General Radio Type 502G Vernier Dial. It's 2 3/4" diameter,
and marked 0-100 over 270 degrees. Probably sold with a bushing
for the pinch roller shaft, but missing now. Sold new for $1.25,
later raised to $1.50.
Vernier dial drives have been made in a number of styles. Here are some examples from my collection over the years.
General Radio Type 502G Vernier Dial. It's 2 3/4" diameter,
and marked 0-100 over 270 degrees. Probably sold with a bushing
for the pinch roller shaft, but missing now. Sold new for $1.25,
later raised to $1.50.
Here's one style, made by Jackson Brothers, in England. That's a
quarter in the foreground, for comparison. The dial or pointer is
mounted with a standard 3/8" nut. This style is not found on their
current web site. About 10:1 reduction ratio. Open with a spanner wrench
on the front in the two slots or by grabbing outer edge of large mounting
ring with pliers while gripping back end of case in vice using only enough
pressure to hold, not to deform case.
Jackson Brothers Type 5620 Adjustable Torque Ball Drive. The screws and
springs allow adjustment of desired tension, and disassembly for
cleaning or lubrication. The dial or pointer is attached with two
small screws. 6:1 reduction ratio.
Jackson Brothers Type 4511DAF Ball Drive. These are very common,
often found stacked in dual ratio knob sets. About 6:1 reduction
ratio. While not intended to be user servicable, some have reported
opening them for cleaning and relubrication, and adjusting tension
by bending the assembly tabs.
This style, manufacturer unknown, I first saw as a teenager in the 1950's. It can be completely disassembled for adjustment of desired tension or cleaning and lubrication. It can be assembled with mounting ears forward or to the rear. About 5:1 reduction ratio.
National Company Velvet Vernier dial. National made a number of
different dials based on their drive reduction mechanism, some with
calibrated knob skirts, as shown, some with pointers, paper scales
marked by the user, and frames. About 5:1 reduction ratio.
Here's another unknown, removed from a homebrew audio oscillator
with mostly Japanese parts, so I presume it's from Japan. Complete
with cracked glass, but missing any sort of calibrated scale or
pointer. Nice black crackle paint on the bezel, which is about
1/2" (1.27 CM) deep, and more professional looking than the National
dials with frames. About 8.5:1 reduction ratio.
Here's a variable capacitor with the reduction drive built in.
This has a 5:1 reduction ratio.
Here's an example of a pointer used with the flat flange of a
Jackson Brothers Ball Drive.
A pair of variable capacitors with built in reduction gears as well
as an additional ball drive. The reduction gears provide about a 2:1
reduction. The small variable cap requires 6 turns of the shaft to
move the VC through 180°. A dial attached to the ball drive flange
would have the calibration spread over about 360°. The ball drive
on the dual gang VC is not the usual 6:1 reduction; it only takes
about 3½ to turn the VC through 180°.
Copyright © 2003 John Kolb Last revised July 5, 2003