Patrick Schaefer writes ...
> 
> 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.
>

     What neat info! Thanks for posting it.

 
> 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.
>

     Using a 74LS00 in a non-crystal RC circuit is a pretty poor way to get an
oscillator to do refreshing. It's fairly likely that the dumb thing is not
oscillating at all.

     Since individual chips vary as to how well they work as oscillators, swapping
in a new 74LS00 or 7400 or 74S00 could fix osc startup problems. With VR1 being
part of the osc control, a spritz of Control Cleaner could help a lot, too.



> 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.
> 

     Yes; this 'magically' fixes all sorts of computer stuff.



Rubywand