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Simple Drake R4C Receiver Power Supply Modification
By: Ronald Baker / WB4HFN
This modification allow you to upgrade the low voltage power supply in the Drake R4C receiver utilizing the existing circuit board. This eliminates installing an after-market circuit board and replacing the old audio amplifier circuit because of its high current requirements. During the construction you may want to refer the before and after circuit diagram links below to compare. First you will need to remove several components, the diodes CR18 and CR19, resistors R115 and R115 these are the two 5 watt resistors, capacitor C47 and the pass transistor Q2. Next remove the wire from the +150VDC supply feeding the PTO circuit, and remove the white wire from the regulated low voltage source feeding the audio amplifier, make sure you cut the right wire since there several wire at this point. Next make two circuit board cuts as shown in Figure 1. Next install the LM7812 IC regulator and heat sink exactly the same way the pass transistor Q2 was mounted, see Figure 2 for details. Next install a ground wire jumper as shown in Figure 1 to provide a ground path for the regulator, and install the circuit pad jumper between the unregulated voltage supply to the unused circuit trace as shown in figure 1. The unused trace you jumpered to will be the input connection to the filter capacitor, that wire in inserted to the board from the component side of the board.
Next connect the wire you removed earlier which goes to the PTO circuit, this now attaches to the +12VDC regulated output from the regulator. Next attach the wire providing power to the audio amplifier board and the wire from the filter capacitor output, to the unregulated voltage input to regulator as shown in Figure 2. Figure 3 shows the power supply low voltage filter capacitor with the two wires from the power supply board and the 1 ohm resistor between the two filter sections. The last step is to install an additional 2700uf/25Vdc axial lead filter capacitor from the output connection of the LM7812 regulator to ground. This filter capacitor provides extra filtering in the +12Vdc regulated supply to eliminate hum in the audio. This now completes the actual modification, by referring to the pictures and diagrams someone familiar with circuit board work and basic troubleshooting skills can easily figure out this modification. The LM7812 IC regulator and small heat sink are both available from Radio Shack for under $10.00. Since the audio amplifier is no longer connected to the regulated side of the circuit the remaining current draw is small requiring a minimum amount of heat sink. Even though the audio circuit is now unregulated I've detected no difference in the operations or audio quality. The unregulated side of the circuit has plenty of current handling capability so voltage fluctuations with changing audio level is hardly noticeable and offers no change in the audio performance of the radio. If you can not find the LM7812 regulator a NTE-966 is a direct replacement. When selecting a heat sink make sure its small and that the fins do not protrude very far below the bottom of the circuit board because when the bottom tray is in place there is very little space between the circuit board and the bottom of the radio. If the heat sink touches the case bottom this will not present any problems since both are at ground potential.
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