[Radio Wrangler]

DO NOT as in NEVER EVER try forcing a knob on an 8640. The plastic of the knobs and the gears behind them are polycarbonate and have gone brittle.

Dismantle the thing so you can get in, clean and lubricate.

The tuning knob oterates a special screw. The nut is a nylon piece made to have no backlash by a spring pressing the segments of the nut onto the screw. This drives a piston to and fro in a stainless steel cylinder.

The end stops for the screw shaft are done with a stack of plastic discs on the shaft with overlapping protruding segments. At each end of travel the protruding segments hit each other and stop the shaft turning. Turn in the other direction and you get about 300 degrees rotation per disc before the segments hit in the reverse order, forming the other end stop. There's a conical lump of diecasting supporting a reamed bush around this mechanism. Gears off of the main tuning shaft turn a pot. The pot triggers switching of the filters that give a sinewave out of the divider chain. The basic oscillator runs 256 to 512 MHz. A long chain of divide-by-two stages creates the lower bands. Each divided band has to be split in two for a pair of filters in order to get harmonic removal at the band ends.

There is a plastic differential gearbox driving the deviation range attenuator, by subtracting the angle of the freq range switch (which sets the divider factor) from the angle of the deviation knob on the front panel.

Very nice generators once going. Keep the room warm, too. Beware of knocking the counter reference int/ext switch on the back panel.

David