From: phil_a@tpg.com.au   
      
   "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   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)