PET composite video adapter

Ethan Dicks ethan.dicks at
Sun Oct 12 23:08:31 CDT 2008

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.  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.

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.

Oh... in case anyone is wondering why I'm bothering, I have a couple of
naked PET boards here, no chasses, no CBM monitors.  I did cobble up a
DC-based harness (+5, +12, -5V, no tape motor voltage, but then no tape
drive, either) for testing, and I do get "interesting" looking pips on
a 'scope when I check for video, so I'm hopeful that the boards are
working well enough to at least give me a screen full of _something_,
even if it's the classic "garbage" screen you get when the machine
isn't running well enough to zero video RAM.

So has anyone here made an external PET video adapter and have any
experiences to share about what worked and what might not have worked?



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?

Ethan Dicks, A-333-S     Current South Pole Weather at 13-Oct-2008 at 03:50 Z
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Ethan.Dicks at  

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