Replacement solution for GR55 expression pedal potentiometer

Started by ogerard, November 27, 2021, 08:26:02 AM

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ogerard

One year ago, I needed to replace this pot because it was out of order. I tried to find the Roland ref as mentioned in the service manual but as it is a specific manufacturing for Roland it is  impossible to find it at electronic suppliers (RS, farnell ...). The Roland pot has a specific curve. So I realised a simple electronic design to get the same behaviour than the Roland pot with a standard 10k linear pot. So I post pictures and a schematic diagram to explain how DIY. Once you have change the pot, you just need to calibrate the     
Expression pedal

Harry1227

Hello. Thank you so much for sharing this idea.
I changed that pot this summer. I found only such a RK11K114001Z on sale. To increase the pedal stroke, I had to remove the rubber limiter at the bottom of the pedal. I also turned the pot to a certain angle to make the Calibration pass. Under the rubber cushion on top, I installed a button that shunted the 2 and 3 pins of the potentiometer through the resistor. That's how I got the expression pedal mode switching. It seems that somewhere on this forum such manipulations have been described.
I really liked your idea with the connection to the output - 7 V. I made a diagram according to your description in the program EVERY CIRCUIT. It really should work. The main thing is to choose a resistor (you have 2.9 kOH) so that with the available potentiometer, the measured voltage does not fall into negative values.

How does this scheme work in practice? Are there any digital noises? After all, 7V is analog power supply. But 3.3 V digital power supply

gumtown

I would probably put a diode in there, as a little protection to prevent the wiper voltage at the input from going <0 volts
Free "GR-55 FloorBoard" editor software from https://sourceforge.net/projects/grfloorboard/

Harry1227

Quote from: gumtown on November 27, 2021, 11:51:02 PM
I would probably put a diode in there, as a little protection to prevent the wiper voltage at the input from going <0 volts
The diode will lower the measured voltage by 0.5 V. I think that such protection will not be superfluous, if it does not disrupt the performance of the circuit

gumtown

Quote from: Harry1227 on November 28, 2021, 12:02:02 AM
The diode will lower the measured voltage by 0.5 V. I think that such protection will not be superfluous, if it does not disrupt the performance of the circuit
A shunt diode across the A/D converter input, will stop the input going lower than -0.6 v
Free "GR-55 FloorBoard" editor software from https://sourceforge.net/projects/grfloorboard/

Harry1227

Quote from: gumtown on November 28, 2021, 01:18:28 AM
A shunt diode across the A/D converter input, will stop the input going lower than -0.6 v
Thank you. With schematic I understood your idea.

yogin

Thank you for
detailed instructions!
How does it work in practice with a shunt diode over time? Any problems arise in comparison with the native version from Roland? - This is also good and not dangerous, for example, if the GKC cable is accidentally damaged?

Harry1227

I also had problems with the GKC cable. With a faulty cable, there were many glitches up to freezing. With that experience, I didn't take the risk of using this scheme to replace the EXP pot.
I bought a potentiometer with a logarithmic characteristic. I installed it in its proper place, I did not need to turn it to a certain angle, as was the case when I tried to calibrate the linear potentiometer. Removed the rubber damper from the heel side of the pedal. After that, the pedal was calibrated and works without problems. I did this last December. I bought PTV111-1415A-A103.
In my opinion, it makes sense to take risks if you do not find at least a similar logarithmic potentiometer.

Will

I had the same problem. Jumping expression pedal, drop outs through the pedal sweep. I decided to take a risk and dismantled the pot.
I cleaned the inside, the wipers and resistor strip, put it back together, re-fitted it and it works fine.

JEBULA

HOLA SOY DE LIMA PERU, PUEDES ENSEÑARME A COMO USAR ESE REPUESTO DE POTENCIOMETRO .. PORFAVOR. GRACIAS MI CORREO JEBULA3@GMAIL.COM

compusic

Quote from: gumtown on November 28, 2021, 01:18:28 AMA shunt diode across the A/D converter input, will stop the input going lower than -0.6 v


I just encountered the same problem. I'will ask my friend, a technician, for help. And what's the best value of the added diode?

Thanks.

Elantric

Quote from: compusic on March 25, 2024, 08:20:29 AMI just encountered the same problem. I'will ask my friend, a technician, for help. And what's the best value of the added diode?

Thanks.
A common switching diode

1N914, 1N4148

https://en.wikipedia.org/wiki/1N4148_signal_diode


compusic

Just FYI, I have successfully ordered an offcial replacement pot through local authorized service center. The price, including shipping fee, is abut 8 US dollars.

Now my Gr-55's expression pedal works like a charm.

BTW, I'm living in China mainland.