[sebhc] H89 strange problems....

Robin England robin.england at dial.pipex.com
Wed Jan 18 10:32:57 CST 2006


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,
Robin


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