I wonder if someone else who has one of these beautiful beasts can tell me
how many turns their Dial Control for capacitnace bridge balancing takes
from stop to stop. Mine is just under six and I suspect it should be 10 as
I don't get a complete drum scale. There were signs that the drum had been
out and I have it out now but don't want to screw anything up. Any tips
appreciated.
73
Hank WD5JFR

I had to trash oklahoma.net because of poor cus svc and am trying SBC. For
several hours the cus svc from India was horrible but on the seventh attempt
I got to Austin TX and 15 minutes I was up and running. That SBC outsourcing
cost me 3 hours.

Hi

Thanks to Larry and Steve for checking and telling me that it takes 9 revs.
.. I took the drum out and apart yesterday and determined that it would do 9
turns, there's ten tabbed washers.. The dial drum mechanism is well made
and quite precise. Someone else had had this unit apart and screwed up the
timing on the washers. My crank had some binding that wouldn't go away. I
disassembled the drive unit and found some galling in the rear bearing.
After cleaning out a few brass filings, smoothing the rough spots inside the
bearing and applying a very light coat of grease it didn't bind and is
velvet smooth for the whole nine turns. The worm drive is well made with
dowel pins for alignment and a ball bearing thrust washer to adjust. Since
I moved both the capacitor and Helipot I now have to figure out if I can
recalibrate it. I think I've got the Helipot part thought out at least, but
I need more time on the capacitor. Since it's a ten turn pot I assume that
250 ohms must be left at each end and the drum centered at 2500 ohms. On
the lowest range, 5 to 100 pF, the Helipot isn't used to balance the
bridge. A 15 to 150 pF variable capacitor paralleled with a 1.5 to 7 pF
trimmer is used. The substitution method is used and I assume that the dial
is calibrated so that the unknown capacitor and the variable capacitor add
up to 200pF so that when the bridge is balanced the ratio of the fixed arm
at 2000 pF is ten times the sum of the unknown and the variable. The other
leg is a 75K and 750K fixed resistor giving a ten to one ratio. Any tips
appreciated.
I originally thought the 5K Helipot was bad because when I checked it
with my Fluke 87 the resistance of the unit appeared erratic and jumped as I
turned the wiper. (I only had 6 turns too!) After I got the drum out I put
50 volts on the Helipot and the voltage varied smoothly and didn't jump.
Had I better eyes or watched my Fluke more closely I would have noticed that
it changes scale automatically and randomly somewhere before 500 ohms and
starts readings in decimal kilohms. So at say 398 ohms it jumped to 0.56 K
ohms as I turned the shaft. I'm relieved to find that the Helipot, a SC-234
is good because I couldn't find anything on the Internet as a replacement
and it has the shaft at both ends!

73
Hank WD5JFR