exxos's Atari, Amiga & retro forum

So I have a 520STM that has some video noise that, while not too significant, was bothering me. So I took a chance with a seemingly tested NOS ebay board. It arrived yesterday and first thing I did was plug it into power and a monitor to see what the video quality looked like.

Unfortunately, just got a blank white screen. I've done some basic troubleshooting, reseating all the socketed chips, eventually swapping the glue, mmu, and rom chips with known working ones. Still a blank screen.

One thing I noticed is a bunch of diodes with some clumpy solder and flux residue next to the RF modulator. Anyone see anything like this before? My working current board doesn't have these. Just looking for some areas to start the more detailed troubleshooting.

Just for clarification. I've left it running for 5 minutes (still white screen). Also no "pinging" noise with the keyboard disconnected.

What tools, in terms of diagnostic equipment, do you have?

By the way, those things look like Zener diodes... very strange. What board rev is it? Maybe we can compare it to an image in the archive.

P.S. Looks like C070243 board. Those diodes don't appear on anyone else's board. I would remove them. (The one on the corner of the chip behind is normal.)

I'm suspecting bad RAM as that will often give a white screen as the machine boots far enough to reset the graphics sub-system but then crashes.

stephen_usher wrote: 28 Jan 2023 16:23 What tools, in terms of diagnostic equipment, do you have?

By the way, those things look like Zener diodes... very strange. What board rev is it? Maybe we can compare it to an image in the archive.

P.S. Looks like C070243 board. Those diodes don't appear on anyone else's board. I would remove them. (The one on the corner of the chip behind is normal.)

I'm suspecting bad RAM as that will often give a white screen as the machine boots far enough to reset the graphics sub-system but then crashes.

I have a decent multimeter and a logic probe, and plenty of other ST's to play around with for subbing for working spare parts and trying things out.

It's a Rev E board. I've never seen diodes like that before either...that was my thought too, but wanted to ask around before removing them.

Thanks Stephen!

Without an oscilloscope or diagnostic cartridge it's going to be tricky working out what's wrong I'm afraid.

If your STM is an European version and your monitor can display some information, you can check if it switches from 60hz to 50hz right after power up. This would mean CPU is executing some code. With a multimeter and logic probe, I would suggest checking voltage on various ICs and also the checking the reset circuit.