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