Dragon 32 RGB SECAM and video tests

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Azerpy
Posts: 29
Joined: Tue Apr 09, 2019 1:16 pm
Location: France Pyrenées

Re: Dragon 32 RGB SECAM and video tests

Post by Azerpy »

Hi Stew and John
Thank you for your advices.
For the moment, I can't reduce the oscillation which causes the vertical lines by putting caps on the power supplies near the 6847 circuit. I will carry on my investigations.
My two Dragon RGB have the same behaviour.
In RGB output, the vertical lines are not so bad on all screens, variable with colors, and can be totally reduced on the green, by setting brightness, contrast and gamma when possible.
The RGB circuit is very simple, much more simple than the Dragon Plus card, John !
I will post the schematics of this RGB conversion, it is a four stages circuit using transistors.

The SECAM circuit is a nice relic. The Voltage Controlled Oscillator is also made with transistors on CA3046, there are several caps and selfs for tuning and filters. The quality is not so bad with my little amplifier (see photo).
Last edited by Azerpy on Sun May 09, 2021 6:25 pm, edited 1 time in total.
Azerpy
Posts: 29
Joined: Tue Apr 09, 2019 1:16 pm
Location: France Pyrenées

Re: Dragon 32 RGB SECAM and video tests

Post by Azerpy »

Dragon 32 SECAM OUTPUT.jpg
Dragon 32 SECAM OUTPUT.jpg (2.69 MiB) Viewed 607 times
Azerpy
Posts: 29
Joined: Tue Apr 09, 2019 1:16 pm
Location: France Pyrenées

Re: Dragon 32 RGB SECAM and video tests

Post by Azerpy »

I have looked on my Dragon 32 SECAM at the RGB conversion which is part of the PSU board.
On my schematics below, we can see that phi A (R-Y) and phi B (B-Y) signals of the 6847 are mixed to make the (G-Y) information.
The LUM, which is a clone of (- Y) information is inverted and then combined with (R-Y), (B-Y) and (G-Y) to get the three R G B colors. The Synch signal is the same as the Green.
D9 and R20/R21 give the bias of Q5.2 transistor and the voltage references. The offset of the outputs are different but that has no importance on the (modern ?) screens.
The tiny capacitor C11 is important for the contrast, I had to replace it by a 47 pF.
As I wrote before, all the machines that I know give too much blue, that can be corrected by the output resistors or by the settings of the screen.
Gérard
Azerpy
Posts: 29
Joined: Tue Apr 09, 2019 1:16 pm
Location: France Pyrenées

Re: Dragon 32 RGB SECAM and video tests

Post by Azerpy »

Here are the schematics.
Dragon 32 SECAM RGB part of PSU Board
Dragon 32 SECAM RGB part of PSU Board
circuit rvb.jpg (5.26 MiB) Viewed 433 times
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