Garbled display when certain keys are pressed.

[Lardo Boffin]

I just got a zeddy and it has a weird issue.

If I press some of the keys it works fine but others cause the screen to become garbled. The computer has not hung however and mostly continues to work!

10. So far so good.

Press REM and this!

Press return and the line goes to the top of the screen as expected!

If I press NEW and enter I get the cursor back.

Not all the keys cause this issue, for example PLOT is ok.

If I plug in an external keyboard it does the same.

I plugged in my ‘spand and it loaded and displayed garbled stuff which should have been the menu. I could see blobs of black moving up and down when pressing the arrow keys.

All of the chips are socketed so I will change them to see which one is dodgy but any guesses at this stage? I’m guessing (hoping) ROM.

[Moggy]

With the xpand you are running on its ROM (albeit with the chr table from the zeddy ROM) so I suspect it could be that old issue one(158) ULA partly giving up the ghost, but as everything is socketed then, as you suggest, try chips from another zeddy.

[Lardo Boffin]

With the xpand you are running on its ROM (albeit with the chr table from the zeddy ROM) so I suspect it could be that old issue one(158) ULA partly giving up the ghost, but as everything is socketed then, as you suggest, try chips from another zeddy.

Bang on the money. In a ‘it was the butler’ stylie it was of course the ULA. I was hoping it was to do with the characters stored in ROM (as I have a couple of spare ROMs) but no.

[1024MAK]

This fault is almost always due to the ULA...

Mark

[Moggy]

It's weird how the ULA can partly fail. I had one that caused a white screen when certain keys were pressed but not others and was ok with a Memotech keyboard. A change of ULA curing this completely and testing the ULA on other systems which then exhibited the same fault confirmed a partially faulty chip.

[Lardo Boffin]

Yes. This is second time I have had a ULA fail in this way recently.

The first one failed when any key was pressed and then locked up the computer completely. However it would boot up the ‘spand without issue (so long as I did not use it).

This one just garbles some stuff and continues to work but does garble the output regardless of key presses. The ‘spand menu is a mess.

Strange.

At least now I have two ‘danger ULAs’ that I can happily use to test other zeddies without worrying too much about them getting damaged.