exxos's Atari, Amiga & retro forum

Sorry, long post, you can skip to "The problem" below if you don't want all the background.

I recently got a Mega ST 4 MB for a few euros as it had heavy battery leakage. The seller told me the it was booting but floppy weren't working.

I first cleaned corrosion as much as I could and tried to boot it up: As said, it was booting but no floppy detected. After a few seconds the screen started acting strangely, and finally decided to lose the Blue signal, giving a very yellow tint.

This motherboard had some re-work done in the past (don't know if's recent as it's not only in the corroded area), and I had to reflow most of the solder joints :pullhair:, which gave me the opportunity to socket the CPU (for easy upgrade) and the WD1552 and replace the internal ACSI connector (lot of corrosion on both) .
I started troubleshooting the floppy issue and quickly discovered the WD1772 was dead, replacing it brought back to life the floppy.
Although I was hopping to get it to boot off an external UltraSatan, the machine kept locking up and did not recognise the US :(
Four corroded traces and a few bodge wires later, I finally got the ACSI to work. In the meantime I discovered the cartridge port not working neither => Some contact cleaner, inserting and removing a diagnostic cart a few times, cleaning the diagnostic cart with alcohol each time and it finally decided to work.
:cheer:

Finally, on to the video issue, using the scope to probe the video signal between the shifter and the DIN-13 connector showed the 2N3904 transistor was dead on the Blue signal, I replaced it and did the Red and Green too, just in case. Just to be on the safe side, I also replace the associated diode and one of the resistor which didn't look good (each time on the 3 signals)

I rebooted to a very Pink screen :dizzy: ... Broken trace on the green signal, another bodge wire.

So I finally have a Mega ST 4 mother board, with a working Floppy, cartridge port, ACSI and video signal.. kind of.

The problem: The video signal is kind of washed out and I can't find why.I don't know if it was already an issue before I worked on the machine or not as the Blue signal failed only a few seconds after I initially booted.

Here are a few examples of the issue:


Using the same PSU, same cable, same floppy, connected to the same OSSC a 1040 ST gives the following picture:


So I scoped the video signal for Red, Green and Blue (Using the COLOR option of the Diag Cart, 50hz), but there is nothing obvious:


The only slight difference is the red signal being a bit above the green and blue ones, but apart from that, everything looks good...on the scope, not on the screen. :-? I can reduce the gain on the OSSC and I end up with a normal picture, but pretty dark.

Any clue to what can give this result ?

I think it would be better if you use the colour test on the diagnostic cartridge and posted that result.

Kind of looks like brightness is way too high. but do both of your machines have the same shifter revision in them as that could be a factor.

Also you say you replaced the diodes, but they can act as a voltage drop which depending if you replace them with the exact part number could also be a factor. But tolerances across diodes can happen per batch as well which complicates things.

I don't think you mention exactly what resistors you changed either.

Also really you should try with a normal TV with scart as maybe the OSSC simply doesn't like the MEGA ?

Is the PSU correctly outputting 5v ? A lot of ST's output 4.8V which could also be a factor.

Also checked L47 inductor isn't blown ?

Are you running that on an LCD or CRT. It looks like a CRT, but if it's an LCD I have seen some variance between machines on my LCDs. Auto adjust on the display usually fixed it for me.

Sorry late reply, I was on vacation for the past few weeks.
I’m running on an LCD as my CM8833 monitor just died a few weeks ago and I have no more CRT.
I checked voltage on the PSU, which is a steady 5.10V under load, the inductor is fine too. I checked a few transistors from my spare box and it seems some of them are a bit out of spec, i should have checked before soldering. Anyway, I went the lazy way and ordered the ST video DAC buffer from the shop, so it will bypass this circuit entirely and give a nice output.

Sorry to be slightly off-topic, but your OSSC output at medium res looks excellent and much better than mine... What settings are you using?

Ok, so have you tried it direct to monitor/TV and not via the OSSC?

It's be interesting to see if there is a difference. I tend to run my STs (Megas, STF/Ms, MSTE) all on VGA and not via my OSSC which was always a bit meh.

OSSC works great for everything else though.

Re: OSSC.

I take it that you all have "Allow upsamplex2" turned on? If not you won't get a good input sample as you need twice the input frequency to sample the waveform correctly. Basic signal processing.

Also, depending upon the display, the output option for multiplying the resolution can make a massive difference as many TVs will try to stupidly upscale and ruin everything if the resolution it's given isn't close to its native resolution.

stephen_usher wrote: 29 Aug 2023 22:52 Re: OSSC.

I take it that you all have "Allow upsamplex2" turned on? If not you won't get a good input sample as you need twice the input frequency to sample the waveform correctly. Basic signal processing.

Also, depending upon the display, the output option for multiplying the resolution can make a massive difference as many TVs will try to stupidly upscale and ruin everything if the resolution it's given isn't close to its native resolution.

I do have allow upsamplex2 for mono, the picture has really bad moire patterns otherwise (which happens going straight through to my old, 15khz capable LCD, interestingly) but I don't have the option available for colour as the monitor I'm currently using only seems to like passthrough for low/medium res. I'll be getting a different LCD in soon that'll hopefully do a better job (and possibly support 15Khz...), so I'll see then I guess. Thanks for the info!

Quick update on this one: I finally installed the video DAC and ran in a different issue. The picture was now correct in terms of colors but very very dark. The original video circuit was disconnected at the diode level, but still not a good picture at all. One strange thing, when reset was pressed I had a perfectly fine white screen but right after that the boot screen turned to a grayish white and after boot white were light gray and colours pretty dark.

At the same time the floppy (gotek or real floppy) started acting strangely, sometimes not working at all, so I decided to stop working on this motherboard, I suspect the damage done by battery leakage and the rework done by a previous owner on many solder joints might be causing all these issues.

As I had a Mega ST 2 with its own problem (damaged floppy connector and two bombs on boot), I repaired the floppy connector, transferred all DRAM from the Mega 4 motherboard, added a socket for the CPU and I now have a fully functional Mega 4.
I didn’t even reinstall the Video DAC on this one as the picture is perfect with the original circuit.


After soldering sockets and replacing all the chips I had a small error


Which was quickly diagnosed

And finally everything works

It was quite tedious, over 1300 solder joints to remove and more than half of that to solder afterward… at least now I have sockets for all DRAM.

I also replaced the original PSU with a Meanwell, next steps are ST536, VGA card and maybe 1.44 floppy.

:bravo: