Re: Chroma/Luma (or s-video) Problems on 128DCR

On Wed, May 2, 2018 at 11:12 AM, Francesco Messineo <
francesco.messineo@gmail.com> wrote:

> On Wed, May 2, 2018 at 4:55 PM, J. Alexander Jacocks <jjacocks@gmail.com>
> wrote:
> > All,
> >
> > I've asked a number of places, but I thought that I would get the
> thoughts
> > of the fine folks, here, as well.
> >
> > I have a C128DCR NTSC that has issues with signal level on chroma, from
> the
> > video connector.  Signal levels seem to be just fine at the RF connector,
> > and the picture is fine, from that connector.  From the video connector,
> to
> > a modern TV or monitor, the picture is excellent quality, but monochrome.
> > Checking the output pins on the video connector, the level is very low,
> on
> > the chroma signal.
> >
> > I have changed:
> > 1) VIC-II (8564)
> > 2) Clock generator (8701)
> > 3) Crystal (NTSC/14 MHz)
> > 4) RF modulator
>
> according to the schematic, there're two jumpers that select different
> levels of the chroma signal:
> J10 and J11 and associated resistive divider R13-R14. I would check
> these components as second step after having excluded EMI42 filter.
> The filter should have basicly 0 ohms dc resistance between input and
> output and basicly infinite resistance between either input or output
> to GND.
> Of course, you should make sure the "about 0 ohms" dc resistance is
> present from the J10/J11 jumpers to the chroma output pin too, to rule
> out any track/solder problem.
> Almost all you changed, couldn't have been the problem judging from
> the tests you describe.
> I would have first checked with another monitor/TV.
>

Frank,

I did indeed check with at least 2 TVs, and both showed the same issue, so
it's not the TV/monitor.  I will open up my 128DCR this evening, and check
the levels on those resistors and jumpers.

I didn't think that any of the parts were the problem, except for the RF
modulator, which since the levels are low after the modulator, I thought
might be faulty.

Thanks!
- Alex