Guitar Synthesizer Repair Tips

My GR-100/300 has suddenly stopped working

GR-300 string LEDs stay on constantly

What seems to be the problem?

One day your GR-100/300 just stops working. Or, the GR-300 powers up, but does not make any sound. The string level LEDs may light all the time, regardless of whether or not a guitar is plugged in.

In one case, I happened to buy a GR-100 off of ebay. The unit powered up, the lights flashed, but it did not make a sound. Also, it got very, very hot. And quickly! I had another case of a GR-300 that powered up fine, but the string level LEDs stayed on constantly, and it did not make any sound at all.

Fortunately, I called my buddy Marty, a guru of musical electronics. He also looked at a photo I sent of the interior of the GR-100, and immediately told me to check the blue tantalum capacitors in the power supply. On Marty’s advice, I unsoldered one leg of each of the caps, taking it out of the circuit. I powered the GR-100 up, and it worked perfectly. Marty told me he had seen many failures with this kind of capacitor, popular in power supply designs of the eighties. At the time, these tantalum capacitors offered high capacitance in a small package.

I also followed Marty’s sage advice and replaced the bad capacitors with new, 10 uF, 50 volt electrolytic capacitors. The power supply of the GR-100/300 is +/- 15 volts, so any capacitor with a rating above 15 volts would work as well, like a 10 uF, 25 volt capacitor. Just to be on the safe side, I went with 50 volts for extra protection. If you want to stick with tantalum capacitors, the modern version is much more reliable than the ones found in the Roland gear. And finally, and this is important: be sure to check the polarity of your capacitors! You will notice that one side of the capacitor is marked negative, or "-." When installing the capacitor for the positive side of the power supply, the "-" leg of the capacitor will go to ground. For the negative side of the power supply, the "-" leg of the capacitor will go to the power supply. If you are unsure how to proceed, get some help! Otherwise you may find yourself with a noisy, smelly explosion!

By the way: in my case only the capacitor on the -15 volt supply was bad. That is why the lights came on, but the unit did not work! The heat in the unit was from the voltage regulator being shorted out by the failed capacitor. If I had kept the unit on for too long, I would have likely destroyed the voltage regulator.

Testing the GR-300 Power Supply

Before you warm up the soldering iron, it is a good idea to test the GR-300 power supply to determine if you do have a power supply problem. To do this test, you need a basic multi-meter that can read DC voltages, in the 0-20 volt range.

Here are the photos of testing the power supply:

Negative test probe attached to grounding bus wire between jacks.
Testing +15 volt supply on voice board.
Testing -15 volt supply on voice board.
Multi-meter display for +15 volt supply.
Multi-meter display for -15 volt supply.
Repair Photos:

The yellow arrow points to the bad capacitors.
New, 10uF, 50 volt capacitors.
Problematic blue tantalum capacitors in the GR-300.
Completed repair with new, 10uF, 50 volt capacitors.
Problematic blue tantalum capacitors.
Back