Ebay Board 2 : The Mystery of the failed 6 chip to 2 chip TOS upgrade

[Elethiomel]

I think I’d start by metering out all of the address lines from the ROM to the cpu. It’s likely low ROM areas are fine as it boots to diag cart but something is corrupted after that.

Yup, I guess I'm just going to have to trace all of this out. I was hoping someone might say "Oh, yes, on that revision board you need to do X" :D
Right, if one of the high address lines is borked then the 6 chip might work as it doesn't use those high lines, just the lower ones.

Also the black foreground and white border also points to ram, so give the address lines and mad lines a text. I’m presuming ram tests ok with the cart?

The black and white is mono mode on another system with an RGBHDMI installed which is working AOK thankfully.

[rubber_jonnie]

Yup, I guess I'm just going to have to trace all of this out. I was hoping someone might say "Oh, yes, on that revision board you need to do X" :D
Right, if one of the high address lines is borked then the 6 chip might work as it doesn't use those high lines, just the lower ones.

Unfortunately there are very few scenarios where you can just do 'X' and it will fix the problem, and it comes down to just good old fashioned troubleshooting.

I'd agree with the others that tracing all the connections to the ROMs is a good place to start. On these old boards they are very mechanically fragile, and things like vias can be cracked, giving you an invisible fault, so you will only ever get results by buzzing out.

The cart starts early in the TOS boot sequence, so there is some positive in that because you know the ROMs are active to a point. I know yours is an ST, but I know from fixing Bretts STE, that there was a problem with the LDS line at the CPU, and it turned out to be a bad connection in the CPU socket, so if you've not done so already, then consider this too: https://www.exxosforum.co.uk/forum/viewt ... hilit=plcc.

[frank.lukas]

The Atari Service Diagnostic Rev 4.4 does not know about TOS 1.04 in two chip so the behave is normal and not an error !

Diagnostic Rev 4.4 does not know anything about TOS 1.04, not in two chip and not in six chip ...

520st.jpg

520st_crc_error.jpg

The Computer works fine with TOS 1.04

[Elethiomel]

Well guys, it turns out that this IS one of those "Oh, yes, on that revision board you need to do X" things!

Armed with some schematics, a multimeter, some tape and a pen, I traced out the decoding logic.

Everyone knows that to upgrade from 6 chip to 2 chip, you desolder the 256k pads, bridge the 1M pads, pop a 7411 in U68 if needed and plonk your ROMs in to sockets U67 (LO-0) and U63 (HI-0) right?

Nope! Not on a C070789-001 REV B you don't. You see, the output of the 7411 (the 1M CE line) doesn't go to pin 20 of those sockets to enable the ROMs.
It instead goes to the HI-2 (U48) and LO-2 (U53) sockets! Exxos notes in The LaST Upgrade Part 1 that this can be achieved on REV C boards by doing a mod. No mod has been performed on this board. That's just the way it's factory wired!

20220502_163045.jpg

ROMs installed in the correct place for this board revision

20220502_163151.jpg

Looking good!

20220502_164232.jpg

There's that Rainbow I was chasing :D





She's only got 512K of RAM so I think an upgrade might be next :)

[exxos]

Nope! Not on a C070789-001 REV B you don't. You see, the output of the 7411 (the 1M CE line) doesn't go to pin 20 of those sockets to enable the ROMs.
It instead goes to the HI-2 (U48) and LO-2 (U53) sockets! Exxos notes in The LaST Upgrade Part 1 that this can be achieved on REV C boards by doing a mod. No mod has been performed on this board. That's just the way it's factory wired!

:WTF: :dizzy:


Now I gotta go look for one of those revision boards now..

[exxos]

@Elethiomel Yes indeed it's reversed on the REV B :stars:

I think the ROM Order was reversed on some other machines but I thought this was only on the STM or something.. cannot remember offhand now.

Great find!

EDIT:

oh look https://www.exxosforum.co.uk/forum/viewt ... =56&t=2290 :lol:

[Elethiomel]

oh look https://www.exxosforum.co.uk/forum/viewt ... =56&t=2290 :lol:

Yup! Wish I'd seen that earlier :D Oh well, it was a fun problem and I worked though it and got the solution in the end. If everything just worked, hardware would be kind of boring :D

[JezC]

@Elethiomel Glad you sorted it...and doesn't it give you a sense of satisfaction to have worked it out yourself?

I'd noticed on a few of my boards that the two ROMs were in different sockets, so I'd (wrongly) assumed that it didn't matter :roll:

At least now I know as well! ;)

[atari030]

Well done. I've been watching this as one of my Mega's has the reverse problem, two chip works and six chip 1.04 doesn't. We could have just swapped ROMs. :? I might revisit that next, it's gotta be a trace.