PET composite video adapter

Ethan Dicks ethan.dicks at
Sat Oct 18 22:05:01 CDT 2008

On Sat, Oct 18, 2008 at 10:44:42PM -0400, Roy J. Tellason wrote:
> 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?

As has been hashed over before, I'm more inclined to think some hand-drawn
original was marked 2200pF (and then poorly transcribed to a formal print
drawing).  The graphic clearly shows "mf", though, not "nf" or "nF".

In practice, though, since I didn't have anything quickly at hand, I
gave the circuit a try with no cap (based on Tony's comments about a
similar circuit for the TRS-80 that had no RC on hsync).  I did get
recognizable video, but I couldn't get a perfect horizontal lock.  The
screen slowly (at best) slid left or right.  The text was perfect and
the pixels were perfectly aligned, at least.  I'm beginning to think
(based on another circuit) that this cap and the attendant pull down
resistor are a simple delay circuit to allow one to phase-shift the
hsync relative to video-out, but perhaps that's an incomplete
understanding of it.

> > 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.

Having now built it, it seems to work, but I still think it's awfully large.
> > 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).

To confirm - yes.  This circuit produced video on a security monitor, but
a modern LCD wouldn't sync to it.

> 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.

It's clearly designed for the PET, so 1960s is right out, unless that
was the mindset and sematic context of the designer.
> I suspect that pot might be to set black level?

I would doubt that, since the pot is long before anything that mixes
with video-out.  Based on comments from another design, it's part of
a horizontal sync compensation circuit (along with that "2200 mf" cap,
I suspect).
> It's really hard to say much more without looking at the schematic you're 
> looking at.

I posted the URL in a followup once I rediscovered where I found it...

As I've said - that circuit basically works, but I do get a slight
horizontal roll with the one monitor I have access to (and its
h.hold knob is quite twitchy - probably a dirty pot).


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