[sebhc] H89 strange problems....

West, Ronald S. RONALD.S.WEST at saic.com
Tue Jan 31 09:45:53 CST 2006


Any luck in solving this problem? 


-----Original Message-----
From: sebhc-bounces at sebhc.org [mailto:sebhc-bounces at sebhc.org] On Behalf Of
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 have
booting to HDOS ok. 

I do, however, have a really strange problem with both of them though and I
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 makes
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 flicker
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 floppy
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 separate
switch-mode PSU. 

All this would suggest to most people that there is a definite problem with
the 5v supply to the main CPU board, however I can see no problems with the
'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 both
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 home
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 I
could try to resolve it?

Thanks & regards,

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