be a dc voltage referenced to chassis ground.
If they are o/k, then I would suspect C26 (right above TP 13).
Maybe it is "sticking"
I mean leaking.
The best bet there is to replace it.
A very strong possibility also exists that the power amp could use some resoldering.Comment
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The schematic shows Q11 +40V, my readings were +36.1V, Q15 was -36.1V. This was when the amp was not drawing current. I understand what you're saying, I should take these measurements again in Vdc mode. That might show me which cap is letting the Vdc through. Cap #26 & #59 are the ones I tested good with the analog meter. I will just replace them to be sure. Yeah, sticking wasn't a good choice of words. I will test this tomorrow. Thanks again for your help.
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UPDATE: THE AMP IS FIXED.
Thanks Jazz P Bass for having me look at C26 again. I just replaced it. The grounding resistors all tested good. I'm still confused why the cap would work after unplugging/plugging back in the speaker cable. I switched the amp on and off several times with short breaks in between and it is still working perfectly. Thanks again. This is an awesome site. Take care.Comment
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