Roland CR-78
Roland CR-78
The Roland CR-78 is a pre-set rhythm box, with 4 additional memories for storing patterns created by the musician. The Roland CR-68 is the same unit, but without the additonal memories. All of the drum sounds are analog (no sampling), and the functions of the machine are controlled digitally with a microprocessor.
A synth-collector friend of mine brought me his CR-78. Everything was fully functional, but it sounded a bit... weird. It was hard to put a finger on it, but it was a bit muted. You might call it a 'smeared' sound.
He was also worried about the backup battery. He had the unit for over a decade, and the battery was never checked, or changed out. If it leaks, battery acid can etch away traces on the PCB, and the repairs can be difficult and costly.
CR-78 Without The Front Panel
CR-78 Without The Front Panel
Cleaning The Buttons
Cleaning The Buttons
Vintage synth gear that has been sitting around in the studio, or in storage, tends to gather a lot of dust and dirt. This CR-78 was no exception.
Nice And Clean
Nice And Clean
Cleaning The Faders
Cleaning The Faders
The first thing I did was to remove the case and front panel, and clean all of the buttons and knobs. At the same time I cleaned the level faders with contact cleaner.
Power Supply Recap
Power Supply Recap
Next up on my list was to replace the power supply capacitors. This is mandatory maintenance on anything older than a few decades.
Solder Bridges On PCB
Solder Bridges On PCB
While poking around the inside of the CR-78, I noticed some solder bridges on the PCB. This doesn't look good! Since the patina of the bridges looked the same as other solder elsewhere on the board, I concluded that this was probably a manufacturing defect.
I traced backwards from the solder bridges, and found out that they were shorting the trigger lines for the hi-hats, cymbol, and snare. Aha! Perhaps this was what was causing the weirdness in the sound of this CR-78.
Clean PCB Traces
Clean PCB Traces
After a little bit of work with some solder wick, the traces cleaned up nicely.
Old Rechargeable Battery
Old Rechargeable Battery
The original battery in the CR-78 is a rechargable NiCad. Fortunately, this one had not started leaking yet, so everything was clean and dry.
New Battery
New Battery
Rather than replace it with another NiCad, I decided to change it out with a longer lasting lithium battery. Since the new battery isn't rechargeable, it's important to disable the charging circuit, otherwise the new battery will definitely rupture and leak (or explode!).
Charging Circuit
Charging Circuit
Charging Circuit Removed
Charging Circuit Removed
The charging circuit in the CR-78 is composed of several isolation diodes, and a current limiting resistor. Removing these components is all that's required.
Alignment And Calibration
Alignment And Calibration
Before I put everything back together, I powered up the CR-78, and calibrated all of the drum sounds to the specifications in the service manual.
Everything sounded clear and crisp. The mushiness in the sound had vanished. Obviously the solder bridges were causing triggering problems.
There is a bit of mythology around old analog gear. Sometimes two identical units can sound subtly (or not so subtly) different. One of them may be more warm or fat. This can be due to components that have drifted in value, or variations in calibration.
This CR-78 now sounded great, but it was a bit on the cheesy or clinical side of things.
Bass Drum Decay
Bass Drum Decay
The service manual recommends a bass drum decay setting of 100 ms. I think this sounds a bit dry. To fatten things up, I extended it to 200 ms.
This is an oscilloscope trace of the bass drum output, after I made my changes. I have the vertical sweep set to 20 ms/div. The total decay is about 10 divisions long, or about 200 ms.
An optional programmer was available with the CR-78, called the WS-1. It interfaces to the back of the unit through 1/4" jacks, and provides clock pulses and memory write signals. It makes it (relatively) easy to program the user memories. However, not many of the programmers were sold, and they are very hard to come by today.
It's possible to connect a standard footswitch to the memory write (TS-1) jack, and program the CR-78 in real time. However, since there isn't a click or metronome, it's very hard to create patterns with any accuracy. One trick is to use the tamborine sound to keep track of the beat, but even that isn't enough to keep things in sync.
Another, rather complex, way to program the CR-78 is to use clock and trigger signals from a second drum machine, and 'clone' patterns from one to the other.
In this video I demonstrate the footswtich method, and also cloning a pattern from a Kawai R-100 drum machine to the Roland CR-78.
Roland CR-78 Programming Without A WS-1
Roland CR-78 Programming Without A WS-1
