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600XL and 800XL red screen repair


plazma

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I've had 600XL and 800XL units laying in my garage for years.

Today I soldered a video and PSU cable for them.

 

Both give a red screen only. The 800XL now gives a grey screen.

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First I started to debug the 800XL. A simple touch test followed by measuring with an IR thermometer revealed two hot DRAM chips (9C above other chips) and a hot OS rom chip (13C above the other ROM). I replaced the three chips. I only had 27C512 EEPROMs so I made an 64kB image by copying the original image four times. The address pins 14 & 15 are high on the MB.
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There was a change. Now the screen is grey. A second round with the IR thermometer also found the U3 MMU to be hot.

I found PAL & GAL chip sources from here:
https://www.atarimax.com/jindroush.atari.org/achmmu.html

Have anyone tried these?

Do anyone have compiled files?

I could buy a GAL16V8 from a local store and flash it.

 

I measured the ICs on the 600XL and they were all cool. Some were only 5C warmer but I consider it to be normal. The ICs are socketed so I will try the MMU from the 600XL on the 800XL.

 

If I get one of the machines running it would help with the second one with IC swaps.

 

Any hints or tricks? I also have an oscilloscope and a logic analyzer.

I have fixed multiple Commodore 64s but Atari machines are new to me.

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MMU always tend to run hot, even when 100% healthy so that's not necessarily an indicator of anything. 

 

Query: what PSU are you using? I hope you're not using the "Ingot" model (see below). If you are, please measure the output voltage - if it's high or low, it has likely failed and damaged one or both machines in the process.  Even if it's presently healthy, discard and get a safe replacement. Any of the below are fine, especially for stock machines with no internal mods. Some folks simply cut off the DIN plug and cord and solder to a modern +5VDC wall wart supply. 1.5A or better should be fine.

 

 

XL Power Supplies.jpg

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Very good detective work! You have narrowed down what seems to be 3 very common failure points (at least common on this forum over the last few years) - RAM, ROM, MMU.

 

Yes, a programmed GAL16V8 can replace the MMU. I was curious about this myself and bought a few to try, and can confirm it works in an 800XL after programming with a TL866.

 

Compiled files are posted here:

XL/XE except XEGS and stock 1200XL: https://atariage.com/forums/topic/155863-mmu-chip-needed/?do=findComment&comment=1911484

XEGS: https://atariage.com/forums/topic/290832-atari-xe-game-system-help-with-keyboard-issue-not-the-mylar/?tab=comments#comment-4271375

 

Edit: For completeness - Original MMU chips are priced on the order of $5-$14 from MyAtari/B&C or Best Electronics, but best has a minimum order GAL's from eBay/China are much cheaper if you have a programmer though. And yes, All of these failures together would lead one to suspect to suspect the power supply as @DrVenkman has noted. Those ingot supplies fail by putting 9+ or more volts into components designed for 5V.

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Since you have a scope and logic analyzer, you can help narrow things down even further -get a PDF of SAM'S COMPUTERFACTS for the 800XL - there is a copy available at Atarimania.com; there are a couple minor typos but they should be obvious if you happen to run across them; there isn't a 600XL version I don't think, but they electronics are very similar between models even if the layout and precise details are a bit different.


SAM'S has logic charts for each pin of each IC. Between those charts and the schematics, you ought to be able very quickly to track down whatever issue you might run into.

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11 hours ago, DrVenkman said:

Since you have a scope and logic analyzer, you can help narrow things down even further -get a PDF of SAM'S COMPUTERFACTS for the 800XL... 

I'll get that document and check it for diagnostics hints. 

 

I used a 5V 4.5A PSU I found in my garage. I measures it to give 5.15V and negligible ripple. I soldered a DIN connector to the cable. 

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So with the scope, first ensure the CPU is starting up properly - check pin 39 on SALLY for the Phi2 signal (part number CO14806, IIRC). Verify you’re getting a relatively clean clock output, then check the same signal on all the rest of the main chips (SAM’S has all the pinouts for the rest of the main chips). Use the scope or - since you have it - your logic analyzer - to check the signals on the data and address lines. 

 

And of course, there’s always the simple dumb and easy step of simply removing each chip and reseating them. Sometimes the cheap single wipe factory sockets wear out and lose their spring tension, and in some rare cases the machine has been stored so poorly that surface corrosion can cause them to break off completely. A 1200XL of mine required me to replace three sockets to get it working correctly (6502, POKEY and PIA). 

 

Good luck! We look forward to hearing your progress. :) 

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I found an ATF16V8B chip from my C64 VSP bug fix circuit. I flashed it with my TL866II+ programmer with the jed file from "comment-1911484".

It worked! Also tested with the chip from the 600XL and it also worked.

Hint for others: the ATF16V8B chip is cheap and readily available. It is compatible with the GAL16V8 jed file!!! Make your own MMU chips :)

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OK. So this 800XL unit was easy since all hot ICs were faulty. Next I'll try to fix the 600XL. All ICs are rather cool. But now I can swap ICs from the working 800XL.

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Did some IC swaps between the 800XL and 600XL.

The CPU seem to be dead. In the 800XL it gives mixed yellow/brown screen. Working CPU in the 600XL still gives red screen.

 

All chips have been removed and reseated. DRAM was replaced with tested chips.

372436948_600XLchips.thumb.png.7f25d48786c21f2e3d4e3d077fecd91d.png 

 

BTW I ran the memory self test again a few times on the 800XL. Why does it sometimes test three complete rows of RAM and sometimes only two and half rows?
PhotoCollage_20190623_170242786.thumb.jpg.c53733460f06833fb58d76fc9397a87d.jpg

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Still some kind of addressing problem there. Each block represents 1K, but because of the way the DRAMs are connected to the bus, you can’t isolate things to any one chip with the built-in self test.  So the first pic shows only 40K, the second pic shows the proper 48K being tested; even though the 800XL has 64K, the space above 48K is of mixed utility because of space reserved in the memory map for OS ROM, BASIC, etc. There are numerous chips involved in addressing, both at the physical switching level (the MMU and the 74-series logic) and the OS level (the 6520 PIA).

 

Do you have an SIO2PC-USB device or some other way to load files into the 800XL? If so, search for forums for a more thorough memory test utility. XRAM is a good one and can help isolate issues to specific DRAM chips or at least tell which which parts of the address range are causing issues.

 

EDIT: It’s been forever since I paid a lot of mind to the built-in test, but I think you can get different results if you access the test by booting with OPTION held down to disable BASIC, or by typing BYE at the READY prompt from BASIC after you’ve already booted.

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3 minutes ago, plazma said:

That memory test difference seem to be related how the self test program was started. With the command BYE you get two and half rows. Power up with OPTION-key down gives three rows in the test.

Yep, see my edit to the post above. That’s normal then. Good deal! :) 

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Yeah BASIC enabled replaces 8K of RAM with the BASIC ROM, leaving 40K of RAM to test. (ie after typing BYE) Holding OPTION on powerup disables the BASIC ROM, and self test will see 48K RAM. Interestingly, with the BASIC ROM physically removed it will always go to self test, but still only test 40K without OPTION pressed as the 8K is still mapped to the empty ROM...

 

Somewhat unrelated observation, but that black bar at the top of your 1702 display can be eliminated by taking the back cover off and adjusting some trimpot knobs on the back of the tube. One of them adjusts the vertical "stretch." Unless you set it that way intentionally... Another will adjust the horizontal centering, yours looks offset to the left.

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That's the normal for PAL although the one at top is bigger than the bottom so v-position isn't right.

I've found that about a character line or slightly more top/bottom is good, any less and the aspect ratio is wrong.

Though that said, "wrong" just means it would be closer to what NTSC gives by default.

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@plazma How did you determine the 4416 DRAM's in the 600XL are good? They are not compatible for swapping with a 800XL. (Maybe you swapped with another 600XL?)

 

Also you havent noted the status of the PIA in the 600XL (bottom right IC). Its PORTB lines control the MMU, so could be suspect.

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22 minutes ago, Nezgar said:

Somewhat unrelated observation, but that black bar at the top of your 1702 display can be eliminated by taking the back cover off and adjusting some trimpot knobs on the back of the tube. One of them adjusts the vertical "stretch." Unless you set it that way intentionally... Another will adjust the horizontal centering, yours looks offset to the left.

Don’t mean to thread-jack - perhaps a PM would be better but I prefer to share the info - can you post a photo or point me to a good source showing the locations of the trim pots? I need to tweak my own 1702. I have an 1802 as well, but the service manual makes no mention of similar trim pots, which is sad because it needs the same kind of tweaking. :(

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28 minutes ago, DrVenkman said:

Don’t mean to thread-jack - perhaps a PM would be better but I prefer to share the info - can you post a photo or point me to a good source showing the locations of the trim pots? I need to tweak my own 1702. I have an 1802 as well, but the service manual makes no mention of similar trim pots, which is sad because it needs the same kind of tweaking. :(

I need to replace the power cord in my other 1702 so I will take some pics while I have it open. This will prompt me to get to it sooner than later. :)

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30 minutes ago, Nezgar said:

@plazma How did you determine the 4416 DRAM's in the 600XL are good? They are not compatible for swapping with a 800XL. (Maybe you swapped with another 600XL?)

Also you havent noted the status of the PIA in the 600XL (bottom right IC). Its PORTB lines control the MMU, so could be suspect.

I replaced the DRAM with 4464 chips which have been tested on a C64.

PIA was soldered to the MB on the 800XL so I did not swap it.

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2 minutes ago, plazma said:

I replaced the DRAM with 4464 chips which have been tested on a C64.

PIA was soldered to the MB on the 800XL so I did not swap it.

For what it’s worth, I’ve never had a PIA go bad in any of my 12 - 14 A8’s I’ve got sitting around here. The one time I had trouble with one it was due to a bad socket in the afore-mentioned 1200XL. Once I pulled the socket and put in a quality dual-wipe, that problem went away. 


But since you already mentioned the differences in your boot procedure (holding OPTION at boot versus typing “BYE” at the READY prompt), I think that explains the differences in the block counts for the RAM tests.

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29 minutes ago, plazma said:

Can the CPU be replaced with any 6502?
R/W is on different pin but that would be an easy fix.

No, I don’t believe so. The 6502C “SALLY” that Atari had made for them adds a HALT signal as well.  The good news is that it was used interchangeably in nearly all Atari machines from 1982 or so onward, so they’re not rare or hard to find. 

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On 6/23/2019 at 9:35 AM, DrVenkman said:

Don’t mean to thread-jack - perhaps a PM would be better but I prefer to share the info - can you post a photo or point me to a good source showing the locations of the trim pots? I need to tweak my own 1702. I have an 1802 as well, but the service manual makes no mention of similar trim pots, which is sad because it needs the same kind of tweaking. :(

Power cord replaced, and picture of the 1702 innards as promised with annotations from my observations adjusting each knob.

1702annotated.jpg

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