Re: Apple II plus hardware problem

[Rubywand <rubywand@swbell.net>]

Allen Bong writes ...
> 
> Hi All,
> 
> My Apple2+ recently developed an intermittent problem.  When I switched it
> on, It went straight to low res graphic mode and the whole screen was
> scrolling up accompanying with the constant beep sound.
> 
> I checked with a logic probe and found F14 74ls259 pin 4 to pin 7 were all
> L, so I suspected that F14 chip was bad.  I changed the chip and the fault
> disappeared for a while and as soon as I plugged in the keyboard the same
> fault came back again.  I'd removed all the cards plugged in the slots
> when I did all the test.
> 
> Is there anything other chips that would caused this problem ?  I'd also
> soldered the 6 eproms directly onto the PCB as the sockets
> were giving other
> problems last time.  Please help me if you'd any clue for me.
> 
 ....

     Check pin 15 on the 74LS259 at F14. If it is low, the problem may be at
the 74LS00 at A2-- it may be poorly socketed or need to be replaced. Other ICs
you could try pulling and resocketing-- with power Off-- (or replacing) are the
74LS138 at F13, the 74LS02 at B13, and the 74LS174 at B8.

     In case the above do not help, a different approach is to consider that
you are getting a lores graphics display because the display-related hardware
is being told to give you one. The 74LS257 IC's at B6 and B7 could be poorly
socketed or defective. Or the 6502 may need to be pulled and resocketed.

     Beyond the above, a standard fix is to pull and resocket each socketed IC.
(It's a good idea to do this in a low-static environment.)  During this
process, if you notice that an IC has really groady looking pins-- like they
are black with corrosion-- you can do some light sanding to clean them or dunk
the IC in Tarnex.



Rubywand