Frying tonight
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Grif

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OK...not too clever, but whilst trying to measure the input voltage on one of the regs in my amp (G&W TWJ1) I slipped and shorted accross in/out. I turned off the amp quick, but the op amp after the power supply still went up in smoke
.....yes, thankyou, I know I'm a numpty


Anyway, I've replaced both regs and the op amp and the amp is working and sounding good...BUT....when there is no source connected to the RCA's I'm getting quite a lot of noise coming through the headphones (buzz) which increases in volume when I turn the pot...as soon as I connect a source to the rca's the noise dissappears entirely.

Is this some kind of ground issue? As I say, with a source connected you'd never know there was a problem


Any ideas gratefully received!!
 
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rickcr42

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terminate the input jacks !

Use shorting plugs for testing

BTW-the regulators should have been fine.They have auto shut down.The opamps no
 
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Grif

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Thanks rickcr42....I thought the regs would be OK, but as they were so cheap I replaced them anyway.
Turns out it was a ground problem....the headphone socket is chasis mounted as opposed to pcb mounted and I was testing it with it hanging off....as soon as I mounted it on the face plate of the amp...noise gone!! Told you I was a numpty


rickcr42, what did you mean by..."terminate the input jacks"
 
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rickcr42

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when testing an amp you need to terminate the input by using a shorting plug.
Simply take a couple of cheapy RCA plugs and solder the center hot terminal to the ground terminal then plug them in.
you never want to have an output load attached (speakers or headphones) with unterminated inputs so unless you have a signal generator hooked up to a "Y" connector or have an actual source plugged in you need to make up some "blind" plugs for testing for output noise and hum.
 
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Grif

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I certainly did'nt know that....thanks for the explanation
 
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Garbz

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Indeed in some cases this can also be dangerous. For instance a Gilmore Dynalo with no volume pot attached will produce about 1V of DC offset very erraticly if the input's aren't shorted or loaded.

If in doubt always measure offset before you plug anything in, while you plug in a dummy load, and then get on with the measurments.
 
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