Troubleshooting help needed - Maxon Bass EQ

[DIY Bass]

Schematic Here:  http://www.freeinfosociety.com/electronics/schemview.php?id=953

When I first got this, no signal was passing through at all.  I sounds probed it and signal was first being made noisy by one of the electrolytic caps and blocked entirely by another.  I have replaced pretty much all of the electro caps already, because I assumed that if 2 were gone for sure then the others are sus as well.  The only one I didn't replace was C23, because I don't have any non-polarised electros handy so I didn't have a replacement.

I am getting clean signal up to the junction of C2/R3/R4.  At pin 3 of IC1 I get a loud hum.  This type of hum I have come to associate with a dead capacitor (although there may be other causes).  Just to be safe I replaced R4 but the problem didn't change.  I am guessing there must be some component gone bad in the gyrator circuitry somewhere.  Maybe C23 could be bad.  Is there some way I can test for this without having a non-polarised cap to sub in?  The hum gets louder at the output if I raise the 250Hz, 500Hz or 1kHz sliders.  Most change is with the 250Hz slider.  I am not sure if that is because the problem is in that filter, of if the hum is in that frequency band though.  I have probed all the op amp in/out pins though and the hum is present on all of them.

Any ideas about what to do next?

Thank you

[Slowpoke101]

Measure the voltages on all of the ICs. Pins 1, 2, 3, 5, 6 & 7 should be near 4.5VDC Pin 8 should be at 9VDC and pin 4 should be 0VDC.
If pin 3 of IC1 is a voltage other than 4.5VDC (plus or minus 0.5VDC), replace C30 (47uf 6.3V) - if you haven't already done so. Also check R42 and R43 (both 10K). If other pins are also other than 4.5VDC check C8 (10uf 16V), R12 and R13 (both 10K). C31 (10uF 16V) also should be checked.
Make sure that the capacitors that you have already replaced are correctly polarised.

[DIY Bass]

IC1

1  5.3v
2 5.3v
3 5.3v
4 0v
5 2.4
6 3.1v
7 3.2v
810.6v

Voltages measured with all sliders centred

I have replaced C30/C31/C.  The polarity matched what was marked on the PCB screen print for each one and I also watched the polarity of the original ones that I took out.

[Slowpoke101]

Interesting. The voltage that R12 and R13 supply is rather low as shown by the low voltage of IC1 pin 5. Measure the voltage across across C8, it should be near 4.5VDC (or 5.3VDC). Then measure the voltages on pins 3 and 5 of all the other ICs. One of them may be a lot lower than 4.5VDC (or 5.3VDC). Or you may find that all measure low. If that happens you are going to have to remove each IC one at a time until the voltage comes up to 4.5VDC (or 5.3VDC).
You may wish to remove C23 just to see if the voltage on pin 5 of IC1 comes up to 4.5VDC (or 5.3VDC). It is unlikely that C23 is faulty but you never know.

[DIY Bass]

OK, I haven't had a chance to run audio through it again, but may be on to something.  C8 was suspiciously flexible, and it turned out that the pad under one of the connections seems to be toast.  Nice solder blob not connecting to anything much.  I used an offcut to run it through to the next solder pad and now the voltages on the IC1 signal pins are all between 4.8 and 5.2 volts.  That seems much better.  Hopefully it will also sound better when I get the chance.  I'll post back and let you know.

[DIY Bass]

Still getting hum - even with no audio going through the audio probe shows it clearly :-(

[DIY Bass]

I just tried removing C23, The hum seemed to be a bit quieter at pin 3 of IC1, but was instead audible at the Q1/C2 junction quite loudly, where I haven't heard it there before.  What should I try next?

[Slowpoke101]

This is getting to be a very interesting fault.
I suggest replacing C23 with two 47uF capacitors wired back to back in series - solder their negative leads together and install the positive leads into where C23 was. This makes a simple 23uF (approx) NP capacitor and will replace C23 quite well.
Now test the pedal again but use a shorting plug for the input connection and a battery - external noise should now be eliminated. Have a listen - if the fault is still there check all the connections from the main board to the slider sub-board. If all is OK start removing (one at a time and retesting each time - do not reinstall the capacitor just removed - keep them aside) C10, C12, C14, C16, C18, C20 & C22. If the fault is associated with the gyrators the fault should disappear when the associated capacitor is removed.
Hopefully you will have located the gyrator circuit causing the problem. Check all components used in that section including the capacitor removed. The associated IC could also be causing the problem so make sure that you have replacement ICs on hand. As silly as it may seem, IC1a can cause this fault too. If you have suitable replacement ICs and sockets, just replace them - it may clear the fault without too much effort.

Good luck and let us know how you go.

[DIY Bass]

Thank you.  I had thought there was a way of replacing NP caps, but I wasn't too sure how.  That is useful.  I was thinking about removing the gyrator caps one by one, but I would probably have put each one back again.  I'll try your way.  I don't have direct replacement ICs, but I do have other (better probably) OP Amps that I can use if necessary.  Life around here is pretty busy in the next few days but I'll try it when I can and let you know.  Thanks for your help.

[DIY Bass]

OK, so this may end up being a bit embarrassing.  Rookie error.  I have left this one for a bit.  Had a few other toys to look at that looked to be easier fixes.  I now have three of these pedals - 2 bass and one guitar.  Not much difference between them.  I was opening up the guitar one to test it out.  There was a similar noise in that one - even in the input.  I put the oscope on it and there was quite a neat 100Hz Sawtooth wave.  Checked the power supply channels and there was the same wave. Checked the power supply that I was using and that sawtooth wave was in the power supply without anything plugged into it.  I have used that supply to power heaps of pedals in the past without a problem.  Anyway, went and got the regulated bench supply from the shed, plugged it in and the noise disappeared.  Plugged it into this one and there was no noise.  I have disconnected about half the gyrators in this one, so I will need to reconnect this before I know for sure that the problem is gone.  I am not sure whether it is that these are EQ pedals, or the Maxon power filtering, but these pedals are obviously more sensitive to this supply than the others I have used with it before.  At least the mystery seems to be solved for now, and I know that in future in need to not be so lazy and go out and get the proper power supply if I am troubleshooting inside :-)

[PRR]

At first you had "noise", which comes in many forms.

Then you had specific "hum", a clear clue. (But it seems it may be "buzz", hum with harmonics?)

There's only two places hum/buzz come from. All the electricity in the air (need shielding), or Power Supply.

An audio gizmo can be made resistant to power hum/buzz with added filtering. Not always needed, but when you need it, you NEED it. It appears your plan lacked effective filtering. (Your link is not working for me at the moment.)

Many lessons to learn in this racket, too many to remember them all. But one is: if it hums/buzzes, try a battery or a known-quiet wall supply.

Just because a supply has "powered heaps of pedals in the past without a problem" is not proof it is blameless. Much maybe most of that "heap" has self-defense against crappy power.

If we can find that Maxon plan again, it may not need a huge bunch of added filtering; sometimes one un-filtered bias-line is the main sneak-path for power crap.

[DIY Bass]

I have the schematic saved.  I will try and upload it.   There is a fair bit of filtering on it.