A Vox Continental from mid 1963
Electronics (mostly)
For general orientation: at back the twelve note generator-divider assemblies (finished in silver/grey Hammerite paint). The detached assembly on top is the power section / vibrato board. At right, the drawbar assembly.
The main purpose of this page is simply to provide an overview of the electronics of the organ. Some of the material will necessarily overlap with that published and illustrated elsewhere - but it seemed useful to document it nonetheless, for the time being under fairly general headings.
Further details will be added in due course. General views of the organ can be found here.
Drawbar assembly
Designed and patented by Derek Underdown and Geoff Harris of JMI. The footings at left (four drawbars) are: 16' pitch; 8' pitch; 4' pitch; Mixture IV (reeds). The two drawbars at right enable relative volumes to be controlled: "Foundation" and "Mixture".
The JMI circuit diagram is OS/030. The copy generally circulated, drawn by Mike Turner, bears the date 2nd April 1963.
The two 100uf capacitors on the rear board are, in the picture above, new. The originals had slipped upwards to 200uf.
Power and vibrato section
The JMI circuit diagram is sheet 0S/028, drawn by Mike Turner, with changes noted at its top right. ISSUE 1, 28th February 1963: - further filtering added; ISSUE 2, 21st March 1963: - two resistors added, three deleted; ISSUE 3, 21st May 1963: - change to the value of a resistor on the vibrato board; ISSUE 4, 25th September 1963: - no changes.
The organ has the changes introduced in ISSUE 1 and ISSUE 2 (February and March), but not that of ISSUE 3 (21st May). It should perhaps also be said that the dropper resistor (R1) for the zener diode is not 270R as marked on OS/028, but 200R. In practice this makes little difference.
The zener, which clamps the HT voltage at -8.2v, is an early germanium part (part number 1S7082B). The one in the organ below has since been replaced though, having slipped to -9.5v.
The power section contains the latest date stamped inside the organ - 8th May 1963, once on the assembly itself, again on the aluminium-topped bridge rectifier.
The Continental power section assembly. On top the Mullard OC35 transistor and two preset controls for the vibrato (speed and depth). Early power sections have simple switches to select the required voltage (115v/230v) and paper handwritten stickers. Later on red plastic voltage selector plugs (as used on the Vox AC4 and Domino amplifiers) were introduced.
A general view of the power section and vibrato board, a new zener diode in place (see below for a picture of the original). When the new voltage selector plug was introduced later in the production run, it was positioned behind the mains transformer (to the right of where the switch is in the Continental above). The switch was done away with and the vibrato board lengthened to 15 tags. These changes by serial number TC1094.
Date on the power section assembly.
The value of these early zeners was indicated by bands of colour, much as a resistor. The bands on this one are: grey (8) and red (2) for 8.2v. The pots are nicely encrustulated with rust externally but work perfectly well.
The main filter capacitors are T.C.C. and Radiospares. The one on top is the 1000uf cap. intoduced in ISSUE 1 of the circuit diagram (February 1963). The date code "UA" indicates that it was made in January 1963.
Vibrato board. Note the presence of the 100R resistor between the small grey Radiospares capacitors. ISSUE 3 of the circuit diagram (21st May 1963) prescribes the new value: 200R.
Note generators / dividers
The assemblies are supported at the rear of the case by a wooden runner and at foot by a metal panel that contains pass-through connections to the keyboard. The metal sleeves and circuit boards are stamped 29th April 1963 and 30th April.
In terms of the complement of capacitors employed for voicing, the generator-dividers fall into three groups: (i) those for C-D sharp; (ii) E-G; and (iii) A flat to B. The circuit boards were numbered (on paper stickers) - "1", "2" and "3" - to ensure that the units were mounted in the right order. The musical note handled by the assembly was written on a further sticker placed on top. In late 1963 JMI had circular black stickers printed up
It may be worth setting out the values of the three series of capacitors at some point. The JMI circuit diagram - sheet OS/027 (first version, February 1963) - does not specify them. An asterisk refers the reader to a further sheet.
In terms of manufacture, signal capacitors are a mixture of Hunts (only the 0.04uf coupling caps), WIMA, and Mullard mustards. The Mullards have date codes ranging from A2N (first quarter of 1962) to A3N (first quarter of 1963). In late 1963 / early 1964, JMI dropped WIMAs in favour of red Hunts resin-dipped caps.
The 100uf capacitors required by the boards are gold-coloured Dubiliers, date code "TE" = May 1962. The sliding resistors for the setting of bias were made by Egen of Canvey Island. JMI had used Egen-made potentiometers from the 1950s.
A number of assemblies removed to show the wiring looms and metal panel with pass-through connections to the keyboard. The springs for the keys can be seen above with their tensioning plate.
A detail of four assemblies with their hand-written paper stickers.
The rear of one of the number "2" circuit boards - for notes E-G.
One of two generator-divider assemblies stamped "29th April 1963". The other ten have "30th April".
One of the clearer stamps on the circuit boards.
Detail showing the red-print 2200pf WIMA capacitors used in the dividers (bypass).
A WIMA still with its +/- 1% tolerance sticker.
Wiring looms
More will be said on a forthcoming page, just to note that the wiring looms in the Vox Continental are simply works of art. Below, a shot taken underneath the keyboard - the other side of the pass-through sockets can be seen in the first picture in the previous section.
Some of the looms were produced at the West Hill Hospital, a short distance away from the Dartford Road Works.