13 Oct
2008
13 Oct
'08
12:38 a.m.
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
Do you have a URL for said scheamtic?
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
IO think we can instantly eliminat 'mf' == millifarad (which is what it
should be!), since that's then 2.2F, which is rediculously large
2200uF (microfarads) is also IMHO too large for a non-polarised
capacitor, I've not seen non-polarised electrolytics that large, and
anyhting else would be essentially hopeless. 2200uF as an aluminium
electrolytic would be very possible.
2200nF (==2.2uF) would be possible, but it's a very odd way to write it.
2200pF (==2.2nF) is quite possible, for some reason some people
(particularly in the States) don't like the unit 'nF'. That would also
seem to be a suitable value for coupling a 15kHz signal.
large lying around. 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
Well, tatalum caps tend to be in the range 0.47uF to 100uF (yes, they
exist outside that range, but they're less common). so 47uF would be
possible.
inverted video out/composite out via a 470 Ohm
resistor). I have 47uF caps,
but that seems pretty large to me.
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
Whenever I've wanted to combine syncs and video, I've used a circuit
rouhly based on the TRS-80 Model 1 video output and/or the IBM PC CGA
composite output. No capacitors involved. An XOR gate to combine syncs, a
couple of transistors and a resistor chain. Never had any problems with it.
-tony