Excerpted from an email from with Bob Pease, in his own words.


*** Most  choppers  got  retired  when working well. If they failed, they were tossed out long ago, so you are unlikely to find a bad one!


*** There  are  several things  that  can go bad  with choppers:


- -  After  several years, they can wear out and really fail.

- -  If overdriven at 8 Vac, they can hammer the contacts into bad shape, and wear off the gold plating.

- -  Even when new, a poorly designed  chopper can go  "open" or  

intermittent  due to its poor metallurgy  for  working in "dry"  

circuits  with i <<  1  UA. - -  A chopper  can have  bad  phase  shifts between the 2 throws and  

fail to demodulate  properly. Not a problem at Philbrick.

- -  It can pump out a lot of current, in a clean null circuit.  The best ones  were better than 10 pA;  poorer ones  went  above  40 pA.

- -  The better ones had low dc offset voltage errors, below  5 UV.  

Low "triboelectric" effects.

- -  The good ones had low induced (magnetic) noise,  due to a good layout. 

At Philbrick, we liked  the expensive Airpax ones  the best, except  

when they "went dry", and  drove  our  customers  crazy.

(Think of a nuclear power plant  where a lot of alarms  go off, and everybody marches  to the exits. Is it  really a runaway reactor - or just another bad chopper?)


We also used a lot of Oak Choppers,  and  we  experimented  with CSI  

(Cambridge Scientific) in Cambridge Maryland,  a splinter off of Airpax.  


Choppers were used in  vacuum-tube amplifiers such as  K2-P and -PA, and USA-3 and USA-4 amplifiers.

They were also used in solid-state  amplifiers such as SP-656, SP-456,  


Note that originally, choppers were designed for use as a sub-miniature "vibrator" in a car  radio, to boost the 6 VDC  up  to 90 volts.  There was  never any problem with "dry" contacts in that work.


And   remember  what George Orwell said in  "1984"....

"Here comes a candle to light you to bed;

"Here  comes a chopper to CHOP OFF YOUR HEAD!"     



Bet regards. / rap