Mystery Mod

[Maxlean]

Hi there everyone! Got my first 800xl new in '85, and I'm trying to revive it.  Seems that it has a memory related problem, but I'll save that for another thread. I installed a Newell 256kxl way back in '86, and in the interest of diagnosis, I'm going to remove it, but I found that I had done another mod that I don't remember doing and I don't know what it's for.  I don't think its related to the 256kxl install as I've re-read the install instructions, and there's no mention of this.  It concerns chip u20.  It seems that I took the 74ls258 that is removed from u27 for the 256kxl, and piggy backed it on u20, utilizing only a few of the pins, and attached a bodge wire to the GTIA. any idea what this is for? I'm attaching pics of this. 

 

Also, on a different subject, can I just remove the 256kxl, reinstall a 74ls258 in u27, reinstall the 3k resistor in r32, and keep the 256k ram chips (I don't have the 64k chips anymore), and they'll just function like the 64k chips?

Thanks in advance!

Scott

 

[reifsnyderb]

Maybe one of these?

 

https://www.atarimax.com/freenet/freenet_material/12.AtariLibrary/2.MiscellaneousTextFiles/showarticle.php?105

 

 

Edit:  Never mind.  I was just looking at the pictures and didn't read.  I don't know about the u20 connection as that's video related.

 

Edited April 27, 2022 by reifsnyderb

[reifsnyderb]

Ok.  I looked up the datasheets for the 4264 DRAM chips (original) and the HM50256...which is what you appear to have.  Re-installing R32 will add +5 to pin one of the DRAM chips.  I think you can do as you suggest as re-installing R32 will just set address pin 1 to high.  But if your problem is one of the HM50256 chips your computer still may not work.  

 

That wire connected to the GTIA pin appears to be connected to pin 30 which is phi2.  The system clock.  I don't know what that mod is about.  Maybe someone else has seen it before.

[_The Doctor__]

mod may have been for extended mmu/memory selection...

[Maxlean]

Thanks for the replies everyone. Like reifsnyderb noticed, it appears that I tapped into the system clock on the GTIA and connected it to the 74ls258 that was piggybacked on chip cd4050 which is u20. And like it was mentioned, u20 handles video processing.  I still have no memory of doing this, but since no one seems to know what it is either, I should probably remove it too. Maybe it was something that I read in Antic magazine. At some point I'd like to install the ultimate 1mb. I assume this would mean I'd have to remove the 256kxl anyway? (I'm guessing they don't coexist)

Scott

 

[reifsnyderb]

While I don't have an ultimate 1MB (U1MB), the U1MB does add memory.  From what I remember reading about it, I think it needs the first 64k then adds the rest as extended memory.  Removing that 74ls258 shouldn't hurt anything, either.

 

[_The Doctor__]

There was a GTIA fix for certain chips that were manufactured out of spec... this doesn't look like it exactly ... I'd document and take pictures anyway just in case , you never know what someone did and for what reason... but it sometimes turns out to later to help solve more than just the mystery of what it was.

[ledzep]

Your other personality is going to be very pissed when he sees what you did to all his hard work building the first 8-bit A.I.  He's almost worked out all the bugs!  He might even prevent you from returning to consciousness next time he takes over your body.  Just leave a note on the odd wiring, ask if it's ok to reverse it.  If you see a "Don't touch it!" response in your handwriting, back away.

[Maxlean]

21 hours ago, ledzep said:

Your other personality is going to be very pissed when he sees what you did to all his hard work building the first 8-bit A.I.  He's almost worked out all the bugs!  He might even prevent you from returning to consciousness next time he takes over your body.  Just leave a note on the odd wiring, ask if it's ok to reverse it.  If you see a "Don't touch it!" response in your handwriting, back away.

I like that! Not surprising that I can't remember the specifics (or anything about this mod) after 36 years.  Heck, I'm grateful that I can still remember how to turn the darn thing on!

And on a related note, I de-modded the board.  Removed the 256kxl, reinstalled the 74ls258 that the 256kxl replaces, reinstalled resistor r32, and removed the mystery mod. Powered it up, and it's working pretty much as it was before demodding, as in it's randomly failing the memory test.  I had ordered 24 replacement 256k memory chips (cheaper in bulk) a couple of weeks ago and picked 8 to replace the ones that I had on the board from the 256kxl mod.  As expected, they seem to work fine on a stock board, at 64k of course.  However, I still was getting random memory test failures.  Tried reinstalling the original 256k ram chips, and was greeted with a black screen.  Installed 8 of the new chips, and it functions but still has the random errors. Eventually found a combination of chips that didn't produce memory errors, and then methodically swapped one chip at a time until errors returned. Thought I had settled on two bad ones.  One from the original set, and one from the new set.  Success, right?  Not quite.  While I'm still getting no errors, I can no longer replicate a faulty chip no matter what combination of suspected bad chips I substituted.  It almost seems that the longer I let the machine run, the more and more stable it became.  Now before anyone says it was a poor contact on one of the sockets, I had already used Deoxit on ALL the chips (fully socketed board).  Removed each chip, sprayed the socket and chip, and then reinserted multiple times.  All this on each and every chip.  So I don't think this latest round of memory swapping and success were a result of a poor connection, but you never know.  Now as luck would have it, I have a keyboard fault. The Q key seems to be electrically stuck.  I've never had an issue with this keyboard, and all of a sudden, the Q key is stuck. WTF?

Scott

[reifsnyderb]

Did you re-seat all of the other chips?  i.e.  GTIA, ANTIC, CPU, etc.

Edited May 3, 2022 by reifsnyderb

[Mclaneinc]

 

Retro computers, a wonderful doorway to the past with rich memories of the machine and of past happenings and yet, despite all this, they can drive us to despair at the same time

 

[+DjayBee]

3 hours ago, Mclaneinc said:

 

Retro computers, a wonderful doorway to the past with rich memories of the machine and of past happenings and yet, despite all this, they can drive us to despair at the same time

 

Quite similar to long-lasting wives.

[Mclaneinc]

Ha ha...I'm over 40yrs married, as the old joke goes, you get less time for murder..

 

I can't complain (regardless of the knife that is being placed near my testicles by her now)

 

 

 

 

Seriously, it is all good here, I found a good un...

[Maxlean]

6 hours ago, reifsnyderb said:

Did you re-seat all of the other chips?  i.e.  GTIA, ANTIC, CPU, etc.

Every chip on the board got the deoxit and reseat before any serious diagnosing was started.