Mac Plus with (minor) issues

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu Apr 29 17:59:34 2004

>
> 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.
[...]

> 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.

It's just like a B&Q TV (OK, I am probably the only one to remember
working on those :-)). On the back of the yoke are 2 metal plates.
Rotating them, either together or in oposite directios, will move the
picture (strictly the raster, but anyway..) around on the screen.

Is there a 'horizontal phase' control? That has the effect of a
horizontal shift, at least over part of its range. I really can't be
bothered to move the 20 or so cameras that are on top of my Mac+
schematics folder...

> 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?

You could easily have knocked the centering rings.

>
> 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

A Mac should run at 4.95V...
 
> 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).

OK.. My first guess is that your meter is not telling the whole story.
You've got ripple, or more exactly spikes, on the PSU outputs. The
average voltage may be around 5V (that's what the meter shows), the
spikes trip the crowbar.

I would start checking capacitors on the output side of the power supply
(look for electrolytics with a 16V or 25V voltage rating). If you've got
high ESR here, you will have that sort of ripple.

> 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"?

I don't believe the 5V line is that critical unless you've got a
seriously marginal chip in there. My guess is that the PSU is not
behaving, and that the output voltage is not telling the whole story.

In general, adjustments do not drift!. YOu rearely, if ever, cure a fault
by tweakinga preset. More likely some component has failed.

> Bonus Q5) Is that right? Isn't that a recipe for thermal and
> electrical disaster? (I guess not, it runs, but....)

I would assume the Brainstrom 'requests the bus' from the 68000 (AFAIK
nothing else ever does on the Mac+, so there's no conflict here),
effectively disabling the 68000. All the 68000's outputs will be
tri-stated (high impedance) in that case.

-tony
Received on Thu Apr 29 2004 - 17:59:34 BST