I need help troubleshooting Boss TW-1 T Wah

[BDuguay]

The sensitivity control seems to be non-functioning on this old TWah I have. As a result, there is no wah swell either up or down. Can someone point me in the right troubleshooting direction please?
Thanks,
B.

[R.G.]

I'd start with "What to do when it doesn't work."

[BDuguay]

I realize my initial post was short and therefore probably not detailed enough but that's because I somtimes feel I go on a bit too much. Sorry about that.
I will try and cover as many points in your guide as I can later but, in the meantime, I can say this about the pedal.
Although a little beat up cosmetically, it appears all original and is sound mechanical shape.
Bypass funtions as it should, as does all other controls except for the aforemetioned sensitivity control.
The only measurement I've taken so far, which is the only measurement I can think of that would make any sense to me without having the schematic, is the the dc voltage at the first power filter cap. 5.7vdc.
As always, I thank everyone for their help and input and will do my best to cover all the steps next time.
B.

[geertjacobs]

Possible solution in recent post on page 2?
http://www.diystompboxes.com/smfforum/index.php?topic=84715.0

[Mark Hammer]

I think your first step is to verify that something is or isn't getting rectified.  To that end, measure the AC voltage on the output of the rectifier stage while providing some sort of audio input (guitar, radio, drum machine, signal generator, etc.).  That will be the inside leg of R29 (where the "outside" is closest to the edge of the board), a 1k resistor that immediately follows the output of IC2.

Are there any potentially dried-up electros that might compromise this?  The only one I can see is a .47uf cap, labelled C2, going to the input of the sensitivity pot.  It can be found adjacent to  solder pad 4 at the edge where all the wires come out.

Once a functioning rectifier is verified or disconfirmed, we'll see what to do next.  Keep in mind that a seemingly nonfunctioning sensitivity pot could mean the rectifier isn't working, or that the rectifier is working, but what it drives isn't.

[BDuguay]

I will check that tonight Mark.
Geertjacobs, there is no page 2 in the link you've posted that I can find. Are you sure that is the right link?
Thanks again all.
B.

[BDuguay]

Okay, I started with the .47mf cap but that didn't solve anything. Then I checked for AC signal at R29 and there was nothing at all.
What's next?
Thanks,
B.

[Mark Hammer]

Okay, now check for AC signal on the emitter of Q1, and on the wiper of the Sensitivity pot.  If the Q1 emitter shows you AC in response to audio input, but the wiper of the pot doesn't, then you have a bad pot, or perhaps an unseen fracture in on of the wires connecting it to the PCB.  I am assuming you have some sort of audio signal coming through in both bypass and effect mode?

Assuming there is audio successfully passing from input to output, another thing you can do is manually manipulate the filter frequency.  The photocell, that the envelope follower drives, is in parallel with a 330k fixed resistor in the feedback loop of IC1, between pins 1 and 2.  Find the corresponding pads on the copper side and tack on leads to a 1M pot, to be placed in parallel with the photocell and 330k resistor.  In theory, you should be able to sweep the wah manually with your offboard 1M pot.  If that works, then you know there is nothing wrong with the audio filter portion.  You should be able to hear the difference between engage and bypass.  With that confirmed, unsolder the added leads.

[BDuguay]

Yes, audio passes both in bypass and when engaged. Just so I know we're on the same page, I have my guitar plugged into my audio probe and the output of the Twah plugged into my amp. That's how I had it set up for testing as you suggested. That is the correct way, yes?

[Mark Hammer]

I'm not sure what you mean by "plugged into my audio probe".  The guitar is providing the audio input signal to the pedal?

[BDuguay]

Yes, my guitar is the signal provider.

[Mark Hammer]

Well, then, now would be a good time to try the parallel pot trick I suggested, and also verify that there is audio on the wiper of the sensitivity pot.

[BDuguay]

I finally had a chance last night to troubleshoot further as per Mark's suggestions. I got signal off the wiper of the sens pot and again from the emitter of Q1. I didn't have time to try the 1meg pot in parallel with the photocell cuz I had to meet some friends at the local watering hole to watch the Blackhawks hammer the Flyers.
I do have some updates to offer.
Both the peak pot and the sens pot have no effect on the signal actually. However, with the toggle sitch in the 'up' positon, the signal sounds like a regular wah in the toe down position and when switched to 'down' it sounds like a wah in the heel down position. There's just no enveloping if you will.
Does this trigger any other suggestions?
B.

[BDuguay]

Bump and update. I forgot to mention that I removed and socketed C2 but it's okay. Also, I removed the sens pot and checked that it's functioning properly. It is.
B.

[Mark Hammer]

Both the peak pot and the sens pot have no effect on the signal actually. However, with the toggle switch in the 'up' position, the signal sounds like a regular wah in the toe down position and when switched to 'down' it sounds like a wah in the heel down position. There's just no enveloping if you will.
Does this trigger any other suggestions?
B.

Typically, in the absence of any envelope signal, it ought to sound like a wah in the heel position for up, and the toe position for down; the opposite of what you document.  The envelope signal is supposed to be added to whatever basic DC offset is present for the "up" sweep, and subtracted from whatever the DC offset is for "down" sweep.  What you depict makes it seem like the envelope signal to be added or subtracted is at some maximum value.

So here is what we know:

• There is nothing wrong with the path between the input jack and the wiper of the Sensitivity pot.
• There doesn't appear to be anything wrong with the opto-isolator since it responds to voltage changes coming from R32 on the output of IC2.
• There doesn't appear to be anything wrong with the filter itself since you get signal passing and evidence of a filtering effect.
• There is nothing wrong with the up/down switch since it provides some discernible effect.
• The optoisolator is not changing in response to input signal, only to the offset DC voltage from the op-amp stage immediately before.
• The LDR is behaving as if the envelope signal is holding at maximum value, as if the input signal were a signal generator that provided a steady signal.

So, in view of all of that, I am going to go out on a limb and suggest that either something is amiss with trimpot VR3, such that the unit has been tweaked to be impervious to envelope signal, OR that something is amiss with the rectifier stage based around the other half of IC2.  In view of the additional fact that signal in evidence at the wiper of the Sensitivity pot yields diddley at the output of IC2, something is wrong with your rectifier.  That could be a cracked diode, or burnt resistor, or a hairline crack in the board, or simply a fried IC2.

That's where I'd concentrate.

[BDuguay]

Oops.
I mentioned C2 earlier but forgot to put the 'I' in front of itso, to clarify, IC2 is working fine.
I mucked about with the trimpot and sure enough, it looks as though it had been tweaked at some point. This pedal belongs to Luke Doucet who has mentioned to me in the past that he likes using a '%^&*ed' wah sound from time to time. He may have had this adjustment done to this pedal and forgotten about it.
After adjusting the trimpot, I've got it working as it should now.
As always, thanks for your generous help Mark.
B.

[Mark Hammer]

Excellent!

If he likes to use the pedal for that sound, you should modify it for him so that he can revert from the triggered to the fixed mode.  If you install a 1M-2M pot in parallel with R14 (and the optoisolator), all he has to do to switch to "%^&*ed-wah" mode is to turn the Sensitivity down to eliminate sweep, and adjust the added pot to the frequency he wants.

Luke has some good tunes, and knows how to work a Gretsch properly.  Nice to be able to help someone like that out. 

[BDuguay]

I didn't realize there's a filter of sorts that blocks the use of words that ryme with rock, but start with the letter C. I take it you know what I meant though, right?

[Mark Hammer]

Right.  In light of that, I think I'll put off posting any recipes that involve a certain type of dried Japanese mushroom.

When I was 12, my buddies and I were seriously into building airplane models, often WW I vintage.  One day several of us were gathered in the living room at one guy's house.  One of the gang, of Dutch extraction, was going on about a plane he had built, and it was Fokker this, and Fokker that ( http://en.wikipedia.org/wiki/Fokker ).  After he left, my friend's mother came over and, steeling all her self-restraint, said "Andy, I don't think you should play with Peter any more.  I don't think he's a very nice boy."

Gotta love those filters.

[BDuguay]

I guess we have a lot more in common than just all things guitar and guitar pedal related. I too, built models as a kid, including balsa wood model airplanes.  I had that same fascination with WW1 planes and I was always careful when referring to that particular German aircraft.
And yes, I was very fortunate to be able to do some work for Luke D. I gaurantee you will never meet a nicer guy and wow, what a monster player.
I'll have to post pics of his pedalboard that I did some work on.
B.