Osborne Executive CRT circuitry

Roy J. Tellason rtellason at blazenet.net
Tue Feb 14 13:35:09 CST 2006

On Monday 13 February 2006 02:34 pm, David Griffith wrote:
> I'm trying to restore an Osborne Executive, but there seems to be
> something wrong in the CRT circuitry.  The machine powers up and beeps,
> but I see nothing on the CRT.  Fiddling with the CRT pots does nothing.
> I hear none of the expected noises from the CRT circuitry, nor do I see or
> smell anything burnt.  Can someone here point me in the right direction?

First question is,  do you have that jumper plug in place?  The one that bears 
a sticker about not removing it while power is on,  or something to that 
effect.  This is visible on the front panel,  and if it's not there you'll 
get exactly nothing from the monitor at all.  The plug I refer to is visible 


just to the right of the two monitor knobs and to the left of the reset 

If it's missing (I've encountered this on some O-1s),  you can possibly jump 
from top to bottom of the board in a few places and get things working.

If this isn't the problem,  then what you're most likely seeing is bad 
capacitors in the monitor itself.  Mine worked the last time I had it on,  
but there was just a little bit of jitter to the display.  I wouldn't be 
surprised if it didn't work the next time I tried it,  but then I have a 
spare monitor tucked away that I picked up some years ago.

There's also that composite video output over on the right -- try plugging 
that into something,  either a TV with composite input or if you don't have 
one handy the video input of a VCR to use its modulator to feed into the TV. 
I happen to have a Zenith 12" amber monitor that I used for this purpose as 

If you don't get any display at all,  then there may be problems with the 
logic board,  though getting the beep makes this sound less likely.

If the monitor itself is the problem,  I remember reading in some issues of 
"FOGHORN" years back that there were certain capacitors in the monitor 
assembly that would cause problems,  though I don't remember which ones they 

The monitor itself was apparently made by Zenith,  but I have not yet been 
able to locate a schematic for that portion of things,  though I have 
complete tech info on the rest of the computer.  If anybody knows where I 
might find a schematic of the monitor as well I'd sure like to hear about it.

Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin

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