Troubleshooting a 5V Apple III

[Tony Duell]

> Understood.  Thanks for the explanation.
> 
> I'll check the PSUs with the multimeter tonight.

It never hurts to be sure of something like this :-)

> 
> There's a set of Apple III schematics here:
> 
> http://apple3.org/iiischematics.html
> 
> I appreciate your assistance.

OK, I managed to take a look at them...

Assuming you can get a logic probe, my first test would be on the reset 
circuit -- say on the rest pion of the 6502. Since you mention the 
machine does something, I would be suprised if it was being held in the 
reset state, but I've been cuaght by that once (a leaky capacitor in the 
reset circuit..). So again, it's best to check.

Then look at the 'timing' schematic (there's one with a name something 
like that anyhow). That seems to be where the video sync signals are 
produced, the CPU clock, etc. Make sure they are. 

Another word of warning (*becuase I've been 'bitten' by this). A logic 
probe will tell you if a sighnal is doing something, but in general it 
won't tell you what it's doing -- the relative timing with other signals, 
etc. I onve had a fault where a 16 bit shift register (made of 4 4-bit 
shift register chips) had only the first 4 bits changing. So I assumed 
the second chip in the chain was dead, and thus not responding to the 
input from the first chip, and not passing anything on to the next chip i 
nthe chain either. Well, I changed it, only to find no change in the 
fault. So I tested again, this time using a logic anaylser to find it was 
the _first_ chip in the chain that had died, the signals from it were 
completley mis-timed, Replaced htat (Acutally using the chip I'd taken 
out when doign the first 'repair'), and everything was fine.

But anyway... If you're getting the main clock signals, I would then 
start looking at the 'video' schematic. You know the video output is 
missing, let's find out why... Of course it may be a fault elsewhere 
(CPU/ROM/RAM, for ecample) which is causing some enable signal not to be 
present, but that's what youneed to establish. 

-tony