1. Remove the four black screws and plastic washers holding the front panel to the radio frame. Remove the front cover by gently pulling it away from the radio frame. Watch out for the wires going to the speaker and microphone.
2. Remove the OPTION switch by unscrewing the tiny black screws mounting it to the bottom panel. Cut the white wire that goes up from the switch to/ the top of the board, and unsolder or cut the resistor going from the switch to the grounded transformer can nearby. The removal of the OPTION switch makes room for the installation of the ten volt regulator.
3. Locate a 7810 three-terminal regulator and bend its heatsink to fit against the bottom panel while the regulator sits parallel to the front panel. This means that the heatsink will have a 90 degree bend. Attach the heat- sink to the bottom panel -- either with a tiny machine screw and nut, or by loosening the other hardware on the bottom panel, and slipping the heatsink between the layers of sheet metal. Then retighten the other hardware. I found that some versions of the 10 volt positive regulator come with a very thin heatsink that makes this sort of installation possible. The usual 7810 regulator heatsink is too thick, and screw/nut mounting is the only way.
4. Locate the orange wire running from the center pin of the external power jack (with diode installed) to the power switch. Cut this wire where it passes the new regulator, and solder the end from the power jack to the INPUT terminal of the regulator.
5. Install a diode (1N4003 or higher) on the OUTPUT terminal of the new regulator, anode toward the regulator (arrow pointing away from regulator, in other words). Now attach the other end of the orange wire (going to the power switch) to the cathode of the new diode.
6. Nothing need be attached to the GROUND pin of the regulator if the heatsink is grounded well. If in doubt, add a wire from the GROUND pin of the regulator to the spot where the OPTION resitor was grounded to the transformer can.
How it works... The old diode on the center pin of the power jack protects against accidental application of reverse polarity. The incoming 12v goes to the regulator and is dropped to 10v. The new diode drops the 10v down to 9.3 volts (ideal for the radio), AND blocks drainage of the internal batteries by the new regulator.
Steve Johnston, WD8DAS
September, 1990
All rights reserved to Steve / WD8DAS
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