repairing early HP calcs

[Tony Duell]

> I think for example in the TRS 80 /I  a 7400 with a defective unused
> gate was used because it was cheaper than a working 7400.

I don't recall that, and I've worked on most TRS-80s, and read the 
schematics. Of course there are unused gates that could be defecive.

BUT...

I remember a UK company called Bi-Pak. They bought up defective ICs, 
tested them [1], and sold them. They did sell things like TTL quad-gate 
packages with only 3 good gates. Problem was, as we all found, those 3 
gates quickly failed in use.

[1] The tester was built from telephone relays and selectors. It ran at 
about 1Hz. Plenty of chips passed that test and failed at more reasonable 
speeds.... Don't ask how I found that out.

-tony