PET composite video adapter

[Roy J. Tellason]

On Saturday 18 October 2008 23:05, Ethan Dicks wrote:
> 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?

That should've said "2200pF" there...

> 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.

Yes,  perhaps.  Now that I've seen the thing maybe figuring some time 
constants is in order.  (But it's awfully late here at the moment.  :-)

> > > 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.

Probably fussier about what it was being fed.

> > 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...
>
> http://oldcomputers.net/pics/pet-video.jpg

That "black level" comment was just a guess based on what I'd read,  before I 
saw the diagram.

> 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).

Or nowhere near enough horizontal sync amplitude.

No cap in there now?  That'd give you some different waveform coming out, 
probably.

-- 
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"
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