HP info and corrections

[Tony Duell]

A few things relating to HP machines, particularly the 9845 that I'm 
working on.

Firstly, a correction about dismantling the tape drive (this also applies 
to the 9825 drive that Sellam was working on). I mis-remembered how the 
motor is fixed -- the screws don't go in from the bottom, they go in from 
the top (cartridge side), go through the chassis and tap into the plastic 
disk that holds the motor. 

Two of them are obvious, where you'd put the cartridge. To get to the 
other two, you have to remove the head (outside 2 screws on the mounting 
block, this shouldn't muck up the alignment) and remove the microswitches 
(tape inserted, write protect), again 2 screws, but the actuating pins 
fall out, so be careful. When you've done that, it's easy to remove the 
motor.

Secondly, the keys do seem to work as I dsecribed, with one correction. 
There are 2 lopps through the torroid, and there is a magnet that affects 
the magnetic properites of that torroid, and thus the coupling between 
the wires. But the magnet is normally near the torroid and moves away 
when you press the key. Makes sense, I would have guesed that the magnet 
saturated the torroid and reduces the coupling between the loops, so this 
way, uou get a signal when the key is pressed.

Thirdly, if you're working on the keyboard encoder board, note that the 
keyboard controller chip (with a custom number on my board) seems to have 
the same pinout as the chip in the HP250 (schematics on bitsavers). The 
LM311 comparator (8 lead TO99 case) is nothing to do with detecting the 
return pulse from the key matrix, it's used as a relaxation oscillator 
for the contoller clock. The return signal is detected by a couple of 
transistors in the CA3046 array (long tail pair circuit).

Finally, I've typed out the DIO bus pinout from that HP9826/9836 Pascal 
system manual that's on bitsavers (this being the piece of info that's 
going to be the most useful to me at the momnent). If anybody wants this 
(a plain text file) to avoid downloading/reading the whole pdf, I can 
post it here.

-tony