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CBM 4032 display problem

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bolideuk

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Mar 27, 2011
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I am currently restoring a CBM 4032 that I bought as a faulty unit. It had some sticky keys which I have sorted by cleaning the keyboard. It had the common bad Character ROM chip which I have now replaced with a 2532 eprom. The final two problems I am still working on.
First an odd one? All text lines start at column 40, and then carry on from column 1 on the next line below. The cursor is two character spaces ahead of any typed text, meaning there is always a space between the typed text and the cursor. Anyone have any thoughts on what would cause this?
Secondly some of the charcters sparkle, in other words some of the screen bits flash on and off.
Otherwise the computer is working and will run programs.
 
bolideuk said:
I am currently restoring a CBM 4032 that I bought as a faulty unit. It had some sticky keys which I have sorted by cleaning the keyboard. It had the common bad Character ROM chip which I have now replaced with a 2532 eprom. The final two problems I am still working on.
First an odd one? All text lines start at column 40, and then carry on from column 1 on the next line below. The cursor is two character spaces ahead of any typed text, meaning there is always a space between the typed text and the cursor. Anyone have any thoughts on what would cause this?
Secondly some of the charcters sparkle, in other words some of the screen bits flash on and off.
Otherwise the computer is working and will run programs.
Click to expand...
First suspect IMO would be the video RAM.
Is the CG ROM properly decoded (since you're using a 4K chip instead of a 2K xx16) - no floating address inputs?
 
MikeS said:
First suspect IMO would be the video RAM.
Is the CG ROM properly decoded (since you're using a 4K chip instead of a 2K xx16) - no floating address inputs?
Click to expand...

I suspected the video ram was the cause of the the sparkly text, the fault only appears as the chips warm up. I stood a heatsink on top and the problem goes away.
I'm not so sure about the character rom though. I took a tip from others who recommended 2532 eproms, and as it happens these are the only ones I currently had. Of course these are good replacements for the 4k basic roms, I did not check what size the character rom was! When I grounded A11 on the 2532 it did not make any difference to the display layout
 
The E000 and CG ROMs are 2K-2716 compatible and the rest are indeed 2532-compatible.

Grounding A11 on the CG wouldn't change the layout but it might remove the sparkling; then again the VRAM might be responsible for both.

BTW is this a 9" screen 4032 or the 'FAT40' with the 12" screen and CRTC?
 
MikeS said:
The E000 and CG ROMs are 2K-2716 compatible and the rest are indeed 2532-compatible.

Grounding A11 on the CG wouldn't change the layout but it might remove the sparkling; then again the VRAM might be responsible for both.

BTW is this a 9" screen 4032 or the 'FAT40' with the 12" screen and CRTC?
Click to expand...

It's a 12 inch fat40. the sparklies are caused by the 2114 ram chips. I will get some 2716 chips. The UD7 E000 ROM and the UD6 Rom look like they have been replaced before as they are socketed. However they look like genuine pet roms and have the correct markings 901498-01 and 901465-22 respectively.
 
bolideuk said:
It's a 12 inch fat40. the sparklies are caused by the 2114 ram chips. I will get some 2716 chips. The UD7 E000 ROM and the UD6 Rom look like they have been replaced before as they are socketed. However they look like genuine pet roms and have the correct markings 901498-01 and 901465-22 respectively.
Click to expand...
Those two 2KB ROMs are normally socketed to allow for different countries' keyboard layouts and character sets.
 
Just to finish this thread; I received some 2716 eproms yesterday, and having replaced the character rom again with a fully compatible Eprom, it worked perfectly.
 
Congratulations on the fix. The 2532 EPROM should have work also as the high order address line (A11) is tied to CHR Option from the 6545 CRT Controller and should be always low.

I suspect there was a problem programming the 2532 or the chip had a fault.
 
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