Re: Setup of the Apple 2 16K memory card - anyone know??

[Patrick Schaefer <patrick.schaefer@topmail.de>]

CuteGirl schrieb:

> I have an apple II with a 16k card. The card used to work ONLY after
> being in the computer for an hour or so (to warm up?!) and now does
> not work at all. There is a variable resistor ('pot') on the top that
> can be moved 1/2 turn, labled VR1. 

This is a self-refreshing 16k card, I own 2 of these.

The classic 16k card made by Microsoft grabbed its refresh adresses from
the mainboard. You had to remove a 4116 RAM chip, put it onto the 16k
card and run a ribbon cable from the card to the empty socket on the
mainboard.

Your 16k card has its own refresh counter, therefore the flat cable is
missing. VR1 controls the refresh frequency. You will find an oscillator
(based on 74LS00 or so) that drives a 7493, 74393 or something like
that. Multiplexers (74LS157) feed the counter's outputs into the adress
lines oh the 4116.

Frequency too low-> RAM looses information due to leakage currents
Frequency too high -> System hangs because refresh interferes with
regular access.

Your system hangs when cold and runs when warmed up, therefore I guess
your refresh frequency is too high. The RAM cells get 'faster' when
warmed up.

And at first you should follow Joseph's advice to re-seat all chips. 99%
of all Apple II problems are contact problems within the chip sockets.


Patrick