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Date Posted: 22:00:33 12/27/01 Thu
Author: lcs
Subject: Re: re: Instability - edge detectors mod.
In reply to: Potter 's message, "Re: re: Instability" on 04:31:38 12/17/01 Mon

As I said before, changing the the two 1k resistors
to 4k7 made things worse in my Canton4.2. But to test
it again, i took a different approuch and replaced
the other 1k resistor (the one that feeds Vref. to
the comparitors) for a 5k pot. This way i can adjust
the signal level instead of increasing the sensitivity
of the detectors. This worked better and it has a
considerable effect. I can fine center the picture
horizontally and even sincronize --better-- the
Hline with the burst trend. It also helped stabilizing
the Hsynch in channels that were losing lock. I've been
using the board with the pot adjusted to about 2k7.

lcs



>I initially had some problems with this and what I did
>was leave out the 2 resistors in question at first and
>wire in a dual 10k linear pot so I could adjust for
>the best result, measure the pot setting and replace
>with the nearest preferred value. My earliest layouts
>had the resistor and capacitor feeding video to the
>edge detectors reversed because it made a neater PCB -
>this arrangement needed a very high resistance in
>place of the 1k (2k7-4k7)to work reliably (using an
>LM339). I've corrected the layout now and a much lower
>value is required (using LM393 which should be
>identical to LM339 comparators). The best ratio seems
>to be 100:1 eg 470-4.7-4.7-470 or 1k-10-10-1k etc when
>feeding video via a 470ohm resistor in series with
>220pF from an LM733 video buffer's output. I also
>used a much larger capacitor on the centre of the
>divider for Vref ie 47uF instead of 0.33uF.

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