[vcf-midatlantic] Mac IIci, dead (PICS!)

[Herb Johnson]

On 5/14/2021 9:13 AM, Ethan O'Toole wrote:
>> Does it handle Sync on Green?  I found out the hard way that the 
>> IIci's built-in video is SoG ONLY, which was a big surprise to me.  
> 
> Hmmm not sure. But I didn't know that the IIci internal only did that. I 
> can ask a friend to bring over something compatible. 
> 
> I *think* the machine isn't thinking because all the data pins on the 
> PROMs are dead on power on. I just see no activity. I got schematics and 
> found the reset circuit loops through the audio chips or something. I 
> see that signal is high until I hit the reset button then it does go low.
> 
> Was looking for a power on reset circuit that might not be letting the 
> CPU run? That is my "next check." 

What I get from this response, is simple. "no activity from data lines 
on PROM" says the CPU isn't likely running - game over. (I presume he's 
using an oscilloscope, not a logic probe.)

I'd say follow the reset line to the CPU reset pin (it's a tiny 
surface-mount package, hard to probe). You can check the data lines on 
the on-board RAM. Look at the chip enable on the ROM. Things like that. 
Looking at fundamental signals - reset, chip enables, general 
data/address line activity - is a good "cold" approach.

As for monitoring the video signals: The IIci/cx will "run" without a 
monitor connected, that's my experience. So, without a Mac monitor, one 
can simply monitor the video outputs (say some sync line) for general 
activity. Possibly a good use for a logic probe, or one can make up a 
R/C circuit on an LED just as a convenient 'dongle'.

While it's possible Ethan can get lucky, and find some opens or shorts; 
I don't know of a clear path to diagnose other Mac problems likely due 
to corrosion and spread of capacitor-goop. Any physical fault is likely 
to be unpredictable, arbitrary. There is likely several and not just 
one. One would have to be very methodical, to track them all down.

But who knows? Maybe the ROM has a POST program that goes to HALT if 
there's any detected faults? I certainly lack complete knowledge on 68K 
processors and Apple ROMs. In that particular case, one might replace 
the ROMs with wired-up IC packages with pullup pulldown diodes to 
simulate a 68K do-nothing instruction. That forces the processor to run 
the address space. It's an old-school diagnostic.

All that said - it may be, someone who has done a lot of Mac recapping 
*and* component repair, may have established common faults for that 
model. But that's a big set of if's.

By the way: this is the difference between 1990's computing and pre-1980 
computing. With 1980 computing, component level repair *is* possible 
because most 1980 computers were intended for chip-level repair and 
contain human-sized components of a simpler nature; and have circuits 
one can isolate and diagnose.

The Macs, all post 1984, were built by robots, with surface mount, on 
multilayer PC boards. If they were serviced at the chip level that was 
done "at the factory" and not by service techs in the field. I've seen 
no Apple documents on chip-level repairs (but I've not looked hard). But 
that reminds me ...

"Sam's Photofacts" were third-party field-service booklets in the era. 
Checking ... Nope, no Mac IIci or IIcx books, in fact no Mac repair 
books from Sams at all (samswebsite.com). I'm aware of them for Apple 
II's and for other consumer computers (C64's and such).

What remains, are Mac schematics and repair activities established over 
time by techs; and "how to recap" activities. To be honest, I've not 
looked deep on the Web, to see how far those resources can take someone 
for chip-level repair on Macs. (shrug) if Ethan finds resources I'd like 
to hear about them, as I work with 68K Macs all the time (but not to 
repair components for reasons cited).

I hope it's useful to discuss chip-level diagnostics and 
compare-and-contrast in this list. It may be too much detail or tech for 
some.

regards, Herb

-- 
Herbert R. Johnson, New Jersey in the USA
http://www.retrotechnology.com OR .net
preserve, recover, restore 1970's computing
email: hjohnson AT retrotechnology DOT com
or try later herbjohnson AT comcast DOT net