RCA 1861, NTSC and a DEC VR-201 - test-drb@ccmp.vtda.org

29 Sep 1999

--- Tony Duell &lt;ard(a)p850ug1.demon.co.uk&gt; wrote:
...
   So how can I
soup up the output of the 1861 to drive a 75ohm input?  
 Well, if you want the easy way, there are some lovely Elantec chips 
 (EL2001, EL2002, EL2003, IIRC) that are unity-gain buffers with some 
 high bandwidth that are designeed to drive video cables. They're pretty 
 easy to use, but you probably put them on a ground plane, and you 
 _certainly_ need to decouple them. 
I am unfamiliar with these chips.

...
  If you want a kludge, how about opening up the VR201
(it's one screw on 
 the back, extend the leg, and pull the shell off) and looking for a 75 
 Ohm resistor between the video input and ground.  
I have the PCB for a VR-201 right here (I dropped one a year or so ago,
blowing the CRT.  :-P  )

...
  Try removing it -- it 
 may increase the impedance enough for it to work, although the VR201 is 
 now non-standard. It's been a long time since I've been inside a VR201, 
 so I can't be sure this resistor exists, but it's worth a try. 
Looking around, I don't see anything that is obviously that value.   Going
around the board, since we've been discussing internals, besides the external
bright/contract pots, there are pots labelled (on the insulating paper) VH,
H, VL, HH, PH, FOCUS and CUTOFF.  There is an unlabelled variable resistor
near the input connector.  On the input connector, it looks like this from
the PCB...

              |  |  |  | |
     9 10 11 12 13 14 15 |
                 |       |
   1  2  3  4--5--6  7--8

Pins 4, 5, 6 and 13 appear to be ground
Pins 7 and 8 appear to be +12V
Pins 14 and 15 go off to the keyboard connector

Pin 12 appears to be video.  It goes off under the connector, through a 0 Ohm
jumper, across a Yellow-Purple-Gold-Gold resistor, then tied to ground via
a Grey-Red-Black-Gold (might be Sky Blue-Red-Black-Gold) and then through one
of the rear pots.  I would spec the values, but I'm not used to such odd colors
in the significant digits of resistors (just as tolerance bands).  Would a
EE care to translate these?  Might that second one be 92 Ohms?  Could the
first one be 7.5 Ohms?

I'd rather not have to mod a VR-201.  I want to have a nice, portable solution,
even if I have to make a mod or two to the VIP.  Any easy transistor/resistor
hacks to drive more video current?

Thanks,

-ethan

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