Short Atari 520ST vsync problem

[lopezpb]

Hi,

I am trying to solve the problem with vertical lines on the screen displayed by a short Atari 520ST w(C070243 rev.H) on my LCD TV (on 520STFM, 520STE these lines are missing and the picture is very nice)

I tried to record what it looks like:






So far I have:

• removed the RF modulator, according to the diagram, I soldered 2xdiode, transistor and 3x resistors in its place - no change
• removed the memory and inserted the 4mb xtra-ram extension - no change
• change power-supply - no change
• change CPU - no-change
• change MMU/GLUE/Shifter - no change

I checked on an oscilloscope and vsync on CR10, GLUE (pin 38) i MMU (pin 51) looks very strange:

vsync 520st

vsync.jpg (87.71 KiB) Viewed 4123 times

[stephen_usher]

That's normal system noise. It's coming from the address/data buses and shifter itself. All STs have it to a greater or lesser degree depending upon the board and chip revisions. Some displays will show it more than others too.

The v-sync is a red herring as the video circuitry in the display will merely using it as a trigger once the voltage goes above/below a certain value and shouldn't have any effect upon the video signal at all.

[lopezpb]

I understand that this is a dead end, I thought I found the reason for these vertical lines I have two motherboards C070243 rev.H, and both have the same issue.

[stephen_usher]

That's the way most 80s computers are. Pretty well every machine from that period I have show that sort of artefact in one way or another. The ST is in the middle of the road when it comes to the intensity. Sinclairs seem to be the worst and Amigas near the least, along with the Amstrad CPC.

[rubber_jonnie]

I have something like 12 or more ST's, ranging from the ST to the MSTE, and the artifacting, or jail bars varies dependent on the machine and motherboard. I have, and have had, Spectrums too which varied massively, with some having really bad jail bars and some pin sharp. In fact the ZX81 I own now has a perfect display, but the one I had as a child growing up had very faint bars.

I've seen some videos trying to fix this using additional capacitors, or replacing old capacitors, but the success seems variable.

I suspect that since a lot of these machines were built to run on TVs using modulators, where it wasn't as obvious due to the image quality back in the day, that they didn't pay much attention to fixing it and only made changes with different board revisions.

It's probably something you could chase for a very long time with no improvement.

[exxos]

As others have said these are old machines built cheap and are riddled with various faults. The same subject appears on here every couple of weeks in fact. If you want perfect video I would suggest investing in H5 motherboard where all these issues have been resolved.

[lopezpb]

I would like to check the H5 board, but as far as I know it is matched to the STF and does not fit to the short ST

[exxos]

I would like to check the H5 board, but as far as I know it is matched to the STF and does not fit to the short ST

ah yep. Though @Icky's HDMI shifter may actually solve such problems as well thinking about it.

[commander]

any way to bring the sync signal to the proper level and curve to make a great rgb output picture
eg like using some TTL Logic Chips?
Any suggestions?

[Steve]

If you're interested in adding a HDMI output, I can say that putting an RGBtoHDMI Pi device in my short-ST worked perfectly.