11 Apr
2012
11 Apr
'12
12:22 p.m.
Darlingtons are not 100% replacements for standard transistors. Among
other things, the voltage drop across the base will be twice normal,
which could screw up the biasing for something designed around 0.7V
drops. There's a reason (well, a few) that they're not used everywhere.
Looking at that page of the schematic, it looks carefully biased.
A casual examination leads me to believe that it's biased to trigger
the discharge of the 555 when its base voltage reaches a certain level
I beleive it's part of the regulation loop of the bias supply. This PSU,
overall, is rather complciated, the part in question is a small (fairly
conventional) SMPSU tthat runs all the time that mains is connected to
porvide power to the chopper control ICs for the nmain outputs, and also
to the control logic.
The associated 555 timer drives the chopper transistor for this little
PSU. Q21 is some part of the regulation loop, it controls the duty cycle
of that 555 based on one of the voltages in the PSU.
(i.e. a crude switching power supply), but I could be
entirely wrong
about that; I only looked for a few minutes. In any case, given how
expensive 1% resistors were in 1978, I'd venture to say that you
probably wouldn't want to upset the bias point by swapping in a
Darlington pair.
I would be very suppirsed it this was originally a darlington, and as
such it shouldn't be replaced by one.
-tony