#1
Alright, so I was part of the Wah modding thread a while back, but I can't go through 180 pages to see if this problem has ever happened, and search bar gave me nothing. So here goes.

I tried a while ago to do the true bypass mod to my standard Dunlop GCB-95. Something didn't work, and I gave up, and I just came back to it today (years later, more experienced) with ideas for a couple of mods... and it still doesn't work. I've learned a lot since I originally did it but still can't figure out what's wrong. It passes signal in bypass mode, and is totally dead (no hum, hiss, noise, RF, anything, it's as if it wasn't getting power) when switched on.

Here's what I know:
  • Resistors all measure correct values.
  • Transistors have ~8v when measuring collector-ground.
  • True bypass part works. Circuit does not.
  • Neither effect input nor output short to ground when activated.
  • Power stage diode works properly.
  • Checked all connectivity with pot and switch, no issues.
  • Pot and switch both funciton properly.
  • No traces have ever been cut on the board.
  • No components have been swapped, only the pot and switch.
  • Wah functioned perfectly (with new pot) before I attempted the mod.

I tried two different switch wiring methods, shown here and here. Neither worked, and I tested with a multimeter and made sure all connections were solid. For the bypass mod I pulled the parts listed at the latter link (and many others) and checked all connectivity.

I'm stuck. Photos below if they help. Can anyone give me any ideas, places to measure something, anything at all?






#2
Which true bypass circuit did you use? I did one on mine a few years ago (GCB-95). It works great and I definitely remember having to cut a track on the under side of the PCB. This is in contrast to one of your dot points that you have not cut any tracks.

One of the links you posted details in a photograph which track is to be cut. It also goes on to say you have to do it else the switch will mute the sound all the time.

I would double check your mod information.
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Last edited by Phoenix V at Oct 19, 2015,
#3
I did the one where you remove the buffer, and according to step two of the second link (http://stinkfoot.se/archives/546) you don't need to cut any traces. The spot he shows to cut the trace in step 2b is showing a component I removed, so it wouldn't make sense to cut the trace since it no longer connects to anything.

If anyone else has done this and definitely cut a trace, please let me know where, as I hope it is a fix that easy. At the moment everything looks like it lines up, and as I said, nothing grounds out where it shouldn't, transistors get voltage, lines are connected (though sloppy from reconnecting 5-6 times), etc.

Honestly I'd put the buffer back in and try the trace cut, but I used the parts for something else and I don't think I have any more MPSA13 lying around.
#4
Never tried the method you followed. Don't know if it works or not and not really going to go check the circuit diagram mod to see if it does or not.

I just know this one worked for me. Still using it. Watch this video.

https://www.youtube.com/watch?v=kqyFsuC9fk4
'It takes 100 guitar players to change a light. One to change the light and 99 to stand around pointing, saying..."Yeah man, look...I can do that too"...'

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#5
The method he uses retains the buffer, which I wanted to eliminate. The one in the video matches the directions on my second link (http://stinkfoot.se/archives/546) and if I had the buffer parts still, I'd try it, but unfortunately they've been scattered. As noted on much of the literature about this mod, it shouldn't cause an issue like I've found though.

I do appreciate the info! If I ever have to do this again, I'm probably going to do it that way and retain the buffer.
#6
Just as closure for anyone else hopefully finding this with issues:

Check the DC voltages from each leg of your transistors to ground. They should be in the ballpark of:
Q1 (filter stage MPSA18):
C: 3.5-4.5v
B: 0.6v
E: 0.08v (tiny but normal)
Q2 (output stage MPSA18):
C: 8.0-8.5v
B: 3.6v
E: 3.0v

I'm not sure of the buffer stage since it's a different transistor in a different role, but google what MPSA13 should be at and you'll figure it out.

I had a dead Q1, and it threw me off because Q1 was reading C-8.0 B-4.0 E-3.3 which is a normal set of values, but not what it should be at, and Q2 was reading C-8.3 B-7.0 E-7.6. Emitter should be lower than base (in this application), so I knew something was wrong. I originally thought Q2 was dead, swapped it and no change, then I swapped it back and swapped out Q1, and boom, back in business.

I did notice as well that years ago when I attempted the mod, I also switched out a cap and a couple resistors for the vocal and Q mods, and looking at the schematic, the .01uF cap is the feedback path for the output stage, so make damn sure if you do that mod that you have a good cap in place, otherwise that will knock out your output as well.

I'll still have notifications on here, so post if anyone has issues and I'll try and point you in the right direction.

TLDR: Check and swap all of your transistors, especially when there are only two. Didn't take long.
#7
I have done this mod and the worse part is scratching out a bit of the circuit board. If you get that wrong things can't be good. It can be solved with a re-solder if you are lucky.

I have done this several times. Twice it worked. Once it didn't. So its just a matter of doing it again and checking your soldering. Practice soldering helps before you take a solder to the unit.

Sometimes people get a Carling DPDT (316-B-PP) but what was recommended was the 3PDT not the DPDT, a mistake that can be made.
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