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More about the GR-100/GR-300/GR-33B Power Supply Failure Repairs:

Introduction
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GR-100/GR-300/GR-33B Power Supply Failure - Replacing Blue Tantalum Capacitors:

As a safety precaution, I strongly recommend replacing the original blue tantalum capacitors. These capacitors, while very popular in the late seventies, are known to fail. Here is a list of typical failing blue tantalum capacitors symptoms:

  • GR-100/GR-300/GR-33B suddenly stops working
  • Unit powers up, but does not make any sound
  • GR-300/GR-33B string LEDs stay on constantly.
  • GR-100/GR-300/GR-33B gets hot very quickly.

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:

Power test Power test Power test
Negative test probe attached to grounding bus wire between jacks. Testing +15 volt supply on voice board. Multi-meter display for +15 volt supply.
Power test Power test Power test
Power supply ribbon block is just in front of the 24-pin connector Testing -15 volt supply on voice board. Multi-meter display for -15 volt supply.

If you get a reading of "0" volts, or anything less than +/- 14 volts, you most likely have a blue tantalum capacitor failure. As a quick check of your GR-100 or GR-300, you can simply unsolder one leg of the capacitor to remove it from the circuit. You can power your synth up and confirm if the capacitor is the source of the failure. As I mentioned, I recommend replacing these tantalum capacitors with newer capacitors, since a capacitor failure can destroy your entire power supply. A great replacement part is any modern, 10 uF electrolytic capacitor, with a rating of 15 volts or higher. Feel free to use a 10 uF capacitor rated at 25 volts, or 50 volts.

Typical replacement part: 10uF, 50 volt general purpose aluminum electrolytic capacitor at Mouser.com.

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!

Power test details Power test details Power test details
Blue tantalum capacitors in place next to TA7179 voltage regulator chip Blue tantalum capacitors in place next to TA7179 voltage regulator chip (GR-100) . Blue tantalum capacitors in place next to TA7179 voltage regulator chip (GR-33B)
Power test details Power test details Power test details
Replacement 10 uF electrolytic capacitors installed (GR-3300) Replacement 10 uF electrolytic capacitors installed (GR-100) Replacement 10 uF electrolytic capacitors installed (GR-33B)

Roland GR-100 GR-300 GR-33B Power Supply Repair