ST536 + mandatory fixes = help needed

[szeremiocki]

Hi,
I am one lucky owner of Mega4 with st536 built for me by @LarryL, who not only built it, but also had the great patience to support me in it installation. But I think it is time to take this to the forum :-).

Because removing of 68000 CPU went somewhat difficult for me (with motherboard trace needed repair) I was hesitant to desolder ROMS, affraid of ruining them (1.4 UK in two chips). So, I installed ST536 with onboard ROMs in place; and it worked! Not everything worked, I was having some issues, but most of thing of interest worked, including STGA+ET4000. Also worked: floppy drive, cartridge slot (my only cartridge is 'sidecartridge multi'), ST536IDE, ACSI UltraSatan... Next week I decided to desolder ROMs and install sockets. It went pretty well, so now I have some flexibiity here, which will get important. Since that weekend, my Mega 4 was working equally well with or without onboard ROM chips and I was eager to start more soldering practices: mandatory fixes!

So, this Saturday I did:
* Bus resistors (viewtopic.php?f=17&t=2235#p27504)
* 1772 pull ups (viewtopic.php?f=17&t=1395)
* DMA Pull up on databus (viewtopic.php?f=17&t=1417)
(in this order)

BUT already after bus resistors replacement (4.7k->2.2k), my Mega4/ST536 started misbehaving: floppy stopped working, cartridge stopped working also. And it wasn't better after next two fixes: both fdd and cartridge stll invisible.

So then I removed ST536 and inserted OG 68000; of course, it needed ROM chips also... Powered up, FDD AND cartridge worked! So I eagerly inserted ST536 back again, not removing ROM chips, and:
* without ROMs onboard, both FDD and cartridge are NOT WORKING
* with ROMs onboard, and without cartridge, FDD is NOT WORKING
* with ROMs onboard, and with cartridge inserted, FDD is WORKING (cartridge itself seems not working -- can't force it to whatever function).

I am really lost here. I see two options:
1. bring back 4.7k resistors on RP2/RP3/RP4/RP5/RP6
2. ask you guys if removing ROMs from socket was sufficient as a enabler for ST536 (maybe there is something more to it)
3. (because cartridge port obviously has some say here) bring back only RP3/RP4RP5/RP6 (these are for d0..15 and a0..15, going also to cartridge slot; RP2's a16..23 is not in cartridge's reach)

What would you propose as a next step?

Thanks in advance
Regards,
Przemek Szeremiota

[exxos]

You should remove the MB ROMs and use the ST536 ROM for booting. Make sure you use my build of TOS206. https://exxosforum.co.uk/atari/last/TF536/ST536a2.zip

I don't have any suggestions for the MEGA ST unfortunately, I gave up supporting MEGA ST with projects as there was endless issues, more than the STFM and STE. IMP chipsets will likely end in endless problems.

You could try the STE536 firmware ( I haven't worked on it for some months, so the ones below *might* be a later build, I don't remember the difference) as its updated over the ST536 firmware, but it's totally untested on the ST536 and may malfunction worse as its designed for the STE.

STE536_UNKNOWN_VERSIONS.zip

Also note if you had a blitter patch, there is a cut trace under the CPU which needs fixing.

[szeremiocki]

You should remove the MB ROMs and use the ST536 ROM for booting. Make sure you use my build of TOS206.

Yep, ST536 uses your TOS206; but without MB ROMs fdd is not working; and I would ask: „remove MB roms” means just removing them? No jumpers to close, CE signals to cut etc?

Also note if you had a blitter patch, there is a cut trace under the CPU which needs fixing.

It is fixed (but I will check it again)

I’ll try reversing RPs to 4.7k and report back

Thanks for quick reply,
Przemek Szeremiota

[exxos]

and I would ask: „remove MB roms” means just removing them? No jumpers to close, CE signals to cut etc?

Just remove the ROMs.. They are not in use really because they are TOS104 in general and TOS206 decoder will not activate them. But its better to remove them anyway.

I’ll try reversing RPs to 4.7k and report back

Currently bus resistors changes haven't been tested with 030 boosters. The mandatory fixes mostly assume a stock machine, or use one of my 68000 boosters. Unfortunately I haven't had time to do such tests with 030 boosters with 2.2K. It shouldn't matter... but it's only a guess. I would guess 4.7K would work fine with the 030 boosters.

[Steve]

My biggest tip when dealing with bus resistor changes is: install sockets. Just normal turn pin sockets. The amount of people breaking the traces leading to the bus resistors is very high. I always say: if the resistor literally doesn't fall out without any force, you've probably broken a trace.

Then with sockets installed you can experiment with different values without any fear of breakage.

[szeremiocki]

That is actually a great tip! — and a starting point to carefully check in my case, thanks!

[szeremiocki]

Just to report:
I have managed to restore 4.7k(D-lines)/10k(A-lines) combination on RP3/RP4/RP5/RP6 (with sockets!) and fdd and cartridge slot are back in operation

In the meantime, trying 4x4.7k for all of theese, I regained fdd+cart but lost video :) So for now its back to schematics values — except for RP2 still hanging on 2.2k (will do it today)

Regards,
Przemek Szeremiota

[exxos]

I suspect the changes are pushing something else over the edge rather than the changes directly effecting things. Like I've said before , the chipset and clocks are bad on the MEGA and some have resistors in series with the 16mhz clock and others don't which further complicates things.

Have a look here..
https://exxosforum.co.uk/atari/last/MEGAST/index.htm