Obsolete op amp?

[mwint50]

Hi folks. Before I begin I am a complete amplifier amateur. I learn by trial and error and helps from others. Always have.....that said.....

I have, I believe, a fried op amp preventing my preamp from passing signal to the power amp stage of my guitar amplifier.

This exact IC seems to have gone the way of the dinosaurs. How would I begin to decide what would be a suitable replacement?

[oc disorder]

Hi Mwint50 +welcome..... you're in luck still in production

TL072CN | Maplin

What makes you think it's fried?

Measuring each pin of the IC with a multimeter set to measure DC with the negative probe on
the main ground or chassis.

Pin 1 is beside the dot and count anti clockwise. (pin 8 is opposite)

[mwint50]

Thats great news. Does Maplin have a reseller in Australia. Thanks so much for your reply!

[km6xz]

ANY electronics parts house will have it, the most common general purpose dual op amp available. Agree, check the voltage on each pin and list it here by pin number. Pin 1 is the one with the dot.
1o 8
2 7
3 6
4 5

[oc disorder]

Geezz another bloody Auzzie .. as above "ANY electronics parts house will have it" like

TL072/LF353 Dual Low Noise JFET Op-Amp Linear IC | Jaycar Electronics

and

Tl072cn Ic | Wagner Online Store

and

TL072CP | Texas Instruments TL072CP, Dual Op Amp, 3MHz, 8-Pin PDIP | Texas Instruments

finally

from this site it seems ST Microelectronics no longer make the 8pin version only the tiny surface mount one.
Just an observation as your one is made by ST ... any TLO72 is fine the trend these days is to make them so small
the old wave of techs cant see them !!

TL072IN - STMICROELECTRONICS - IC, OP-AMP, 4MHZ, 16V/ us, DIP-8 | element14 Australia

[mwint50]

Thanks OC. Your a bloody ripper mate! (just kidding).
km6xz - I took voltages and they are as follows

1=0
2=0
3=0
4=0
5=0
6=0
7=0
8=17.35v

[olddawg]

Thanks OC. Your a bloody ripper mate! (just kidding).
km6xz - I took voltages and they are as follows

1=0
2=0
3=0
4=0
5=0
6=0
7=0
8=17.35v

Are you missing the negative rail and have no regulation on the positive rail? Schematic?

[mwint50]

No Schematic….

Are you missing the negative rail and have no regulation on the positive rail? Schematic?

Schematic is impossible to find so far. Its a Carlsbro Sherwood Classic 100w combo. Flying in the dark at the moment. I also need to learn a lot more about circuits in general which I am currently through you generous folk. Cheers!

[jmaf]

Geez the day when the TL072 goes out of production will be earth shattering for guitar amps and effects

[J M Fahey]

Weird values.
To shine some light, not asking you trace and draw *all* of that amp circuit (although it would certainly help ), but at least trace the nominally +15V (pin 8) and -15V (pin 4) rails all the way back to the power supply.
Not such a big deal, not worse than tracing a maze appearing at a Sunday paper.


We want to know 2 things:
1) whether other Op Amps in that amplifier receive +/-15V , real quick and easy to check in 5 minutes, and:
2) whether said voltage is lost along the way (bad connector/wire,broken track or solder) or the supply itself is not generating them.

Again: why do you think that particular Op Amp is bad?

[oc disorder]

And when you have finished drawing that out we have an atomic fission problem you may like to get involved with !
Juan , it may be rather daunting for him to draw the schematic .. he did say "I am a complete amplifier amateur".

But... us Aussies are resourceful people and the proud inventors of the stump jump plough! (https://en.wikipedia.org/wiki/Stump-jump_plough )

Mwint50 you will have to undo all the knobs to remove the circuit board to see the reverse side.

That way you can see where the tracks go.

A multimeter with a continuity "beeper" (makes a beep when you touch the leads together) is also a very helpful tool
when checking what goes to what.

I recall servicing a similar amp... the pots were shocking ..the parametric eq burst into oscillation and was so loud I had to dive for the power switch !

I do recall failure of part of the power supply a zener... looks like this


See if you can find 2 with 2 slightly larger resistors which may look like they get hot.

They are sorta identical .. one is for a positive supply and the other for a negative supply.

They hold the voltage to a particular value and dissipate the excess in the form of heat.

Probably found down near the power switch or fairly close to the transformer secondary low voltage wires.

[mwint50]

Weird values.
To shine some light, not asking you trace and draw *all* of that amp circuit (although it would certainly help ), but at least trace the nominally +15V (pin 8) and -15V (pin 4) rails all the way back to the power supply.
Not such a big deal, not worse than tracing a maze appearing at a Sunday paper.


We want to know 2 things:
1) whether other Op Amps in that amplifier receive +/-15V , real quick and easy to check in 5 minutes, and:
2) whether said voltage is lost along the way (bad connector/wire,broken track or solder) or the supply itself is not generating them.

Again: why do you think that particular Op Amp is bad?

Im not sure that it is now. I am so impressed with the generosity of this forum. Thanks again. I'll have a go at drawing the circuit. When measuring the other op amps do i check the pin of each inverting and non-inverting input? (4 for each IC?)

[olddawg]

As was mentioned before. Measure pin 8 and pin 4 with respect to chassis ground. You should have something close to +15 vdc and -15vdc. Also, with the power off, measure the resistance of pin 8 to ground. If it's a short, suspect that Zener diode. Could also be a shorted filter cap or an open trace. But that -15vdc has to be there. It's called a symmetrical power supply. That 17.35 vdc measurement suggests it's unloaded or there is ripple on it.

[J M Fahey]

I didnīt ask for the full circuit, just mentioned it as something "ideal" , but DO insist on checking whether we have approppriate voltage where needed.

If that canīt be checked, no big deal, an end user is not supposed to, I canīt repair a modern car which I happily drive nor my own teeth which I know and use better than any Doctor, bu then itīs a clear indication that a Pro must get his paws on it, just like that.

[km6xz]

Agree, supply problem at the chip means it can't work. The pin 4 problem needs to be traced back to the power supply portion. You have been given good advice to check the resistance, with the power off, to ground from pin 4 and from pin 8.
The most likely case is that you have a short to ground on pin 4 somewhere between that pin and the power supply, or there is no short to ground but an open circuit between that pin and the power power transformer low voltage winding. In either case look for a burned current limiting resistor in series with the -15volt supply