exxos's Atari, Amiga & retro forum

Instead of posting various messages when I break stuff(mostly Atari), let start a topic.

I’m currently trying to build my “main” machine in order to replace my current Mega STE which is both too big for my desk and lack alt-ram. I settled on a Mega ST as I like the format and it’s easier to customize than the Mega STE.

The story so far:
I got a Mega ST 2 without floppy and bombing on boot, most likely due to some RAM issue. This Mega 2 is one of the later revision with room for extra memory chips, so I socketed everything and replaced all memory with chips from a Mega 4 donor board.

The motherboard came with a French “Mode HD” from 96 to allo HD floppy, so I installed a Sony MPF 920 with proper changes to get it to work on the ST and in HD. Instead of recapping the PSU, I swapped it for a Meanwell RD-65A and changed the fan for a Noctua.

After checking everything was OK, I installed a ST536 and thanks to Exxos’s help got it to work after doing the mandatory fixes. I still lack the custom TOS, but will install it as soon as I receive new EPROMs to burn it.

Now, starting the next (and hopefully last phase of the hardware mods: Installing an STGA I got from Frank Lukas a few weeks ago. I tried to get it working but no luck so far, no signal from the VGA port. :(

An ET4000 (with MUSIC ramdac) is plugged in the STGA with -12V and power connected from the auxiliary power connector on the motherboard. The Auto folder contains in the following order:
- Maprom
- Blitfix
- Emulator
- Menu_e
- STA_VDI

But no signal is detected by the VGA monitor. I tried a few different resolutions but no success so far.
Next steps will be to check the ET4000 on the MegaSTE currently using a Mach32 and if it’s confirmed working(it was working in the MegaSTE until I got the Mach32), start hardware troubleshooting.

The only potential issue is the -12v which drops to -9.9v when plugged to the STGA, but I v’e read somewhere it’s kind of normal and shouldn’t be a big deal.

I’ll look into that this week or next weekend.

There are T0, T1 and so on to T8 NOVA driver packages for differnent DACs and Cards.

When you start the emulator program manual with the emulator.bib in the same folder you become a 640x400 Pixel mono Desktop over the graphiccard if you have the right one ...

Don't know any "MAGIC" RamDAC? There are MUSIC RamDAC´s that usually need the NOVA T0 driver and work with 256 Colors max..


The NOVA driver do not use the Blitter (same with NVDI) and switch it off.


-12V break down to -9V and that is OK for the graphiccard to work.

Yes, it’s a MUSIC RAMDAC and I’m using the T0 version, but there’s no signal on the graphic card. I will also try with a different monitor, this one can be a bit capricious with some frequencies but it usually says it can’t display the signal.

Start with 640 Pixel and may be 1024 Pixel. I have a ET4000 that put no picture on the Monitor with standard 60Hz 800 Pixel, it works only with 40Mhz Pixelclock at 800 Pixel ...

You can boot with nothink and start the VMG and load the sta_vdi bib ...

Ok, new day, new tests on the STGA/ET4000:
1. Tested the ET4000 in an old 486 - Works fine.
2. Continuity tests between 68000 pins and Mega ST bus - All OK
3. Continuity tests between the STGA 5v/GND/12v/-12v and the ST - all Ok
4. Tested plugging in the STGA without IDE connector (it’s really in contact with STGA card when fully inserted) - no change.
5. Used the scope to check Hsync and Vsync on the ET4000 - NO SYNC, one is 0v, the other is 5v, but no sync signal at all.
6. Detected a missing capacitor ont he STGA board, soldered a 4.7uF as per schematics - no change.
7. Tried different screens, including OSSC - no change.
8. Tried different resolutions using VMG.PRG - no change.


So ET4000 is fine, connectivity to the ST motherboard and power are fine too but there is absolutely no signal. I also noticed that the power on the STGA is very very noisy. Before adding the 4.7uF capacitor I had nearly 2v peak to peak on the GAL VCC pin (and others).
The capacitor helps a bit, but it’s still very noisy while on the ST motherboard, the peak to peak on ICs VCC is barely a few hundreds mV. I tried disconnecting the -12v but it doesn’t change the noise.

No more time to look into this before Friday, I’d like to find a way to reduce the noise in order to discard this as a potential cause.

New round of tests today:

Installed the STGA and ET4000 in a stock Mega ST 2. It works !… kind of… there’s actually some video corruption (see below) but it might be due to the same issue as the one preventing it to work on the Mega 4, or simply because it’s a stock machine without mandatory fixes.

It depends on how and where you measure. The mass/GND point is important where it lies.

There is a saying here: "If you measure a lot, you often measure nonsense"...

Noise on the 5 volt VCC is overrated and not the problem.

Because of the pixel garbage on the graphics card please test...

- Run the STGA with the original 68000 CPU
- if you use a relocator CPU board for the SF536, test without it. (you can build a Mega ST bus extension to try it out)

I just tested on the Mega 4 with Original CPU (No relocator) and same result as with the stock Mega 2 : I get a picture, but again with pixel garbage.
To give further information, as I have no decoder in the Mega ST, when using original CPU it uses TOS 1.4 while with the ST536 it's a vanilla TOS 2.06. The Mega ST 4 has mandatory fixes applied.

PS: I get your point on the GND point and measuring non-sense, trying desperately to find a fault leads to bad interpretations, my bad.

How much ET4000 ISA cards do you have for testing ?