[vcf-midatlantic] Mac IIci, dead after power on

Herb Johnson hjohnson at retrotechnology.info
Wed May 12 14:31:24 UTC 2021


I'm a little confused by the telegraphic (short) descriptions that Ethan 
gives for what he's repaired on and the symptoms of his IIci. (Hint: I 
think his Mac may be running but it's not producing a VGA-compatible 
video.) Here's the translation and the gloss to explain it.

My read is: the Mac when bought, was dead, no power no video no audio. 
(Macs "chime", a musical chord, on power up. He describes "chime" but 
maybe he means "when the power button is pressed to start the Mac".)

He then recapped the what? motherboard? power supply? - still no video 
or maybe no audio. But he says he measures DC power.  The 68K Macs (ones 
with a 68000 series processor), like PCs, operate the power supply from 
a start-up circuit which is activated by a push-button to the 
motherboard (mobo) starting circuit. There's a button on the 
motherboard, and there's also a power key on the Mac ADB-type keyboards 
(as he describes).

There's a way to short two pins on the DC power connector to force the 
power-supply to "wake up" - Web search for that. The IIci, IIcx and 700 
models use the same power supply. Look it up.

"screen is black, monitor wakes up" decrypts to "maybe my VGA monitor is 
showing there's a signal, but it doesn't display it". That may be 
correct! The Mac has a video connector which needs a quote "Mac to VGA 
adapter" which mechanically rearranges signals but does nothing to them. 
The IIci native video, is *not. VGA. compatible.*. The native IIci video 
(on mobo) however, will never EVER run a VGA only monitor.  So it's 
possible the IIci is producing a video signal.

On my Mac IIci, I have a Nubus video card and use that video for VGA 
compatible results. The Macs "sense" which video connector (on mobo or 
NuBus) has a cable attached, and route video (and video signal rates) 
*according to the connector that's cabled up* (which video you connect 
the monitor to) and select video mode (horizontal vertical signal rates, 
  resolution modes) according to *specific grounded pins on the Mac 
DB-15 video connector*. The "Mac to VGA adapter" selects those grounded 
pins.

What about the chime? Well, it's common for these Macs to lose their 
audio. The circuits which produce the audio, tend to corrode out, from 
both the failing electrolytics PLUS actual corrosion as a consequence of 
being near the clock circuits. Huh? Well, the Mac time-of-day clock, has 
constant battery power. That constant DC current creates a galvanic 
copper rust. Some of that current "creeps" over to the audio circuits. 
There may be some audio but it may be very quiet.

Of course the power supplies fail too, like other switching power supplies.

One can research these 68K Mac behaviors on the Web, and Apple had some 
internal repair diagnostic documents, and "Apple service manuals" per 
model. Look on the Web for these. But Apple did not document what 
happens 30 years later; that gets described on the Web by those who 
repair these things. I don't have much on Mac repairs on my site, I just 
sell Mac parts, like these.

regards, Herb Johnson
unexpected Mac geek


-- 
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

-- 
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


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