A view of some of the tone generator cards in the V301H organ
The V301H tone generator card: tuning coil and master oscillator on far end,
six serial divider circuits, and sockets to install into the organ distribution board
The pins in the distribution board receive the V301H tone generator card
|When developing the V301H "California" Continental, Thomas Organ identified a few problematic design issues in the original British V301J Continental. Thomas had considerably more experience designing organs that JMI, and the Thomas Organ 1965-66 V301H Continental Organ redesign addressed these issues. To better understand the V301H circuit redesign, I will explain the differences between the British and American versions of the Continental tone generator cards.
Both the JMI and the Thomas produced Vox Continental organs had 12 individual "tone generator" cards, one for each note of the musical scale.
Each of these twelve tone generator cards had a "master oscillator" and a six stage "divider" circuit.
On the UK made V301J Continental organs, the master oscillator circuit used a pair of germanium transistors, several capacitors and a tuning coil. This was the frequency tunable part of the circuit. The pitch or frequency of the master oscillator is tunable by adjusting the tuning coil.
The output from the master oscillator is fed serially through six slave dividers, each dividing the input frequency to the divider in half. In the British version of the organ, each of these six divider circuits had a pair of germanium transistors and a network of resistors and capacitors to accomplish this task. The output from these dividers provided the tones, divided by octaves, that were controlled by the drawbar circuit of the organ.
The JMI or British tone generator card also had a small adjustable bias potentiometer that was used to adjust the bias voltage to the slave dividers. If improperly biased, the divider circuit malfunctioned.
When JMI first introduced the Continental Organ in 1962, germanium based transistors were the norm. Germanium transistors were not very dependable, as evidenced by anyone in the early 1960s that had a small transistor radio that failed. The germanium transistors were also prone to drift, causing both tuning and slave divider instability. For these reasons, JMI Vox Continentals manufactured between 1962 and 1970 with germanium transistor based tone generator cards tended to visit the repair shop often.
In their 1965/66 redesign of the Continental, Thomas Organ had the benefit of the new technology of silicon transistors. Silicon transistors were much more stable than germanium, and rarely needed replacement. Thomas Organ eliminated all of the germanium transistors from the British circuit in the new US design. The stability of the silicon transistors also allowed the slave divider bias adjustment potentiometers to be eliminated, yet eliminating another service amd dependability issue.
For further ease of assembly, the network of resistors and capacitors used in each of the six divider networks were encapsulated into a flat brown molded capsule called a "couplate"....sort of an early integrated circuit.
The tone generator cards plugged into a ten pin socket on the "distribution board." Unfortunately, it is this ten pin socket that seems to cause the most service problems with the V301H. The solder joints on the distribution board pins that receive the tone generator card have tended to crack over time, losing conductivity and causing partial tones to dissappear. Removing the distribution board to resolder loose pins requires all twelve tone generators to be removed, and the process of removing these will likely damage additional socket pins. I have found that the best course of action in repair is to install a small "jumper" wire from the distribution board to the tone generator to repair this problem.