Disclaimer: Don't try this at home unless you know what your doing. This will void the warranty. It may possibly turn your box into a doorstop. There are electric voltages inside the unit. Unplug unit before taking apart and be careful.
Now that that's out of the way, Here's what I found.
To Enable SPDIF:
This looks relatively easy. There are 2 jumpers, 1 electrolytic capacitor and an Rca jack missing. The jumpers are J652 & J666, the electrolytic capacitor is C679. Im thinking the value is 10uf @16v looks like a standard coupling capacitor. (I think I will attempt this one soon) You will need to add these and an Rca jack to the back of the unit.
To Enable S Video:
This don't look as easy since I don't have a schematic. But this is what I found. Looks like C643 (possibly 100uf@16v) needs to be added also C680 a chip capacitor (value ?). J630 needs to be added and possibly C689 (Value ?). This modification will require more homework.
I tried the SPDIF mod and it didn't want to pass audio to my Yamaha HTR. So more research is needed. I think I'll have to scope the video to find the separate Y & C for the S mod. Nothing ever seems as simple as one thinks.
Anyone else with an electronics background with any suggestions feel free to add some input.
The firmware/software in the box also needs to support SPDIF and S-Video (They could have disabled the feature in the software). If the firmware supports it your hardware modification should work.
You know I was thinking about that too. There are no selections in the menu to turn these on. So unless you can dump and modify the software the SPDIF may not work. The S video on the other hand may be able to work because Im sure the composite signal has to be derived from Y & C digital video output. One would have to find that point and tap off of that adding a buffer amp if necessary.
I did some deeper digging in the Magnavox this past week. I found the composite video is coming directly off the tuner. All the pad's for S video on the board are dead. Which leads me to believe that you would need to crack open the tuner module to dig deeper. At this point I don't see alot of hope for the S video mod. Determined to get rid of the dot crawl in the composite signal I built a tuned L/C notch filter that removes 99.9% of it. I constructed it with 4 parts. 1 inductor, 2 capacitors and a pot. It turned out very well and I see a picture as clear as S video would give. There is no dot crawl on color bars and less than 00.1% in some graphics. I've been messing with video filtering circuits for a few years and all my knowledge was a plus when I designed it. If you want to build one, scope the composite output and play around with filtering the color burst frequency from interfering with the luminance. You have to know what your looking at and understand passive filters to get the job done right. The results can be quite pleasing.
As far as SPDIF I checked the pad on the pc and a voltage is present. So the firmware needs a mod to get it running.