CED VideoDisc and Player Discussion

[toasterking]

Hi there!

I added audio and composite video outputs to my SFT100 via the method described at http://www.cedmagic.com/home/cedfaq.html#twofifteen .

My problem is that I am unable to adjust R3202 (video level) on the circuit board to get a stable picture on the composite video output jack I have added. Either: a) the video level is too high, with an overly bright picture with little contrast, and streaking/blooming, or b) the video level is too low and the picture loses colour, becomes horizontally wavy, and has unstable vertical hold. There seems to be no suitable compromise between the two states. Has anyone tried this who could maybe suggest an alternate adjustment?

It seems to me that the composite video out inherits a much higher video level (picture much brighter) than the RF out when R3202 is at the same level, and when R3202 is adjusted to the point where the picture loses stability, it is still in fact too bright on the composite video out. It would seem then that the overall signal on the composite video out is inherently too high. Can I just attenuate the signal to ground a little on that output to correct for that?

Thank you for any help you can provide!

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BTW, in case anyone is curious about my mod:
I drilled 0.25-inch holes in the back panel of the bottom shell and added the two RCA jacks and a toggle switch which, when closed, puts a 1 K-Ohm variable resistor in parallel with R3202 on the circuit board to act as a shunt for instant adjustment of the video level (preset with the variable resistor and activated via the switch). This way, I can leave the switch open when using the RF out connector, and close the switch for instant video level adjustment for using the composite video out without having to open the case and make that awkward adjustment. (I have tried adjusting R3202 with and without the shunt and it is not causing a problem.) In addition, I wired my jacks and toggle switch/trimmer to a male 5-pin DIN plug, and wired a female 5-pin DIN jack to the circuit board, so that I can easily disconnect everything when opening the case.
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[cedmagic]

Since you can adjust the video above and below the proper setting, I would guess there is something wrong with variable resistor R3202 preventing it from providing a continuously variable range of resistance over its rated limits. If you have a multimeter, try attaching it across the resistor and observe how smoothly the resistance value varies as the screw is rotated.

If this is the problem, the resistor could be replaced, or another resistor could be soldered in parallel to achieve a total resistance equal to the value needed for stable video. With the installed shunt disabled, use the multimeter to make an approximation of this value, and then use the resistance product-over-sum rule to determine the value of the parallel resistor:

Total R = R1 * R2 / R1 + R2

--Tom Howe

[toasterking]

Hi Tom, thanks for replying!

I guess I could have been clearer in my original post. I see now how my wording left the problem a little open to interpretation.

The variable resistor R3202 seems to be working just fine, i.e. there are no "dead spots" in its adjustment range. My actual problem is that when decreasing the video level via R3202 (with my shunt disabled) and using my composite video output, the picture starts to become unstable, wavy, and monochromatic before I actually get down to the proper video level. Another odd quirk: If I retard R3202 just a bit further, the front-panel buttons stop working and sometimes the CED plays the same few frames over and over, like it's locked in page-mode, but without the sound muted.

In the way I worded it before, the video level is either too high or I have the effects of it being too low. The adjustment of R3202 works very smoothly and the variance in resistance seems completely linear, so I don't think that's my problem. By contrast, I start to see anomalies before the video level is decreased enough.

Perhaps I have a bad capacitor somewhere?

[toasterking]

Regarding the addition of the composite video output jack to my SFT-100W, I checked into it a little more today. It seems that the video level (R3202) can be adjusted so that it is probably just about right, and the color looks fine besides being a little washed-out, but it seems the luminance component of the signal is way too high at the composite output. (I'm not using any testing equipment to measure this, but have concluded it by observing the result visually and tweaking various other internal adjustments.) I can dampen luminance by attenuating the signal at TP3302 (Luminance output) and can get it to look pretty close to optimal, but then the picture becomes unstable. I can attenuate the signal at TP3401 (Luminance input, or luminance output from video converter IC U3402), but then the picture looks washed-out with low contrast, and unstable. It would seem to me, in my somewhat inexperienced mind, that the picture is becoming unstable because I am probably also attenuating a sync signal.

So, I think I need to find the luminance signal before it is actually modulated with sync, and attack it from there. Can anyone with an SFT100 service manual assist me with this?

Thanks again