Clyde McCoy clone issues

[harrisxr650]

Hi everyone,

Its been a long time since i last posted something here and also a while from building my last pedal .

I've just finished my clyde clone with a custom pcb layout bought from a supplier in Europe with a Whipple inductor and ICAR pot. The only change i've made to the original components values is the 1k5 resistor to 1k8

i've used mostly modern components (except for the huge PIO caps i had from an unfinished project ).

The issue i've run into is that, when the pedal is rocked to the toe position and when hitting with no excessive force the strings, the sound it gets distorted from the middle of the pot travel and on,

It doesnt happen when i play softly,,but very very softly..

 i've tried the pot and the inductor to my old crybaby and they work fine.

Q1 & Q2 are both BC 108b and i didnt measure the hfe on them before solder them to the board.

what i should start looking to get it debugged?

By reading in the forum my first thought is to check trannies hfe and replace them accordingly if needed?
Or maybe a faulty transistor?

I've attached a pic of the guts also.

BR
Harris

[Rob Strand]

If you have a multimeter, measure the voltages on the transistors.    That's the best place to start.
If the biasing is off it will clip earlier.

As to the cause, one possibility is  you could have E and C flipped on the transistors.   However, I'd expect the wah behaviour to be off if that's the case.    Wrong part values or shorts is another reason.

You can knock-back the gain by increasing the 68k base resistor perhaps better the 470 ohm emitter resistor.  Unfortunately this will mess with the wah characteristics.

[harrisxr650]

If you have a multimeter, measure the voltages on the transistors.    That's the best place to start.
If the biasing is off it will clip earlier.

As to the cause, one possibility is  you could have E and C flipped on the transistors.   However, I'd expect the wah behaviour to be off if that's the case.    Wrong part values or shorts is another reason.

You can knock-back the gain by increasing the 68k base resistor perhaps better the 470 ohm emitter resistor.  Unfortunately this will mess with the wah characteristics.

Hey Rob, thanks for the feedback
forgive me for my ignorance,,,do i measure voltages with pedal on or off?
br

[Rob Strand]

forgive me for my ignorance,,,do i measure voltages with pedal on or off?

You need to be powered but in this case it doesn't make much difference if it's in effect or bypass mode.
Good idea to plug in a guitar (to enable power) and turn volumes to zero.
Turn amp down to stop "bangs" coming through if you slip.  No amp also fine.

[harrisxr650]

You need to be powered but in this case it doesn't make much difference if it's in effect or bypass mode.
Good idea to plug in a guitar (to enable power) and turn volumes to zero.
Turn amp down to stop "bangs" coming through if you slip.  No amp also fine.
[/quote]

From what i read, in other posts about wah trannies, voltage in Q1 is totally off?

Q1
E 0.11v
B 0.69v
C 4.33v 

Q2
E 3.24v
B 3.68v
C 9.15v

[Rob Strand]

From what i read, in other posts about wah trannies, voltage in Q1 is totally off?

Q1
E 0.11v
B 0.69v
C 4.33v

Q2
E 3.24v
B 3.68v
C 9.15v

They all look spot-on text-book wah voltages!

If you want to read a bit go to this page,
http://www.geofex.com/Article_Folders/wahpedl/wahped.htm

Scroll down about 12% down the page to "The Vox Mystery" then a few paragraphs  in RG shows some bias calculations.

This page has the voltages on the schematic,
https://www.electrosmash.com/vox-v847-analysis

It seems your input signal might be a little hot.   Would you consider your signal hotter than normal?

If you want to drop the wah gain/level you will to makes some mods.   It's a little unconventional to have to do that.

[Rob Strand]

As a sanity check, check the values of your 68k and your 1k8 (modded from 1k5 as per first post).

[harrisxr650]

If you want to read a bit go to this page,
http://www.geofex.com/Article_Folders/wahpedl/wahped.htm

Scroll down about 12% down the page to "The Vox Mystery" then a few paragraphs  in RG shows some bias calculations.

This page has the voltages on the schematic,
https://www.electrosmash.com/vox-v847-analysis

It seems your input signal might be a little hot.   Would you consider your signal hotter than normal?

If you want to drop the wah gain/level you will to makes some mods.   It's a little unconventional to have to do that.

i dont consider signal to be that hot, although there is no distortion when i hit the strings softly, but i can hear a slight ''whiss'' whenrocking a the pedal but not hitting the strings. the pups on my guitar are burstbuckers 1&2  and on the other guitar are dimarzio 36th anniversary (both). i can try lowering pups although i'm not sure it will have an impact. Also i might be able to record a clip later on to make you understand better.

As a sanity check, check the values of your 68k and your 1k8 (modded from 1k5 as per first post).

i did and the values are correct. Since i'm a bit above unity gain can raise a bit the input res value? lets say 75k? or it would be better to rise the 470ohm to 510 ohm?

Also by mistake i wrought that i used bc 108b transistors. I used bc 109b for both Q1 & Q2

BR

[Rob Strand]

i dont consider signal to be that hot, although there is no distortion when i hit the strings softly, but i can hear a slight ''whiss'' whenrocking a the pedal but not hitting the strings.

The "whiss" doesn't sound good.   You can try a 47pF from Q2's base to ground.  This come up on a recent thread by anotherjim.   It seems very unlikely the same obscure problem occurred on two units about the same time(!)  but you can see if it removes the "whiss".

i did and the values are correct. Since i'm a bit above unity gain can raise a bit the input res value? lets say 75k? or it would be better to rise the 470ohm to 510 ohm?

You would probably be  best just increasing the 68k as that will reduce the gain with the least impact on the wah characteristic.
You will probably need to at least go to 82k or even 100k.

The 470R messes with the sweep and it's effect on signal gain depends on the transistor gains.

Also by mistake i wrought that i used bc 108b transistors. I used bc 109b for both Q1 & Q2

Any 'b' transistors will pretty much do the same thing.

[harrisxr650]

i dont consider signal to be that hot, although there is no distortion when i hit the strings softly, but i can hear a slight ''whiss'' whenrocking a the pedal but not hitting the strings.

The "whiss" doesn't sound good.   You can try a 47pF from Q2's base to ground.  This come up on a recent thread by anotherjim.   It seems very unlikely the same obscure problem occurred on two units about the same time(!)  but you can see if it removes the "whiss".

i did and the values are correct. Since i'm a bit above unity gain can raise a bit the input res value? lets say 75k? or it would be better to rise the 470ohm to 510 ohm?

You would probably be  best just increasing the 68k as that will reduce the gain with the least impact on the wah characteristic.
You will probably need to at least go to 82k or even 100k.

The 470R messes with the sweep and it's effect on signal gain depends on the transistor gains.

Also by mistake i wrought that i used bc 108b transistors. I used bc 109b for both Q1 & Q2

Any 'b' transistors will pretty much do the same thing.

Ok then i guess the best thing to do replace the 68k res with a 100k pot and adjust the input gain first, and check everything else after that.

what might be the cause of the''whiisss'' effect?

Thanks

[Rob Strand]

what might be the cause of the''whiisss'' effect?

In the context of my last answer it would mean the circuit is oscillating at high frequencies.

It could be normal as well, I would have to hear it.

Another possibility is there something wrong with your build and for some reason (don't know what at this point) the gain is high.   The high gain would then make the pedal seem noisy.   The high gain would also explain why you are getting overloads.
The way you would normally test that is to put in a sine wave and adjust the level so it doesn't clip.  Set the pedal down, then measure the level on the output of the wah, adjust the sine-wave frequency to find the maximum level.   Repeat the test with the pedal up.   You can then compare gain at the peaks with what you expect from a wah (around 18dB or so, depending on your transistor gains and inductor resistance).

[harrisxr650]

i'm attaching the schematic from fuzz central on which the pcb is based and the layout of the pcb,
Pcb is fabricated form a pro which makes and sells a lot of them so i doubt the issue is on the pcb, however i did check the traces with a dmm and it seems fine. i will try to record a clip later on and post it here in case it helps more

[Rob Strand]

One very subtle point is there was two versions of the 1967 McCoy Wah with the Halo inductor.    The earliest one had halo inductor with a small centre hole in the ferrite core.  Typically the resistor across the inductor was 100k (the red 100k on your schematic).   The next had a halo inductor with a large centre hole in the ferrite core.  Typically the resistor across the inductor was 33k.  [There a few examples of small centre hole inductors with 33k's as well.]

The two inductors had difference specs.  Especially the DC resistance.    The Whipple inductor is a Halo clone.   It clones the halo with the large centre hole in the ferrite core.    That means to mimic the correct vintage inductor the 100k resistor should be 33k.

The 100k will in fact boost the peaks.  I don't know if its your problem but it is a difference.

Having said that, there's plenty of people out there who take any old wah and tinker with the parallel inductor resistor.   I'm not aware of them complaining of overloads.

Please don't think I've dissected hundreds of vintage wahs.   All I can say is I have collected some info on the early Halo inductors and my findings match what others have said.  (I have only found one possible contradiction and that may have been an error on the authors part.)

[Rob Strand]

Another observation is you DC voltages are very much as expected.   That means a considerable fraction of your build *must* be correct.  It wouldn't take much of a fault to put those voltages off.   So things that would affect the (AC) gain but not the DC voltages would be a short across the 68k.    You could measure the resistance in-circuit (un powered).   There's not much else.

[harrisxr650]

Another observation is you DC voltages are very much as expected.   That means a considerable fraction of your build *must* be correct.  It wouldn't take much of a fault to put those voltages off.   So things that would affect the (AC) gain but not the DC voltages would be a short across the 68k.    You could measure the resistance in-circuit (un powered).   There's not much else.

Ok.. here is a weird thing,

i just plugged in to record a short clip with the issue, i had removed the bottom cover from yesterday in order to check the input and the 1k5 to 1k8 resistors. but i didnt test it once i;ve put them back.

Now the pedal is sound as it should be. no clipping or whiss on clean channel or with my overdrives etc...
could it be the input resistor, since its socketed, it didnt make 100% contact on the socket and thus it left all the signal flow through the circuit?

i'm gonna  test it more the next 2-3 days and see if the issue appears again.

Thanks for the help Rob,

I hope that this was the problem.

BR
Harris

[Rob Strand]

Ok.. here is a weird thing,

i just plugged in to record a short clip with the issue, i had removed the bottom cover from yesterday in order to check the input and the 1k5 to 1k8 resistors. but i didnt test it once i;ve put them back.

Now the pedal is sound as it should be. no clipping or whiss on clean channel or with my overdrives etc...
could it be the input resistor, since its socketed, it didnt make 100% contact on the socket and thus it left all the signal flow through the circuit?

i'm gonna  test it more the next 2-3 days and see if the issue appears again.

Thanks for the help Rob,

I hope that this was the problem.

Well, all I can say is bad connections can do some weird stuff and they do come and go without any pattern - probably the most annoying types of faults in the electronics are intermittent ones!

However, it's unlikely a bad connection would *increase* the gain.

[anotherjim]

With a new build, nothing can be ruled out.
The Q2 b-e voltage at 0.44 looks marginal, but it could be the meter loading on the 470k causing a lower base voltage reading.
Try tapping around with a coqtail stick or back of a pencil to catch a bad connection. These can be inside components. Be gentle with the inductor - you don't want its coil winding fractured.

[Rob Strand]

The Q2 b-e voltage at 0.44 looks marginal, but it could be the meter loading on the 470k causing a lower base voltage reading.

It's definitely meter loading (10M ohm input impedance meter).   Q1 gain of around 320, comes out at Vbe1=610mV to 620mV.
All looks good for BC109B.

[harrisxr650]

With a new build, nothing can be ruled out.
The Q2 b-e voltage at 0.44 looks marginal, but it could be the meter loading on the 470k causing a lower base voltage reading.
Try tapping around with a coqtail stick or back of a pencil to catch a bad connection. These can be inside components. Be gentle with the inductor - you don't want its coil winding fractured.

That was the first thing i did, triple checked and found no bad connection.

didnt had time to play around a bit today will do tomorrow and will check again for bad connection in case i've missed something.

Im going to check the 100k resistor parallel to the inductor as Rob suggested, maybe put a pot and see if lowering the resistance  eliminates the issue.

Thanks both for the help

[Rob Strand]

Im going to check the 100k resistor parallel to the inductor as Rob suggested, maybe put a pot and see if lowering the resistance  eliminates the issue.

Going from 100k to 33k will knock peak levels down by about 3dB.

It only affects the peaks but maybe that's enough to stop overload.