|Under the left front corner of the organ is the V303J power supply. Vox officially called this the Block 7300 Power Supply Unit. The purpose of the V303J power supply is to convert the local AC mains voltage to the small DC voltages required by the organ.
The power transformer of this power supply featured a 240 volt primary with a center tap that allowed operation at either 240v or 120v. Figure 1, at left, shows the red capped voltage selector plug that manually selects between the 120v or 240v input voltage. A 2 amp mini fuse is mounted below the removable red cap.
The V303 power supply provides two different voltages to operate the organ electronics. To this end, the power transformer has two secondaries, each rated at 12 volts, 5 amps.
One of these two secondaries is connected to circuitry in the power supply that provides a tightly regulated -8.2VDC for operation of the tone generators. To accomplish this regulation, this leg of the power supply includes a traditional M160 diode bridge, several large filter capacitors (C1 and C3, as shown in Figure 3), a zener diode, and a regulating transistor.
The second secondary provides a non regulated +7.5 VDC to operate the bass divider circuitry. This leg of the power supply also employs an M160 diode bridge along with a 5000 uf 15v filter capacitor.
Diagnosis of power supply problems:
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What malfunctions in the power supply unit? If the organ doesn't turn on when connected to local voltage and the switch in the "on" position, the problem could be as simple as a blown fuse.
The fuse lives under the red cap of the voltage selector plug (see Figure 5). If the organ was turned on with the voltage selector in the wrong position, the fuse might blow. Replace with a 2A fast acting fuse and reinstall the voltage selector plug selecting the correct local voltage.
If you find that the fuse blows again when you turn on the power switch (and the correct local voltage is selected), the problem is most likely in the power supply unit.
Unfortunately, the power supply needs to be taken out of the organ to continue with the repair. This requires that the four screws that fasten the board that supports the twelve tone generators, preamp, and vibrato boards must be removed. This board then needs to be slid to the right to provide clearance to slip out the power supply. The power supply itself is held in place with four 1/4" nuts.
It is very important to understand that potentially lethal voltages can be stored for weeks inside capacitors C1, C3, and C4. Unless you are knowledgeable in the precautions necessary working inside power supplies, you should stop here and refer the repair to a professional.
I would suggest the following sequence of parts exchange. Filter capacitors C1, C3, and C4 can easily cause the fuse to blow, and if original, are at least 30 years past their life expectancy.
If replacing these capacitors doesn't correct the problem, I would next replace the OC35 regulating transistor. It is fastened to the case with two screws and requires a mica insulator between itself and the case of the power supply.
If the power supply still blows a fuse, then one or both of the M160 diode bridges has probably failed. I am almost certain that the M160 is an obsolete part. An experienced electronics tech should be able to build a discrete diode bridge to replace the M160.
You might wish to purchase the Vox V303J schematic to help you repair the power supply circuit. Parts values for all capacitors, resistors, and diodes are included in this schematic. Click here to purchase this from North Coast Music.