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Sick 800XL - No Display - Where to troubleshoot ?


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I'm restoring a completely socketed 800XL.

When I power it up, all I get is a quick flash of a light, pale green screen, then that very first low peep 800XL start with, then just a black screen. No IC's get warm. I reseated all the large IC's, no change.

Suggestions on next steps?


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I just finished a chip by chip swap with a good 800XL. After each swap I rebooted and got the same result. The original working machine now has ALL the IC's from the dead board, and it boots just fine. All Except U29 as it is soldered into the bad board.

So it could be U29, or the next thing could be bad caps. I did a close visual inspection of all caps and they look good. No leaks or bulges on top.

There under side of the board at U29 is showing dark spots on the solder pads. The good board dose not show this. Time to breakout the soldering iron?

The PSU I use is in good working order.


Edited by KLund1
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Wouldn't be the first U29 gone bad, just not as

common as CPU, MMU, and the rest of the usual

suspects. Just have to do it and then you can

know for certain. Put in a socket and eventually

with another good U29 Delay Line you'll have two



He wants a picture of the Power Supply so he can

warn you about keeping and using an ingot type.

They do go bad more often. And when they do go

bad they will take out your ram chips.


95% isopropyl alcohol will make short work of the

brown flux residue under U29, you may have to look

in several stores before you find it right beside the

50% rubbing alcohol, but it's worth the leg work

since it cleans several times better and faster.

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Many thanks. Ya, I thought so.

Just put in a socket, swapped U29 is's and get the same results. iso cleaned it up well. thx.

So check caps? All I have a analog multimeter. Not sure hows to check caps with one.




I got this on evilbay about 10 years ago. Works great! Small footprint, low heat. I should have purchased more, only about $20.


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Wasn't there a guy from Poland or somewhere that made a little module to replace the C060472 delay chip? I would like to know more about that. It looks like Freddie did it using multiple inverter gates to delay the signal. He must have done something similar.


I saw this a long time ago, and it disappeared.


Start a NEW thread if you know about this.


Sorry to derail this.



Edit: Or was that something for the 74LS08?



Edited by Kyle22
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I don't think your caps are gonna be the problem; unless they're leaking or swollen, they're almost certainly just fine. I've had one tiny 47uF cap go bad in one 800XL video circuit, and it had obviously split and leaked splooge down the side. Never had one of the larger internal caps go bad on 14 A8 machines and two 5200's.


When the chips were removed from the suspect machine, did you look closely at all the sockets? I'm gunshy about crappy single-wipe IC sockets after restoring that troublesome 1200XL of mine last year; ended up having THREE bad sockets - two of them had bent some wipes that had nearly no spring tension in the metal and would only make contact with the chip leg when pressure was applied or if the chip was wiggled, and the third had had wipe broken clean off due to oxidation (this machine had been badly treated in its early life).


So anyway, that's my next suggestion: remove all the socketed chips again one by one, examine each socket under magnification to see if any of the wipes is bent in or broken, and just see what you see. If you have a logic probe or oscilloscope, I'd suggest checking all the pins of each main IC with the system powered up - look for power and ground, clock pules, activity on the data and address lines, etc.

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I'm a bit confused by the original post: "quick flash of a light, pale green screen, then that very first low peep 800XL start with, then just a black screen." Does it still do that? I can interpret this as almost a normal startup where the color pot is off making it green (or some other power issue) then the 800XL buzz/fart noise... then black. I'd check to make sure you have proper voltages on all the chips.. assuming you replaced every chip.. EVERY chip, then you are down to generic components. After the black screen can you hear when you type on the keyboard? Does a power off (count to 10) and then back on produce the exact same result or is it always black and you have to wait a few minutes?

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kheller2, Yes it behaves the same way with a complete chip replacement. (all 28 chips). I get the same result after a 10 count wait. I do not have the keyboard attached. I guess I should to test for keypress sounds.

The_Doctor, I do not have a scope, just lowly analog multi-meter. I do not know how to check voltages on the chips. I mean what pins to check with a meter.

DrVenkman, I will check the sockets, and report back


Thanks everyone, I do appreciate all the help!!

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SUCCESS!! It Lives!!

I did find a bad socket pin at U27. I fixed it and it boots to BASIC!!!


Wow that was frustrating. I have not had that much trouble fixing an 800XL before. I learned a lot again!!

Thanks again everyone!!!!


Congratulations! Fantastic to hear of another machine saved to live another day. :D


Nice also to see I'm not just typing words into the ether about some of my personally hard-won lessons.

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Nice one, well earned win too.


NO INGUTS!!! Got it. :)

Kind of weak sauce but if it's enough then yeah, I'd

be wishing I had bought more at $20 too.


Wasn't there a guy from Poland or somewhere that made a little module to replace the C060472 delay chip?

I saw this a long time ago, and it disappeared.

Me too, but from what I could tell it was impossible

to be an actual replacement. Thread kinda died

at that point so I'm thinking well it didn't work

and those that didn't want to rake him over the

coals publicly just let it die like that. A non-

working scam gone full tilt eBay, how unusual.


I was born at night, but it wasn't last night.

3 AM, what a PITA huh?

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