Hey guys, new here as I just picked up my first MSX System. I bought it as dead and brought it back to life, mostly, lol. Have to say I really like it the more I'm researching the system and it's history. Very cool hardware. I do arcade pcb repair dso not a shock to work on these.
Here is the repair vid link to my channel:
https://www.youtube.com/@arcadelaboratory
Ok, Opened it up when I got it on the bench and after an inspection noticed it had been worked on previously. No biggie I deal with that all the time. This is what I found:
No boot. Black screen
They had a jumper from the V9938 clock going directly to the Z80 clock. No clock present at the LS367 or the HIC. Found and repaired the gouged trace and removed the jumper.
Now showing signs of life. data and address buses are now alive. Horizontal red scrolling bars on left side of screen. Still no boot.
Noticed the Z80 Socket has been replaced and the duel wipe contacts on one side were spread open. The socket also was not seated properly when it was resoldered. I removed the socket, repaired all the lifted traces and missing pads from the previous tech and replaced the socket.
System is Booting!! Looks like the original firmware and programing is intact. But the display is out of sync, badly. Only rolling and vertical waves, but you can see the image is there. No sound.
Double and triple checked all clock signals. good to go.
RGB and Csync look awful coming out of the C9938. Isolated the HIC to double check the origins of the dirtty signal, yep at the V9938.
Recapped the entire board, the HIC and the Composite / audio output board.
Sound now works
Picture now stable mostly, still have vertical waves and red scrolling bars on the left.
Reflowed the power and component\audio board connectors the HIC connectors and anything I thought needed it.
Csync and RBG still dirty. Csync has ringing. like its jumping in and out of medium res frequency.
Built a component circuit to grab the H and V sync signals from the V9938 since they were clean. Vsync out of the Z9938 is 600khz witch is 10 time faster than I need and was unsure it would work or would display anything. Now with 2 monitors, one for RBG with composite sync and one with component sync I powered on.
Results surprised me. Yes, not a locked image on the composite monitor due to the 600khz but now the component image is rock solid. At this point I'm sure its a ground issue since the ground has had a half volt of noise and with the added composite circuit there is no noise on ground. Cool, but RGB and Csync still look really bad.
Continued to probe with the scope and when probing blue and green on the HIC I apparently shorted them and lost B and G signal from the Z9938. (I isolated the HIC from the V9938 to be sure) Bummer. BUT now the Csync and Red signals are crystal clear. Weird.
My theory is the V9938 was failing and induction from the probing finished it off.
Ordered some new V9938s so will swap it when I get them.
Will recap the power supply too.
Questions:
I read somewhere that clocks are a problem on these system? True?
Heard that video is an issue as well? Is that because the V9938 has know issues?
Anyone have any extra V9938s to sell or is this a good time to upgrade?
These boards have known ground issues?
Any words of wisdom or experience for me?
How do I add pics into the thread? lol The url thing is throwing me. Shared pics have to be online? Once I sort that, I'll drop some pics.
Thanks!! Keith
