18 Oct
2008
18 Oct
'08
7:44 p.m.
On Monday 13 October 2008 00:08, Ethan Dicks wrote:
Hi, All,
I'm just finishing up assembling a PET "Video Mixer" from an old JPG
of a much older scan of said circuit. This particular one is annotated
as being from the "Commodore Pet Users Club of England - Newsletter
issues 1 & 2, page 9". It's the variant with three NOR gates ('LS02),
with a few resistors, capacitors, and a diode. The part that I'm
curious about is how much variation is allowed in the caps.
The schematic shows the horizontal sync coupled to a gate via a "2200 mf"
non-polarized cap. I don't know that I have any disc or ceramic caps that
large lying around.
That value definitely doesn't sound right to me. Maybe 2200 nF?
The output of that gate (which NORs the inverted
vertical sync with the
massaged horizontal sync) feeds unto a "47mf" tantalum cap (then pulled down
to ground by a diode, and wire-ored to inverted video out/composite out via
a 470 Ohm resistor). I have 47uF caps, but that seems pretty large to me.
For a coupling cap? Maybe not. Particularly if that's coupling vertical
sync.
I have a basic understanding of RS-170-type video, and
I know this circuit
is probably going to produce a signal that "modern" IC-based video inputs
will not like (much the same way the classic RCA CDP1861 video behaves),
but fortunately, I have an old B&W security-type monitor to plug this into
(which has already been tested with a CDP1861).
I'm just curious why the caps are so large. I think I get what's going
on with this circuit (the way it combines the syncs and video), and I've
heard of similar circuits being used successfully by others, so with
luck, it'll work the first time.
With the right parts in there, yeah. The abbreviation "mf" is not common on
stuff coming out of Europe, it sounds more like early us-based tech, 1960s
or earlier, but that doesn't agree with the rest of what you say here about
the origins of the circuit.
(Snip)
P.S. - I found another type of composite circuit using
a CD4011 as an
inverter and a CD4066 to mix the signals, but for some reason I don't
have all the components labelled on my drawing - the cap on hsync in
particular. One interesting variation with that one is that the 'LS02
circuit has a 1.5K pulldown on hsync between the cap and the gate,
while the CD4011/CD4066 circuit has a 10K pot. How much adjustment on
hsync is likely to be needed between the PET and the first gate of the
composite circuit?
I suspect that pot might be to set black level?
It's really hard to say much more without looking at the schematic you're
looking at.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin