On 17 Jan 2010 at 18:40, Tony Duell wrote:
3) There is resistor in parallel with the
MITS did this routinely and it wouldn't surprise me if they did this
on the 680. In particular, I remember that the +5 supply for the 4K
Oh, right... The OP says he has the schematics, do they show any such
DRAM boards had this dirty little trick. Anything, I
shave a few dimes off the manufacturing cost.
I came across it in the HP59500 HPIB interface for the multiprogrammer.
This is basically a unit which send/receives ASCII strings over HPIB and
has 16-out, 13-in TTL level interface to the multiprogrammer.
It's a large board of TTL (something like 70 chips) and a little PSU
board giving the necessary 5V (that's the only supply it needs). The PSU
board has 2 TO3 componnents on it, one is a regulator (I forget it it's a
fixed of adjustable one) and the other is an SCR  for the crowbar
circuit. Anyway, there is a reistor shunted across the regulator to carry
most of the load current. If you try to test that supply without a dummy
load oe with too small a load, the crowbar will fire and the resistor
gets hot and bothered.
 We (UK) often call that component a 'thyristor' by analogy with
'thyratron' (a gas-filled triode or more complex valve, line the 2D21
(EN91), etc). Is that name recognised elsewhere?
Everything's fine with a full load, but if the load should drop for
any reason, any IC connected to the regulated (?) line were toast.