Mouarf.... trying to fix a VCR, first one in my life, never worked on these things before...
It was given to me and I thought hell, would be cool to (try to...) fix it as an exercise and to have at least one VCR around, you never know when you might need one... seeing as I tend to like old stuff, who knows, might come handy one day.
Anyway. First it did not power up at all. Was bloody RIFA. Removed it, works fine now. don't have VHS tapes but the old guy who donated the VCR also donated a tape of Star Wars the Phantom Menace.
Anyway it plays the tape... audio is just fine, I can hear the movie sound track just fine, but no video !
I tried using SCART and video composite on two different LCD monitors equipped with these I/O, and every time the same answer : " No sync ", and a black screen, so not happy.
Looked at the schematic, have partial ones, not the main board sadly... but have the one for the rear panel PCB that carries the I/O connectors, and plugs right angle to the mother board.
I see that the video composite signal comes from a chip on this board, and is served to both the RCA jack AND the SCART connector. Apparently SCART has both RGB and composite signals available. I don't know which one the monitors actually use when I ask them to display SCART. Anyway, at least if make some sense that both SCART and composite would not work...
Anyway, I scoped the composite signal on the RCA jack, and to me looks like a I do have my composite signal ?! So why doesn't it work ?!
I know squat about video let's be clear.... I simply looked at Google to find diagrams of what it should look like. I have my negative sync pulse, about 5us as it should. Total frame length is about 64µs as measured with the cursors, which is what it should be about. So as far as sync goes, the LCD monitors should be able to detect a signal eh ?!
My two main worries that might explain the problem (?!), tell me what you think :
- Noise... as you can see from the video clip below and my screenshot, there is lots of noise... but that could just be my probing setup which is more than awful of course, not having a BNC to RCA adapter nor even a coax cable.. so just using my probe in x1 and a little contraption made from recycled RCA jacks. It's too ugly to be showed here, don't want to stoned.
- Looks like there is a HF signal superimposed on everything, whose vertical excursion spans the entire composite signal... looks like faint squiggly lines at the top and bottom of the waveform. This does not show on wave form diagrams I see on the net.. however I understand there are two combined signals on these things, "luminance" and "chrominance", so maybe what looks like squiggly lines is just one of those two signals and everything is fine . I am so clueless it's painful.
- Voltage levels : 99% of the diagrams I found do NOT indicate voltage levels. Instead they use funny "IRE" units, whatever that means, to give the proportions of the various parts of the signal, but not actual voltages. Proportions look fine on my signal, I find. Video content is about 2+ as high as the sync pulse.
The rare diagrams I found that did indicate voltage levels, showed a 1Vpp excursion, but I get as you can see 2.5Vpp or so !
It's so horrible to hear the sound track of Star Wars but not being able to see the video even though I can see it on the scope !!!
Anyone here with working knowledge of composite signals ? I am in Frog land so I guess we are talking SECAM, though from what I see, SECAM / PAL / NTSC have very similar looking wave forms...