How do you check opamps?

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in a previous post i was trying to fix two of my orion 225 hcca's (thanx for all the help by the way-got one them to work again)...but the other one still has low output... :(

what ive done since was checked/replaced all :

1.bipolars...fets....hi speed diodes-ok
2.changed all drive transistors-A06...A56...2n5539's...all of them
3.changed all caps big and small
4.checked all the mylars-ok
5.changed all diodes,regular and zeners
6.rewound a new transformer, getting 21.XX volts at the secondaries
7.replaced all the tantalums

jezus christ i just realized how much ive done to find the problem and still to no avail....ahhhhhhhhhhhhhhh
:bawling:

still the same problem....'extremely' low output...

only things left to check are the opamps and the 100 million resistors left on the board as well as the pot....

i really do think that an opamp is bad, one of them is mounted unevenly (sloped) from the factory..the opamp is a 2N5532N with a big 'S' before the part number...know what manufacturer this is?

so my question is..."How do you check Opamps" in or out of the board....and what is a suitable 'drop in ' replacement...cuz im not capable of putting in a different opamp and compensating for it...

thanx
 
Extremely low output accompanied with a little too much background hiss is one possible outcome when an op amp is damaged, I've done it to a couple of 5532's before, doesn't mean that has to be the problem though.

If it were me, I would replace the op amp(s) before trying to replace all the resistors (for sure). The 5532 is a dual op amp which is already compensated for a certain gain (can't remember how much, not important here). Many duals are factory compensated due to not enough pins in the package (the 5534 single is not comp).

Use any brand, and if using something else, make sure it is pin compatible and can handle the same standing current. Some options are the TL072 low noise or the LM833 audio pre. I would stick with a 5532.
 
Hi,
you can do a few checks while the opamps are on board.

Using a DMM set to voltage;- check the voltage on the supply pins.
Check the DC voltages on the input and output pins are close to zero offset from ground and that they are approximately mid way between supply rails. Ground might not be car chassis ground.
 
thanx for the help fellas, kinda slow to reply...been busy hooking up systems lately...

Andrew-I'll check the Opamps and let you know what i come up with...where do i put the negative lead of the meter?

i can change the pot, not easily but i can do it...only thing is, the new pot wouldnt be the same brand, and im not sure of the value.

Perry-if i pull the muting transistors from the board can i safely power up the amp? i've changed them already but still the same low output.

i'll pull them and power up if you verify that i wont fry anything...but, if i already replaced them and still no progress...wouldnt that mean that thats not the problem? :confused:

Indm- thanks for setting my mind at ease, ive been freaking out and thinking the 'p' means that it is a 'p' channel...

if i can fix this thing, i'll probably be able to fix anything...:D
 
To test the opamp you have there, short both inputs to ground and check for 0 output. If it's bad, you will probably have a substansial DC voltage at the output.

When you remove it use chem-wik desoldering braid and suck all of the solder out of the row of pins and holes at once. I always spend 20 cents to install a socket after that, and plug in the new one.
 
It's safe to power up the amp with the muting transistors removed.

The muting transistors may not be the problem. Something driving them could be defective causing the audio to remain muted. If the amp plays at normal volume with them removed, you know that all of the op-amps, drivers and output transistors are OK. Then you will have only about 10 components to work with to find the problem.
 
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Hi commsysman,
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
 
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.
 
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.
 
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...



;)
 
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!!!

:D
 
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... :xeye:

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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