All,
Two separate (?) issues with my Mac Plus.
1) Screen display is too far to the left.
2) System seems *very* sensitive to voltage setting.
What I've got:
Mac Plus (upgraded from 512k), original mouse/KB
Brainstorm 16MHz 68000 upgrade
Kensington System Saver Mac (switch, fan, and surge protector, mounts
on top of system).
La Cie external SCSI hard drive, 230 MBytes
Analog (Radio Shack, automatically suspect) VOM.
Mac OS 6.0.8, 7.0.1, 7.1 (chosen by System Picker utility) all on HD
Screen Display issue:
Everything (all pixels) is shifted uniformly to the left
about 5 mm. I found the "width" setting on the analog board (well
duh, read the label on the cardboard shield) and fooled with it
(system on, plastic tool). Works fine, image gets narrower and wider.
I can set it narrow enough that pixels are all on the phosphor, so
the system is usable - but it's aesthetically slightly annoying to
have everything to the left of where I expect it. I could not find a
setting to shift left/right.
Problem is present either with or without the Brainstorm
installed, so I don't think it's related to that. I *think* the
problem first appeared when I took out the "Elf Armor" mu-metal ELF
shield that I got back when that was hot psuedo-science, my wife used
the machine a lot, and we were expecting our first child (see the
connection? :-) ). Significance of that is I had to unplug the cable
from the back of the CRT, so might I have knocked something askew?
Voltage sensitivity:
System started occasionally resetting itself. Opened it up,
stuck the VOM on ground (frame) and 5 V (using the pin in the middle
of the Analog -> Digital board connector, at the digital board) and
found it reading a *little* (maybe 0.05 V?) shy of 5 V. OK, (duh,
read the label on the cardboard analog board shield again) set the
voltage higher, with system running. I can set it quite a bit higher,
up to around 5.1 V. System keeps running. However, when I power-cycle
at that setting, it goes into an endless loop of "chirp, chirp,
chirp..." and the VOM stays near 0 V (wiggling at each chirp).
I assume that means there's an overshoot on 5V coming up
which triggers the crowbar, which sets up for a repeat.
Q1) Is that supposed to work that way? I'd expected the 5V rise to be
critically damped or better, so no overshoot. In other words, I'd
expect if it'll continue to run at a given Voltage setting, it'll
start at that setting. Have I got a latent problem forming?
OK, sooo... set the Voltage back down some, try the "reset"
button.. (repeat a few times) ... "Bong" ... starts up nicely ...
runs a while and then freezes. (Here and below, "freezes" means some
sort of bad instruction. Could result in lock-up, could mean drop
into MacsBug, unable to continue, could mean reset.) This behaviour
is not present unless the Brainstorm is installed (at least not
obviously. It could be true but with a longer time constant between
freezes). Sigh.
So try setting Voltage barely higher. Works, but freezes
*sooner*. Hmmm. Voltage lower. Works better. Voltage lower. Finally
found a setting very slightly below 5V, (mmmm..maybe 4.98 V?? But
remember the instrument I'm reading from here...probably about 1
significant figure) where it seems to run indefinitely ('til bedtime,
anyhow).
Q2) I thought that all digital stuff basically ran better until it
over-voltaged - which the crowbar was supposed to prevent. So I'm
surprised to see freezes/crashes at *higher* voltages (but still low
enough to avoid the start-up overshoot) but see them less frequently
at lower voltages.
Q3) I assume there is a minimum voltage at which I'll also start to
see freezes/lockups. True? Am I better off hunting for the lowest
possible voltage, or try to find a "happy medium"?
Q4) Have I got some incipient problem (dying isolation cap?) that is
closing my window of working voltages?
Warning: I'm on digest mode, the system is at home and I'm usually at
work while accessing email, so my response time is likely to be ~24
hours for anything that requires tests/measurements. And then there's
the kids' YMCA soccer (football) league ....
TIA and no urgency, as the system appears to be running at
the moment (raps forehead soundly) 'Knock on wood'. Kids are getting
hooked on "Sim City" while I'm at work. But the Brainstorm may cure
that...Sim City appears to run twice as fast with the 16MHz upgrade
as it did with just the original 8MHz processor.
Which reminds me: The Brainstorm is on a little daughterboard
with 2 other IC's. The daughterboard plugs into a socket which is
installed piggyback atop the original 8MHz 68000. There is also a
flying resistor that solders to one leg of another component a short
way away on the motherboard. If I pull the Brainstorm daughterboard
off, and desolder the resistor lead, the system works just as before,
with the 8 MHz CPU. If I plug the Brainstorm into the socket and
connect the resistor, the system runs (see above, sometimes) at 16MHz
(apparently). (FWIW, there are a couple of other modifications to the
motherboard. I *think* they are just IC replacements.)
I *think* this means that when the Brainstorm is plugged in,
both 8MHz and 16MHz processers are running simultaneously.
Bonus Q5) Is that right? Isn't that a recipe for thermal and
electrical disaster? (I guess not, it runs, but....)
--
- Mark
210-522-6025, page 888-733-0967
Received on Thu Apr 29 2004 - 14:14:49 BST