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800XL blank blue screen


hueyjones70

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Is it entirely blue or just a blue window?

What happens if you hold Option when turning on?

 

Blue window only without "READY" probably means the Basic Rom is bad.

Troubleshooting for that in order I'd suggest swap out Basic Rom, MMU, PIA.

Edited by Rybags
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  • 2 weeks later...

The 800XL tech manual says to replace U2, that is a 74SL138 chip. Should I be able to use one from an 800 ROM. I check voltages on each pin on the chip in the blue screen computer and they were all significantly lower than the same voltages on the pins of a chip in a working computer.

Edited by hueyjones70
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Yes to using any other 74LS138 for testing purposes.

 

Remove keyboard and try it without.

SignGuy81's #3 might be the issue where SELECT and

START are stuck on and you are looking at the

Cassette boot screen. Which is waiting for you to

press the spacebar IIRC, then it should beep to

signal that you need to turn on the cassette player.

 

Try swapping out 4051 chips before tearing apart the

keyboard, they go bad often enough to be the real

issue here too.

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Unless you've got some smutz on the keyboard jack

around the area of pin 18 and higher and that

errant drop of 25 year old soda is emulating SELECT

and START for you without your knowledge. And you

can't know that until you press the spacebar, and

that circuit actually still works and only then

does it go BEEP after a few seconds. Correction

to my original proposition is that 4051 is not

involved in SELECT or any other function button,

that is GTIA's job but smutz on the motherboard

top and/or bottom or damaged resistors or bent pins

on bottom side might still be pressing a non-existent

pair of buttons for you. Since GTIA has been changed

it's unlikely that the fault is there.

 

Standard working blue color is not the norm for

a non-working machine. Can't ignore this so you

really should plug in the keyboard and give the

spacebar a press.

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  • 7 months later...

800XL boots to a blank white screen, no border; carts boot OK. What is most likely culprit?

 

Could be a bad MMU, could be an issue with the PIA .. when you say "carts boot OK," how many and what types did you try? Do you have any larger games or utilities like OSS carts that would exercise the bankswitching?

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Star raiders, pole position, tbis computer has standard memory.

 

Do you have something that uses more memory? Even a 16K cart might behave differently. If you have something like an UNO or Ultimate Cart, you could try a .CAR for Action! or MAC/65. They should work on any 48K or greater system.

 

If those work okay, it would help eliminate the MMU, PIA and probably the OS from being the likely cause.

Edited by DrVenkman
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