|Under the orange lid of the organ, near the power supply, is the preamp printed circuit card. It has ten contact pins and is fastened to the organ with one screw.
The preamp board is comprised of two circuits. The first circuit of these circuits preamplifies the output from the fundamental and overtone drawbars before returning the signal to the input of the black "flute" (foundation) and "reed" drawbars.
The output of the foundation and reed drawbars are then fed into the second stage of the preamp. In this second stage of the preamp, a series of capacitors and resistors are used for "foundation" and "reed" tone shaping. Finally, the organ signal is passed through another stage of preamplification before heading to the output jack.
A circuit board style potentiometer allows the output level of each keyboard in the organ to be adjusted to avoid input overload distortion.
Diagnosis of preamp problems:
The Vox Showroom and North Coast Music accepts no responsibility for personal injury or damage to property while using this information to attempt an organ repair. Proceed at your own risk.
What malfunctions on the preamp boards? The most likely culprits are the six electrolytic capacitors on each preamp board. Two of these are 100 uf (the light blue devices in the photo at left) and four 25 uf (these say 25/25, Ducati), all rated at 25 volts. Electrolytic capacitors typically have a life expectancy of ten years, so forty plus years after their original installation on these circuit cards, these are the prime candidates for replacement.
The capacitors needed for this repair are commonly available at a Radio Shack or Tandy. You may substitute a 22 uf capacitor for the 25 uf part originally installed in the crcuit boards. The replacement capacitors will probably have a 50 volt rating, but that will not be a problem. Please observe the location of the + end of each capacitor and install the replacements the same way. The cost to replace all six capacitors will be around $5.
If changing these capacitors does not fix the problem, then one would next look at the two germanium transistors as the other part most likely to fail. A shot of "freeze spray" or "component cooler" (again available at Radio Shack for about $10) on each transistor should reveal which of the two transistors is bad. Either may be replaced with an NTE 102A transistor.
You might wish to purchase the Vox V301 schematic to help you repair the preamp circuit. Parts values for all capacitors, resistors, and diodes are included in this schematic. Click here to purchase this from North Coast Music.