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Atari 520ST, C025912-20 vs C025912-38 and Z-RAM expansion?


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Hi!  So I'm finally installing NOS of the Z-RAM expansion kit, after finding recommended memory on EBay.  I agonize over the manual, lined up the notches for all chips, carefully insert the MMC piggyback...  The 520 (with Rainbow 1.04 ROM) comes up with a white screen with wavering lines.  No GUI desktop. Sometimes I'll boot and it's all black, but reseating helps, but it does seem to be more finicky than I'd anticipated.

 

The manual states "C025912-38" for the MMU.  Mine is C025912-20.  Does the 20 signify only a manufacturing week, or could this version of the chip not be compatible with the kit?  (Granted, I see "8515" below, suggesting it's made on the 15th week of 1985...)

 

I looked for "C025912" in Search, and only one post - citing "C025912-38" - appears so I'm at a loss right now.

 

The manual also states to clip the 9 resistors and wire elsewhere if one has problems, but it's emphasizing that on the 1040ST models. I have not yet done this, as I want to rule out basic chip compatibility and to test putting back the enclosure to make sure the RAM assembly fits. I'd be worrisome if I clipped the resistors, redirected them with cable, and all for nothing.  

 

Addendum/on edit:  I have not yet tried removing half the chips to try this in 2MB mode. I can't imagine any problems if I try this device in 2MB mode. Will test that and write an updated reply afterward...

 

 

 

 

 

 

 

Edited by CommodoreDecker
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37 minutes ago, snarkdluG said:

Is it this? https://docs.dev-docs.org/htm/search.php?find=z-ram

 

Or did different z-ram exist? 

 

That looks the manual I've got, but with some minor alterations and omissions to verbiage, as my copy of the manual includes direct references to the 520 and 1040ST, not just the Mega line, and the troubleshooting section had similar verbiage, but with some differences, including mentioning the bombs during boot, and the PDF has omissions (my copy refers to the 520 and 1040 numerous times, including photo references).  Mine's probably a newer copy. I'll try to take photos the next time I'm in the work area.

 

I do have an update from additional testing:

 

I removed the second bank on the ZRam, leaving it at 2MB populated (for 2.5MB total). After plugging back in the ZRAM piggyback, the 520 boots up without wavy distortions -- so the wavy lines are likely not due to the ribbon cable causing interference. Either the ZRAM board has bad RAM, or the c025912-20 is incapable of addressing 4MB and I'd need a c025912-38.

 

That, and chances are high that I'd only need to clip the nine resistors and rewire - I think that's a part of the instability as well.  But before I do that, a RAM test is something I want to do and see pass as, sadly, TOS 1.4 doesn't show the amount of found RAM in the Help screen...

 

...but the disk that came with the kit cannot be read by my ST354 or Indus GT-100 drives, so I cannot confirm if the new RAM installed is good... But before I can confirm how much RAM is seen, I will have to get the Gotek up and running then find a disk image to copy...   I might have a PC floppy disk somewhere for my old laptop; if the ST can read PC-formatted disks, or if Hatari emulator can write to a floppy in ST's format then that'll be a little easier in expediting doing this RAM test...

 

 

  

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Isn't each chip 1-bit? So filling the whole pcb would be 2mb. Since is seems that it is a solderless upgrade you can only upgrade bank 1 and bank 0 is filled with 0,5mb on the mother board. 

 

Otherwise if you got a 1040 you need to disable bank 1 first. 

 

Could be that the first version of MMU couldn't handle 2,5mb.

 

But ram chip fault is a very common problem as well. 

Edited by snarkdluG
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On 11/25/2023 at 2:07 PM, snarkdluG said:

Isn't each chip 1-bit? So filling the whole pcb would be 2mb. Since is seems that it is a solderless upgrade you can only upgrade bank 1 and bank 0 is filled with 0,5mb on the mother board. 

 

Otherwise if you got a 1040 you need to disable bank 1 first. 

 

Could be that the first version of MMU couldn't handle 2,5mb.

 

But ram chip fault is a very common problem as well. 

 

 

I believe each chip is 1bit.  The board is also double-sided; each side having 16 sockets.  I went by the list of compatible RAM modules and opted for the Samsung KM41C1000 due to its recommendation, noting other brands exist.  When only the top 2mb is populated, the system boots and without screen distortion or problems that I currently know of. Once I configure it for 4MB, then the issues begin. I'm leaning toward the manual stating to clip off the resistors and redirect them as my other 1040's memory upgrade cited various resistors had to be clipped because of the 4mb limitation.

 

Until I get a disk that works or a Gotek image for a RAM diagnostic, I cannot confirm how much RAM is available right now.  As it's a 520STM, it's possible that the MMU I have in it is an older revision and not compatible with a full 4mb complement...  The ZRam manual indicated that a newer revision of the MMU also exists; I still haven't photographed the manual but it's a very different number (C100109?), with these newer MMUs designed for the 520 just to more cheaply get inventory sold out...   regardless, it might be best or simplest that I should scout around for the c025912-38 and equally suffixed c025915-38 GLUE chip... 

 

In re-researching, there's precious little about the -20 vs -38 suffix, it's as if 25912-20 doesn't really exist and only 25912-38 does...  I've a couple spare 1040STFM units - I can pop those out and see what happens...  Especially as the only thing I could find was this:  Atari 520 ST video issues - exxos's Atari, Amiga & retro forum (exxosforum.co.uk)  The -20 does indicate an earlier revision of these chips and there are some issues with some applications, and while that thread discusses video issues, one response piqued my interest:

 

The other chips may need to be -38 types as well.. 

 

So, yup, it's eBay time or I'll cannibalize my least-favorite 1040 that has the -38 types.  That won't solve my disk read issue, but I'll be looking up image repositories to find a replacement disk image so I can test the chips, and finishing the Gotek install... the chips were listed as operational, am hoping they are...!

 

  

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  • 2 weeks later...

A quick update-

 

 

Are there other 520ST upgrades that don't use the MMU piggyback? I loathe the idea of soldered options as all the on-board RAM has to be desoldered, which means a risk of frying the fragile trace contacts...

 

After hours of cleaning contacts and re-seating the chips, I do see 2.5MB of RAM, but the screen is loaded with artifacts, and the piggyback for the MMU easily wobbles and loses contact. 

 

 

Thankfully most game titles were made with 512MB in mind. A couple needed 1MB, but I don't think I've found them.  I think Captain Blood and SimCity worked well enough on 512K...  

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  • 6 months later...

I responded on another thread as I have this same board. It came in a 520 STFM I picked up on a Goodwill Auction and It seems to work fine on it. I see 2.5MB of Ram and everything is working fine.

 

I would like to get a copy of the manual you have and that diagnostic disk as I am getting ready to up it to 4MB.

 

Darkstar2k

 

On 12/10/2023 at 10:55 AM, CommodoreDecker said:

A quick update-

 

 

Are there other 520ST upgrades that don't use the MMU piggyback? I loathe the idea of soldered options as all the on-board RAM has to be desoldered, which means a risk of frying the fragile trace contacts...

 

After hours of cleaning contacts and re-seating the chips, I do see 2.5MB of RAM, but the screen is loaded with artifacts, and the piggyback for the MMU easily wobbles and loses contact. 

 

 

Thankfully most game titles were made with 512MB in mind. A couple needed 1MB, but I don't think I've found them.  I think Captain Blood and SimCity worked well enough on 512K...  

 

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