[sebhc] H89 strange problems....

Robin England robin.england at dial.pipex.com
Tue Jan 31 12:47:29 CST 2006

Hi Ron

As luck would have it I haven't had the time to look at these machines since
my email! Work has got in the way of fun! :-(

As soon as I have a chance I'm going to replace the bridge rectifier and
solder the connections as per the suggestions. I'll keep you posted as to
the results!


----- Original Message ----- 
From: "West, Ronald S." <RONALD.S.WEST at saic.com>
To: <sebhc at sebhc.org>
Sent: Tuesday, January 31, 2006 3:45 PM
Subject: RE: [sebhc] H89 strange problems....

> Robin,
> Any luck in solving this problem?
> Ron
> -----Original Message-----
> From: sebhc-bounces at sebhc.org [mailto:sebhc-bounces at sebhc.org] On Behalf
> Robin England
> Sent: Wednesday, January 18, 2006 11:33 AM
> To: sebhc at sebhc.org
> Subject: [sebhc] H89 strange problems....
> Hello All
> Firstly many thanks for your response to my previous request for MTR ROMs
> for the H/Z89 machines. I have now been able to get both the machines I
> booting to HDOS ok.
> I do, however, have a really strange problem with both of them though and
> was wondering if anyone has any ideas, or previous experience of this.
> After a short while of being powered up (a few minutes usually), a
> horizontal bar starts to move slowly from the bottom of the screen to the
> top. When the bar reaches the top, another one starts from the bottom. At
> first I thought that this was due to ripple on the CRT electronics 85v
> supply (suspected leaky capacitor). However, a quick check with the 'scope
> proved otherwise and even replacement of the cap with a brand new one
> no difference.
> The really weird thing about these bars, is that the characters on the
> screen directly under the moving bar become faint and even appear to
> and break up at pixel level. At the same time, the main logic board
> sometimes goes crazy; often it locks up, crashes or you can hear the
> drive start clicking.
> Even stranger, is that if the main logic board is removed and the terminal
> board is used alone, the problem does not occur at all - you can fill the
> screen with characters and they all remain bright and stable. I have also
> been able to make the problem go away even when using the main logic (CPU)
> board provided that the 5v supply that powers it is obtained from a
> switch-mode PSU.
> All this would suggest to most people that there is a definite problem
> the 5v supply to the main CPU board, however I can see no problems with
> 'scope. I've even replaced all the capacitors on the separate PSU PCB plus
> I've even replaced the 5v and 12v regulators (TO3 package) that supply
> the CPU board and terminal board, but no difference.
> The machines have both been in storage since the early 1990s and were
> working fine back then. One is a Z89 (factory built) and the other is a
> built H89, one has a paper-white CRT and the other a green CRT. Both
> machines have a hard-sectored floppy drive built-in.
> Is this a known problem? Does anyone have any suggestions of other things
> could try to resolve it?
> Thanks & regards,
> Robin
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