Help: Is this opamp acting normal?

[TGuitarman]

Hi ,
I wonder if anyone can tell me if this op amp is working normally from the diagram I've posted?
I'm trying to fix a malfunction on a pre-amp/comp unit.
The part that controls the compressor 'ratio' is not working properly.
I think it might be an op amp that has gone wrong. ( see diagram)
I don't really have enough electronics theory to know for sure if this IC needs replacing, so can anyone tell me if the voltages I measured are normal in the schematic that I've posted?
I would appreciate any help.

Thanks

TG

PS The voltage at pin 14 varies smoothly as the 20k pot is rotated from the 'pink' end to the 'blue' end. The voltage at the point between the pot and the 30k resistor is only 1mV to 2mV as marked.
The voltage at TP2 is normally -400 mV, but gets pulled down as the pot is turned towards the 'blue' end ( and that shouldn't happen.)

[Matt Syson]

Hi
I would suggest that this part is probably OK. Two parts shown as 0 volts are at 'virtual earth' points so would be at very close to zero volts. The fact that the output varies smoothly also suggests it is OK. Would need more info to suggest anything useful.
Matt S

[JohnRoberts]

Originally Posted by TGuitarman

Hi ,
I wonder if anyone can tell me if this op amp is working normally from the diagram I've posted?
I'm trying to fix a malfunction on a pre-amp/comp unit.
The part that controls the compressor 'ratio' is not working properly.
I think it might be an op amp that has gone wrong. ( see diagram)
I don't really have enough electronics theory to know for sure if this IC needs replacing, so can anyone tell me if the voltages I measured are normal in the schematic that I've posted?
I would appreciate any help.

Thanks

TG

PS The voltage at pin 14 varies smoothly as the 20k pot is rotated from the 'pink' end to the 'blue' end. The voltage at the point between the pot and the 30k resistor is only 1mV to 2mV as marked.
The voltage at TP2 is normally -400 mV, but gets pulled down as the pot is turned towards the 'blue' end ( and that shouldn't happen.)

Perhaps you can clarify an apparent discrepancy (or I may be misreading your data).

When the pot wiper is at the blue end. Is pin 13 of the opamp at 0V or 2 mV?

Ohms law suggests that there will be a current flowing in the 20k pot (.125V/20K). An equal current should be coming out of the mode by the blue dot on the pot. If the other end of the 30K is indeed 0V that current could only be coming from pin 13 but since bifets have only miniscule input currents that isn't a valid possibility.

I would ignore difference between pin 13 and pot wiper as measurement slop, but if you confirm that other end of 30K is indeed 0V than you need to figure where the current is coming from to cause that voltage drop across 20K (note: could be something like a solder bridge to ground but that's pretty rare).

It looks like the opamp is working OK and has a nominal 2mV input offset voltage. Do a little Sherlock Holmes and find the missing current.

JR

[TGuitarman]

Thanks Matt and John for your help.

Originally Posted by JohnRoberts

Perhaps you can clarify an apparent discrepancy (or I may be misreading your data).

When the pot wiper is at the blue end. Is pin 13 of the opamp at 0V or 2 mV?
JR

I just checked the forum and saw your repies - I'll post back with more data after I've had a bite to eat...

TG

[TGuitarman]

.

I was just about to measure those voltages again when I noticed the 'gain reduction' meter on this unit was showing the correct reading - for the first time since I started looking for this fault.
Sure enough, the unit is working correctly.

Voltage at pin14 :
1.6mV with pot wiper at the 'pink' end
2.0mV with pot wiper at the 'blue' end ( this one was going up to .125V)
Voltage at pin13 : 1.6mV at any pot position

As the unit is now working perfectly correctly, these voltage readings must be what they're supposed to be.

So I suppose that the IC (TL074), has an intermittant fault... Does this make sense?

Bearing in mind I haven't altered anything at all except to power it up again...

TG

[the scum]

So I suppose that the IC (TL074), has an intermittant fault... Does this make sense?

Is the opamp in a socket? if the contacts to the socket were marginal (dirty, corroded, r generally a poor fit), and your poking around fixed the contact, that could explain it.

IC's don't often go intermittent by themselves. They'll attempt a heroic job up until they die in the process...but that doesn't keep the environment they're installed in for being intermittent: bad sockets, leaky caps, etc.

[JohnRoberts]

Originally Posted by TGuitarman

.

I was just about to measure those voltages again when I noticed the 'gain reduction' meter on this unit was showing the correct reading - for the first time since I started looking for this fault.
Sure enough, the unit is working correctly.

Voltage at pin14 :
1.6mV with pot wiper at the 'pink' end
2.0mV with pot wiper at the 'blue' end ( this one was going up to .125V)
Voltage at pin13 : 1.6mV at any pot position

As the unit is now working perfectly correctly, these voltage readings must be what they're supposed to be.

So I suppose that the IC (TL074), has an intermittant fault... Does this make sense?

Bearing in mind I haven't altered anything at all except to power it up again...

TG

No the opamp appears to be OK and it's very unlikely for the bifet front end to dump that kind of input current, especially intermittently.

If you are confident that the other side of that 30k was really at 0V, what shape is the 20k pot in? Good solder, good mechanical condition?

Sometimes DC measurements can be confounded by AC if something is oscillating upstream.

Probe around and wiggle stuff to see if there's something mechanical intermittent.

Good luck

JR

[TGuitarman]

Originally Posted by the scum

Is the opamp in a socket? if the contacts to the socket were marginal (dirty, corroded, r generally a poor fit), and your poking around fixed the contact, that could explain it.

IC's don't often go intermittent by themselves. They'll attempt a heroic job up until they die in the process...but that doesn't keep the environment they're installed in for being intermittent: bad sockets, leaky caps, etc.

Hi,

No sockets - all these ICs are soldered direct to the circuit board.

I have been trying to find the problem on this unit for a few days now, on and off.
I've checked for all the obvious things - dry/bad joints, suspicious-looking components etc. - found nothing...
I can't find what was causing that +125mV on pin14 .

If the TL074 is not faulty, replacing it won't change anything.


TG

[TGuitarman]

Originally Posted by JohnRoberts

No the opamp appears to be OK and it's very unlikely for the bifet front end to dump that kind of input current, especially intermittently.

If you are confident that the other side of that 30k was really at 0V, what shape is the 20k pot in? Good solder, good mechanical condition?

JR

It was definitely at 0v on the right-hand side of the 30k resistor.
Funny thing is, now, with the unit working properly, it's 0.7mV or 0.9mV (depending on the pot's wiper position)

I had some doubts about the pot, but every time I tested it, I couldn't find anything wrong.

TG