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Atari 800xl repair log / bad video output

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Thought I'd share my recent 800xl repair log from a different site, re-posted here in case someone needs help in repairing their 800xl.

 

 

Although I grew up with a Commodore 64, I have a soft spot for Atari 8 bit machines. My mission in life is to save them all from going into landfill. I spotted this grimy Atari 800XL on Gumtree very recently as untested, it came with an Atariwriter Word Processor Cartridge. I met the friendly chap selling the item and we did the exchange for the computer at Town Hall steps, which is a popular meeting place in Sydney.

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Taking the machine apart, I was happy to see that the machine was fully socketed and that the PCB was in excellent condition despite it needing a good clean. Actually, I was almost expecting a fully socketed machine because I also had a made in HK 600xl which was fully socketed as well. Another pleasant surprise was the brand of DRAMs used ( OKI ) instead of the mT variety, which have a bad reputation for reliability and were used extensively in the Atari 8 bit and C64 line.

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I had a close friend of mine over after work and we went through the troubleshooting together. Unfortunately, not much happens after power it up. A black screen most of the time and sometimes an intermittent picture, obviously this is a sync issue.

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Sometimes I manage to get a screen that looks like this. This is a good sign. At least I know the CPU, ROMs and DRAMs are working to some extent to produce this screen.

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Luckily I have a fully socketed 65XE to swap parts to and from. I also tried a known semi good GTIA for troubleshooting purposes, which was bad on one output only but otherwise produced a nice clean picture in a working machine.

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Replacing it doesn’t change a single thing so I consult the schematics, the signals go directly to a hex non inverting buffer ( 4050 ) so I switch my logic probe to the CMOS setting and start probing the chip. Output 6 ( lum0 ) is good, lum1, 2 and 3 were all bad and all outputs were floating. I wanted to check the CYSNC line ( composite sync ) and that was also floating ( no signal at all ).

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I short the input and output pins of the 4050 ( pins 15 & 16 ) with my logic probe briefly which restores the picture to the screen.
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Replacing the chip completes the job!
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Edited by shoestring
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Nice job on the diagnosis and fix.

 

I've had the 4050 go bad on an 800XL. They are evidently rather susceptible to damage from static electricity. In fact, in the original VCS Sixers (CX-2600 models), the 4050 is sort of a "sacrificial" component in the video circuit. They take the hit from a shock rather than the TIA or 6532 RIOT. Might explain why I've picked up two 4-switchers (CX-2600A) over the years that each had a bad RIOT chip (on the 2600 RIOT reads input from the console switches) and another with a bad TIA.

 

As for the memory in your machine, I've got eight XL machines and none of them use mT DRAMs. Only my 130XE uses them and, perhaps shockingly, they're all still okay. Someday I'll desolder them and socket the board but today is not that day, lol.

 

Anyway, now it's time for you to pull that board out of the bottom case and clean up those plastics! :)

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Yeah the 4050s are CMOS so it doesn't surprise me that those are susceptible to static.

 

As for the case, key caps and keyboard. I've already finished cleaning, it's disgusting but they've come up looking rather nice so far.

 

There's some yellowing on the top so I'll need to retrobrite as well, it's now winter.. so I don't hope for much sun.

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As for the memory in your machine, I've got eight XL machines and none of them use mT DRAMs. Only my 130XE uses them and, perhaps shockingly, they're all still okay. Someday I'll desolder them and socket the board but today is not that day, lol.

 

Anyway, now it's time for you to pull that board out of the bottom case and clean up those plastics! :)

 

Interesting. I repaired a made in Taiwan 800xl about a year ago which wasn't socketed, this came with mT DRAMs. I also had a very clean 800XLthat also had the mT DRAMs and was also made in Taiwan. Both machines had the 4 & 5 keyboard variant type. So maybe there's some correlation between where they are made and the brand of DRAMs put inside. I actually made a mistake above, the input and output for CSYNC is pin 14 and 15.. not sure how to edit the first post.

 

Here's what I've cleaned so far. Just the bottom half of the case for now, some light cleaning with tooth brush, running warm water and hand soap.I don't think the bottom half needs any retrobriting, it's the top half that's quite yellowed. Pulled key caps off, cleaned all the dust out with a small paint brush and scrubbed the key caps with tooth brush in a basin of running warm water and hand soap.

 

As you can see I'm missing a rubber foot but I saw these on ebay. Just wondering what everyone else thinks about them before I go ahead and order some.

 

https://www.ebay.com/itm/28-Rubber-Feet-Atari-2600-7800-800-XL-XE-Joystick-1050-/130899455339

 

 

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Edited by shoestring

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Ages old B&C Atari vendor's eBay storefront, pretty

much can't go wrong with those rubber feet. Same ones

Atari used. They are Atari NOS.

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Well, no: the rubber feet in the eBay listing will fit the XE, but are of a larger diameter than those used on the 800XL.

Edited by flashjazzcat
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Well, no: the rubber feet in the eBay listing will fit the XE, but are of a larger diameter than those used on the 800XL.

 

Huh. I have a strip of those rubber feet Bruce sells. I've put four of them on a WICO Command Control bat joystick that I bought new-old-stock; the feet had melted to goo right inside the still-sealed box. When I restored my Ugly Duckling 1200XL I used one or two more as well and didn't notice any issue about the size.

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You won't have issues with the size on a 1200XL since the 1200XL's feet are the same size as those of a 65/130XE. But in this instance the target machine is an 800XL, whose rubber feet are much smaller.

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Currently in the process of doing the Super Video 2.0 mod. Noticed a big difference after doing the first 5 steps, much cleaner.

 

Does this mod really need the CV disable switch ? I didn't not notice any difference last time I added this switch to one of my other machines. I might try it again on this machine and use a scope to see if I can spot the difference in the actual signal.

 

 

Ages old B&C Atari vendor's eBay storefront, pretty
much can't go wrong with those rubber feet. Same ones
Atari used. They are Atari NOS.

 

Do you have a link to the store ?

 

I'm not able to find it.

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Nice work shoestring on the repair (and greetings from down Geelong way).

I have a few XLs in need of repair (I bought them as projects from New Zealand) but finding the time to make a start is my biggest challenge.

May need to read up on using a logic probe as my abilities are limited to doing basic soldering tasks following step-by-step guides...

 

 

Sent from my iPhone using Tapatalk

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Nice work shoestring on the repair (and greetings from down Geelong way).

I have a few XLs in need of repair (I bought them as projects from New Zealand) but finding the time to make a start is my biggest challenge.

May need thttps://m.youtube.com/watch?v=omq42Rh4rBQoread up on using a logic probe as my abilities are limited to doing basic soldering tasks following step-by-step guides...

Sent from my iPhone using Tapatalk

Thanks. I did make a mistake in the repair log though, corrected in my other post . So just be aware of that before you destroy the chip.. lol

 

You're really blind without a scope or at least a logic probe. Logic probes are cheap enough, the one I have is good ( Elenco Lp-550 ) and easy to get off eBay. Avoid the worthless piece of crap you can get at Jaycar.

 

Check out Adam's tutorial on using logic probes and troubleshooting logic devices. His channel is more about arcade repair but it all applies here as well.

 

https://m.youtube.com/watch?v=7l039y8dX5A

 

https://m.youtube.com/watch?v=omq42Rh4rBQ

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Currently in the process of doing the Super Video 2.0 mod. Noticed a big difference after doing the first 5 steps, much cleaner.

 

Does this mod really need the CV disable switch ? I didn't not notice any difference last time I added this switch to one of my other machines. I might try it again on this machine and use a scope to see if I can spot the difference in the actual signal.

 

 

 

I've never done a SV 2.0 mod. Instead, on one of my 800XL's, I did Bryan's Quick & Easy Video Upgrade for 800XL. It made a tremendous difference in the stock XL output.

Edited by DrVenkman

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Thanks for the link. It's a very similar mod but I'm keen to try adding the new power lead instead of using a cap to filter the noise from the existing 5v supply line since I don't have a 10uf cap lying around anywhere :)

 

Looks great so far on my 1084s so far, I think the biggest difference was made when I removed that cap at c56.

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I am currently working on an 800XL with a UAV-D device and unstable video. The monitor screen looks like an old style TV with the vertical and horizontal sync out of adjustment. I get a rolling picture with diagonal bars of varying color. I tried a different UAV and got the same results. I have disconnected chroma and composite video and luma gives the same unstable video without the color. I haven't started swapping chips yet because I am hoping that someone has had the same or a similar issue and I can at least narrow down the possibilities.

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I am currently working on an 800XL with a UAV-D device and unstable video. The monitor screen looks like an old style TV with the vertical and horizontal sync out of adjustment. I get a rolling picture with diagonal bars of varying color. I tried a different UAV and got the same results. I have disconnected chroma and composite video and luma gives the same unstable video without the color. I haven't started swapping chips yet because I am hoping that someone has had the same or a similar issue and I can at least narrow down the possibilities.

I'd double check the jumper configs on the UAV. Is it possible you may have them set for the rev C?

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I found the problem, I had pulled a trace when I desoldered the 4050. Replaced the trace with a jumper wire on the back of the MB, now everything looks great.

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I recently purchased an 800XL for parts or repair. It will not boot to basic. It will boot with a PAC MAN cart and a POLE POSITION cart. It will not boot with a STAR RAIDERS nor a BASIC cart. With no cart it boots to a blue screen with a white square but no READY prompt. The RAM and the 40 pin chips are the only ones with sockets. Does anyone have a hint where I should start desoldering chips and installing sockets?

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NEW PROBLEM--I have an 800XL that doesn't boot. When I turn it on I hear a click like it is going to boot but nothing else happens. I have swapped out every IC, it will not boot with Star Raiders and I get no video using SYSCHECK.  When I was swapping out ICs the ROM was bad and 1 RAM IC was bad. I also believe someone had tried to work on it before because most of the capacitors did not have the same voltages as my stock test machine.

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when you see that, I always start with looking for broken traces, pads, and messed up sockets... then I move to values on capacitors... if it's the correct or better spec... can't hurt to make sure the color pot is okay as well and the related transistors are still working... are you are checking for video from a DIN cable... or RF modulator?

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I discovered the problem. I found that the SALLY was bad so I set it aside. Then I checked the PIA. The PIA was good but when I replaced it, I accidentally replaced it with the bad SALLY. When I started looking for broken traces etc. I noticed that I had two SALLYs. All is now good.

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