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Old 23 July 2017, 01:32   #1
CFOU!
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SCART problem on CRT TV (philips 21PT5408) dull/bad colours

Hi quys,

i have a curious problem:
If i use my both a1200 on Commodore monitor with (DB23=>DB9 connector) or conposite connector, there are no problem.

but if i use on TV via original DB23=>scart connector colours are bad.
Symptoms (see joined picture):
=> colours are very dull
=>red is orange
=white is a bit green
(in fact all coulors seems dull and have too green colours)


I test with two A1200 (classic and tower), two Scart connectors and two differents PSU.

all time colours are bad... (exactly same problem: see picture)

It's not monitor because it work fine with DVD player and groovymame (connected via ATI GPU and VGA-SCART adapter)


i read this :

http://eab.abime.net/showthread.php?...+color+problem


SOLVED: 07/24/2017:
Solution:
=>just add 110 ohm resitance on scart pin 20 (TTL CSync) and it's working


regards
Bertrand.
PS: see too photo of SCART opened
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Old 23 July 2017, 11:53   #2
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Hi Bertrand,
something wrong from photo.. both rèsistance are connected to blue cable? (it may be that I do not see well)

From Stedy's scheme amiga pin 23 going to SCART 16 through a R=75ohm and amiga pin 22 through R=1Kohm to SCART 8.
"The BLANK signal (pin 16 of the SCART) is fed from +5V via a 75 ohm resistor, which together with the 75ohm terminator in the TV, creates a voltage of 2.5V, adequate to RGB input mode. The AV mode signal (pin 8 of SCART) is fed +12V via a 1K resistor. The resistor is present to limit the current in case of a short circuit."

12V on pin 16 is a symptom, there is no switch to RGB..
How did you calculate the values for resistances to obtain 12V/2,5V from the single divider on Amiga pin 22?
Inside TV pin 8 of SCART have a 75ohm terminator like pin 16?

Cheers,
ross

[i'm reading the mentioned thread to get some hint]

Last edited by ross; 23 July 2017 at 12:05. Reason: []
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Old 23 July 2017, 12:19   #3
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Quote:
Originally Posted by ross View Post
Hi Bertrand,
something wrong from photo.. both rèsistance are connected to blue cable? (it may be that I do not see well)

From Stedy's scheme amiga pin 23 going to SCART 16 through a R=75ohm and amiga pin 22 through R=1Kohm to SCART 8.
"The BLANK signal (pin 16 of the SCART) is fed from +5V via a 75 ohm resistor, which together with the 75ohm terminator in the TV, creates a voltage of 2.5V, adequate to RGB input mode. The AV mode signal (pin 8 of SCART) is fed +12V via a 1K resistor. The resistor is present to limit the current in case of a short circuit."

12V on pin 16 is a symptom, there is no switch to RGB..
How did you calculate the values for resistances to obtain 12V/2,5V from the single divider on Amiga pin 22?
Inside TV pin 8 of SCART have a 75ohm terminator like pin 16?

Cheers,
ross

[i'm reading the mentioned thread to get some hint]
i use 2 scart adaptors 'not home made' buy in 80's and 90's.

in fact blue wire is conneted with resistance on pin 16 & 8 (12v)!!

Sadly on db23 the are no pin (5V)!!

I will compare with second adaptor to try to build Stedy's connector

Last edited by CFOU!; 25 July 2017 at 09:28.
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Old 23 July 2017, 12:52   #4
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Quote:
Originally Posted by CFOU! View Post
i use 2 scart adaptors 'not home made' buy on 90's.

in fact blue wire is conneted with resistance on pin 16 & 8 (12v)!!

badly on db23 the are no pin (5V)!!

I will compare with second adaptor to try to build Stedy's connector
A comment from Stedy's thread:
"Pin 8 has an input impedance of >=10K ohm according to several references."
So to respond myself: no, for PIN8 and PIN16 cannot be used a simple divider
Maybe in '90 there was some 'standard' about it..

If you don't have free cable to separate 22/23 (+5/+12v) you can use an Amiga RGB video ground cable, from schematics are all connected together.. and use the right R.
[EDIT:mmh, no, from your photo seems that video grounds is not separated, there is only composite ground connected .
My advice: remake this 'not home made' a 'home made' ]

Regards,
ross

Last edited by ross; 23 July 2017 at 13:27. Reason: [EDIT]
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Old 23 July 2017, 13:25   #5
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thanks for your help.

here you can see second scart in photo (similar connection), only resistance are a bit different.

i will try to modify second adapter which have pin 23 (5V) on DB23.

Best regard
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Old 23 July 2017, 13:32   #6
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Quote:
Originally Posted by CFOU! View Post
thanx for your help.

here second scart in photo (similar connection) only resistance are a bit defferent.

i will try to modify second adaptor which have pin 23 on DB23

Best regard
In the latter there is a shield cable (yellow), maybe usable differently..

Regards,
ross
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Old 23 July 2017, 15:16   #7
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Originally Posted by ross View Post
In the latter there is a shield cable (yellow), maybe usable differently..

Regards,
ross
In fact, it is the ground wire welded on the chasis of the scart.

No ground wire is welded on the chassis on first scart.

But it's not the problem indeed both adaptor have exacly same problem.

in my opinion, i have just to change pin 8 and pin 16 connection.

I will connect it as Stedy's schematic.

but, I am looking for good resitances without success (i must buy it... it's seems)

Last edited by CFOU!; 25 July 2017 at 10:02.
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Old 23 July 2017, 19:02   #8
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for information, my both scart adapters use following schematic but not working with my 2 CRT.
http://amigamuseum.emu-france.info/F...miga/cable.htm

but i don't understand why voltage divisor seens not works (12V read at vit 16 and 13 of scart

Last edited by CFOU!; 25 July 2017 at 09:58.
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Old 23 July 2017, 22:07   #9
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Quote:
Originally Posted by CFOU! View Post
for information, my both scart adaptors use following schematic but not working with my 2 CRT.
http://amigamuseum.emu-france.info/F...miga/cable.htm

but i don't understand why voltage divisor seens not works (12V read at vit 16 and 13 of scart
Well, if you take as valid the stedy's thread in regards to TV pins impedance the R values on this schematic is not the bests but do not explain your detected values..
For a 12V source your R1 need to be smaller (300 ohm for 2,4V on pin 16) and R2 can be much higher (1kohm).
There can be some explanation: resistance broken, pin16 SCART terminal resistance not 75ohm (measure it )..

Best regards,
ross
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Old 24 July 2017, 00:58   #10
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Quote:
Originally Posted by ross View Post
pin16 SCART terminal resistance not 75ohm (measure it )..
It can be AC coupled and btw seem plenty of mixing impedance with resistance... there is subtle but important difference.

I always recommend using diodes (Green LED, Zener or even few normal Si diodes to provide proper level on pin 16)
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Old 24 July 2017, 01:54   #11
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hi guys,

first thanks for your help

I finnaly found the solution.

Both adaptors used Amenos 2013's schematics (working on LCD) see here:
http://eab.abime.net/showthread.php?t=70944

but it's must be old version because no resitance on scart pin 20 (TTL CSync) on my both adapters.

so i have just add 110 ohm resitance on scart pin 20 (TTL CSync) and it's working

I recall that the two adapters (DB23 => scart) were purchased in the 80s or 90s.
(Without having been modified and worked only on LCD screen).

The symptoms of restistance missing:
- dull colors
- green components too present
(White appearing green / red appearing orange)


Best regards

Last edited by CFOU!; 25 July 2017 at 09:58.
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Old 24 July 2017, 11:58   #12
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Quote:
Originally Posted by pandy71 View Post
It can be AC coupled and btw seem plenty of mixing impedance with resistance... there is subtle but important difference.
Hi pandy71, yes i know the difference , but i've considered totally negligible the j part (if present).
But... why AC coupled? there is not a simple IC on TV side for the SCART management? decoupling C is in every discrete circuit..
Anyhow who know, without a scheme is impossible to say.
Quote:
I always recommend using diodes (Green LED, Zener or even few normal Si diodes to provide proper level on pin 16)
You are right but too many SI diode needed for a 12V source
[in fact the simplest thing could be a diode and a resistance in series]

Bye!
ross

Last edited by ross; 24 July 2017 at 12:39. Reason: []
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Old 24 July 2017, 12:08   #13
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Quote:
Originally Posted by CFOU! View Post
so i have just add 110 ohm resitance on scart pin 20 (TTL CSync) and it's working


The symptoms of restistance missing:
- dull colors
- green components too present
(White appearing green / red appearing orange)
Good that you have solved!

Best regards,
ross
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Old 24 July 2017, 21:14   #14
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Quote:
Originally Posted by ross View Post
Hi pandy71, yes i know the difference , but i've considered totally negligible the j part (if present).
But... why AC coupled? there is not a simple IC on TV side for the SCART management? decoupling C is in every discrete circuit..
Anyhow who know, without a scheme is impossible to say.

You are right but too many SI diode needed for a 12V source
[in fact the simplest thing could be a diode and a resistance in series]
AC coupling due of fact that video signal source may have DC offset (fully legal and allowed by SCART standard up to +2V).

As i said LED can be used as Zener diode (LED in normal, conductive way, btw LED are very stable as Zener diodes - they provide better stability than Si Zener diodes with such low Zener voltage), SCART standard says: Logical 1 is between +1 and +3V - as such any LED with sufficient forward voltage drop between 1 and 3V should be fine and additionally can provide visual confirmation for active output.

As Si diode has typically 0.6 forward voltage drop then by connecting 4 in series (trough current limiting resistor) they should provide 2.4V - 4 small SMD diodes anyway will be bigger than one LED.

Latest SCART standard (hq PDF) attached as reference - may be useful.

remark about signal level - CSYNC with 5V TTL level may disturb or even corrupt modern digital display input (latch-up effect) - SCART at video input expect no more than 1V (can be superimposed over up 2V DC offset so in total 3V maximum).
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Old 24 July 2017, 23:35   #15
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AC coupling due of fact --- cut ---
Thanks for all the informations!
Well, péritélévision is a "libertarian" standard

Regards,
ross
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