520 STFM - no video sync - red screen when it syncs

[amazin]

Hi all, this is my first time attempting to repair an Atari ST, and although I can do a decent soldering job (including micro soldering) i'm not a technician, so my electronic debugging skills are pretty limited.

I was given a few Atari STs and I'm slowly trying to get them back to life, having succeeded with some STEs, but somehow, all but one of the STFMs are working: all of them have the same symptoms: NO VIDEO, and when connected to the OSSC Upscaler, I can see the frequency go all over the place.

This one in particular that I'm working on now, motherboard C070859-001 rev 2, with a RAM expansion by Frontier Software, model XtraRAM, had the same symptoms: no video AT ALL, video frequencies going all over the place.

I saw a post from Exxos that it's common for STFMs to have bad/old/dried capacitors in the video area, so I've replaced them all, no changes. Then replaced the ones on the left side, next to the power connector. It started to be more consistent after that, but all I can get is a red screen. I've already tried to remove the ROMs (original Mask ROMs), spray some contact cleaner, clean the ROM legs, reinstall them, no changes whatsoever.

I noticed a leaked (?) ceramic capacitor (C116), replaced it, no changes.

I'm waiting for two new PSUs to arrive (one from SideCartridge, one is a Meanwell RD50A -- plus I'll be ordering a few capacitor kits from Exxos later today), but I'm not entirely sure this can be a PSU fault...

Oh, one thing you'll see in the photos is that when it syncs to OSSC and display the red screen, it shows 15khz, that's correct, but it's synced at 60Hz, and this is an European machine, not sure it's relevant.

So please let me know what else to check, if this is something common. Anything helps.

Thanks in advance

Pics below:

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[olivier.jan]

Hi,
The 60hz is normal as that’s how ST boots, 50hz is set for European machines by the ROM in its early stage, but if ROM code is not running it will stay at 60hz.
I would recommend reseating all ICs, potentially removing the RAM extension and see how it goes.
Then you can check reset circuit if you have a multimeter or oscilloscope.

[stephen_usher]

Of course, there are the standard tests to do on the CPU:

• Power
• Clock
• /Reset
• /Halt

If all of these pass look for activity on the address and data buses.

Go over to the ROMs and check to see if they are being selected. After reset the CPU should read four long words (eight 16 bit word accesses) starting at address 0x00000000. After reset the MMU should enable the ROMs for these first reads from the low memory addresses. If the ROMs are no being selected check that all the address lines are low except for the bottom three. If they are and the ROMs are no being selected then suspect the MMU. It's probably just a bad connection, so just reseat it and use contact cleaner as you do. Test again.

If the memory expansion is clipped on over the top of the MMU then quite often this can be the problem.

[rubber_jonnie]

BTW C16 is not electrolytic, so I'd suggest that there may have been some other contamination, maybe a liquid spill because C16 simply can't leak, it looks like some sort of monolithic cap.

You may want to look around further for other signs of liquid damage like that, and possibly consider it may have come from a cap on the Frontier board.

[amazin]

Friends, some updates.

1) The memory expansion is a Frontier Software Xtra (non Deluxe). I found the install manual here: https://docs.dev-docs.org/htm/search.php?find=xtra-RAM

2) By reading the install manual, apparently it used to come with an adapter for the MMU chip, but perhaps not to the version on this motherboard, which has a SMD version (C101601-38) soldered directly to the motherboard. The original adapter, as far as I can imagine, used to go over the socket, on the socket version of the MMU. So whoever installed it, soldered wires directly to the resistors next to the MMU, except for a single pin, directly to the MMU.

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3) The installer also did something quite lovely: the shifter board was installed on top of the shifter, being soldered directly to it's legs... I...didn't like that, so I desoldered the Shifter (no damage whatsoever), and cut the pins connecting to the the Shifter's legs. Installed a socket and that's it.

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4) So the problem persists, it's only showing the red screen, sometimes the video sync frequency goes all over the place (from what I can see from the OSSC upscaler's LCD screen), but most of the time, 80%, it syncs and displays the red screen.

So at this point, without an oscilloscope, I can't measure the clock on the CPU pins. I'll try to get one of those cheap portable ones.

So I think that's it for now, thanks all for the help and suggestions!