EMB Rack Wah controller

[Sowhat]

Does anyone know how to wire up a controller for a EMB Rack Wah?

I have not found any schematic or wiring diagram or much information at all, I hear the inventor is in France with his new company called "amplifier" or something like that but I don't have much on that. The controllers plug into standard XLR connectors and lore has it that when somebody would buy one of these things back in the 80's they would send their favorite wah or volume pedal to him and he would modify it into an EMB rack wah controller for them.

I've done a little reverse engineering on it but not enough to determine exactly how the controller interface works yet, maybe someone here has dealt with these before and can give me a heads up.

[The Dude]

I'll preface by saying I don't know anything about it.

Now that that's out of the way- thinking "out loud". There's only a pot in a basic volume pedal. I'm guessing voltage in to the high side of the pot and voltage out of the center tap- and of course ground. So (again guessing), one pin of the xlr would be feeding the pot, one pin exiting the pot back to the rack, and ground. Ground would be easy enough to find checking continuity to chassis (probably pin 1- assuming standard wiring of a mic cable). Measure the pins of the rack device to see where there is something coming out to find the "send". That "something" could be DC if the thing is voltage controlled, or AC if they actually send signal in and out. Either way (signal or DC), the wiring would be the same.

You could grab some alligator clip leads and a pot to see if that would work.

[Sowhat]

You could grab some alligator clip leads and a pot to see if that would work.

This morning I had a theory... how many times have we heard that before, anyway, Pin 1 of the XLR is chassis ground which also appears to be board ground through the wiring harness because the deck of the chassis is made of wood! Focusing on the first XLR only, Pin 3 is a + input to an opamp Pin 2 measures out to be +12 volts but I have not traced that back yet. My though was if I connected a pot from Pin 1 to Pin 2 and hooked the wiper to Pin 3 I should get some action, well... there ain't. I tried pots from 5K to 500K and none worked. The higher ohm pots however would cause it's cable fault light to come on at higher settings.

I thought that maybe the +12 volts and ground allowed me to make a variable voltage divider using a pot from 0-12 volts. I guessed that's what the op amp wanted. I guess I'm wrong about that.

I need to trace back pin 2, it goes to another patch of digital control stuff... maybe it's not +12 volts at all... I never scoped it, maybe I should... maybe it's expecting a pure resistor load of some specific range across pins 2 and 3.

[The Dude]

Well. It sounds like you've already tried a multitude of things. There's always the possibility that they install a whole little circuit board in the pedal when they "modify" it. Maybe someone who knows about these things will chime in. I'll be following along for my own curiosity. Good luck!

[J M Fahey]

Ouch!!!
Homemade typically French style design at it worst.

Each Country has certain traits that show up here and there.

I used to buy tons of paper Electronics magazines from all over the place, all which reached Buenos Aires, including French ones.

Even today, just looking at a schematic/PCB design from one of them I can tell you where it comes from ... quite an easy feat in fact, a no brainer to distinguish from Hobby British ones (all based on Veroboard) , USA ones (blobby ink brush hand drawn ones or first with Bishop Graphics black crepe taped ones) to Elektor distinctive (Orcad?) octagonal pads and CAD designs to always printed in blue (to avoid photocopying) Italian Nuova Elettronica ones to ... French ones.

Which were based on very narrow tracks and very small pads (suitable only for computer boards and such, not abused Guitar stuff) drawn with some kind of Letraset (sheets full of pads, IC shapes and narrow tracks) which were taped to a transparent Mylar sheet and photo transferred on UV sensitive boards using a special exposing device.
Such PCBs were very easy to recognize.


Note very small pads which make reworking a nightmare, be very careful because they are very easy to strip away.
Also some tracks pass *very* close to pads they are not supposed to contact.

And the French typical over over over complication to do anything, this is an egg timer :


(my pet theory is that they envy and try to surpass Germans, which are already complicated but in a good way).

THIRTEEN dual or quad ICs, plus half a dozen mystery chips, a dozen transistors, FIVE (count´em) multiturn trimmers to replace a 1 (one) transistor , one pot, one inductor circuit? .... gimme a break !!!!!!!

Plus soldering is shoddy.
I´m amazed one of these units survives Tour duty.

Back to your problem: remote controller *should* be a plain potentiometer (maybe a Linear 10k or similar to keep things easy and allow for long cables), get 5V (or 12V) and supply a variable control voltage, BUT being a French design I would not be surprised at controller sending some FM modulated frequency to the pedal, it sending back another frequency one, or some other nonsensical complicated way to do things.

But give the plain linear mid value pot a new chance.
Or maybe the remote pot is wired as a rheostat and provides variable resistance to ground
or .... or .... or .....

[Sowhat]

Ouch!!!
Homemade typically French style design at it worst.

Yeah, I don't even want to breathe on that board... you probably didn't see all the blue wires and patches on it like that one piece of perf board standing up on it's edge with a few components on it... yikes!!!

Still, all in all, it's an interesting puzzle. Today I was busy getting things out the door that I could get out the door and this one was not one of them. I did power it up and check that pin 2 with a scope and I got a pulse train riding on it (in the neighborhood of a few microvolts) but it was so small it could have been crosstalk or old caps, it was way down near the noise floor... actually it is composed of modulated noise, I don't see it being strong enough to trigger anything, I don't think it's intentional.

Tomorrow, I'm going to go the rheostat type wiring seeing that the voltage divider idea did nothing, I'll start off with a 10K like you suggest.

[J M Fahey]

Cool.
Scope and measure what is on each XLR pin, relative to ground,that should give us a clue.

[Sowhat]

Thanks to a little Photoshop magic, here's an xray view of the circuit board... you can see the bodged in parts and blue wires in this one.



I just noticed that the two images I posted at the beginning of this thread are upside down... that's not what was sent... maybe they are too big or something and the sites graphics engine is having trouble displaying them.

[J M Fahey]

Ouch!!!
That design is *crazy* , the unnecessarily narrow tracks PCB is a time bomb, just look at the two large orange axial caps, unglued and hanging from tiny pads.
The moment that rack unit takes a sideways kick, any kick, those pads will be torn off.
And now I see they used SEVEN multi turn pots.

Currently busy developing my own "Belton Brick" but "maybe" when I have some free time I will develop a "remote control Rack Wah" ... which should need not much more than a real Wah circuit (including potcore inductor), some "electronic potentiometer" IC or a couple matched and tracking FETs or even LDRs or, leaving inductor aside, some bandpassfilter, remotely variable (one or two LM13700?) with proper Q and tuning range to mimic what a Wah does.

Worst case: 5 to 10% of the parts count and complication I see there.
And maximum two trimmers: one to select Q and the other to shift range up/down so user can tune his Wah to suit personal taste.

Oh well.

[Sowhat]

Ouch!!!
That design is *crazy*

It reminds me of working on space invaders or pac man arcade mainboards in the day... very fragile. This thing is supposed to be one of those "holy grail" rock star pieces of gear from the 80's... Metallica used it so it must be good right? My customer is a collector and brings me the most whacky prototype, custom made unsupported gear he finds out there to get working again. Right now in the shop I have 3 jobs from him. First is a very rare Redmere modeling amp from the late 1970's with a destroyed power amp, a MicroMoog with no main oscillator and a broken pitch finger slide pot kind of thingy and then this thing. He told me that he has more and while he is willing to pay whatever it takes, these things are always quite challenging requiring a lot of research and head scratching... Give me a red plating Super Reverb any day, now that's easy money!