Vox 847 Wah Troubleshooting

[Philippe]

Completed a Vox wah circuit retrofit on perfboard for a friend & this oddball symptom reared its ugly head...probably should mention that this particular mod project is a first-timer for me.

The wah effect itself functions properly (i.e. bypass/true-bypass modes, sweep & overall tone) thanks to the online information & insights provided by RG Keen, Paul Marossey & Fuzz Central.

The problem is that there seems to be some sort of microphonic 'pop' going on when the circuit is being activated. A 1M5 pulldown resistor was installed between the input cap & ground & I disconnected the on-off LED indicator to confirm whether it was the source of the problem...it wasn't.

Should also add that in lieu of a fixed Re1 (biasing resistor) on Q1, there is a 1K pot instead. This actually came in quite handy towards dialing-in the overall wah tone as once measured, it turned out to be something like 170 ohms on a 210Hfe BC109. It is noticeably noisy when being adjusted & tapping on the pot (or knob) reveals an audible resonance of sorts. The 100K sweep pot is quiet.

Questions:
(1) Is there some way of eliminating this resonant 'pop' when the wah circuit is being activated?
    Should the 1K pot maybe be replaced by a fixed resistor instead?
(2) Could the inductor (halo) be a possible culprit as I understand they can pick up some noises.
    Incidentally, I've got the inductor on sockets & when I removed it, the 'pop' seemed less audible.

Only when the 1K biasing pot is at 1K (fully CCW & wah tone nonexistent) does the pop disappear & it seems pointless (as well as impractical) to have to manually advance it CW to a precise point every time the wah is activated.

THX for any input/suggestions/solutions.

[LucifersTrip]

THX for any input/suggestions/solutions.

since it's a microphonic 'pop', it would seem the solution would be a soft touch switch. there were a few discussions here, but i don't remember if there was a final answer...here's one. smallbear mentioned combining switch w/ an actuator.
http://www.diystompboxes.com/smfforum/index.php?topic=88411.0

[joegagan]

if you have another inductor handy, would be worth a try to swap another one there to see if there is a difference. also, you could remote mount the inductor(s) with long jumper wires to physically isolate it from the body of the wah, this might tell you whether inductor microphonics are the issue.

[Philippe]

since it's a microphonic 'pop', it would seem the solution would be a soft touch switch...

Interesting point as there is some inherent/natural play within the switch shaft itself & when moving it marginally to the side or upwards during activation, the pop is barely audible or gone...perhaps this symptom might be due to the questionable construction & QA protocols of a typical Taiwanese Blue.
While promoting their own proprietary & significantly more expensive switch, the Fulltone site pretty much denegrates these imported blue 3PDTs as absolute garbage both in design & over the long haul.

if you have another inductor handy, would be worth a try to swap another one there to see if there is a difference. also, you could remote mount the inductor(s) with long jumper wires to physically isolate it from the body of the wah, this might tell you whether inductor microphonics are the issue.

Will also give this diagnostic approach a try. It's too bad that wah inductors can be cost-prohibitive from the standpoint of simply having a few extra on hand for comparative purposes.

[Paul Marossy]

The problem is that there seems to be some sort of microphonic 'pop' going on when the circuit is being activated.

I had a sort of similar problem once. It was all because of a microphonic inductor. I could hear it in the wah pedal if I tapped on the enclosure or whenever you put your foot on it. Replaced the inductor and the problem went away.

[Philippe]

A most baffling scenario...

Distancing the inductor from the enclosure itself (as per Joe Gagan's suggestion) yielded no difference in terms of the popping sound.

Removing the socketed inductor alltogether reduced the pop but negated the wah effect *duh*

Tapping on the enclosure yielded no peculiar microphonic sounds. Tapping on the 1K pot & knob while it was set to >170 ohms did...probably due to the increased gain setting *another duh*

Possible moral of the story...
(1) buy a Teese RMC (or Area 51) & devote the time to actually playing guitar rather than dicking around with irritating stuff like this.
(2) don't accept (or offer to) work on someone else's effect...let it be their f#*^ing problem/issue to deal with.

[bluesman1218]

(2) don't accept (or offer to) work on someone else's effect...let it be their f#*^ing problem/issue to deal with.

Exactly why I do pedal mods for people, and not repairs, unless very basic. I can't get paid enough for troubleshooting time compared to the cost of a current production pedal.

[Paul Marossy]

A most baffling scenario...

Distancing the inductor from the enclosure itself (as per Joe Gagan's suggestion) yielded no difference in terms of the popping sound.

Removing the socketed inductor alltogether reduced the pop but negated the wah effect *duh*

Tapping on the enclosure yielded no peculiar microphonic sounds. Tapping on the 1K pot & knob while it was set to >170 ohms did...probably due to the increased gain setting *another duh*

Huh, that's just weird. Not sure what to make of it. 

[Philippe]

Huh, that's just weird. Not sure what to make of it. 

No kidding!

This saga (along with any updates) will continue until the problem is either solved OR in a worst case scenario, the effect is tossed-out along the side of a county highway.

Next up...eliminating the Taiwanese Blue 3PDT a potential source of the problem. Afterall, they aren't exactly the finest-constructed/most reliable components we've ever encountered.

[zombiwoof]

Did you use the true-bypass wiring scheme that grounds the board input in bypass?.  Many times that will eliminate any popping, without the need for pulldown resistors.   The diagram is available at several sources, including Fulltone.   I always use that method for TB wiring, and haven't had any popping problems yet.

Al

[Paul Marossy]

Did you use the true-bypass wiring scheme that grounds the board input in bypass?.  Many times that will eliminate any popping, without the need for pulldown resistors.   The diagram is available at several sources, including Fulltone.   I always use that method for TB wiring, and haven't had any popping problems yet.

Al

Hmm... worth a try. I hadn't thought of the bypass switch being a potential candidate. Maybe that is the culprit?

[Philippe]

Did you use the true-bypass wiring scheme that grounds the board input in bypass?.  Many times that will eliminate any popping, without the need for pulldown resistors.   The diagram is available at several sources, including Fulltone.   I always use that method for TB wiring, and haven't had any popping problems yet.
Al

Interesting you should mention that Al. Initially the 3PDT was wired with a grounded input & upon circuit activation, there was popping. Afterwards, I rewired it to the convenitonal wah pattern with the far right column set-aside for LED ground & ground...no difference as the pop re-occured.

As I don't have a surplus 3PDT lying around at present (going to have to order one), I'm currently trying to decide whether to opt for a Carling DPDT & forego the LED function or procure another blue 3PDT...maybe one from a different supplier/distributor. *suspecting that they are all the same* Might even try the Fulltone 3PDT although $15.00 seems kind of high.

[Paul Marossy]

Might even try the Fulltone 3PDT although $15.00 seems kind of high.

That's a total rip-off. You can get good ones for $5.00 at www.pedalpartsplus.com

[Philippe]

*whew* PROBLEM SOLVED!

3PDT switch & halo inductor were fine.

The culprit was that 'adjustable' 1K pot (Re1) that I ran from Q1's emitter to ground. Substituted a fixed resistor & the pop is now one for the ages.

Lessons learned...(1) while the 1K potentiometer was useful in establishing the ideal Q1 bias, leaving it in creates a potential problem as there is live voltage running through the lugs & any on/off switching creates a voltage surge of sorts. Now why there was a difference with a 'fixed resistor' as compared to the pot is beyond my comprehension as the live voltage is still existent at the circuit connection. (2) like guitar amplifiers, perhaps the fewer the knobs the better!

Tonewise, this particular wah has 'nailed' the Jeff Beck sound from his Truth album (circa 1968)...thanks to the combined mod suggestions from Paul Marossy & Fuzz Central.

[Paul Marossy]

Cool glad you figured out the problem! I would have never thought of that being a cause of it.  

Glad that my mods page helped you out in some way, too.  

[joegagan]

*whew* PROBLEM SOLVED!




Tonewise, this particular wah has 'nailed' the Jeff Beck sound from his Truth album (circa 1968)...

that, my friend, is a one of the all time MONSTER uses of a wah pedal, and crazy good tone. i swear to you that my father and i were just talking about the incredible wah mastery of  beck's " i aint supersitious" off this album at 5 pm today.

*dad is 73 and has for the last 50 years the most comprehensive contemporary record collection i have ever seen.