Voltage scratchiness sound on wah pedal - likely causes?

[Josh_bb]

Hi, I have old 1975 Jen wah/volume and its scratchy... (so what's new I hear everyone say) .. 

I swapped out the pot as you do, but it's still exactly the same so I think there must be some dc voltage leaking to the pot because the sound is similar to when i twiddle the volume on my rangemaster.

My question is what would be the most likely causes?
I swapped out the 4.7uf cap but that made no difference either.

Could it be one of the .22 caps ether side of the pot?

I don't want to start randomly swapping out stuff on a 45 year old board, so any help would be greatly appreciated.

It actually works really well and has a really nice vocal wah, the inductor is a bit odd, its red but like a trash can!

Pics are a schematic that looks to be like mine and a duff (phone) pic of my pcb ( i will take a better one)

Cheers for any thoughts

[R.G.]

My question is what would be the most likely causes?

Anything that touches the pot that has DC on the other side.

Have you used your DMM to measure whether DC really exists on the pot terminals?

I swapped out the 4.7uf cap but that made no difference either.

4.7uf doesn't touch the pot, very, very unlikely for it to be contributing to DC on the pot.

Could it be one of the .22 caps ether side of the pot?

If the pot has DC on it (see above), then yes, they might. It could be. But you need to go do the meter work.

[anotherjim]

As above, but you also need to wonder if any DC is coming back from whatever you have it plugged in to. Some tube amp inputs with direct inputs to a tube grid can have DC leakage there due to a tired preamp tube.

[Josh_bb]

cheers for the relies chaps I really appreciate it.

Having a look now in the daylight as it were.

With it being a wah/ volume its always drawing power, and is always making the static type noise whether or not wah is on.

I dont 'seem' to be getting any MV on any on the pot tabs but i do notice the static is constant even when the wah pot is not moving.

it seems to pretty much disappear when the pedal is at full toe position and at full heal position , anywhere in between is static noise, on top of the usual white noise you get -  just left '%^&*ed' in mid position it's making the noise.  it's the same on 3 different amps and guitars.

[anotherjim]

Could be a bad ground - that could be mechanical since the jacks ground via the shell. Take the jacks off & clean about. Check grounded end of the pot is a good ground. Check battery snap negative contact on the input jack is good.

[Josh_bb]

Could be a bad ground - that could be mechanical since the jacks ground via the shell. Take the jacks off & clean about. Check grounded end of the pot is a good ground. Check battery snap negative contact on the input jack is good.

cheers I will do that now

[Josh_bb]

hmm, no difference as yet,
I do notice the scratchy static goes away completely when i earth the centre lug of the pot  but that doesn't help much

[thermionix]

I think you likely have a noisy transistor.  The "DC on a pot" noise usually only happens when the pot is moving, and those 250V box caps are unlikely to ever leak in a 9V pedal IMO.  Resistors can go noisy too, but my hunch is a transistor.

[Josh_bb]

I think you likely have a noisy transistor.  The "DC on a pot" noise usually only happens when the pot is moving, and those 250V box caps are unlikely to ever leak in a 9V pedal IMO.  Resistors can go noisy too, but my hunch is a transistor.

thanks i will have a look.

there is an old thread about this on a self build and it was never really solved,  the OP ended up swapping the  pcb, but this is different in that it is 45 years old

[Josh_bb]

I think you likely have a noisy transistor.  The "DC on a pot" noise usually only happens when the pot is moving, and those 250V box caps are unlikely to ever leak in a 9V pedal IMO.  Resistors can go noisy too, but my hunch is a transistor.

i think you are correct!  the transistors in it are bc318b,  I don't have any to hand so i swapped them out for some bc183b... ( same number just in a different order, lol)

anyhow the noise has gone completely, unfortunately so has the wah sweep. so i guess i need to get some like for likes, i am not sure what would be a suitable equivalent for this particular circuit

[pinkjimiphoton]

transistors don't matter that much. if its the right polarity <npn/pnp> and the gain is in the ballpark will matter most.

sounds to me like you have a bit of bad solder somewhere. i would start by touching up every connection to the board with fresh solder.

the transistors could be damaged, but changing them out is kinda drastic.

me? i'd put them back in carefully, and try adding a smallish cap... anything really from say 470p to 4.7n or so, and tack them in between b and c of each q first. this should nuke the noise somewhat, and tell ya which transistor is malfunctioning, or both.

replacing the pot was a good call. but i'd replace every electro on the board, and check all the coupling caps for dc leakage too just to be sure.

old carbon comp resistors can sound like freekin bacon frying.

but i think jim is right with the transistors. they can damage easy sometimes by surprisingly small disturbances. i killed two 5089's yesterday on the breadboard just by shorting something accidentally.

i would replace both .022's on there too, as either one leaking would cause the exact problem described as well. when the bandpass is at extremes, the noise stops cuzza the frequencies being boosted by the wah circuit.

once ya get it working, leave it powered on someplace warm and DRY for a couple days... that should help get pops cracks and spits to minimize as the resistors dry out.

[Josh_bb]

cheers Mr Photon,  I will have another look tomorrow, I did reflow all the solder joints but it made diddly squat of difference.

The offending noise of gently frying bacon on a distant barbeque in a mild wind did seem to go when i swapped out the original BC318b transistors for some BC183b transistors I had to hand, they were very similar gain to the ones i pulled.

But unfortunately the full sweep of the wah has been lost and now it sounds lame so I guess the BC183's are just a bit too different from the original BC318's which is a pain.

[pinkjimiphoton]

probably not. check the soldering around where you were tinkering. sounds to me like an open joint bro. reflow with NEW solder anything suspect. i bet one of them caps close to the q's or the traces there are messing ya up. keep at it, you'll find it.
i understand not wanting to disturb the vintage circuit, but at the least i'd do the electros, as memory serves one of them is crucial to the wah sweeping. unfortunately on this computer all i can get in is the thumbnail, it won't display the damn image on imgur.

[duck_arse]

the BC183 has what I'd consider the normal BC pinout, ie opposite to the 2Nxxxx transistors [generally]. the BC317, 318 and 319, however, have the reverse BC pinout, so matches the 2N series. I'm sure you know all this, and have fitted the replacement transistor accordingly.

[Josh_bb]

the BC183 has what I'd consider the normal BC pinout, ie opposite to the 2Nxxxx transistors [generally]. the BC317, 318 and 319, however, have the reverse BC pinout, so matches the 2N series. I'm sure you know all this, and have fitted the replacement transistor accordingly.

hi,, yes the pins are flipped on the bc183 compared the the bc318. i fitted them right i think (famous last words)
It does 'wah' but the wah is significantly less vocal with less range than it had with the BC318's .
All the static noise i originally had the issue with has gone now though it must have been one of the original transistors causing the issue.
I wonder if the 183's need different biasing and whatnot to make the behave like the 318's...  The 183's don't come up as a substitute part when you google it so maybe your can't just do a straight swap

[duck_arse]

there's no chance you have a BC183l in there, is there? I'm looking at datasheets again, one mentions the "l", one doesn't mention a, b or c.

[Josh_bb]

there's no chance you have a BC183l in there, is there? I'm looking at datasheets again, one mentions the "l", one doesn't mention a, b or c.

they are "b" BC183b with a hfe of round 275 . the originals are BC318b ( i think there is a 'b' I dont' have them to hand just now)

[Josh_bb]

Thanks for everyone's help with this

It was a noisy transistor....... and then i fitted one of the replacement transistors in the wrong way round  I could make an excuse about it being late etc, but it was my fault, I thought they both faced the same way on the board but on this one they don't (note to self: take better 'before' pictures next time!).

This wah is actually super funky. I own and have owned many wah pedals and some are better than others, some just seem to have the vibe, this
one really does (imo).

The inductor on it is a bit different than the normal fasel (see pic below), not sure if that adds to it's 1975 Shaft mojo.

Anyhow cheers

[thermionix]

Awesome.  I'm curious about the two 220k resistors on the footswitch.  Everything else looks standard, circuit-wise.

[Josh_bb]

Awesome.  I'm curious about the two 220k resistors on the footswitch.  Everything else looks standard, circuit-wise.

Hi

They are standard for this Jen wah/swell, maybe something to do to help it be a better swell pedal?  There seems to be an extra cap too capacitor too, compared to my standard jen wah.

It does work well as a swell even though the pedal have a short travel compared to say a colorsound wah swell.