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Fixing a Roland PC-200 MIDI keyboard controller

Continuing in my series of keyboard repair blogs, I recently bailed an old Roland PC-200 keyboard out of storage. This old guy is just a 4 octave MIDI controller (no sound generator), but it fit the bill for having a reasonably sized polyphonic keyboard for a NYC apartment. Unfortunately it had long ago developed a problem with an F# key no longer working.

While there are newer controllers with many more buttons and polyphonic aftertouch, it seemed such a waste to replace it just because one key didn't work. After a little net research, a problem with many keyboards is that the carbon contact pads glued onto the rubber deteriorates. Replacements for these are available, but this was not a solution to the problem in this case.

I opened the case up and removed an octave worth of keys. The keys are ingeniously held by a spring, which is removed with a pair of needle nose pliers, and the keys lift off. The PCB has a molded rubber conductor strip which sits into holes on the circuit board, one strip covering an octave. To lift the rubber strip requires removing an octave set of keys. The white keys are numbered both by the chromatic name (A, B, C etc), and also the order in which they should be removed and replaced.

Lifting the rubber contact strip revealed the cause of the problem.

In manufacture, a pad of carbon is deposited onto the printed circuit board (PCB). This carbon pad contacts with the small carbon dot embedded into the rubber. There are four pads per key, the time difference between the rear pads conducting and the front pads conducting allows calculation of key velocity. Testing each pad with a multimeter showed a resistance of 60 Ohms, except the failed key, which no longer conducted.

To fix the deterioration of the carbon pad, I found an online supplier of conductive carbon paint. This comes as a 30g bottle with a paint brush.

The carbon paint was tested on an old circuit board first, painting between two metal contacts that I scraped bare. This requires painting a coat, then measuring the resistance with a digital multimeter, and repeating. Of course, as the carbon builds up, the resistance will drop. After several coats, the resistance of the damaged pad dropped to 94 Ohms, which when I tested with the key reinstalled, worked perfectly.

can you tell me what transister Q1 is?

sorry to be a pain but I need to know.
What is the transister Q1 on the main board
of the Roland PC-200.

Q1 details

A kind netizen passed on the following information to help others in their repairs:

Found your site, while I´m debugging my dead pc200 Roland keyboard.
The Qestion about Q1 (on the MKII) is indeed a power Transistor. My PC200 (without the MKII) used a european NPN powertrans.

BD371C
SI, NPN, 1.5 Amp, 45V UCE , 2.5W max. , Ueb 5, hfe25, case TO237
to use as a 5V stabilzer with the Zenerdiode. Ugly but cheaper than a 7805 !

btw. the other Transistor near the MIDI out is the Midi Out driver Transistor (PNP standard)
so if you get no signal on your Gear, maybe this Transistor is dead or the whole PCB (MPU dead like mine)

Transistor Q1

Offhand, I don't know. I don't have the schematics for the PC-200. You might investigate if you can get them from Roland? If I get the time, I'll try opening up mine to see if there are any markings. Other than that, at best, you could determine if the transistor is NPN or PNP and try substituting typical equivalents? My guess is it's a power transistor for the power supply?

Thank you for the answer

Thank you for the answer.

this is more than I got from roland!!!
please note that the project I'm building will not
need the old keys or springs or the little rubber
pads and the case.
so if ther is anyone who needs replacements
note thay are going on ebay.

Use chip for real piano keys

Dear Leigh,
Thanks for your post - interesting!
I want to try building a customized instrument out of an old piano including MIDI out.
Do you think I can use the chip from, say, the Roland PC-200 and let it be triggered by other keys such as real piano keys, in my case?
With kind regards, Roeloff
info@roeloff.com
www.roeloff.com

Using an existing controller for Piano encoding

Hi Roeloff,

Thanks for your interest and question. My initial thoughts are using a modified MIDI controller could be more trouble than it's worth. You will need to run wires from the original controller PCB to each of the keys, create a mount so that conductive rubber contacts attached to each key compress onto your custom PCB which probably needs carbon contacts. There are several kits around for encoding from a number of sensors to MIDI, I have used kits from Highly Liquid and they take a lot of the work out getting something running.

Modifying an existing circuit can be problematic since you have to work around existing design decisions, in many cases, using obsolete parts. This can make sourcing parts to build replacements difficult. In the case of using it for a piano, the velocity of piano keys is likely to be very different from a plastic MIDI controller like a PC-200, and while you may get something working, you may find the velocity curves of the existing firmware don't match that produced by the piano player. Also, the approach of using rubber contacts to compress onto a carbon coated PCB means there is a contact that could well impact the feel and behaviour of the piano keys.

All that said, don't let me talk you out of experimentation! I would suggest working with a couple of keys to test out sensor designs. The trick would be to get the sensor working (mechanically and with analogue electronics) before tackling the digitization, which nowdays is pretty trivial in the age of Arduino's, multichannel ADC boards and many experimenter boards. I would highly recommend getting a second hand CRO for such work.

If you want to get velocity and pressure measures per key, probably a capacitive field approach, where a small metal plate is affixed to the bottom of the piano key and on depression it moves into the field of a square wave projected from a PCB and changes the harmonic content of the square wave is the best approach. Then such square wave generation and ADC can be multiplexed across your sensors.

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