I'm going to do a whole post / video on this at some point, but for now, just going to place this here. Along with this link for a vintage article describing exactly the same thing. 

From some amiga bbs:

> How did you do it?  I mean, connect your Amiga 1200 to your Sony
> KV25-XBR?  Nobody I've spoken with at any computer store or Sony dealer has
> any idea where to get a cable to connect a computer to the 34-pin RGB port
> on my Sony.  I even called Sony customer service, and those folks are real
> polite but even THEY don't have a clue.

I went to a Sony Service Center back in 1985 and purchased the
"KV-25XBR/RM-724/APM-X3U Service Manual".  It has schematics and lists
two options for the RGB MULTI INPUT:

  1) The PX-34 RGB multi connector plugs into the back of the TV.
     It has to be soldered on to the end of a cable that brings RGB
     data to the TV.

  2) The SMK-0001 cable has a PX-34 at one end and a "microcomputer
     connector" at the other.  (From the drawing, it is not obvious
     whether this is a DB-9 or some other style of connector.  The
     microcomputer in the diagram looks more like a TI-99 than an IBM-PC.)

The PX-34 was listed for more than $30, and there was no guarentee that the
SMK-0001 had the connectors I needed.  So I built my own.

I purchased a standard 34-pin connector with flat ribbon cable from a
computer store, and wired up a box that has an Amiga-compatible 9-pin
video connector, 6 RCA phono jacks (red green blue sync left right), and
2 switches.  One switch selects sync-on-green versus separate sync, the
other enables TV audio while in RGB mode.  The Amiga has a 23-pin video
connector, so I had to use either a DB-23 to DB-9 cable, or use a
genlock (which uses a 9-pin output connector).

9-pin Video connector (used by A2002 and A1084 monitors, AmiGen genlock):
  01 = Sync ground	06 = no connection
  02 = RGB ground	07 = combined Sync
  03 = Red		08 = no connection
  04 = Green		09 = no connection
  05 = Blue
	07+01 go to a black RCA jack labeled "Sync"
	03+02 go to a red   RCA jack labeled "Red"
	04+02 go to a green RCA jack labeled "Green"
	05+02 go to a blue  RCA jack labeled "Blue"

On a 23-pin Video connector, 03=Red, 04=Green, 05=Blue, 10=Csync, and
16-20 are all ground.  Do not use 11=Hsync or 12=Vsync in this case.
(On the 9-pin male to 9-pin female cable I was using, I took some wire
cutters to pins 06, 08 and 09, since their signals confused my A2002
monitor.)

I planned on adding a second DB-9 for CGA (TTL) monitor, but never got
around to it.  The original IBM PC used 1=GND, 2=unused, 3=RED, 4=GREEN,
5=BLUE, 6=INTENSITY, 7=unused, 8=H-SYNC, 9=V-SYNC.

34-pin RGB connector:
  01 = +5V		18 = no connection
  02 = +5V		19 = no connection
  03 = ground		20 = Audio, Right
  04 = ground		21 = Mode Switch (digital RGB vs analog RGB)
  05 = Remote Ctl gnd	22 = no connection
  06 = Composite gnd	23 = Video (composite)
  07 = Audio ground	24 = Audio, Left
  08 = Red ground	25 = Red
  09 = Green ground	26 = Green
  10 = Blue ground	27 = Blue
  11 = Ground		28 = no connection
  12 = Blank ground	29 = Blanking
  13 = Hsync ground	30 = H-sync
  14 = no connection	31 = V-sync
  15 = Vsync ground	32 = no connection
  16 = Ground		33 = RGB vs Normal
  17 = no connection	34 = Audio Select (+5v enables audio)

01+34 go to a SPST switch (to enable audio while in RGB mode)
20+03 go to a red RCA jack labeled "Right"
24+07 go to a white RCA jack labeled "Left"
25+08, 26+09, 27+10 go to the Red, Green, Blue jacks (and Amiga connector)
13 goes to the Sync jack ground, 30 goes to a SPDT switch, which connects
it to either the Sync jack (separate) or the Green jack (sync-on-green)

The phono jacks are useable by a "GIGI" (DEC VK100 terminal with BASIC).
The 9-pin connector is useable by an Amiga 23-pin to 9-pin cable, or
by the 9-pin output of an AmiGen genlock.

See also the "Sci.Electronics.Repair FAQ on Pinouts" 
http://www.repairfaq.org/REPAIR/F_Pinouts3.html#PINOUTS_015
for "Sony RGB Multi Input (found on KV-25XBR TV's).

Links to other documents:

 *) http://www.hut.fi/Misc/Electronics/circuits/vga2tv/ for details
	on converting VGA video (15 pin) to NTSC (CGA, 9 pin).

 *) http://home.att.net/~billhudson/rybyrgb.pdf has a component (YPrPb)
	to RGB converter.

 *) http://elm-chan.org/works/yuv2rgb/report.html - look for "Circuit Diagram
	3 (Simplified Lv.2) BG part is omitted (Sync on RGB)".

-- 
Joe Smith  <js-cgi@inwap.com>  CA license plate: "POPJ P,"  36-bits forever!
Humorous disclaimer: "My Amiga 3000 speaks for me."    http://www.inwap.com/