Mega ST and TOS 2.06 decoder board

Upgrade or replace the ROM.
coonsgm
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Re: Mega ST and TOS 2.06 decoder board

Post by coonsgm »

Thank you for the help with this. Yes, it appears that my mega is littered with IMP chips (yes to the GLUE and MMU).

The board is a Mega 4 Rev 5 board. I have a 520ST (not STF) parts board with working non-IMP GLUE, MMU, DMA, and Shifter chips.

I've swapped out the IMP chips for the non-IMP. Error is different now, 3 bombs with blitter in or Atari splash screen that doesn't move past with blitter removed.
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exxos
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Re: Mega ST and TOS 2.06 decoder board

Post by exxos »

coonsgm wrote: Tue Mar 02, 2021 11:39 am Thank you for the help with this. Yes, it appears that my mega is littered with IMP chips (yes to the GLUE and MMU).

The board is a Mega 4 Rev 5 board. I have a 520ST (not STF) parts board with working non-IMP GLUE, MMU, DMA, and Shifter chips.

I've swapped out the IMP chips for the non-IMP. Error is different now, 3 bombs with blitter in or Atari splash screen that doesn't move past with blitter removed.
When you remove the blitter, are you soldering the 2 jumpers over ? Also what blitter have you got ? Only the later ones can work without a blitter patch.

See bottom of page https://www.exxosforum.co.uk/atari/last/MEGAST/index.htm
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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coonsgm
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Re: Mega ST and TOS 2.06 decoder board

Post by coonsgm »

Just soldered the 2 sets of jumper pads...bombs.

In answer to your earlier question, when I removed the blitter patch (and added the bodge to the bottom of the board) the computer worked just fine (as did the blitter)...rock solid. The blitter is a National S8912

Looks like my attempt with the Mega isn't going to work out.
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exxos
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Re: Mega ST and TOS 2.06 decoder board

Post by exxos »

TBH the MEGA ST's are the most unstable machines Atari produced. While we have the mandatory fixes, that covers all the known common faults I have come across many times and its fixed those. Unfortunately, Such issues are accelerating to the point its a fulltime job to keep diagnosing them all. Its why I designed a new motherboard.

I'm sorry I can't really suggest anything else, unless you send you machine to maybe @PhilC as I assume he is who you got the TF536 from and knows about them, and see if he can find anything amiss. Though it really boils down to endless troubleshooting, or get a different motherboard and try again.
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
Just because a lot of people agree on something, doesn't make it a fact. ~exxos ~
People should find solutions to problems, not find problems with solutions.
coonsgm
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Re: Mega ST and TOS 2.06 decoder board

Post by coonsgm »

Hey no worries at all! I'm just glad you and others are out there trying and making things better for many people keeping these fun computers going.

I really appreciate the help in troubleshooting and making certain I didn't miss something obvious. Your guides, boosters, and insight through the forums have helped me through quite a bit on my other ST's.

This is the risk we take when working on these computers...if anyone wants to buy an Atari flashed TF536 + TOS decoder board, I now know someone willing to sell at a low combo price :D
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PhilC
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Re: Mega ST and TOS 2.06 decoder board

Post by PhilC »

Just to add to this, I've had 3 out of 4 Mega STs work with a tf536 but some needed a bit of persuasion, ie caps fitted to the TF536.

Also I've noticed that to get the TF536 working on one of my boards I had to get rid of the motherboard ram and fit something like the 4mb exxos upgrades.

I'm happy to have a look at your Mega for you but with all reasonable costs paid obviously. Presuming you're in the UK?
If it ain't broke, test it to Destruction.
coonsgm
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Re: Mega ST and TOS 2.06 decoder board

Post by coonsgm »

Phil,

The main issue I'm experiencing is that I can't get the TOS decoder to work with the original 68k CPU. The TF536 seems to work just as well as the 68k does....which means that I can get to the 2.06 desktop but as soon as the computer tries to access the disk drive I get bombs. If you have other ideas on getting the decoder board to work, I'm 100% willing to try them out. IMP chips for everything but the blitter. I can swap for non-IMP chips from a donor 520ST

Unfortunately, I'm located in the US so shipping would just be too much for the whole computer or even just the board.
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Re: Mega ST and TOS 2.06 decoder board

Post by exxos »

coonsgm wrote: Tue Mar 02, 2021 2:40 pm Unfortunately, I'm located in the US so shipping would just be too much for the whole computer or even just the board.
Shipping will cost a fair bit, but it might be worth just looking up the cost of sending the board ? they don't weigh that much, if you get the whole thing under 2KG it really helps keep costs down.

I guess potentially if @JezC wants to buy it from you.. then the stuff will be in the UK anyway...
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
Just because a lot of people agree on something, doesn't make it a fact. ~exxos ~
People should find solutions to problems, not find problems with solutions.
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PhilC
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Re: Mega ST and TOS 2.06 decoder board

Post by PhilC »

@coonsgm sorry, being thick here but does the Mega floppy work ok with tos1.04? I've had all sorts of floppy issues on Megas.
If it ain't broke, test it to Destruction.
coonsgm
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Re: Mega ST and TOS 2.06 decoder board

Post by coonsgm »

The floppy works flawlessly so long as:
  • the 68k CPU is in the original socket directly attached to the motherboard
    With either IMP chips or replacements(MMU, GLUE, Shifter, and DMA)
    With the blitter patch installed or removed (with the appropriate bodge wire)
It is only when the decoder is installed that the floppy access causes the bombing to occur.

I've rebuttoned up the mega in it's previous working state and all works just fine as shown in the photo...this is the baffling part to me at least.
IMG-1004.jpg
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