[Coco] S-Video mod for Coco1

[Gene Heskett]

On Wednesday 09 October 2013 10:36:35 Ken H. did opine:

> I tested LM1889 pin 13 on my Dragon with a multimeter and it's 10V :(
> 
> THANKS Luis!  Your persistent warnings probably saved my
> monitor/composite adapter.
> 
> It's strange though because the Spectrum guys are clearly sending LM1889
> pin 13 directly to the s-vga chroma input.
> http://www.retrowiki.es/fororw/viewtopic.php?f=667&t=29522

They probably got by with that because the monitor has a coupling capacitor 
in its input circuitry.  The chroma input is only interested in the chroma, 
which at this point is an AC signal with a nominal 3.58 Mhz center 
frequency, and an occupied bandwidth of perhaps 1 Mhz either side of that.

So I would not worry about the 10 volts other than to protect the LM1889 
from a short circuit by putting a .05 capacitor in series with the line 
going to the monitor.  Without it, the LM1889 is in far more danger than 
the monitor probably would be.
> 
> Is anyone familiar with the s-vga specs? Maybe it can take 10V??
> 
> 
> 
> 
> 
> 
> On Tue, Oct 1, 2013 at 4:45 PM, Luis Antoniosi (CoCoDemus) <
> 
> retrocanada76 at gmail.com> wrote:
> > according to the scheme you post the output is NOT 12V otherwise it
> > would burn your monitor.
> > 
> > just measure it with a multimeter before.
> > 
> > This should work on a dragon.
> > 
> > On Tue, Oct 1, 2013 at 8:44 AM, Ken H. <dragon.atv at gmail.com> wrote:
> > > The input for the LM1889 is 12V.
> > > 
> > > But I think that the chroma subcarrier output on pin 13 is not, if
> > > we can trust the Spectrum service manual
> > 
> > http://www.1000bit.it/support/manuali/sinclair/zxspectrum/sm/sec4_4.ht
> > ml#ic14
> > 
> > > On Tue, Oct 1, 2013 at 2:45 PM, Retro Canada
> > > <retrocanada76 at gmail.com
> > > 
> > > >wrote:
> > > > but is 12v right ?
> > > > 
> > > > On 2013-10-01, at 3:40 AM, "Ken H." <dragon.atv at gmail.com> wrote:
> > > > 
> > > > Luis, your bet on pin 13 of the LM1889 is right on target!
> > > > 
> > > > The ZX Spectrum also has the LM1889 (IC14) and pin 13 is indeed
> > > > used
> > 
> > for
> > 
> > > > its s-video mod:
> > > > http://www.retrowiki.es/fororw/viewtopic.php?f=667&t=29522
> > > > 
> > > > 
> > > > 
> > > > On Mon, Sep 30, 2013 at 10:18 PM, Luis Antoniosi (CoCoDemus) <
> > > > 
> > > > retrocanada76 at gmail.com> wrote:
> > > > > Your answer is on IC39. LM1889: It has several chroma inputs.
> > > > > Need to
> > > > 
> > > > know
> > > > 
> > > > > if one of them is useful. I bet on pin 13 Chroma Subcarrier.
> > > > > 
> > > > > 
> > > > > 
> > > > > 
> > > > > On Mon, Sep 30, 2013 at 3:09 PM, Ken H. <dragon.atv at gmail.com>
> > 
> > wrote:
> > > > >> Looking at the Tano motherboard I can't see an MC1372
> > > > >> http://archive.worldofdragon.org/images/4/40/Tano64.png
> > > > >> 
> > > > >> 
> > > > >> 
> > > > >> On Mon, Sep 30, 2013 at 9:58 PM, Luis Antoniosi (CoCoDemus) <
> > > > >> 
> > > > >> retrocanada76 at gmail.com> wrote:
> > > > >>> Is there a chrominance output on it ?
> 
> --
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Cheers, Gene
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