Rockman Acoustic Guitar Pedal clone (Dunlop version)

Cybercow

Cybercow

Well-known member
After meticulous tracing both the Rockman (SR&D - gray button) Acoustic Guitar Pedal and the Dunlop Acoustic Guitar Pedal (blue button), and all the heavy lifting in KiCAD for PCB layout work from @rwl, we've accomplished a working PCB for this oldie goldie. We went with the Dunlop blue button version because the JFET biasing scheme is a much easier setup than the gray button version.

After initial testing the first rendition (ver 0.90), I called it a success and built a few out fully. And when A/B'd with both original pedals, this clone is indistinguishable from the originals. (Except for the buffers and low battery indicator sections of the originals.) Since then, @rwl has tweaked it for the final working version - just to jockey some connection points around. (I imagine he'll be posting that info sooner or later.)

Meanwhile, here are some shots of the finished schematic and build stages. This rendition, (and @rwl's latest 1.00 version), were designed to fit into a 125B enclosure. Hopefully, as time allows and if @rwl is so inclined, a 1590BB sized project will be done up so the control pot layout will match the original pedals.

AP_Blue_Button_Schematic_April_2025.png

Rockman_Acoustic_Guitar_Pedal_Done.jpg Rockman_Acoustic_Guitar_Pedal_Guts2.jpg Acoustic_Pedal_PCB_Populated_01.jpg Acoustic_Pedal_PCB_Populated_02.jpg
 
I’ve built this and am in the process of debugging my build. I am getting 3.8 volts Vref from a 9 volt supply, about 8.8 volts on the TL072 power pins, and the same 3.8 volts on the test point regardless of the trimpot setting. Should Vref be 4.4 volts? Something funny is going on with my build, most probably my fault.
 
IanMerley said:
I’ve built this and am in the process of debugging my build. I am getting 3.8 volts Vref from a 9 volt supply, about 8.8 volts on the TL072 power pins, and the same 3.8 volts on the test point regardless of the trimpot setting. Should Vref be 4.4 volts? Something funny is going on with my build, most probably my fault.
Click to expand...
Yeah. VRef should be ½ the voltage of whatever is on the Cathode side of D1.
 
OK, I tracked the problem down. I used the Gerber files from Github and unfortunately the polarity of C101, the 10uf capacitor going to pin 5 of U4 is reversed on the silkscreen. I swapped it around and I am now getting the correct Ref of @ 4.4V.

I cannot tune in the correct voltage of 1.25V on the test point though. I find the voltage stays at @4.3V regardless of the trimpot adjustment. When I measure the voltages on the empty transistor socket I get 2.97V on the gate and 4.33V on both the source and the drain. I am using a J201 with a measured VGS(off) of -1.1 volts. Any help would be appreciated. Thanks.

P.S. the pedal does work otherwise.
 
IanMerley said:
OK, I tracked the problem down. I used the Gerber files from Github and unfortunately the polarity of C101, the 10uf capacitor going to pin 5 of U4 is reversed on the silkscreen. I swapped it around and I am now getting the correct Ref of @ 4.4V.

I cannot tune in the correct voltage of 1.25V on the test point though. I find the voltage stays at @4.3V regardless of the trimpot adjustment. When I measure the voltages on the empty transistor socket I get 2.97V on the gate and 4.33V on both the source and the drain. I am using a J201 with a measured VGS(off) of -1.1 volts. Any help would be appreciated. Thanks.

P.S. the pedal does work otherwise.
Click to expand...
Depending on your DMM, it might be loading the measurement. To set it by ear, start with the trimmer set in the center and slowly test for squishiness as you slowly turn it to one way or the other with the Gain control turned to full CCW. With a Vgs(Off) of -1.10V, I'm guessing the trimmer will end up at around 66% to 75% of CW or CCW.
 
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