[Collins] 51S1 recuperation

[Dr. Gerald N. Johnson, electrical engineer]

Why replace the transistors? If they have proper bias voltages and have
any gain at all they are probably the same as when they were made.
Transistor don't tend to age like the wear mechanism of a tube cathode
losing emission or the tube getting leaky from losing vacuum.
Transistors tend to fail all the way from melting the junction, then
they act like a three legged short and have no gain. Also no forward
bias from base to emitter.

2N647 crosses to an ECG (NTE) 103A. A germanium NPN transistor. 2N637A
crosses to an ECG/NTE 104, a germanium PNP transistor in TO-3 case.
Discontinued.

Because germanium transistors tend to have a great deal of temperature
sensitive leakage, can't take either a lot of voltage or heat, they
aren't in favor these days and makers don't remember how to make them.
Other than maybe one maker of replacement transistors they are gone from
the market. With a little bias change, the 2N647 could probably be
replaced by a 2N2222 and the 2N637B by something in a PNP silicon like
something complimentary to a 2N3055. Might be ECG/NTE 219 or 290.

Germanium transistors need a lot of bias feedback (large emitter
resistor) to work predictably and need 0.2 volt forward bias base to
emitter. Silicon can get buy with less bias feedback and need 0.6 volts
forward bias, base to emitter.

73, Jerry, K0CQ

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.