What to do with this Frankenstein Mega ST4

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theimann
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What to do with this Frankenstein Mega ST4

Post by theimann »

Esteemed community,

I recently purchased a MegaST 4 from eBay. I knew that there was something going on with it since one picture (https://photos.app.goo.gl/HpC8QAfjWPAasnGi9) indicated a modified TOS. Lo and behold, I had no idea how much of a Frankenstein I had bought.

Here are some images: https://photos.app.goo.gl/AP8FH6VrRaNfeFwL8

My hardware knowledge is limited but here are my preliminary findings:
  • This Mega (well, the CPU clock) is being accelerated, possibly up to 16 MHz according to the GEMBench Integer Division Benchmark results. Not by some fancy board but by a homebrew kit, possible along the lines of the instructions given in this article (https://www.stcarchiv.de/stc1992/09_gehtdoch.php)
  • For reasons I can not explain the RTC battery has been refactored into a makeshift battery pack. Well, at least it did not leak.
  • As expected the machine features custom ROM EPROMs to deal with the higher clock speed
As you can see this was not a professional job. I'm actually surprised that the system made it so far. So, my question is: what should I do with this machine?

Originally I planned to do a recap and fit some nice board giving me TOS 2.06 and IDE support (i.e. MonSTer or Storm/Lightning ST).
I'll definitely hold on to a recap, but I don't want to keep the makeshift accelerator as-is. So either I'll get rid of it, or I'll rebuild it. But I'm concerned that it wouldn't play nicely with a MonSTer or Storm/Lightning ST.

Since the speed gain from the existing kit is minimal at best (i.e. RAM access is still at 8 MHz) I have a small preference towards getting the board back into a stock state, but I might be missing some parts (GALs, oscillator?).

Your advice on the best path forward with this machine would be highly appreciated.

Best
Tom
Atari 520ST | Atari 1040ST | Atari Mega 1 | Atari Mega 2 | Atari Mega ST 4 |Atari 1040STE | Atari MegaSTE
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Arne
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Re: What to do with this Frankenstein Mega ST4

Post by Arne »

theimann wrote: Thu May 13, 2021 9:38 am
  • This Mega (well, the CPU clock) is being accelerated, possibly up to 16 MHz according to the GEMBench Integer Division Benchmark results. Not by some fancy board but by a homebrew kit, possible along the lines of the instructions given in this article (https://www.stcarchiv.de/stc1992/09_gehtdoch.php)
Your quotes don't match the pictures!
The CPU Speeder looks like a AdSpeed16. I guess that you need some SW to enable its L1 cache. Then the RAM access in GEMBench will rise to approx. 170%.
theimann wrote: Thu May 13, 2021 9:38 am
  • As expected the machine features custom ROM EPROMs to deal with the higher clock speed
:?: :?: :?:

The homebrew PCB with two GALs looks like an OverscanST extension.

Edit:
No, it's not an AdSpeed it's some Hypercache. But you may try the attached CPX.
MSCACHE.zip
Check out these links:
http://www.atarimania.com/utility-atari ... 33137.html
http://www.atarimania.com/utility-atari ... 35082.html
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theimann
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Re: What to do with this Frankenstein Mega ST4

Post by theimann »

Thanks a lot! I'll check out the cache and overscan capabilities this weekend.

Sounds like the add-ons actually provide some value, so I'll probably just clean everything up and not install any additional boards.

Re: overscan, I'll give it a try with a SM 124, according to http://www.atariancomputing.com/blog/ov ... icsupgrade I might be able to achieve 672x480. Would I need any software for that?

Edit: Found some overscan software at https://www.atari-forum.com/viewtopic.p ... 72#p322472
Atari 520ST | Atari 1040ST | Atari Mega 1 | Atari Mega 2 | Atari Mega ST 4 |Atari 1040STE | Atari MegaSTE
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theimann
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Re: What to do with this Frankenstein Mega ST4

Post by theimann »

I got the accelerator and overscan working thanks to your help.
Seeing this Mega ST 4 in its "1989 peak performance" is an absolute delight :D
Added performance results to the album linked in my first post.

As for the accelerator, MS_CACHE.CPX did the trick. In my case is was bit #6.

I was able to turn on overscan using the the OVERSCAN driver by Karsten Isakovic.
It works fine-ish with my LCD monitor (NEC MultiSync LCD 1970NX), I haven't tested it with CRT yet.
I attached the software to make it easier for others to find.

Thanks again, artik-wroc and Arne, you made my day.

P.S.: I'll recap the board and tidy up the extensions. Also, the TOS has apparently been patched in order to fix overscan issues.
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Atari 520ST | Atari 1040ST | Atari Mega 1 | Atari Mega 2 | Atari Mega ST 4 |Atari 1040STE | Atari MegaSTE
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Arne
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Re: What to do with this Frankenstein Mega ST4

Post by Arne »

theimann wrote: Fri May 14, 2021 10:12 am P.S.: I'll recap the board(..).
I really don't understand why people are eager for this task!
Take a scope, check the ripple/voltage and if it's fine then leave it alone.
The so called capacitor plague https://en.wikipedia.org/wiki/Capacitor_plague happened much later.
Just replace the infamous RIFA caps if the PSU has them.
As a bench PSU I use an AT PSU from my mother's 286. It's fine. No leaking caps - nothing. "Never change a running system" does have a clear message.
I never recapped any of my Atari PSUs. I have to recap two x86 mobo's because the caps of the onboard power regulator are definitely bulging but both were manufactured much later (see wiki link above). But no Ataris to this day - and I got a couple of them.
theimann wrote: Fri May 14, 2021 10:12 am Also, the TOS has apparently been patched in order to fix overscan issues.
I wonder why GEMBench just shows 130% for RAM access. Maybe it's a change in code-base but to be safe I suggest to download czietz' CoreMark benchmark https://github.com/czietz/coremark/rele ... atari_port. Your system should be on-par with a MegaSTE (16MHz + L1 enabled). https://github.com/czietz/coremark/wiki/Results
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