Altair 680 power supply...

Tony Duell ard at
Sun Jan 17 13:42:48 CST 2010

> On 17 Jan 2010 at 18:40, Tony Duell wrote:
> > 3) There is resistor in parallel with the regulator.
> 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 suppose, to 
> 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 [1] 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.

[1] 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.



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