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March 2025
At first glance
The unit looked alright. It was not until I looked under the hood that I started to think I was in over my head. The switches of the CR series are notoriously difficult to service and are impossible to buy online.
Faced with this challenge, the previous owner creatively used monetary switches which were able to jump the switch pads with the clever use of staples.
This in addition to a poor soldering job, a custom faceplate to accompany the new buttons epoxied to the case, and some missing LEDs led me to completely teardown the instrument and start fresh.
Keyboard Switches?!
I spent most of the time cleaning the board, removing solder, replacing LEDs, and taking measurements for new switches.
I decided to make my own, taking advantage of the original switches' similarity to mechanical keyboard keys. This involved making a very simple carrier board for the mechanical switch shown here.
To use this adapter..
The switch board comes separate from the mechanical key.
Remove old switches - see https://www.brdwlms.com/repair-diary/roland-cr-5000/switch-cleaning-replacement. Make sure the metal bracket and old switches have been removed.
Install the mechanical keys into the metal bracket. They will click in. Be mindful of the pin orientation.
Flip the metal bracket and mechanical key assembly over.
Install the carrier board to each mechanical key. Align the carrier boards' pins with the four holes on the Control Board. Solder the mechanical key's two pins to the carrier board AFTER verifying the pins align with the Control Board holes.
After soldering all of the carrier boards on, flip over the assembly and align the pins to the Control Board holes. This will require some finessing, use a screwdriver to carefully move stubborn pins into alignment.
Press the entire assembly into the Control Board, flip over the Control Board, make sure the assembly is pressed in snug and solder all of the pins from the back of the Control Board.
Broken Sound Banks
Sound Bank 1 in addition to the first two sounds were not able to be selected. When pressed, the LED would not light and the sound would not switch.
I traced this issue to the control board's IC1 & IC2. These are hex buffer ICs with tri-state outputs which acts as a buffer between the switch matrix and the CPU.
How the "Switch Scanning" works
The CPU holds one of switch matrix rows low through LATCH IC1 and DECODER IC6 on the CPU board. When Sll closes while pin 4 of IC6 (B in Fig. 1) is held low, pin 14 of BUFFER ICl of the Control Board (A in Fig. 1) which is pulled up via Rl, becomes low. This low is read by CPU through data bus.
The CPU continues this sequence for the remaining 7 matrix rows (B, Fig.2). Once the rhythm starts, the time interval between switch scans varies to Tempo Clock rate.
LED "Scanning"
To light LED that corresponds to a sound, the CPU selects the matrix row and column where the LED is connected diagonally. In the above example Dll (corresponding to S11 being pressed) has been on. The CPU the drives the LED driver Q1 through LATCH IC2.
Voice Triggering
The CPU delivers trigger signals (negative going) to individual VOICE Generators on the Voice board. This trigger signal goes negative at the falling edge of tempo clock and stays low until the next falling edge of the tempo clock. That is, the width of trigger signal is equal to period of one clock signal.
Voice Generation
Most voice generators are designed based on a fashion similar to those detailed in the circuit description on the TR-808 Service Notes which is expected to be referenced to as necessary. Exceptions are Cymbal and Rim Shot.
Voice Generation Circuits
All hail the bridged-T
Bass Drum
Constructed from two bridged-T networks. One has a self-dampening frequency of 59.9Hz and the other 87.7Hz. Triggered by the falling edge of a pulse from the CPU through a buffer.
Snare Drum
Constructed from two bridged-T networks. One has a self-dampening frequency of 226.2Hz and the other 335.2Hz.
A noise generator is summed with their output for the snare wire sound.
Low/High Tom
Cowbell
Accent
Handclap
Clave
High-Hat OP/CL Cymbal
Broken Tempo Control
I realized the Tempo Pot was not working after initial testing. The machine would not play at all and when tugging on some cables/pressing on the CPU board I would hear a rhythm at a random tempo.
This led me to think it was a loose connection. It ended up being P8:35-33, a connector that sends the tempo pot's control voltage to some conditioning and then to the uC.
Reversing the yellowed case
The first action I took was to soak the case in 12% Hydrogen Peroxide for several hours in the sun. This did an alright job. I noticed the whitening isn't consistent and there are some faint yellow blotches.
I ended up sanding the case(s), starting with 100->400->1400 grit sandpaper. I then sprayed Krylon Acrylic Coating to improve the appearance.
I also removed the makeshift panel that was installed to accommodate the DIY drum pad switches. I was able to pull off most of it without damaging the enclosure. Below are the before & after images
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