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Old 1st April 2006, 10:44 PM   #11
anatech is offline anatech  Canada
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Hi commsysman,
Quote:
short both inputs to ground and check for 0 output
I wouldn't do that. If you were to put the op amp into a test jig, the positive input to ground, the negative input is then connected to the output for negative feedback (voltage follower). The output pin should then be within mV of ground. With both inputs shorted, there is no feedback and with the extremely high DC gain your output voltage is undetermined.

I normally measure the inputs and output, taking into account the circuit. If there are other circuits enclosed in the feedback loop then your mileage may vary (widely!).

-Chris
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Old 2nd April 2006, 06:19 AM   #12
Eva is offline Eva  Spain
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Clipped:

As you probably know by now, replacing components blindly is a wasteful and very time consuming practice that hardly produces any good result. Furthermore, any mistake that you made this way or any wrong part replacement will introduce new and hard to detect faults in the amplifier. You should rather avoid doing that in the future.

What you have to do is:

1- Get either an oscilloscope, or some kind of audio probe connected to a test speaker trough another amplifier, so that you can view or listen to the signal present in any circuit node.

2- Feed your amplifier with some known signal.

3- Follow that signal through the circuit until you find where the signal gets distorted or attenuated. Compare the results from the good and the bad channel in order to know how the bad channel should behave if it worked.

This method usually allows to identify the exact faulty part quite quickly.
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Old 2nd April 2006, 06:31 AM   #13
lndm is offline lndm  Australia
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Beware shorting the inputs to a stage that you may inadvertently stress the stage before.

I happen to agree with adding sockets when pulling an op amp. Yes, it introduces connections but quality sockets don't tarnish. It will allow you to try different devices, and to aid replacement if this happens again.
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Old 2nd April 2006, 08:56 AM   #14
AndrewT is offline AndrewT  Scotland
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Hi,
I don't think he meant the opamp input pins
Quote:
short both inputs to ground
It sounded more like inserting shorting plugs into the 2channel amplifier input RCAs.

This second procedure is quite safe. No risk to any internal circuits.
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regards Andrew T.
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Old 2nd April 2006, 02:55 PM   #15
anatech is offline anatech  Canada
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Hi Andrew,
I hope that's what was meant. The other idea is not safe.

Eva is quite right, troubleshoot this properly before more damage is caused.

-Chris
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Old 2nd April 2006, 03:51 PM   #16
poobah is offline poobah  United States
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There is one basic check that has been overlooked here. With things powered up, and no input signal, and after checking the power rails, measure from the positive input to ground, then repeat the process for the negative input. The voltages should agree within 0.01 Volts or less.

1) This test will ONLY work in circuits designed to operate in linear mode (amplifiers/filters).

2) Failing this test more or less tells you that the op-amp or surrounding circuitry is bad.

3) Passing this does NOT tell you that all is well.

By no means as good as tracing a signal through with a scope... but, if all you have is a voltmeter...



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Old 2nd April 2006, 04:18 PM   #17
anatech is offline anatech  Canada
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Hi poobah,
Quote:
positive input to ground, then repeat the process for the negative input. The voltages should agree within 0.01 Volts or less.
So true. If they aren't equal, expect the output voltage to be railed one way or the other.

All an op amp "wants" to do is make it's inputs the same voltage.

-Chris
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Old 2nd April 2006, 04:40 PM   #18
poobah is offline poobah  United States
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Yes Sir,

Not a conclusive test... many tests aren't anyway... but fast!

Sometimes I get to build a hundred of "that"... or hundred of "those". Bench testing something as simple as a hundred flashlights gets depressing real quick... so anything for SPEED!!!

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Old 2nd April 2006, 05:06 PM   #19
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Must be using those flashlights for rattlesnake hunting at night.

Its always the one you don't see that bites you.

Oh Opamps... Ummm if there is voltage on the goes outa with nothing on the goes ina its a bad sign.
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Old 6th April 2006, 06:22 AM   #20
Clipped is offline Clipped  Thailand
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finally got a chance to check the opamps, there was definately one bad....reading a bit high....changed it but still no sound....

then

i took out the muting transistors and the sound came

now i just have to find out what was messing with the muting circuit...

ive been meaning to get a scope, but just kinda afraid it wont see enough use to make it feasible....any recommendations for a good cheap one?

on another weird note, i have two identical amps, but the pots attentuation, arent identical to one another...is it possible that the muting circuit..is causing one to attentuate faster/slower than the other?

thanx
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