How I modded my IIgs for Y/C (SVideo) output
-- brief version.
Note: these mods will void your Apple IIgs warranty.
For NTSC video encoding from analog RGB, the IIgs uses the 20-pin DIP
Motorola MC1377 "Color Television RGB to PAL/NTSC Encoder".
Of interest for Y/C are pins 6+8, 10+13, and 9. As implemented in
the IIgs:
- 6+8 are a Y out/in loop used for providing external delay /
filtering prior to C combination.
- 13+10 are a C out/in loop used for bandpass filtering prior to Y
combination.
- 9 is the buffered Y+C CVBS output.
For Y/C SVideo output, there's a variety of ways to go about it, but
here's what I did:
A. modified the pin 9 Y output.
Since this mod makes useless the RCA CVBS connector, I decided to
have Y only on pin 12 of the RGB connector, with 75ohm impedance. No
changes to the IIgs are necessary, but the following makes the signal
path cleaner:
- removed R8 75ohm
- removed R9 220ohm
- swapped C6 1000uf for a wire jumper.
B. modified the 13+10 C loop for a C output.
- removed R17 1kohm (first break after Chroma Out pin 13).
- removed C19 .001uf (last break before Chroma In pin 10,
input no longer needed, left floating is ok)
- soldered a wire wrap wire to the pin 13 active side solder pad of
removed R17 above. To be amplified later.
C. modified the 6+8 Y delay/filter.
Since I elected to not bandpass filter the C output at all (thus
creating no C signal delay), I removed the entire Y delay as well:
- removed L3, L4 10uh, 10uh.
- removed C1, C2, C3 56pf, 18pf, 18pf.
- installed wire jumpers in L3, L4 positions. Important to leave
in the impedance matching resistors R18, R19.
D. amplified Chroma Out signal
- wire from R17 above tied to one side of a .001uf ceramic cap.
- other side of .001 uf tied to:
- one lead of a 16k resistor.
- one lead of an 8.2k resistor.
- base of BC547 transistor.
- other side of 16k tied to +12VDC
- also tied a 1.0uf ceramic from the +12VDC point to ground.
- other side of 8.2k tied to ground
- BC547 collector tied to one lead of a 33ohm resistor.
- other side of 33ohm tied to same +12VDC point as above.
- BC547 emitter tied to:
- one lead of a 75ohm resistor.
- one lead of a 6.8k resistor.
- other side of 75ohm resistor tied to pin 14 RGB connector.
- other side of 6.8k resistor tied to same ground as above 8.2k.
All resistors are 1/8W; all components glued to bottom of IIgs
mobo. If you wish to see a schematic of this amp, see page 12 of the
Motorola MC1377 Spec sheet at bottom -- everything to the right of the
1000pf cap. They suggest (and show) a bandpass filter but I didn't
find this to be necessary or helpful.
The end result is that the Y+C signals are now available on the RGB
port, both 75ohm impedance. I cut one end off an SVideo cord and
wired it to a DB15 plug. Sound is also available at the DB15.