By Paulo Scheidegger
I'm trying to bring my old MSX back to life but it shows nothing on screen, maybe the 9128 is damaged as it is responsible both for video output and clock generation. Any help? Thanks in advance.
Ask Repair Bas, he sure got a leftover of any vdps ever made.
Ebay € 4,05 free shipping ..
^^^ Got a link? I see some € 4.xx MSX1 VDP's there, but no 9128 for that price...
Welcome here, Paulo!
I wouldn't go & replace random parts on a gamble. See what you can find out first:
Which MSX model are we talking about?
Do you have a voltmeter to check supply voltages on the main board?
What connector are you using for video output? (RGB / composite video / RF). Have you tried another video connector on this MSX?
Is cable known good / have you tested cable (or monitor) with another machine?
Do you have an external RAM cartridge?
If machine is switched on, power LED goes on?
If you hold [CAPS LOCK] pressed, does CAPS LOCK led flicker?
If you press [CRTL-G] on the keyboard, do you hear a beep?
If you switch on with a sound-producing (game) cartridge inserted, can you hear sound?
You see.. lots of things you could measure, test & try before bringing in replacement parts...
Well spoken RetroTechie!
Hy guys! Thanks for the tips.
First wanna apologyze for being absent so long, somehow I messed up my account and was unable to login. Seems to have fixed it now.
About VDP, I found one in local e-store, and sure enough the clock returned to life, but still no video, no sound. Since february I bought a 2-trace scope (old dream realized) and started checking lots of vital signals.
First let me answer your questions in sequence so you can help me with diagnostic:
My MSX is a brazilian Expert Gradiente 1.0;
Have voltmeter (digital and analogue) and 20MHz dual trace scope;
Tried all 3 connectors, none show any useful signal;
Cable tested ok, monitor works fine with cableTV or DVD;
No external cartridge attached;
Power LED lits ok, all tensions measured ok with multimeter and no ripple on scope;
No CAPS-LOCK LED in this MSX model;
No beep at Ctrl-G;
I have no game cartridges anymore.
As said above, I measured many lines with the scope looking for any clue. I found the whole thing kind of froze just after reset or power-on, and RAM control lines get stuck with chip select disabled. It looks like selection circuitry is at fault. First I replaced TTL chips between PPI and RAM but nothing changed, so I suspect my 8255 is damaged. Unfortunately this chip is hard to find in Brazil, at least in DIL-40 package, only SOIC.
While my account was down I found a guy, Werner, with a huge MSX collection and solid technical knowledge who is helping me to bring my MSX back to life. He will send me the PPI and some other hard to find chips (I gotta have a full set of backup since this model have 100% socketed CIs - BTW, this was my first concern, I removed them all to clean contacts and found some broken or corroded pins).
If the new 8255 do not improve things my next suspect is the ePROM. I have no programmer anymore and was building a simple reader when my friend Werner told me he would send a good ePROM. Let's see, it should arrive this week.
For now, thanks again for your support!
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