Troubleshooting an Atari 800XL that loads cartridges, but won't boot to Basic

[peco de guile]

Howdy!  This is my first attempt at repairing an 8-bit computer, so I'm definitely learning and having fun.  My goal is to get this unit running without throwing parts at it, so I'm trying to actually diagnose the faults, and come to conclusions based on what I find.  But I need help with good next steps.  Please let me know if there is any other information I can provide to help!

 

Here's the TLDR:  

Boots to gray screen with square cursor, but never ready's up. Single tone and fuzzy noise play.  All chips socketed, cleaned, reseated. U9-16, U2, and U7 look to be replaced as they're a different sheen than the rest of the chips. Using Sam's Computerfacts and logic probe shows errors on U2, U22, U17, U23, U19, U24-25. Installed cap kit. Option / Start / Select keys work with cartridge games, and register logic but won't boot to test.  Using original PSU and 5vdc seems strong throughout the board. Randomly booted correctly once but many/most keyboard keys were incorrect. Randomly booted to self test once and I wasn't sure what all was happening...

Longer version:
Found a socketed chip with a bent leg, and MMU had corrosion on it. Everything else was clean, but got brushed super clean. 

U2, U7, and the right half of U17 all get hot after a while.

Used to boot to a jarbled / yellow screen with vertical lines, but adjusting R38 fixed it

I played a full game of Joust with a controller just fine.

Logic Probe errors are as follows:

• U2 - pins 10, 12, 13, and 14 are High when they should be pulsing. Seems they go to C50 and C52 but I can't find those on the board...
• U17 - pin 16 and 17 are floating, 16 should be pulsing, 17 should be high. Both pins have continuity from top of chip through bottom of board. 
• U19 - pin 5 is floating, should be pulsing
• U22 - pin 25 should be high, but it is pulsing
• U23 - pin 23 should be pulsing, but is high. Also said to go to C52.
• U24 - pins 1,4,5 should be pulsing but are high
• U25 - pins 1,4,12,13,14,15 should be pulsing but are low.  

Reset button does reset the console, but holding none of the other keys seem to accomplish anything.

Keyboard has original film and ribbon cable on it, though I thoroughly cleaned the port and the cable.  I've disassembled the rest of the computer (case is getting retrobrighted) but I have not removed the metal plate behind the keyboard. 
U17 pins 12, 13, and 14 confirm that Start, Select, and Option keys are registering when pressed.  I tried digging into U17 further but I couldn't find a pinout for it on Sams Computerfacts.

Cap kit came from Console 5

Bottom of board says Revision C

I'm using composite video output from J2 Monitor Output.

No obvious trace or pin damage.

 

Thanks in advance for any help getting this bad boy running!

 

IMG_9996.HEIC IMG_0026.HEIC IMG_9985.HEIC

[peco de guile]

So I've made 2 discoveries so far:

 

The Atari does the same exact thing (booting to blue screen with cursor, but won't Ready) whether the BASIC chip at U4 is installed or not.  No difference at all.  So I might try and procure one of those guys.

Also, I'm assuming since the sound comes out of Pokey, that irregular music/sounds means a bad Pokey chip.  Here's an example: https://www.youtube.com/watch?v=DHGY6LZ1Vf4

 

[Beeblebrox]

@peco de guile  Great it's a Hong Kong made socketed 800xL.

 

You can get a replacement BASIC IC but the machine will boot with or without it installed. 

 

So can you confirm you have replaced/swopped out all the RAM chips? Having a full set of replacement ram established would be my first step given the machine is working for some things. IMHO the instability you are experiencing can be tell tale sign of bad and/or failing ram, even if self test looks ok. (Same for Rom)

 

Also I'd  source and replace the OS rom (U5) and maybe also the MMU (given it had corrision)

 

Maybe replace 4051 ICs given they are involved with the keyboard side of things but definitely might be worth checking and replacing the PIA chip (U23) as well btw. 

 

Have a look at the 74 logic ICs and definitely replace U1 (74LS08) with a 74F08. Replace the 74LS375 and also 74LS138 (both appear to be U2) if either/both are getting hot.  I'd expect U7 (Antic) to get a little warm over time (as will the CPU). - but no really hot. Can you confirm how hot it is getting?

 

U17 (GTIA) would also potentially get a little warm over time. Given you aren't getting any on screen glitches I'd rule this out as problematic at this stage.

 

So you restored colour by tweaking the colour pot (R38) presumably? 

 

EDIT: just watched the video - see what ya mean re the sound. Drops the last tone in Donkey Kong. Big bird cart sounds bad. Hmmm, worth sourcing a Pokey replacement (where get a modern version if you can't source an original)

 

Can you confirm that the CPU (U8) is warmish after a while, perhaps boarderline hot, but not so hot that you can't touch it?

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited June 13, 2022 by Beeblebrox

[peco de guile]

Thank you so much for the reply!  Here are answers to your questions, and more questions 

 

Ah, so the machine behaving the same with or without BASIC chip isn't immediately indicative of a bad BASIC chip?  Will the 800xl boot from a 1010 cassette drive without booking to BASIC?  

 

I have not ordered any replacement chips.  Money is pretty tight, and so I wanted to verify with some certainty which chips were bad, before buying replacements.  Which leads to another question: Where do you purchase new or used replacements? I saw a recommendation on another post to replace U30 with a modern version of the same chip when the 800xl will not boot, and I found that chip on Mouser, but do you guys also purchase new RAM/ROM from mouser?

 

When I say "hot", i mean that it is more than warm to the touch, but it doesn't hurt to leave my finger on it

"scalding" would mean I can't leave my finger on it

"warm" would describe anything I can detect an amount of heat from

 

The consolidated list and info for chips is as follows:

 

U5 OS Rom - C061598B-01 - Recommended replace

U3 MMU - C061618 - Recommended replace (hot to the touch, had a corroded leg)

U24-U25 - MC14051D - Recommended replace (I'd like to clean the keyboard and membrane and retest these when in BASIC before replacing. UNLESS these chips could prevent boot to BASIC)

U1 - 74LS08 - Recommended replace with 74F08

U2 - 74LS138N - Recommended replace (I only see one chip for U2...)

U7 (Antic) - C021697-11 - Gets warmer than others, but not super hot.  Visibly looks different than the other chips.

U8 (CPU) - C014806-03 - Gets slighly warmer than the others, but not as hot as U7. It's definitely not 

U22 (Pokey) C012294-22 - Recommend replace based on bad audio issues.

 

U17 has two floating pins, but you're right, nothing appears graphically wrong.  I'll rule it out.  

 

I have tested two power supplies with this 800xl.  An original PSU that supplied 5.2vdc to the board and made the image yellow.  Adjusting R38 fixed this.

Then I made a USB power supply which supplies 4.8vdc to the board and made the image gray. Adjusting R38 fixed this.  

 

I'll also include a photo of U7, as it looks different than the other major chips.  

[Beeblebrox]

@peco de guile interestingly I've just literally been troubleshooting a stock 800XL that came my way which was crashing in the Side3 menu (weird characters, dropped menu text), and within some games. No ram issues in self test or sys check ii, etc etc. So I just swopped out the 74LS08 chip for a 74F08... problem instantly gone!! 

[peco de guile]

Noob Question about replacing the 74LS chips:
 

there looks to be a 

74LS138N

SN74LS51N

SN74LS14N

DM74LS08N

SN74LS375N

SN74LS158N x 2

 

Are these all manufacturers variations of the same chip, or are these 6 distinctly different chips that I need to order exact replacements for?

[+DrVenkman]

3 hours ago, peco de guile said:

Noob Question about replacing the 74LS chips:
 

there looks to be a 

74LS138N

SN74LS51N

SN74LS14N

DM74LS08N

SN74LS375N

SN74LS158N x 2

 

Are these all manufacturers variations of the same chip, or are these 6 distinctly different chips that I need to order exact replacements for?

They are different chip types. 74-series logic has been used for literally decades now and comes in all kinds of “flavors.” Some are buffers, some are AND gates, or OR gates, etc. The part of the designation that matters for replacements is the “74” part (which tells you it’s a basic logic chip); the “LS” part, which more or less tells you how fast it can react and change states, or how fast it is; and the 2- or 3-digit number that follows, which identifies the exact configuration of the logic gates contained within the chip package. The other prefixes and suffixes don’t matter for our purposes here. 

 

Before you go shotgunning parts, though, you should really get a detailed troubleshooting checklist like the one in SAM’S COMPUTER FACTS (you can get a PDF on Atarimania and probably Archive dot Org) and follow through with the steps.

[peco de guile]

Thanks for the reply!  I do have more detailed information in the previous posts where I followed the Sam's guide, I'm just not entirely sure of what to do with that information...

[Beeblebrox]

@peco de guile 

 

Hmm I'd say that MMU is looking to be a contender there if it runs hot and had obvious corrosion. A failing MMU and also RAM chips can cause all manner of glitches, (instablity/crashes, audio glitches, graphical glitches). My money is currently on that MMU as being at least one of the issues.

 

In terms of 74 logic chips and your question.

 

So as per what Dr Venkman outlined above ....

 

....Where see here in your list:

 

there looks to be a 

74LS138N

SN74LS51N

SN74LS14N

DM74LS08N      This is commonly known as a 74LS08  - as mentioned it can be replaced by a 74F08 for extra stability with carts like SIDE3 (see link below to the one I bought from UK Ebay for example)

SN74LS375N

SN74LS158N x 2

 

https://www.ebay.co.uk/itm/254738994395?hash=item3b4fa094db:g:O78AAOSw0p5fefCe

 

However like DrVenkman says I agree right now ya best bet is to methodically work through and see if you can narrow things down a bit more before trying to source new chips. A lot of the 74 logic chips and better equivilants are very cheap - but it all mounts up. 

 

As I say that MMU sounds very suspect.

 

U7 (Antic) - C021697-11 - Gets warmer than others, but not super hot.  Visibly looks different than the other chips.

 

Antic sounds like it's behaving ok to me. Yes they sometimes look different and I believe there are two different model number ANTIC chips. (C021697 and Co12296) - I've run both models in my PAL XLs before IIRC.

 

For future reference if and when sourcing chips:

 

Ram chip alternatives - many available (just avoid MT branded ram). I've used Mostek and Hitachi brands - seem very reliable.

 

A lot of smaller ICs as Dr V mentioned are still available today to buy, (eg the 74 logic chips, the 4050, 4051, etc).

 

Original Atari OS and BASIC chips are fairly common to source (where I see them often on Ebay UK (OS Chip goes for less than £5). Re OS - I believe you can burn them onto Eproms (as many Atarians put theor own versioons of OS on them).

 

In terms of the custom Atari Chips ANTIC, GTIA, SALLY (CPU), and POKEY -they are a bit tricker, where:

 

ANTIC, GTIA, SALLY (6502 CPU) have no modern alternatives - you can't use off the shelf ICs AFAIK - you need to source originals. SALLY is the more common to come by. ANTIC and GTIA very rare in my experience (in the UK for PAL ones). Might be different for NTSC versions.

 

Original Pokey chips are hard to come by - (where thankfully there are modern custom Pokey stereo alternatives available, (eg PokeyMax)).   Also you can salvage original Pokey chips from old Ballblazer game carts - they had an extra one inside.

 

There is a modern alternative for PIA which works well - I've bought a few of them from Mouser earlier this year: 

Mouser electronics, (WD65C21). https://www.mouser.co.uk/ProductDetail/Western-Design-Center-WDC/W65C21N6TPG-14?qs=opBjA1TV902p0LqHsLVgSw%3D%3D&countrycode=GB&currencycode=GBP

 

For other custom Atari ICs including the MMU (Co61618), it can be sourced (I believe that can be fabricated from burnable GAL eproms too).  

For example here in the UK on Ebay: https://www.ebay.co.uk/itm/203520914694?hash=item2f62cab506:g:w7sAAOSwWeJg6aTF

 

Same goes for the delay line (Co60472)  chip, where a modern new custom fabricated one is available, here from Poland:

https://www.ebay.co.uk/itm/125097401743?hash=item1d2062dd8f:g:B-IAAOSwNgVgqpJl

 

The other thing you can do - which might sound a little crazy - is to source another 800XL, again prefereably one made in Hong Kong so it is socketed. If you can get one cheapish you can obviously salvage chips. Of course that might be a non working machine too, but the odds are it might not be the same issue or failing ICs. The ideal for me was to get to a stage where I had one other working machine which I could test the chips from a non working machine in. Has working for me any many others. It can work also if you have an XE, although obviously XEs aren't socketed - plus certain ICs don't exist on the XE that do on the 800XL line - and visa versa.

 

Of course the other valuble tool in my "toolkit" is the fantastic Sys check II module. 

 

 

 

So - in a nut shell - look into the MMU in the first instance IMHO. 

 

 

 

 

Edited June 14, 2022 by Beeblebrox

[peco de guile]

I had given some thought to purchasing a working 800xl and chip swapping to see where the problem lies, but I don't quite have the funds for that.  So slow and steady must win the race.

 

These chips are way more available in Europe than they are here in the States! A question about sourcing the MMU though: they are listed as PAL MMUs, and I am in an NTSC region. Will a PAL MMU be incompatible with my 800xl?

 

Thanks again for all of your help!

[ugliestgolf]

Last thing to do is buy and try any of the custom chips e.g. the pokey..

u may be cheaper buying another sk than buying a pokey for example..

 

In the UsA best electronics is the source for all atari parts and still have “new” unused pokeys for I think $20 which is incredibly cheap

 

uk its ebay and general scouring atari sites….

 

Basic can def be burnt to eprom (might as well get a Basic C and upgrade)

 

MMU can be programmed to an off the shelf GAL

 

pia is std part 

 

so they are all <$10each

 

sally/cpu is about $10-12 and is a common fault

 

 

memory chips are hit and Miss but ebay you can get for $1-2 per chip

I recently bought 50 memory chips from aliexpress, and 47 were working, and I paid $20
 

 

rule of thumb is start with memory and work from there

 

74s are usually swapped out as a “precaution” / recommendation rather than a repair fix, so prob not essential 

 

 

[ugliestgolf]

Without an oscilloscope it’s hard to pinpoint as my goto would always be follow the clock signal. Although you can at least follow the pulses around the board with a probe.

 

jammed high on logic probe to me means shorting somewhere , probably a burnt out ram chip or 8

 

Edited June 15, 2022 by ugliestgolf

[peco de guile]

Wow, I've never heard of Best Electronics, but I'm glad you referenced them!  Their website is pretty difficult to navigate, so I think I might just give them a call.   

 

Thanks!  

[ugliestgolf]

Yeah website is terrible. Call or email them

they are super informed and helpful for all atari parts and fixing/repairing identifying faults

[peco de guile]

21 minutes ago, ugliestgolf said:

Without an oscilloscope it’s hard to pinpoint as my goto would always be follow the clock signal. Although you can at least follow the pulses around the board with a probe.

 

jammed high on logic probe to me means shorting somewhere , probably a burnt out ram chip or 8I 

I DID actually have some jammed HIGHs when it should be PULSE.  One on U23 PIA chip, and several on U2 and U24. 

I don't have one of those clock-reading instruments, so making-do with what I can with the logic probe. 

[ugliestgolf]

Sound wise there is a whole audio circuit of cheap parts you can look at first before you have to source an expensive Pokey

[ugliestgolf]

Keyboard issue could be a red herring, not related to the actual “computer faults”, often membrane issues etc id worry about that last

[peco de guile]

UPDATE:

 

Replaced all of the 74LS chips with new chips from mouser (except for the 74LS14N which is back ordered), no change.

 

Replaced the MMU and Pokey chips, NO CHANGE!  

 

I couldn't believe it.  Even with the new pokey chip, Donkey Kong audio was still wonky!  And even with the new MMU, it still froze on booting to Basic and became the hottest chip on the board.

 

Since RAM is the common usual suspect, maybe that's what I'll look into replacing next.  

 

I started off this repair saying "I'm not gonna just throw chips at it til it works", but I think without an oscilloscope to actually see pulses, I lack the abilities to more accurately diagnose this guy. 

 

Unless anyone has a suggestion for better steps forward?

 

Thanks for all ya'll's help!

 

 

[+DrVenkman]

6 minutes ago, peco de guile said:

UPDATE:

 

Replaced all of the 74LS chips with new chips from mouser (except for the 74LS14N which is back ordered), no change.

 

Replaced the MMU and Pokey chips, NO CHANGE!  

 

I couldn't believe it.  Even with the new pokey chip, Donkey Kong audio was still wonky!  And even with the new MMU, it still froze on booting to Basic and became the hottest chip on the board.

 

Since RAM is the common usual suspect, maybe that's what I'll look into replacing next.  

 

I started off this repair saying "I'm not gonna just throw chips at it til it works", but I think without an oscilloscope to actually see pulses, I lack the abilities to more accurately diagnose this guy. 

 

Unless anyone has a suggestion for better steps forward?

 

Thanks for all ya'll's help!

 

 

I’ve kind of lost track on everything you’ve done and tried, but your audio problems even despite a replacement POKEY might indicate a problem with the audio transistor (can’t recall the silkscreen designation). You should consider replacing that.

 

As for the issue with BASIC, if a replacement BASIC ROM doesn’t correct it, consider replacing PIA if you haven’t already. 

[Beeblebrox]

@peco de guile  I've lost track too... but yeah, I'd replace all the ram to eliminate that being the cause of all the weird behaviour. Especially since it is so easy to do being socketed. Get yourself a cheap Dram tester so you can test each ram chip you remove and also the new ones you put back.

 

Also follow DRV's advice.

[peco de guile]

Replacing the 74LS chips, MMU, and Pokey is actually all that I have done.  The rest was all diagnostics.  

 

I'll look into a DRam tester and see about ordering a Basic ROM, Audio Transistor, ROM set, and PIA.  

 

And I very much appreciate ya'll hanging on for the ride! 

[Beeblebrox]

@peco de guile ya welcome. That is one of the reasons for AA. I've had loadsa help and advice over the months.

 

IMHO replace or at least test all the RAM chips first. Dram testers are dirt cheap. You Ram is all socketed so dead quick. might save you forking out for other ICs/passive components.

 

Re a dram tester - I have this one - cheap and really good quality.

https://www.ebay.co.uk/itm/124726020732

 

(Where the 16pin Dram chips go in the socket on the right BTW (or they get fried):

Edited June 23, 2022 by Beeblebrox

[peco de guile]

both of those sockets are 16 pin.  Why would the left socket fry them?  

[Beeblebrox]

31 minutes ago, peco de guile said:

both of those sockets are 16 pin.  Why would the left socket fry them?  

@peco de guile Because the left hand socket relates to RAM ICs that have different voltage requirement.

 

Check out the listing link i provided:

 

Supported Parts That Need -5V(pin 1), +12V(pin & +5V(pin 9) (LEFT SOCKET)

• Mostek MK4027P
• Motorola MCM4096
• Mostek MK4108-x0
• TI TMS4108-x0
• Mostek MK4108-x1
• TI TMS4108-x1
• ITT/STC 4116
• Mostek MK4116

 

Supported Parts That Use a Single +5V Supply (RIGHT SOCKET)
Please do not put any of these parts in the left-hand socket. Testing any of these in there could damage the part and/or the tester

• Mostek MK4516
• Hitachi HM4816
• Intel 2118
• TI TMS4532xL3
• TI TMS4532xL4
• OKI M3732L
• OKI M3732H
• Hitachi HM4864
• Micron MT4264
• TI TMS4164
• Samsung KM4128
• TI TMS4256

Edited June 23, 2022 by Beeblebrox

[peco de guile]

ah, sweet.  I went and looked for one in the States, but to no avail.  

Would this DRAM tester also be useful in testing RAM in other vintage computers?  I have a C64, C128, Atari 520st, and Amiga 500 to eventually look at.