Partially-bad chips; was: repairing early HP calcs

[Tony Duell]

> 
> The ZX Spectrum actually has jumpers on the board to specify whether 
> you're using "top half bad" 32Kx1 or "bottom half bad", for the upper 
> 32k.  Of course the ones that always went were the bloody awful 4116es.

There was nothing wrong with the 4116. The problem was that that chip is 
known to fail if it loses the -5V bias supply (this is documented in the 
data sheets, which, IIRC, give a supply line sequencing order), and 
Sinclair (as usual) really cut corners with the -5V supply. It tended to 
fail (or at least fail to start up), and the DRAMs failed

I must have well over 100 4116s in running machines here, and I think 
I've had to replace exactly one of them.

-tony