Thanks to my cellular phone camera the reference Composite screenshot looks almost better... You are maybe able to see a "snake" pattern on the left border of the composite image which is not on the S-video image. The difference is much clearer in real because the pattern is flickering, on both the left and right border.
This is a Dragon 32, main board MKII issue 5 ("Siemens").
Wonderful rat's nest prototyping! I take the S-video Luma signal from the video output of the main board (pin 6 and 5 on the connector going to the power board).
For the S-video Chroma signal, a buffer stage seemed to be needed. I made a simple emitter follower (~unity gain, low output impedance):
I have bent pin 13 of the LM1889 so that it doesn't touch the socket, thus the colour signal is not fed to the normal video output any longer. From pin 13 I have a 56 pF capacitor (C1) going to the base of a C1740 transistor, which is biased by a 56 kOhm resistor (R1) to 5V, where also the collector is connected (tapped from pin 14 on the LM1889 on the photo). On the emitter, a 220 Ohm resistor (R2) goes to ground while a 0.1 uF capacitor (C2) in series with a 56 Ohm resistor (R3) goes to the S-video Chroma input. The components are a bit random, just inspired by similar circuits and what I had lying around. The transistor is a general purpose NPN, you can use for instance a 2N2222 or 2N3904 instead.
Schematic:
Code: Select all
+----------+--- 5V
| |
> |
> R1 |
> |
Pin 13 C1 | |/---+
-------| |------+-----| T1
|\ C2 R3
>>--+----| |-----^^^^^----- Chroma out
|
|
>
> R2
>
|
|
--- Ground