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Dunlop GCB-95 wah pedal

Before 1990, all Dunlop wah pedals had wires between the jacks and the pcb. In mid 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 mid 1991 onwards, a buffer circuit was added before the actual wah circuit (it’s there if your pcb says "Rev F" 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. The current revision is Rev I, which has a different circuit layout around the inductor (which is the red Fasel reissue, btw). Other than that, it looks much like the Rev G and H wahs that has been running since mid 1992 (there is actually no real difference between G and H, other than the colour of the circuit board. Rev F, which appeared in 1991, had a different adapter jack, but that's the only real difference between it and the G/H. Rev E units have a slightly different layout, and have a special section below.

I haven't bothered to write any instructions for pedals that are older than Rev E, and the various Vox wahs (i.e. all pedals with jacks that aren't mounted directly to the circuit board), as there are several other sites out there that covers those. Here's a link to one of the best ones (click "DIY resources", and then click the circuit board that matches yours). I have, however, started work on a page about modding the new Vox V847-A. It's not finished yet, but you can find it here.

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 - it’s a tight fit, but it’ll come loose. Sliding it downwards (away from the pot) usually works for me.

Is your pedal a Rev E model (pcb-mounted jacks but no buffer)? Click here to jump to that part.

 

step 2. mods to the pcb (Rev F, G, H & I)

Start by deciding if you want to remove the buffer or not. I’d recommend you to remove it - you’ll take a transistor and a bunch of other components out of the signal path, and that can’t be bad. But it’s your call - leaving it in won’t 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 be a different colour than this one (circuit boards in Rev H wahs are green on both sides), but the general layout is the same. You’ll see that the input jack has six solder points. Only one of them carry the hot signal when there’s a plug in the jack - it’s 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. You’ll trim it to length later, but you will kick yourself if you’ve made the wire too short... Then carefully solder the wire directly onto the north end of the buffer’s input cap (it’s 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.

On Revision I wahs, the input capacitor seems to be changed for a box type, so you might have to make the connection on the solder side of the board instead. It might just have been the one pedal I saw, mind you - it's still a little early to tell if it is a lasting change.

That’s it! Proceed to the switch if you haven’t changed your mind about removing the buffer. For Rev E wahs, look at the special section here. Rev H wahs are exactly the same as Rev G, but the circuit board is green on both sides. With Rev I, they seems to have gone back to the tan brown colour, and there may also be some layout changes around the inductor. 

 

step 2b. mods to the pcb incl buffer removal (Rev F, G, H & I)

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 don’t have to cut any traces - maybe one day you’ll want to use the buffer section again, who knows? Besides, the missing components make a fine "cut" all on their own, so there’s 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. Don’t 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. You’ll trim it to length later, but you will kick yourself if you’ve 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 you’ll 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 2c. rev E wahs (1990)

Thanks to Erik for the solder side pic!

The first year the pcb mounted jacks appeared, the circuit was laid out differently compared to later models. If your pedal looks like the one in the pic (pcb colour may differ, but look at the component layout), it's a Rev E pedal. You will need to cut a trace on these pcb's too, and we will also alter the wiring slightly. Instead of leaving the input jack -> 8-pin connector trace intact, we will break that connection and use the wire in the harness as the "effect input" wire.

Cut the trace between the input jack and 8-pin connector. It's a fairly long trace, and you can cut it anywhere you like between the point where it becomes thin and where it meets the 8-pin connector. But be careful - it does sit a little close to its larger neigbour, so you can't exactly use a hacksaw... When that is done, you need to cut a piece of wire about 15 cm (6") long, and solder it to the pad right below the input jack (you may have to desolder the pad first). This is your new main input wire. The green wire in the 8-pin connector has now become the input to the circuit instead.

For the switch wiring detailed below, treat the green wire as the effect input, and the new wire you just added as the main input (i.e. the exact opposite of how you wire newer GCB-95's).

 

step 3. the switch

You need a good quality DPDT (double-pole, double-throw) switch. You can use any DPTD you like, as long as it will fit in the pedal. But keep in mind that it has to work with the treadle, so it can't be too small. More on that later. Anyway, we will wire the switch 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.

Now which switch to get? These days, I will almost always use the Carling 316PP DPDT switch. As it is the DPDT version of the stock switch, it will fit right in and "feel" the same when you click the pedal over. For pedals that will get an LED as well, I go for a 3PDT instead. You will find both of them at Banzai Effects - 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 - or Musikding, both in Germany. 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 A’s pole (#2) connect to throw 1, switch B’s 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 you’ll be fine. Just don’t 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 [lug 6 for Rev E wahs]. 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 [lug 2 for Rev E wahs]. 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. You’re done!

adding an LED

Lots of people have asked me how to incorporate an LED into the mods, and while the simple answer is "just steal 9v+ voltage from somewhere, add a current limiting resistor to the LED and switch it to ground when you want it to light", it has proven to be a little more complicated. If you don't do it right, you can introduce adapter noise into the signal, and/or get thumps when you turn the pedal (and the LED) on/off. So here's my "trade secret", for all to see... First off, keep in mind that you can not just add an LED to the existing circuit - you need to replace the switch for a bigger unit (a DPDT if you want the stock bypass arrangement with an LED, or a 3PDT if you want to do the true bypass mod as well).

I only do this mod to pedals with adapter jacks incorporated into the circuit board (i.e. Rev E or later) - not because the LED draws that much power, but because those pedals also have polarity protection and extra filtering built in. The trick is to steal (and return) the power from outside the filtering, so that any thumps created when the LED lights up/shuts down will be eaten up by the filter capacitor, instead of being heard in the amp. The first step is therefore to locate the point where we will steal the voltage needed. On Rev F and later wahs, the positive and negative power can be found at the north end of the resistor and diode (respectively) that is located right below the adapter jack. From the resistor, run a wire to the anode (long) leg of the LED.

Solder the current limiting resistor (1-3K, depending on how bright you want the LED) to the cathode (short) leg, and connect it to the switch. From the switch, run another wire back to the negative connection point by the adapter jack (north leg of the diode). That's it!

On Rev E wahs (pic to the left), the idea is the same, but the points are slightly different. The positive voltage point is still on the north side of the resistor (very close to the adapter jack this time, so be careful with the soldering iron), but the negative/ground point is now on the south leg of the diode.

I usually drill out one of the two pre-punched holes in the case (to the left and right of the switch), to fit the LED. Be careful, though - the drill will not fully clear the treadle, so when you have drilled through, it is very easy to nick the paint on the front lip of it. Also, you have to use a low profile LED holder (the plastic variety as shown to the right), as the treadle will not clear the larger chromed LED bezels. If you want to use one of those, you have to drill the hole from the top, so you know for sure it will clear the treadle. For reference, here's the top section of a GCB-95 with the LED installed and ready to go:

Please note that I'm using a 3PDT switch here - as I said in the beginning of this section, adding an LED requires a bigger switch. As this pedal also has a true bypass mod done to it (the white/black wires are the "new effect input" and ground wires, respectively), I needed a 3PDT to allow for the LED switching. If you decide to keep the stock bypass arrangement (which I'd seriously advice against), a DPDT will suffice. The stock switch can only be used for stock wiring without LED.

Also, note that this switch has been turned sideways, to make it clear the bracket that holds the pot. So to reference it to the DPDT diagram posted earlier, imagine turning the diagram almost 90 degrees to the left. Or - if you'd rather - imagine turning the picture above to the right...


fitting a replacement inductor (Rev E, F & G)

inductor padsI find the stock Dunlop inductor to be adequate for most uses - the true bypass mod coupled with a few choice mods from the list below (mainly the gain, vocal and mid boost mods) will usually be enough for most people. But if you want to go further, or if you have a pedal that has a broken or particularly weak inductor (the mid-to-late '80s Dunlops sometimes came with really shitty inductors), you can also replace it. There are plenty of options out there. Keep in mind that while the stock circuit board has many holes in this area, and the stock inductor (and a few replacements) has four connectors, only two really matter. Basically, there's an "upper" and a "lower" group of holes - marked red and green in the pic to the right - and the inductor should go between any of the upper and any of the lower holes. If yours has four connections, you need to figure out what the pins do, so you can turn it the right way. Simply measure for resistance (using a multimeter) between the pins, and you should find the pins that are on either end of the coil. Sometimes, the two "extra" pins aren't connected anywhere (which the multimeter will show you), and other times, the pins are connected in pairs on either side of the coil.

fasel Here's how a reissue Fasel inductor fits (works for both the red and yellow version). It only has two connections, and will only fit in two specific holes, so it is quite easy to orient the right way. You may have to use desoldering braid first, to clear out the solder pads you want to use. The stock inductor was soldered to the four holes centered around the big hole, and this one fits into the two holes closest to the resistor. Make sure it sits flush with the circuit board - its legs are quite short. Also, they can not be bent to help keep the inductor in place, so when you flip the board over to apply solder, simply let the inductor rest on the bench to keep it seated. 

There are plenty of other inductors out there, and I will be adding pics of those when and as I come across them (either on my workbench or on the web). The Revision I model already comes with the red Fasel, and also has a slightly different circuit board layout around the inductor.

 

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):

Some prefer soldering the trimpot in series with the existing resistor, which will guarantee that you never adjust it below the original value. Personally I’m not bothered by that, as an internal trimpot rarely gets adjusted by mistake. Also, keep in mind that you will be using the trimpots as variable resistors - i.e. using only the center leg and one of the outer legs.

 

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