Re: Problem with Laser 128

[mjmahon@aol.com (Michael J. Mahon)]

sirarlias@webtv.net wrote:

>I picked up a Laser 128 (dated 1985) at a local computer thrift a few
>days ago. Since its original external power supply was missing, I
>substituted a //c brick-on-a-leash.
>
>When turned on, the Laser doesn't want to boot about half the time; the
>initial screen appears, but that's it. After hitting open
>triangle-control-reset a few times the disk drive, which sounds like an
>old Chevy pickup running on sludge, finally starts up. It loads programs
>as expected.

It sounds like the power supply is not delivering stable power as
the CPU comes out of its power-on reset.  See below.

>Another problem is with the display: There are horizontal lines of
>slight distortion running up the screen. The monitor and cable are fine.
>
>Do you think the //c power supply could be causing these problems? I was
>under the impression that they were interchangeable since the ratings
>are the same. Or perhaps some of the chips on the board need reseating.

Very possible.  The horizontal lines moving slowly up (or down) the screen
(I think that's what you describe) are indicative of 60/120Hz "hum" on the
incoming +12 volt line, since that's the only place that hum could exist.
If the amplitude of this hum is sufficient to cause the +12 to go substantially
below +11 or so, that could cause regulation problems for the other voltages.

This could be caused by a filter capacitor in the "brick" getting old and
drying out--which decreases its capacitance.  A quick check (and fix)
is to bridge the output of the supply with a large electrolytic with at least
a 25 volt rating.  Over 3000 microfarad should do.  Watch the polarity
when connecting it, since electrolytic capacitors are polarized (well,
almost all!).  If this fixes the problems, then the brick has a defective
capacitor and the external one is supplementing it.  If it doesn't fix it,
then you have a more bizarre problem, possibly involving poor grounding
or a defective monitor (which has "hum" in its picture).

Older (tube) TVs usually had some residual hum, since eliminating it
would have made their power supplies more expensive.  To make the
hum bars less noticeable, the local TV stations always "genlocked"
to the local power frequency.  This made the bars stand still in the
picture, which was much more unobtrusive.  Modern monitors and TVs
are better filtered, and the creeping hum bar problem is mostly gone
(just as well, since so much programming is not local, and is therefore
not genlocked to local power.)

>I'm asking now because the main board is covered with sheilding which is
>not only screwed on but also soldered together. (Kind of like wearing a
>belt AND suspenders at the same time.) I'd appreciate some input from
>you good folks before I go in further.

Well, yes--but the screws provide mechanical strength and the solder
provides electrical integrity.  EMI avoidance requires good shield bonds.

-michael

 Email:  mjmahon@aol.com
 Home page:  http://members.aol.com/MJMahon/