exxos's Atari, Amiga & retro forum

I picked up a 520 STFM from ebay, advertised as "for parts" because it didn't boot. I wanted to take a look see if it was easily repairable. Presumably it worked, once.

Symptoms: when I flip the power switch, the green power light on the keyboard comes on. My screen registers a power signal and flips to the VGA input (output from the Atari monitor port via a VGA adapter), but it stays totally black. Floppy drive makes no noise, light doesn't light. Pressing the warm reset button resets the screen but it comes back fully black.

The board is reasonably clean:


Initially I thought the PSU was handing out bad voltages and hence no startup, but better testing reveals it gives out +5V and +12V as expected. I have a fairly simple multimeter so I don't know how stable that power is. I would like to re-cap the PSU just because it's old, but there's little point if I can't make the whole thing boot!

The CPU, the sound chip, and the keyboard socket all have 5V across them, so as a starting point I think the board is probably good.

Suspicious things:

The unit had one of the Marpet Xtra-RAM upgrades inside. I've removed the daughterboards for now, though I might replace them later if I can get the base unit to fly. Two things are left behind from that:

This socket over the video shifter; I guess this is how they worked around the surface mounted video shifters:


Resistors R59, R60, and R61 clipped, and possibly a bonus resistor soldered in alongside?:


These arrived as you see, not connected to anything unlike what the Marpet instructions suggest. For those resistors, I'd consider just removing the hot glue and resoldering each of them BUT the Marpet manuals only suggest clipping R60 and R61 to disable memory bank 1, as far as I see. So I'm not sure what the correct approach is with these. Resolder them all back into place?

So before I spend time on dead-ends, opinions on this system, please? What should I poke to test the motherboard, and what should I do with these resistors, and is there likely anything else wrong based on the symptoms?

stween wrote: 22 Sep 2018 19:18 Resistors R59, R60, and R61 clipped, and possibly a bonus resistor soldered in alongside?:

These arrived as you see, not connected to anything unlike what the Marpet instructions suggest. For those resistors, I'd consider just removing the hot glue and resoldering each of them BUT the Marpet manuals only suggest clipping R60 and R61 to disable memory bank 1, as far as I see. So I'm not sure what the correct approach is with these. Resolder them all back into place?

So before I spend time on dead-ends, opinions on this system, please? What should I poke to test the motherboard, and what should I do with these resistors, and is there likely anything else wrong based on the symptoms?

Well those resistors have been cut to disable the on-board RAM, and you have removed the Marpet, so there's nothing strange about the computer not being able to boot without RAM :)

The system didn't boot with the Marpet RAM In place, but that part doesn't surprise me so much.

Is clipping R59 related to that upgrade? I mean, I don't doubt it, I just don't see it in the Marpet instructions :)

Also: that resistor at the front: it's not a custom addition, is it? I don't see it on other images of this board revision that I've seen (but maybe my eye isn't good enough).

Well, if it was me, I'd pop the marpet back in but clean up the connections for the simms and reseat them, then remove the Glue and MMu and check for bent pins in the socket and reseat them too.

Then the rest will depend on whether you have access to spare parts, are good at soldering and fault finding and of course, how much it's worth to you to get it up and working.

The shifter chip is also needed. Remove it from the Marpet board and reseated in the socket in the metal housing in the middle.

DoG wrote: 22 Sep 2018 20:40 The shifter chip is also needed. Remove it from the Marpet board and reseated in the socket in the metal housing in the middle.

The shifter chip is surface mounted, with the empty socket on top for the Marpet board to hook into. Seems that's what they did on this motherboard revision.

Aah, sorry. Didn't see that is was under the socket.

The shifter is mounted normally (THT), the socket is soldered onto it. RAS/CAS resistors are disconnected, and +5V via additional resistor is fed into these lines to keep onboard RAM inactive.

tzok wrote: 23 Sep 2018 11:34 The shifter is mounted normally (THT), the socket is soldered onto it. RAS/CAS resistors are disconnected, and +5V via additional resistor is fed into these lines to keep onboard RAM inactive.

As a quick start, I've desoldered that additional resistor and resoldered all three original 68 ohm resistors back in place. Resistance across the pads looks good, so I think the solder job was okay.

No improvement on the symptoms so far. Haven't popped the MMU or Glue chips out for a look yet though.

My experience with the Marpet style RAM upgrades is that the MMU header can often displace the connectors in the MMU socket, and in fact I had exactly this issue on exactly this model of mainboard not long ago.

One part of the puzzle with this particular board is that the MMU is actually fitted the OPPOSITE way around to other boards I have, in other words, it's a 180 degree flip, so when installing the MMU adapter for the RAM upgrade, you also need to flip that 180 degrees too. That had me foxed for a few moments unti I noticed it was different to other motherboards in my collection.

I also ended up having to do a complete reflow of every last solder joint on the MMU and Shifter adaptors, and also the SIMM board.

Once that was done, everything sprang into life with 4MB of RAM :).

My advice though, is to get the board working with just it's 512kb of ram to begin with. Put everything back to factory normal before even trying to get it going with the RAM upgrade.

So sort out the resistors, clean the MMU pins, check all the MMU socket pins are good, clean them, clean the MMU chip pins, and reinsert, remembering that the orientation is 180 degrees out on this mainboard, so line the dot on the MMU up with the dot on the socket to be 100%.

If the mainboard won't work in standard guise, trying to troubleshoot with extras attached is going to be a lot harder.

You can see more here: viewtopic.php?f=29&t=579&hilit=ram

Good luck