(Almost) Unknown Atari ST schematics

[Mr. Undo]

Hi!

 

A few weeks ago I discovered a pair of schematics of the 520ST on eBay. Looking at them in detail it struck me they still included a 128K ROM in the design and the fact that they were drawn at a very early date, January and February 1985.
These aren't any of the 520ST schematics that can easily be found on archive.org or any of the existing online Atari repositories so, although they are just copies of the originals, I decided to adquire them.

 

 

They have a lot of historical significance as they are the first ST schemes that we have evidence of so far. I've scanned them at high resolution, cleaned and uploaded to archive.org

https://archive.org/details/atari-520-st-schematics-jan-feb-1985

PS: I say they are almost unknown because in 2009 @Cebus Capucinis found these same schematics inside the box of a 520ST that he acquired in 2009 and uploaded some images of them in this forum.

 

 This is the only previous reference I have been able to find on these schematics.

 

 

Edited April 22, 2021 by Mr. Undo

[Cyprian]

nice finding, I wonder what elso was different from the regular 520 ST?

[Mr. Undo]

I let here the schematics I've just uploaded to archive.org (drawn on Jan. 29 and Feb. 4 '85) and the already known schematics for the 520ST/260ST (drawn on June 5, July 16 and July 22 '85). It would be great if someone who can fully understand these schematics could tell us the differences

Atari_520ST_Schematics_Jan_Feb_1985.pdf Atari_520ST_Schematics_Jun_Jul_1985.pdf

[Mr. Undo]

The earliest 520ST motherboard found up to now is labeled as C070115 REV. 2

 

 

The schematics found have drawing number C070112. Really close!.

 

Besides the change of ROM, (192K in 6 32K chips instead 128K in 2 64K chips) I don't know if there's any other variation

[ParanoidLittleMan]

There were much more variations. Even listing them all is long thing.

Just this:  I bought 520 STM in 1987, April, in Germany. It was with 6 ROM chips, TOS 1.00 De, and very similar layout as is shown on photo in previous post.

But there is modulator, PAL encoder, and PCB part where they go is different. Even under TOS ROMs are some added resistor packs.  As I see, most diff. is at mouse and keyboard connectors. REV H.

Then, when TOS 1.02 and blitter came in game - poor 520 ST was still without blitter, but there was added 74LS11 under ROMs - that's needed for 2 chip TOS ROM variant. That would be REV I - you see ? Many letters later in ALPHB compared to 2? And I have 2 of them, with pretty different modulator and PAL encoder placement.

They should do as it is by street addresses:  Bond street, rev I/A, and for other Bond street, rev I/B . Sorry, not possible - no more free space on PCB ?

 

[Mr. Undo]

2 hours ago, Mr. Undo said:

Besides the change of ROM, (192K in 6 32K chips instead 128K in 2 64K chips) I don't know if there's any other variation

Maybe I didn’t explain myself very well here. In this case I’m wondering if there’s any other variation (besides the change in ROM) between this specific PCB (C07115 REV 2) and the drawings of the schematics I’m referring in this topic, which are numbered C07112.

 

Regarding PCB versions, you’re right, the list is long. Here is a good compilation, maybe still incomplete:

https://temlib.org/AtariForumWiki/index.php/Atari_ST_motherboard_revisions

 

Quote

Sorry, not possible - no more free space on PCB

?

 

 

Edited April 27, 2021 by Mr. Undo
pcb labels

[Mr. Undo]

There is also a C07115 REV 2B PCB

 

https://imgbb.com/7KLXgY3

 

not listed here:

 

https://temlib.org/AtariForumWiki/index.php/Atari_ST_motherboard_revisions