TL072 Op Amp problems

[boogielicious]

I'm building the Neutron filter and U2 is causing me some problems.  I'm getting positve voltage on pin7.  Pin 8 is reading 9.3vdc and pin 4 is -9.3 vdc.  When  I put in the op amp, the voltage on pin 7 starts increasing toward +9 vdc.  Are these bad op amps?  U1 and U3 seem to be OK.

Scott

[seanm]

Sounds like your biasing is bad. I know nothing about the Neutron filter but I recently had a problem with an opamp where it would sound ok then the sound would go bad.  Do you have a link to the schematic?

[boogielicious]

http://www.generalguitargadgets.com/diagrams/neutron_filter.pdf

Here's the pdf from GGG.  

Thanks for your help.  I'm just learning about op amps.

Scott

[niftydog]

Pins 4 and 8 are good.

Pin 7 is bad.

What are the other voltages?

The op amps are probably ok.

[boogielicious]

This is what I am getting for Op Amp voltages without the H11F3 in and a new TL072.

Pin     U1     U2          U3 (Up)     U3 (dn)
1         0      1.35^      -7.86        9.12
2        .15    0             -.03           -.03
3        .17    0             -.06          3.82
4       -9.3   -9.3          -9.3          -9.3
5         0       0              0              0
6         0       0              0              0
7         0      9.0^         .36           .34
8        9.7    9.7           9.7           9.7

U2 pins 1 and 7 keep increasing when I read them. I know that is wrong, but I can't find anything wrong with the wiring or the board.  U2 is obviously not working.  When I am in High Pass mode, the signal gets through.  In band pass and low pass, nothing.  They come out of U2.  Where else should I look?  This one has been a real challenge.  Usually I just put them down and come back later and find the issue.

Thanks,

Scott

[boogielicious]

I apologize for the bad formatting.

Scott

[niftydog]

Ok, so we're really only interested in U2 at the moment, so just clarifying;

U2 voltages;
1 ~ 1.35
2 ~ 0
3 ~ 0
4 ~ -9.3
5 ~ 0
6 ~ 0
7 ~ 9
8 ~ 9.7

correct?

Righto. 2,3,4,5,6&8 look good to me. That just leaves those dodgey outputs! Ok, so I'm stating the obvious... just following my nose ok!

First up, have you "idiot checked" C5, C6, C7 and C8 for the correct value?

Is Pin 1 stable at 1.35V or is it slowly rising?

The other one that bothers me is pins 2&3 on U1. Logic says they should be the same at least, and they should also be 0V with no signal. Pin 7 of U1 is zero, but the next stages input shows some voltage. Even U3 pin 6 is zero... so the only possible place that 0.17V on U1 pin 3 is coming from is the peak pot etc.

The feedback path is causing some funny things to happen. The only common thing among all this is the peak potentiometer.

BTW, the unused terminal of those pots should be wired to the wiper to prevent them from picking up noise.

ok, sorry... just putting my thoughts down.

Check the voltages on the peak pot and try disconnecting it to see if anything changes. Looking at the layout there's a chance it's been solder-bridged into the power supply input.

[boogielicious]

Niftydog,

Again, thanks for your help.

I checked the caps, C5-8, and they are correct.  I played with the peak pot and even disconnected it/replaced it.  This is what I get for voltages when the pot is out.

Here are the voltages:

With the pot at 0K

U1
1: 0V
2: .17
3: .2
4: -9.3
5: 0V
6: 0V
7: 0V
8: 9.7

U2
1: .84 and climbing slowly
2: 0V
3: 0V
4: -9.3
5: 0V
6: 0V
7: 1.9V and climbing slowly
8:  9.7

U3 Direction Switch Down
1: 9.13V
2: -.06
3: 3.3V
4: -9.3V
5: 0V
6: 0V
7: .37V
8: 9.7V

U3 Direction Switch Up
1: -7.7V
2: 0V
3: 0V
4: -9.39V
5: 0V
6: 0V
7: .38V
8: 9.7V

With the Pot at 100K

U1
1: -7.8V
2: .5
3: ..34
4: -9.3
5: 0V
6: 0V
7: 0V
8: 9.7

U2
1: 9.0
2: 0V
3: 0V
4: -9.3
5: 0V
6: -7.63
7: 9.0
8:  9.7

U3 Direction Switch Down
same

U3 Direction Switch Up
same

With pot removed

U1
1: -.45 and climbing
2: 0
3: 0
4: -9.3
5: 0V
6: 0V
7: 0V
8: 9.7

U2
1: 1.28 and climbing slowly
2: 0V
3: 0V
4: -9.3
5: 0V
6: -1.7V
7: 9V
8:  9.7

U3 Direction Switch Down
same

U3 Direction Switch Up
same

I DMM'd the board again, and can't find any bad solder.  I'm baffled and about the shelve this one.  What should the voltages be?

Thanks,

Scott

[niftydog]

not knowing the circuit, and not having one here to measure I cannot be sure... BUT it is my belief that the inputs to all op amps should be at zero with no signal. In particular, all inputs of U2 should be at zero because the inverting terminal is tied to ground, which in turn should keep the non-inverting terminal at ground potential. Keeping both inputs at zero should result in zero at the outputs.

Try grounding the input jacks signal pin. Maybe that's drifting up or something.

You didn't check the voltages present on the pot itself?

This is sounding like there's a ground not connected or something. I know it's frustrating checking it over and over, but it's easy not to see our own mistakes as mistakes!

Be absolutely sure that all grounds are interconnected and that they all go back to the power supply ground.

Those voltages are even more baffling. U2 has all inputs at zero, and yet the outputs are positive and rising! Time to take the op amps out of circuit. Do you have new ones you can swap with?

[boogielicious]

Thanks for the encouragement.  I've got a new batch of op amps to try this weekend.  I may try my tried and true method of taking everything apart and rebuilding.  sometimes you can't see the mistake even if you're staring at it.  I did try the pot voltages and they seemed weird as well.

thanks for your help,

Scott

[boogielicious]

Thanks for the encouragement.  I've got a new batch of op amps to try this weekend.  I may try my tried and true method of taking everything apart and rebuilding.  sometimes you can't see the mistake even if you're staring at it.  I did try the pot voltages and they seemed weird as well.

thanks for your help,

Scott