exxos's Atari, Amiga & retro forum

I have been experimenting with the ST/F/M / MEGA ST-RAM 4MB UPGRADE [0035] on my 520STM C070243 Rev H motherboard.

It is working across the bench but there are a number of issues to resolve:

1) After fitting, faint vertical background bars occur on the screen display at 16 pixel spacing (medium resolution) and 24 pixels (low resolution), this is particularly noticable on the bright green background - less so on the dark blue background of the diagnostics rom cartridge. I suspect that the extra noise load on the PSU may be causing this? I have a Cisco 3A 5V, 2A 12v, 200mA -12v psu.

2) There is a physical collision between the ribbon RAM cable as it comes out of the shifter box with the case mounting pillar. The ribbon connector needs to be atleast 3 mm further over to the right.

3) If you socket everything then the shifter displacement board is 21.8 mm high too high for both the shifter shield and for the upper mother board shield. - You can win ~ 6 mm for each socket you remove so if solder shifter IC directly to displacementboard and solder the displacement board directly to motherboard it should fit (it is then 12 mm lower ie 9.8 mm high which will fit under the shifter shield. (using IDC 1 row square headers instead of the round pin would win a couple of mm.)

4) RAM board seems suspect in the PLCC socket, it is not what I would call firm - I suspect that it wil vibrate out if the main unit is shipped anywhere. - perhaps a fixing clamp of some sort would be warranted. -Not the original MMU has a sprung retaining clip to hold the MMU chip in the PLCC.


Has anybody else successfully upgraded their C070243 REV H? What did you do about the above issues?

Thanks

Peter

Interference is very hard to get rid of on these machines. Noise comes from the power supply but a lot is just inherent in the design of the motherboard.

How you fit the shifter board really depends on personal preference. If everything is socketed then of course you cannot close the lid on the shifter can. It would have to solder the shifter directly into the PCB and then solder the board directly into the motherboard. But I don't think people really bother doing that anyway.

Probably would not hurt to put a little bit of insulation tape under the cable when it comes out of the shifter can either.

I'm also assuming you disable the on-board RAM.

@microman noise is inherent to these machines, the output is really bad on all of them. If you think that is bad check anything that has black or dark blue backgrounds, like Frontier, it's horrendous.

The ONLY way of solving the problem is by getting yourself a RGBtoHDMI Pi device for Atari. I use this on my STM machine and it gives pixel perfect HDMI output.

In regards to your concerns around the MMU connector... well, it's just part of having that kind of upgrade. It's solderless and easy to install, but it has it's limitations. Besides hot-glue I'm not sure what to suggest for you.

Exxos *is* producing a 'solder-in' 4MB upgrade kit for the STM soon (if not already) but yes, it is much more difficult to install as it involves removing all the RAM on the motherboard.

Topic on RGB to hdmi:
https://www.atari-forum.com/viewtopic.p ... 4de39f7477
A guy selling built units:
https://www.sellmyretro.com/offer/details/61574
Solder-in board:
https://www.exxosforum.co.uk/forum/viewt ... =15&t=4557

I 'borrowed' the 4Mb board from my 520STFM C103253 Rev.1 and built a separate shifter displacer board as I need to fix a fault on the STFM and am considering the best options - least hassle for best result to upgrade the RAM on both of them. If this had been a total success I would have upgraded the STFM with SM RAM chips to 1 Mb and used the 4Mb on the STM. But at present the jury is out.

Steve, Thanks for your comments on the new solder in RAM board for STM.

Re the video background issue prior to the 4Mb RAM upgrade I had not seen it on the STM nor on the STFM. I feed the SCART into a SCART -> HDMI adapter that seems to work quite well on other vintage kit.

Looks like a long slow process with the scope and looking for earthing issues / decoupling.

Peter

Yes onboard RAM is disabled - R133 & R134 feeding CAS from MMU to RAM lifted at MMU end and a wire added to +5v on the now free end of the Rs.

Peter

BTW I note that several designs do something similar to RAS but on the C070243 RAS goes direct from MMU to RAMs and not via a resistor.

I would just slap some 4700uF caps on the 5V rail on the MB near the MMU somewhere. Also don't forget if you have the old RAM in there still, you are probably overloading original power supply a fair bit as well.

I thought that was the main point of lifting CAS and to tie it to +5v was to disable the onboard RAM and reduce the power consumption.

Peter

microman wrote: 17 Feb 2022 22:54 I thought that was the main point of lifting CAS and to tie it to +5v was to disable the onboard RAM and reduce the power consumption.

Disabling the RAM simply means it does not conflict with the other memory upgrades. This should only be done as a quick test to confirm the new upgrade works before removing the old memory.

The RAM still powered up from the 5V rail.. Still consuming power.. And likely is contributing to your noise problem.

Hey there, so I have just removed my original RAM and capacitors. Do I need to remove any of the resistors or undo the old RAM isolation soldering?

I'm getting a black screen (white border) and am assuming that I missed something. Any help would be appreciated :)

Fixed it....had a thin whisker of solder connecting two (now empty) RAM solder pads that I found while playing with my multimeter.