Jo22 wrote on 2022-05-31, 19:01:

Hello everyone, […]
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Hello everyone,

I wonder what the electrical differences between today's S-Video and the old Chroma/Luma ports are.

My Commodore 1702 had LCA connectors on the back and I used them for my Super Nintendo once.

However, the output wasn't exactly correct.
Maybe it was the cable, but it looked to me as if the voltages weren't in spec.

That's why I wonder - does anybody have old documents of the 1702?
Some that provide a hint, whether or not there were differences between S-Video via Hosiden and LCA (RCA, Cinch, Phono plugs)..?

Thanks in advance.

Jo22

I am in NTSC-land and I used a 1701 or 1702 (don't recall which) with a Norcent DP300 DVD player with custom firmware which, AFAICR, allowed s-video style luma and chroma to be sent through 2 of the component (normally YPbPr) RCA cinch connector outputs, which allowed using standard RCA cables. AFAICR, picture looked great with minimal (if any) adjustments.

Looking at the schematic, the most immediate thing is that there is no 75ohm termination, luma has 150 ohms and chroma has 82ohms, this is enough to make things not right with many devices that expect 75ohm termination for correct operation.

Tiido wrote on 2022-05-31, 19:51:

Looking at the schematic, the most immediate thing is that there is no 75ohm termination, luma has 150 ohms and chroma has 82ohms, this is enough to make things not right with many devices that expect 75ohm termination for correct operation.

This 1701 manual scan from manualslib.com says video inputs should be 75 Ohms and and signal level should be 1 VPP .

Either the schematic or the specs in the manual could be wrong or there might different hardware variants/revisions with differing specs .

Service manual from here definitely shows the values I mentioned for both models : https%3A%2F%2Fgona.mactar.hu%2FCommodore%2Fmonitor%2Fschematics%2F1701-1702_Service_Manual_Preliminary_314004-001_(1984_Oct).pdf

Though maybe there are variations where it changes to what it should be. I have never seen such a monitor in person and likely never will also so I don't have any further comments. It seems to be quite a low end design, without clamping on inputs so rapid brightness changes in video signal cause black level shifts on the image.

Thank you very very much for your help! I really appreciate it!
These 150 Ohms were an impedance value a friend of mine vaguely remembered, as well, btw (I asked him today).

So there's something different, indeed.
Maybe the PAL/NTSC models of the 1702 were different a bit, too.
Or maybe some early 1702 models were modified 1701 models, not sure.

Anyway, knowing these potential values for Chroma/Luma may help fixing S-Video cables.

I also agree that the 1702 wasn't exactly the best design.
People on the net really glorify that particular model.
Perhaps because it's cult, of because of the aesthetics or because it has a nice form factor (stackable).
It also had many habdy controls behind the front bezel.

But visually.. I don't know. When I was a child, it simply was my video monitor that I got from my dad.
The picture always was okay via AV, but I don't remember it being stellar.
- I *think* it might have had an analogue comb filter, because Composite never was "bad" no matter the source material or cable type.

But somehow I always was impressed by the modern TV set my friends had.
Because, in the 90s, their TVs had AV sockets and a clean picture.
It seemed more hi-res than my 1702vs picture.
They had those OSDs rather than knobs.

Our old TV at home in the living room had RF only.
We used a video recorder as a tuner and to have AV ports.
A bit later, still in the 90s, my dad also installed his old satellite receiver besides the attic in the mansard.
The parabolic dish was mounted on a chair near the window. It still used an LNC, the predecessor of the LNB.
The AV cable of it was ~10m cable long and routed down through the house to that 1702 in my bed room.
But strangely, picture quality was still okay.
The receivers RF was fed through coaxial cables, I believe. We still had a huge terrestrial antenna on the roof back then.

English: Small corrections. Sorry for my poor English. I did understand what you guys wrote, but sometimes I have trouble expressing things in English language.

It definitely has no comb filter in it, just normal semi naive direct decoder with bandpass filter and manual alignments.

Another thing I noticed is that part of the deflection circuit is hot and live tests can prove fatal without isolation transformer or you really knowing what you are doing.