Old Dunlop Wah GCB 95 - Not Wahing

[TomCarlos]

I must have a very old Dunlop Wah GCB 95 because this circuit board has just two transistors - the 5117s.

I found an old thread where someone was asking about the 5117 transistors - that is what I have in this unit. Both transistors tested ok on my Atlas meter.

The switch needs to be replaced - for now, I am shorting the output jack to either the input or the circuit.

The pots is ok - but I need to figure out how to properly reinsert it. I am guessing you get the pot to the center position, the lever to center, and then tighten the pot in.

I am not sure about input and output jacks. Maybe there is an issue there.

In any event, I am trying to find a schematic for this thing. The circuit seems simple enough but I cannot get the wah signal to the output.

One of the transistors does not seem to be turned on. So I am trying to figure out what drives the base to get current flowing through it.

Anyone have a schematic or can perhaps point me to a resource for this thing? I am thinking it must be something like version 1 !!!

(And yes, I will soon be closing the loop on my other open threads!!!)

Thanks,

Tom

[TomCarlos]

Ok... just discovered this is a Revision D, 1981. If I disconnect the wires to the input jack (and it is the wrong type of jack), undo the wires at the switch, I still get 9.5 volts at the collectors. So without the ground shorting of the jack, I do not see how the circuit is powering up. More to come...

[The Dude]

I believe that input jack is wrong. It's a switching jack and should be a stereo jack. When a mono plug is inserted, ring and sleeve are shorted together completing the ground circuit and powering the unit. With no plug, there is no ground, so no current is drawn. With the switching jack that is in the unit now, I don't see how a ground path could ever be completed. It's a bit of speculation without a schematic, but that is typically how pedals are configured.

[Jazz P Bass]

Not too sure about the specific transistors but give this a look.

[TomCarlos]

Thanks guys....

Yes, I need a 1/4" Jack, Switchcraft, Stereo, double open circuit. It looks like the case is grounded. So that switch brings ground to the circuit board through the purple wire.

I will also get a new Carling SPDT switch. But for now, it is jumper wires.

As for the circuit, this looks similar to the Vox V847. So I will use this to debug. I'll remove the connector and use some jumpers to open the ground and then reconnect. Let me solve that problem first... then figure out why the wah circuit is not engaging.

[vintagekiki]

Anyone have a schematic or can perhaps point me to a resource for this thing?

http://www.griffineffects.com/content/23-identify-dunlop-wah-circuit-boards

https://www.electrosmash.com/crybaby-gcb-95

https://www.schematicsunlimited.com/d/dunlop

1)

https://en.wikipedia.org/wiki/Dunlop_Cry_Baby
Models

[Helmholtz]

Early Vox V847 and Dunlop GCB95 wahs used the same circuit and board layout. I think the Vox V847 wah was actually manufactured by Dunlop.

Transistor type doesn't really matter. The influence of Hfe on bias points is insignificant.
So you can use the reference voltages shown in the Electrosmash article on the V847.

This might be interesting for you:

https://music-electronics-forum.com/...ge6#post931644


I adjust the gear wheel of the pot to get ~ zero Ohm between wiper and ground in the toe down position.
But make sure to allow for a little extra rotation to activate the switch.

[TomCarlos]

Thanks again everyone.... seems like we are "wahing" once again but I need to do some tweaking.

There was a grounding issue in this pedal. In addition to the parts I mentioned previously, the output jack also needs to be replaced. The simple mono type jack was at fault.

Helmholtz post to the other thread (for the Vox 847) describes the biasing for this unit. Let me reproduce that here -

"The Vox circuit uses an interesting and clever biasing arrangement which utilizes DC voltage feedback to stabilize the collector current(s) against transistor variations. In fact, doubling the current gain (hFE) of Q1 changes the collector current and potential by less than 5%. The effect on voltage gain is even less. The bias for both transistors is derived from the collector potential of Q1. For Q1 the bias network essentially consists of the voltage divider R6/R8. C3 (4.7µ) grounds the signal across R8. This is necessary to avoid undesirable signal NFB.The DC voltage across R8 is then applied to the base of Q1 via the low DCR of the inductor in series with R2. The bias for Q2 is provided via R5, which connects between the collector of Q1 and the base of Q2."

For those reading this in the future, also take a look at this - https://www.electrosmash.com/vox-v847-analysis.

I will dicker with the pot and placing that back into the unit. I am also thinking of changing some wire colors - you can't always assume that black means it is a ground wire!!!

With my appreciation...

Tom

[Jazz P Bass]

"https://www.electrosmash.com/vox-v847-analysis.:
That link is dead.

[Helmholtz]

Try this : https://www.electrosmash.com/vox-v847-analysis