Hey all! First time, long time, etc. Was hoping somebody could give me a hand repairing an oldish MojoHand FX Bayou Trem ( https://mojohandfx.com/products/bayou ). This is a pedal I've had for a while and haven't had any problems with, but sometime I think in the last move I must have rattled something loose, since it now displays a lot of the classic non functional tremolo symptoms - passes clean signal, volume control works as a clean boost, and rate LED flashes with the rate knob, but no tremolo when the effect is turned on.
I don't have a copy of the circuit, but it looks fairly similar to the EA Tremolo design shown here: http://tonepad.com/getFile.asp?id=130, and there are pictures below. Three transistors, Depth/Volume/Rate, etc. I've done a little bit of reading of other threads, particularly this one : https://www.diystompboxes.com/smfforum/ ... ic=82066.0, and it seems like my symptoms indicate a problem with Q2, which notably is a J201 in both circuits. Something I did notice though is that in the Bayou, the Source pin doesn't seem to be connected to anything, and instead the transistor is connected to the depth knob by way of the Gate pin, with the Source measuring 0v. I don't know enough about JFETs to know if thats something that makes any sense? But I see variations in voltage on the wiper of the depth knob, as well as on the gate of Q2, and continuity between the drain and the 22uF cap it's connected to, so I'm a little at a loss here. The second two pictures below are closeups of Q2 on the board. It feels like there's gotta be something obvious I'm missing here, so any help is appreciated!
MojoHand FX Bayou Trem not... Trem-ing
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mauman
- Resistor Ronker
Welcome, and thanks for the post!
It's not a trick, Q2 source (center pin) is connected directly to the ground plane, no trace required. Note that the traces to the gate and drain are separated from the ground plane by the fine green lines...
It's not a trick, Q2 source (center pin) is connected directly to the ground plane, no trace required. Note that the traces to the gate and drain are separated from the ground plane by the fine green lines...
Thanks for having me, and thanks for the response!
Thaaaat makes some sense. Like I said, new to the PCB repair side... So Q2 is working seemingly as it should, it's just not affecting the guitar's signal. Any advice on where to look from here, then? Am I right in thinking Q2 applies volume changes by way of Q1?
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mauman
- Resistor Ronker
Right, Q2 should be offering a variable resistance to bypass Q1's emitter resistor (the 1.2k) and thereby modulate the Q1 output level to produce tremolo. The Rate LED indicates the LFO is working, but Q2 is not modulating the output of Q1. Where in the circuit is that trim pot? Could it have gotten mis-calibrated? You could note the current position, and try adjusting it. Then I'd replace Q2.