Tab Mechanism

The two sets of preset tabs have a mechanism which prevents two tabs being selected at once. This is made from various brass levers and pivots, and is located immediately behind the tab switches.

This mechanism requires lubrication, otherwise the tabs will stick or be very stiff to operate.

Dismantling the mechanism is the best way to apply grease. Only a thin film of grease is required, but it should cover all of the moving parts. Electrical contact grease was used, same as for the drawbars.

The set of images shows the assembly sequence. First, the three small brass levers are placed on the pivots. Two round brass spacers go on the two pivots near the bottom of the picture; note the left spacer is thicker than the right one. A third spacer goes on top of the rightmost brass lever.

In the second image, a larger brass lever is placed over the right and centre small levers.

The third image shows the second large brass lever which covers the rightmost small lever and the centre section of the previous large lever. Two additional spacers are added, these can be seen to the left of the picture.

Finally, the top lever goes over all of the others. Two small brass tabs secure the whole assembly in position. These should be tight enough to hold the levers down, but still allow the parts to move easily when the different preset tabs are pressed.

The Vibrato/Chorus tabs have a similar mechanism, although this is simpler, covering three tabs rather than four.

Vibrato repaired

After some investigation, the failed vibrato was a single 1M resistor which had gone open circuit. This resulted in the vibrato preamp not working at all.

It was possible to replace the resistor without removing the amplifier from the organ, and the vibrato now works perfectly.

Tab contacts cleaned and lubricated

Other items which have been done are the cleaning of the contacts on the tabs. This picture shows the rear of the tab assemblies, the top of the organ has been removed and the frame holding the tabs lifted up and tilted to allow access to the drawbars.

The switch contacts are clearly visible behind the white plastic tabs. The drawbars are to the right of the picture below the 3 white tabs, and the round can object is the variable capacitor for the pedal drawbar.

The contacts are easily cleaned using a contact cleaner spray. Note there are various types available. The first type to use is one which does not leave any residue. This will dissolve any grease, oxides or other dirt on the contacts. A compressed air spray will remove any dust and dirt.

After cleaning, it is essential to apply some lubrication in the form of a contact spray which leaves a lubricant behind. Without this, the contacts will wear away quickly. This must be a lubricant designed for electrical contacts.

The tabs where only one can be selected at once have a mechanism behind them made from various brass levers and pivots. This also needs lubrication, otherwise the tabs will be very difficult to operate. Electrical contact grease is ideal.

The drawbars have also been cleaned and lubricated in a similar way.

Reverb Amp

As mentioned earlier, the reverb amp has a distinct mains frequency hum when either or both of the reverb tabs are on. Further investigation reveals this is from the amplifier itself, rather than any of the associated wiring, reverb tank or the controls.

This will either be a bad valve (tube), or old and faulty capacitors. The electrolytic capacitors are most likely, given they are nearly 45 years old and well past their expected lifespan.

I'll be replacing these shortly, and also replacing the caps in the main amplifier as well.

While the main amp is out for capacitor replacement, it should be possible to fix the faulty vibrato as well - probably a failed resistor.

Further problems

Now that the generator is practically silent in operation, certain other faults are noticeable.

There is a 50Hz hum on the Reverb section, only present when one or both reverb tabs are on. Clearly picked up from the mains supply somewhere so probably a poor connection or failing capacitor in the reverb amp or reverb circuit.

The Pedal drawbar is noisy when moved, probably just dirty contacts.

A couple of keys on the upper manual have intermittent contacts, only apparent on some drawbars. Busbar shifting may fix this.

In addition to these minor problems, the Vibrato system has almost completely failed. With vibrato selected the volume is dramatically lower than normal, and serious distortion occurs when two or more notes are played. This is probably a bad valve (tube) or contact somewhere.

The scanner appears to be working as when playing just one note the sound is correct although much quieter than it should be. (The note is barely audible even with the 'volume soft' tab off and the swell pedal wide open).

The vibrato was working properly only a few days ago, so this is a very recent failure. It's not over oiling either, since only a small amount of oil went into the scanner tray, and the scanner is still working.

Unfortunately, failures like this are inevitable with 40+ year old electrical equipment.

The next repair will be to clean and lubricate the tabs and drawbars once some contact cleaner and lubricant is obtained.