Re: IIgs SVideo Mod Results

[MdntTrain <js@cimmeri.com>]

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.

   JS