RCA 1861, NTSC and a DEC VR-201

From: allisonp_at_world.std.com <(allisonp_at_world.std.com)>
Date: Mon Sep 27 13:41:18 1999

> > No scan or copy capability. The unit I was woking with required a
> > resistor from the video ouput to ground, I'd picked 75ohms for the
> > purpose.
> Everything I've seen is like the basic 1861 design in the Elf-II article in
> P-E: http://www.infinet.com/~erd/retrocomputing/ElfII/ (10K resistor on
> SYNC, 2K resistor on VIDEO, tied together and to ground by 1K resistor).
> Perhaps I just need to play with the resistor values.

It may help. The problem in my case is that output drove an
emitterfollower transistor ckt to buffer the signal but the left out the
load resistor. The bare 1861 resistor outputs can only drive a high
impedence input and some will present far to much load to drive with that
circuit, Vr201 may be one.

> It's the standard COSMAC VIP. I'll scan the 1861 output section and post
> a URL for it later tonight if I get the chance. IIRC, it looks just like
> the Elf-II circuit. Nothing complicated.

Don't bother, have the books. That one does not like low impedence

low impedence inputs are used on better monitors to keep the video signal
from getting smeared by cable capacitance, (undesireable for 80char
lines!) so they terminate the cable at 75ohms. If the drive ckt does not
have the oomph to puch that the signal will drop to nothing nad there will
be liitle if any video.

Received on Mon Sep 27 1999 - 13:41:18 BST

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