[Phil Allison[_3_]]

"patrick-turner"

In a previous post about my radical surgery performed on an
BWD Electronics 141 oscilator, I mentioned I had troubles
with the original Compton pot used to alter the F within each
range. The Compton is a wire wound item with adequate
power ratings, and it reputed to work smoothly without Vo
jitter or bounce, and with linearlity between DC and at
MHz. But I found it performed with jittery Vo for the parts
of the ranges where it did work, and for part of each range
the oscillations stopped or went wild. I decided the pot was
terminally sick, so I replaced it with 12 pos make before
break switch.

I re-examined the 2 gang pot, and although it looks impossible
to separate the two sealed pots, but the outermost has a metal
which can be easily prised off. I plotted the R values for each
and soon found a region where outermost pot had an open
connection between wiper and resistance wire track. Both pots
measured similar R where the wipers of both did connect to the
their tracks.

I was lucky the sick pot was the only one that seemed accessible
I found some grease had been used for lubrication, but no sign of
any corrosion or fault in the wiper operation and all mechanical
aspects looked brand new. BUT, sure enough, wiper contact
wasn't being made at a small length of the track, so I cleaned the
track + wiper without damaging the hair thin wire. I thought a bit
more use might dislodge the tiny spot of hard oxide that may exist
on the resistance wire track. No luck, and I gave up after an hour
or two, and felt vindicated that I'd replaced the darn pot with the
best thing I had laying around.
So when Phil says the Compton Pot is a blessed marvel that
enabled superlative oscillators to be made, it could never have
been alwats true, because pots can become defective, or maybe
even be defective when they are sold.

** You have seen one faulty WW pot in a BWD141 - probably the only faulty
one that ever existed.

" I was cleaning up my workshop when I discovered I had been given an old
BWD Electronics
141A audio oscillator which had many problems. "

There is a big clue there ....


Wire wound pots can be notoriously jerky with Vo levels.
But OK when not being moved, when they are then usually noisless.

** I have always found WW pots to be highly reliable and long lived.

1. The voltage control on my most used bench PSU has a 10 kohm WW pot made
by IRH in Sydney, Australia - been in daily use for nearly 30 years, still
works perfect.

2. The two fine adjust controls on my Wien bridge notch filter are 200ohm 10
turn WW types - allows up to -100dB notch depth at 66, 996 and 5855 Hz.
They are 30 years old too, got them the same time I bought my Sony CD101
player.

3. I have two Compton dual gang WW pots, same as the one PT is so down on.
One is 40 years old and the other 24 years old. They both work perfectly.

To overcome the issue with fine frequency adjustment, I fitted 500ohm, 10
turn WW pots in series with one of the sections of each Comptom pot - which
is 2 x 15kohms. Makes it easy to set any frequency up to 50kHz, +/- 1Hz.

Stability is good enough to work with the -100dB notch filter, which at 996
Hz has a notch width of about 0.02 Hz.



..... Phil