Dunlop GCB-95 wah pedal

Before 1990, all Dunlop wah pedals had wires between the jacks and the pcb. In 1990 Dunlop changed the pcb design, and started soldering the jacks directly on to the circuit boards. If your wah has pcb jacks, it is marginally more difficult to mod for true bypass, as you have to cut a trace on the pcb (instead of moving a cable, as before). Also, from 1992 onwards, a buffer circuit was added before the actual wah circuit (itís there if your pcb says Rev. G or higher on it), to help with some of the tone-sucking. The buffer is no longer needed if you mod the wah for true bypass, and removing it will make the wah a little smoother.

Do you want to skip the true bypass part and go straight to the mods to the wah circuit itself? Click here.

 

Important note: In these instructions I refer to "upper", "lower", "north" and "south". To avoid misunderstandings, you need to orient the pcb accordingly - the "top" or "north end" of the pcb is the one where the 8-pin connector is. Also, even though connections are made on the underside (trace/soldering side) of the pcb, directions like "to the left of..." or "just above..." refer to the front or component side of the pcb. Also, there are two different inputs being mentioned - in step 2 the "buffer input cap", and in step 2b the "input resistor". It is a little confusing, but the buffer circuit is a separate circuit that has been added to the existing wah circuit. The buffer therefore has its input (the capacitor), which then feeds the wah circuits' input (the input resistor).

 

step 1. disassembling the wah

Take the back plate off the wah by removing the 4 rubber feet and remove the battery. Unscrew the input/output jack nuts from the sides [tip: for Dunlop wahs, use a 7/16" socket], then remove the 8-pin connector at the top of the pcb and the single screw that holds the pcb down. Now gently take the pcb out from the casing - itís a tight fit, but itíll come loose. Sliding it downwards (away from the pot) usually works for me.

 

step 2. mods to the pcb

Start by deciding if you want to remove the buffer or not. Iíd recommend you to remove it - youíll take a transistor and a bunch of other components out of the signal path, and that canít be bad. But itís your call - leaving it in wonít make the wah sound any worse than before. Taking it out will however change the sound slightly. To the better IMO, but still a change.

If you decide to keep it, follow the instructions below. If you want to remove it, click here to jump to that part.

Take a good look at the pcb - yours may look different from this one, but the general layout is the same. Youíll see that the input jack has six solder points. Only one of them carry the hot signal when thereís a plug in the jack - itís the innermost one in the lower row. From there the signal flows two ways:

  1. Downwards via a short wide trace to the first buffer component (it is a capacitor, even though it looks like a small yellow blob in the pics).
  2. Upwards via a long, narrow trace to the 8-pin connector. This trace then turns into the green wire, ending up at the switch.

You want to keep trace 2 intact, and break trace 1. With a sharp knife, carefully cut trace 1 between the input jack and the buffer input capacitor. In the pic the place to cut is marked with a white line.

 

Cut yourself a piece of wire (thin, flexible multi-strand) that is long enough to easily reach the switch from the pcb. About 15 cm (6") is a good length. Youíll trim it to length later, but you will kick yourself if youíve made the wire too short... Then carefully solder the wire directly onto the north end of the bufferís input cap (itís right below the white line in the pic). Just slide it in under the capacitor's leg, pull it up and back over said leg. For neatness, you can tuck the wire strands in under the capacitor leg, using a small flat-head screwdriver. When the wire is properly seated, slide it as far from the cap as possible (to avoid overheating it) and apply solder. This wire is now your new effect input.

Thatís it! Proceed to the switch if you havenít changed your mind about removing the buffer.

 

 

 


step 2b. mods to the pcb incl buffer removal

With the pcb out, take a good look at it and compare it to the one in the pic to the left. The buffer components are the ones crossed over with red X:s. It should be six components in all (including the transistor). You donít have to cut any traces - maybe one day youíll want to use the buffer section again, who knows? Besides, the missing components make a fine "cut" all on their own, so thereís no need to be violent. I use a desoldering braid or a "solder sucker" to remove the solder, and a pair of flat-nose pliers is also nice to have for quick and easy removal of components. Donít jumper anything - just leave the holes as they are.

 

Cut yourself a piece of wire (thin, flexible multi-strand type) that is long enough to easily reach the switch from the pcb. About 15 cm (6") is a good length. Youíll trim it to length later, but you will kick yourself if youíve cut the wire too short... Then carefully solder the wire in the far left hole (as seen from the component side) of the three vacated by the transistor. If you follow that trace youíll see that it ends up on the north side of the 68K input resistor. This wire is your new effect input.

In the picture to the right, the orange wire is the effect input. The brown wire is a ground wire from the switch (more about that later). You don't have to solder the ground wire in this place, or twist it together with the input wire - I just do that because I like things neat... But again - more about the ground wire later. For now, just remember that the effect input wire goes in the transistor's far left hole.

 


step 3. the switch

You need a good quality DPDT (double-pole, double-throw) switch. We will wire it in a way that combats crosstalk (when the effect can be heard faintly even in bypass mode), switching noise and clicks. Most switching noises come from the effect input suddenly seeing infinite impedance when it is disconnected and left wide open in bypass mode. This way of wiring the switch (which I believe Jack Orman is the father of) shorts the effect input to ground in bypass mode, making it shut up. End result? No crosstalk in the switch (as there's no signal to overhear) and tiny or no switching clicks. I often use an Apem switch from ELFA (part #: 35-761-47) because it's small and neat, but mostly because itís readily available here in Sweden. Another option is Banzai Effects in Germany - Olaf sells DPDT, 3PDT and even 4PDT switches, and for us europeans it can be both easier and quicker to order from within the EU. Also, you can buy both DPDT and 3PDT switches from Stuart Castledine, as well as from Andi Allan at MonkeyFX. Both are in the UK, and are very good chaps to deal with. Outside Europe, I shop from Aron Nelson, who will happily sell you 3PDT switches at very decent prices, and Steve at Small Bear Electronics. Again, both are good chaps, and will not overcharge you for international shipping either - props to both of them!

Anyway, a DPDT switch consists of two SPDT switches side by side (if you bought a 3PDT instead, just ignore the third row of soldering lugs). The two halves (as marked in the graphic) are independent, but both poles move in the same direction. This means that when switch Aís pole (#2) connect to throw 1, switch Bís pole (#5) connect to throw 4. Click the switch and pole 2 makes contact with throw 3 instead, while pole 5 connects to throw 6. Read this article by R.G. Keen if you want to indulge yourself in all things switching... The actual wiring of the switch is easy, if only a bit finnicky. It requres moderate soldering skills, but youíll be fine. Just donít warm the lugs too much - you can melt the switch if you are too slow. As with any soldering be decisive and quick.

  1. De-solder the connections on the old switch and remove it. Install the new DPDT switch and check that it fits and will work as intended. You should barely (if at all) be able to make it switch over when pressing the rocker pedal down by hand - using your foot and body weight will then compress the rubber stops enough to activate the switch.

  2. Remove the switch and install a jumper wire between lugs 1 (top left) and 6 (bottom right) on the switch. Only solder lug 1 for now (tip: a cut-off leg from a resistor works great as jumper wire).

  3. Reinstall the switch and solder the new wire you previously installed on the pcb to lug 2. Another new wire goes from lug 3 to ground. Any point that makes contact with either the pcb's ground or the wah case will work. One of the lugs on the pot has a black ground wire connected to it - you can try soldering to that point. Or draw the wire all the way back to the pcb (like the brown wire in the pic shown earlier). Anyway, this connection will short the wah input to ground in bypass mode, which more or less eliminates cross-talk and switching noise.

  4. Solder the blue wire (coming from the pot) to lug 4, the purple (coming from the 8-pin connector) to lug 5 and finally the green (also from the 8-pin connector) to lug 6. Note: some wahs (mostly newer ones with the Hot Potz II pot) have two blue wires soldered to the switch. If so, solder both to the new switch as well.

  5. Youíre done!

 


modding the wah circuit itself

There are a million modifications that you can do, of course, but some are more useful than others. Here are a few (pic below):

 

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this presentation © : Andreas MŲller 2002