The V301E Continental - Bias Pot Adjustment/Replacement for the Divider Circuit

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How to: Bias Pot Replacement
Step 1. Usng a solder removing bulb, heat each solder joint on the old bias pot and suck the solder out of the joint. You need a soldering pencil of 40 watts or less for this job. Don't try to remove the old part until you have removed the old solder, you will be likely to damage the circuit board if you don't do this operation first. The photo above shows a V303J tone generator, but the process is the same on all Vox Continental organs.

A view of the bias pot area of the tone generator after the solder has been removed.

Step 2. A gentle pull on the pot should remove it from the circuit card. If it doesn't come out easily, reheat the three solder joints a second time and use the desoldering bulb to further clean the joints.

Step 3. Press the new part into place. Bend the tabs over slightly on the foil side of the board to form a mechanical connection to the circuit board prior to soldering

Step 4. Solder the pot. After reinstalling the tone generator into the organ, readjust the bias pot as described on the right side of this page.

If your V301E Vox Continental has keys that have tones that seem to be an octave or more too high, the symptom will likely be caused by one of two malfunctions. These are:

1) A bias pot adjustment is required for the tone generator divider circuitry
2) There are one or more defective transistors in the tone generator divider circuitry

I will attempt to address both of these subjects in simple terms, and you might even be inclined to attempt some of these repairs yourself. However, the Vox Showroom and North Coast Music accepts no reposnsibility for personal injury or damage to property while using this information to attempt an organ repair. Proceed at your own risk.

The simplest, and most common repair to a British or Italian made Continental involves an adjustment (or replacement and adjustment) of the tone generator divider bias pot. This bias pot is normally located on the end of the tone generator card opposite of the ten connector pins. The bias pot is normally adjacent to the mounting screw that fastens the card to the organ. It is the component shown at the bottom of the photo at left, and the bias pot may vary slightly in appearance from the photo.

To diagnose the problem, remove the orange cover of the organ. Next, turn on the organ, connect to an amplifier and pull out only the 16' and ~ drawbars. All other drawbars should be pushed fully in. Starting with the low C, which is called "C1", test each note from the lowest to the highest key of the organ, which is called "C5". Listen for any note that seems to be an octave or more higher than adjacent notes. This symptom points to a problem in the divider circuitry, and this problem may be corrected by a simple bias adjustment.

Make a note of any key that seems to have a problem and find the corresponding tone generator card for that note. Each tone generator card is labeled for the note is designed to produce (see lower photo below).

While holding down the problem key, use a small screwdriver to rotate the bias pot for the related tone generator circuit card. As you rotate the pot, you wil hear the pitch jump up and down, by octaves. When properly adjusted, the correct octave tone will play on the key that you are depressing. No electronic meters or electrical measurements are necessary, the divider bias pots are adjusted by ear.

If the bias pots in your organ are still the original factory parts, odds are that they have outlived their usable service life. In many cases, simply replacing and readjusting the bias pots fixes divider problems. North Coast Music sells replacement Continental Organ bias pots, and it is generally a good idea to replace these for the most stable adjustment to be made.

North Coast Music offers replacement tone generator bias pots for UK and Italian made Continental Organs. Click on the link below to purchase.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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