Advice Re Dual OpAMP

[thandley]

Hi

I am relatively new to Pedal building, this is my second project.

Project: The Comet By Aion

Issue: Unsure about OpAMP

The project is working and providing me with distortion, however during the process of getting it working (I had an issue with a lifted pad on C15, which I found with my audio probe) whilst using my audio probe I found I only generated a signal on the IC dual OpAmp (JRC4580D) at pins 2 and 5, these are both non-inverted inputs (if I'm correct). I generated no sound at either outputs or inverted inputs. So my question is, is this normal? Or do I have an issue with my OpAmp?

I am currently in the process of making a simple OpAmp tester (waiting on parts) but I thought I'd ask some experts, having never used an OpAmp before.

All Pins have continuity with the board, all solder joints are good – and essentially the pedal works.
All components are standard from the kit.

I have uploaded the schematic and Highlighted where I am not getting a signal from the audio probe around the IC.
This is the link to the build documentation: https://aionfx.com/app/files/docs/comet_documentation.pdf
The rest of the circuit is now fine on probing and testing with a DMM.

Below are my IC1 Pin voltages (JRC4580D):

DC Supply – 9.01v
V @ PCB (1B) – 8.14v

1   4.08
2   4.02
3   4.07
4   0
5   3.53
6   4.07
7   4.07
8   8.14

These are my Transistor & Diode values if needed:

Q1   C    8.13
   B   2.63
   E   3.17

Q2   C   3.16
   B   0.60
   E   0.10

Q3   C   8.13
   B   1.91
   E   2.90

D1   A   9.01
   K   8.75

D2    A   0.59
   K   0.41

D3    A   0.43
   K   0.59

D4    A   0     (Gnd)
   K   3.54

D5    A   3.59
   K   3.90

D6    A   3.49
   K   3.59

D7    A   3.90
   K   3.79

D8    A   3.59
   K   3.49

D9    A   3.90
   K   4.04

D10    A   4.04
   K   3.91

If anyone can advise me on the OpAmp readings/probing I would be very grateful
Apologies if I have forgotten anything.

[antonis]

Hi & Welcome..

Your voltages look fine so could you plz post some pics of your build..??

P.S.1
What is weird enough is the signal you get on pin 2 without signal on pins 7 & 3..

P.S.2 pin 5 is non-inverting input where pin 2 is inverting input..

[duck_arse]

also welcome. also your Q3 base emitter voltages are not quite right. please remeasure. and as antonis always sez, pics please.

[antonis]

also your Q3 base emitter voltages are not quite right.

Same for Q1, but I think we have bigger issues around op-amp..

[PRR]

also your Q3 base emitter voltages are not quite right.

Same for Q1, but I think we have bigger issues around op-amp.. 

Q1 Q2 are what we expect for 1MEG base resistors and a non-magic voltmeter.

Q1  -    B   2.63

We expect 4.5V and find like 59% of that. Something is loading with between 1Meg and 2Meg. The transistor itself is some of that (hFE*Re, say 300*10K). The voltmeter may be a couple-Meg meter.

If we assume a similar loading around the op-amp, it is working fine.

Hard to see how it can have good DC and yet lose signal. I sure suspect solder joints.

[thandley]

Thanks for all your input. When I get a chance I will get the PCB out of the enclosure and post photos as well as rechecking the values and re audio probing.

[ElectricDruid]

The project is working and providing me with distortion, however during the process of getting it working (I had an issue with a lifted pad on C15, which I found with my audio probe) whilst using my audio probe I found I only generated a signal on the IC dual OpAmp (JRC4580D) at pins 2 and 5, these are both non-inverted inputs (if I'm correct). I generated no sound at either outputs or inverted inputs. So my question is, is this normal? Or do I have an issue with my OpAmp?

Welcome!

If the project is working and providing distortion, then it sounds like you've fixed it. Well done!

However, if at some point during the build you probed the op-amp outputs (pins 1 and 7) and got no audio, then something was definitely wrong at that point. For *inverting* op-amps, the inverting input is a virtual ground and you won't hear anything at that pin, but that's not the situation in this particular circuit. Both op-amps are wired up as *non-inverting* op-amps, although IC1B is just a buffer (e.g. no gain, just x1, the same in as out). So you should hear signal on all the pins if they're working.

So to summarise: there *are* situations where no signal at the inverting input is normal, but this isn't one of them, and no signal at the output is never right. But if the pedal is working then that's presumably not what you get if you audio-probe it now.

HTH

[duck_arse]

also your Q3 base emitter voltages are not quite right.

Same for Q1, but I think we have bigger issues around op-amp..

d'y'no - I didn't even see that.

[thandley]

Thanks again for all your input.

I have attached photos of the build as requested.
I re-did the audio probing and I do indeed have signal now at all the IC1 pins except Pins 4 and 8, (which is what I would expect). I must have done something right between my last probing and it working!

I have re done the voltage measurements and my transistor Base voltages on Q1 & 3 remain below the emitter. Is this ok or do I still have a problem? I can't seem to find an issue.

Thanks

DC Supply – 9.01v
V @ PCB (1B) – 8.15v

1   4.06
2   4.06
3   4.07
4   0
5   3.54
6   4.07
7   4.07
8   8.14

These are my Transistor values:

Q1   C    8.15
        B   2.63
        E   3.17

Q2   C   3.73
       B   0.59
       E   0.09

Q3   C   8.14
       B   1.89
       E   2.91

Again Thanks for your thoughts

[antonis]

I do indeed have signal now at all the IC1 pins except Pins 4 and 8,

Well said (and done)..

I have re done the voltage measurements and my transistor Base voltages on Q1 & 3 remain below the emitter. Is this ok or do I still have a problem?

In theory, you DO have a big problem..

In practice, don't bother anymore 'cause if theory could stand, you shouldn't have any signal neither on IC pins nor on OUT..
(you've probably mixed up B and E pinout..)

[thandley]

So went to put it back in the enclosure today but before I did I thought I'd check the audio probing one last time! Again same problem with the IC1 signal only from 2 & 5.
I get distortion and audio output when connected to the amp but it is not as loud as bypass and so not amplifying well.
I did nothing to the board prior to retesting.
Voltages remain the same.
I rechecked the board with my DMM, there are no lost connections, and only audio probe loss is at the IC1 as before!
Any one have advice as to what's going on?
I also built the OpAmp tester and the OpAmp looks good.

Really not sure what's going on, but I am definitely a beginner at this.

[idy]

It is not uncommon to have a problem after boxing.
Sometimes it is an intermittent joint, like bad solder, that "coincidentally" bends a little when you attach the pots etc. to the enclosure.

Sometimes the plugs/jacks touch something, like the back of a pot.

The backs of your pots don't touch the PCB, but that is another common fail.

Maybe show us how it fits in the box. Have you touched up your solder joints? Bad solder joints don't always look bad.

[antonis]

Bad solder joints don't always look bad.

I'd replace "don't always" with "rarely/seldom"..

[idy]

Looking at above pics I see transistors in sockets. They will be loose. You may add dabs of solder to them.

A lot of kits say things like "always socket ICs and transistors." I would say always socket ICs, and socket transistors if you are building a circuit and want to audition Ge transistors, or search for a low gain Si or something (smarter is breadboard but hey...) And for beginners it saves hair pulling if you mess up the pinout. But once you're sure you've got that, sockets not helpful.