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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 12th Jun 2004, 8:21 am   #1
oldeurope
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Location: Solingen, Germany
Posts: 727
Default Re: 405 modulator

I am happy to be able to make my own transistor
design.

It is no problem to be better than
" National " .
The trouble is I am not able to explain
my circuit in english . So only experts can see
what happens here.

Have a look at Q69 and Q670 for example:
They replace several IC's
an analog switch IC, a few opamps, a Schmitt trigger
and a white clip stage ...

The stage Q66 Q67
David only uses one transistor to replace the diode.
Davids stage is 500 times better than a diode!
(500 is the B of BC547C)
and the stage I used is 250000 times better than a
diode. Do you understand this?

The audio modulator Q64 combines a modulator
and a so called " infinitive detector "

I have made a DC-coupled negative feedback from the
detector to the input. The audio stages are Loftin-
White stages.
This is high end hifi design
In the IC applicationes you would need a additional
receiver to make this...

The modulator transistors are similar to the
g1 modulation used in (vintage) transmitters.

What I wounder about is that nobody has seen
the inputs.
At the video input you must have 75Ohm
The video output must have 75 Ohms too.
If you load the output with 75 Ohms you get
1Vpp video at peak white. This is a standard!!!!!!!

The audio input impedance must be more than
50KOhms for scart and RCA plugs.
If there is a 1KOhm poti and a low input resistance
you get loss in bass and the outout stage is overloaded.
Note that a typical audio
output has a 2uF cap!

Sorry for my bad english...

Look here, I drawed a block diagram for the
modulator video section.
The input filter and Blatt 8 will be added soon
in the Normwandler website.
www.pixum.de/viewalbum/?id=1137100

Kind regards

Darius
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Old 13th Jun 2004, 10:59 am   #2
oldeurope
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Location: Solingen, Germany
Posts: 727
Default Re: 405 modulator

Hi all,
the website
http://www.your-normwandler-page.de.vu/
is updated now.

Please have a look at the foto " VCR video in "
You can see the videosignal of a not modified
VCR playing back a 405 video.
The sync signal moves up and down and makes
a yump after the V- puls.
This causes bad syncronisation at the top of the
picture.

In the foto " VCR video out " you can see the
videosignal after passing the clamp stages
at the output of the video amp Blatt 8.
The video signal is perfect now.
The TV has no sync problems any more and
the signal can be recorded.

Can you see the great progress against
the other modulators? :
If your 405 source is a VCR you must try out this!

In the website Blatt 8 is added you can see the
power supply and the scart video output amp.

I added a few ordercodes from RS for you.

Have a look at the bias voltage-generator Basis
Q67. Exept the oscillator frequency the whole
thing needs no agliment!

Please let me know what you think about it...

@Malcolm and Jeffrey
I had a look at the LM1881.pdf
They wrote that the 1881 is made for sync sep. at
good signal quality.

My sync seperator stage is made to get the sync from
badly damaged video signals as well!

This is importand for 405 video playback.

I used a trick to make the seperator's live easyer.

Look at Q69 it is a amp which has a gain of (-)3.
Normally it is not possible to get this gain at an
operating voltage of 5V DC and an input voltage
range up to 2Vpp (1V video+DC level changes).
I moved the bias of Q69 that the sync is in the linear
region and the rest is clippt away.
The DC movements in the signal at the collector
of Q69 are less, because the white levels are clippt.
The sync has a higher level. This makes live
easy for the sync seperator.
If you connect pin 2 of the LM1881 over an inverter to
collector of Q69, you get the same quality of sync
seperation as I have.
But for me it makes no sens to replace Q670 by the
LM1881 an inverter and a output transistor.
Do you agree with this?

Kind regards
Darius

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