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Roland CR-78 Tuneup

Roland CR-78

Roland CR-78

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.

Something Doesn't Sound Right

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.

Cleaning It Up

CR-78 Without The Front Panel

CR-78 Without The Front Panel

CR-78 Without The Front Panel

CR-78 Without The Front Panel

Cleaning The Buttons

Cleaning The Buttons

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

Nice And Clean

Nice And Clean

Cleaning The Faders

Cleaning The Faders

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

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.

I Found Some Bridges

Solder Bridges On PCB

Solder Bridges On PCB

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

Clean PCB Traces

Clean PCB Traces

After a little bit of work with some solder wick, the traces cleaned up nicely.

Changing The Battery

Old Rechargeable Battery

Old Rechargeable Battery

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

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 definately rupture and leak (or explode!).

Charging Circuit

Charging Circuit

Charging Circuit

Charging Circuit

Charging Circuit Removed

Charging Circuit Removed

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.

Calibration

Alignment And Calibration

Alignment And Calibration

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.

Fattening Up The Bass

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

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.

Testing The User Memories

CR-78 User Programs

CR-78 User Programs

CR-78 User Programs

CR-78 User Programs

Back Panel

Back Panel

Back Panel

Back Panel

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

3 comments on Roland CR-78 Tuneup

Steve
Can I ask which VR is each drum sound - did BD ( by ear ) worked a treat
January 8th 2016 12:34 EST
Rob
Hi... Our cr78 drum machine unit stopped working after it had been unused and stored a while...We can't get it to switch on or make any noise ! Was wondering if you or anyone out there reading this can offer any info help and assistance to us to try and get the cr78 playing and operating ... Read More
February 2nd 2017 14:16 EST
Keith
Hi Rob. If it doesn't power up at all (no lights or anything), then it's probably related to the power supply. Try taking it to any local electronics repair shop. It's best if you can find a music repair shop, but most decent repair techs should be able to get it going again, even if ... Read More
February 2nd 2017 19:32 EST
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