Re: Apple II: another project

["RedskullDC" <RDC@oz.org>]

Hi Dirk,

"spacealbum" <dbackenkoehler@t-online.de> wrote in message 
1147164943.904558.84010@j33g2000cwa.googlegroups.com">news:1147164943.904558.84010@j33g2000cwa.googlegroups.com...
> Hello from Germany,
>
> encouraged by Nama's great project, I decided to do some research an my
> dead Apple II again. I bought a II europlus as a donor and started to
> swap the ICs to see if I can trace some defective ones as well
> (cleaning alone didn't help)  In the end it turned out that two of them
> are defective!!!
> I could not test the two little 555`s though as they are souldered to
> the RFI board - btw. the two ICs on the original II read 455, might
> this be a problem? - and I couldn't test the two 8T28s, because Apple
> changed them on the RFI board).
>
> Changeing the two dead ICs I have the follwing situation on my Apple II
> (Motherboard Rev. 800-0001-04, Monitor ROM, Integer Roms):
>
> When I power it on, the power light on the keyboard goes on and the
> computer beeps (it didn't beep before swapping the ICs). If I plug in
> an autostart ROM and a disk interface, the disk drive starts to wirl
> and a the computer loads the disk. If I put in a DOS 3.3 disk the
> computer seems to load it, and I can reboot typing in PR#6.
> But unfortunately on the screen I only see 40 horizontal bars, I can
> move the corsor around in between these line, but I see no characters
> ;-(
> After 30 seconds, the character generator ROM at A5 (that works fine on
> the Plus) gets terribly hot so I have to switch of the machine.

If the Character Generator ROM is a known good part (as you say, works
in another machine), I wouldn't mind betting that the 74166 shift register
at location A3 is holding one or more of it's output pins high or low.

Try a logic probe on pins 4,5,6,7,8 of the Char gen rom to see if
any are being held high or low. In normal operation the should be
changing state constantly.

This would account for the extra heat being generated.

Cheers,
Red