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Old 05 October 2014, 23:43   #1
Ox.
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RGB scart from A1200 to lcd problem

I recently purchased an rgb scart cable from amigakit (modified for use with lcd tv's) and after much deliberation my lcd tv won't work with the A1200, nothing wrong with the cable or tv per se they just aren't compatible, the a1200 boot screen loses the purple background colour and the screen blacks out on and off.

Anyone know specifically what the issue is here (the tv is a Bush 19134HD) and what the possible fix is before I go off and buy another cable to mod?

Last edited by Ox.; 08 October 2014 at 01:49.
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Old 08 October 2014, 01:52   #2
Ox.
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There are some resistors on some lines of the scart connector, I'm wondering if these need to be removed to work with my tv or whether it needs different value resistors?
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Old 08 October 2014, 03:09   #3
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pictures please, before removing anything ^^)
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Old 08 October 2014, 03:29   #4
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Sounds to me that either the Fast Blanking voltage is either too high or too low.

I suspect you have one of "Those" display panels that is some what very temperamental on the input voltage that selects RGB.

Pin 16 is of the SCART is the decider here, you need to know the following

1. What voltage is being supplied (I suspect 5vcc but CHECK with AmigaKit)
2. What resistor is being applies (I suspect something like 330Ohms - check)
3. What load resistor is in the LCD panel (and this is the bit that will require a bit of effort)

From what I had read here, I suspect that this LCD TV Panel has a higher than 75ohm pull down (on pin 16) - if this is so, then its possible the input voltage is too low < 1volt pp.

While unlikely it is possible that there is NO pull down on the input of pin 16 which would mean the voltage could be to high > 3v pp.

I seriously suspect the former and you will have to investigate more.

Firstly I only encourage you to contact AmigaKit where you purchased the cable to seek their advice, for me to attempt a work-around, I would drop the input resistor 100 Ohms and see what happened.
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Old 08 October 2014, 09:47   #5
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Get this on one of my LCD TV'S on A500, A600 and A1200 - the cables are fine (from Amigakit) as I don't have issues on any other screen.

If I manually select an AV channel it seems fine but my other TV'S don't require that...in fairness the TV is only a cheap one and I have others to use....reminds me I must get my spare 1084s back up and running.
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Old 08 October 2014, 14:14   #6
Ox.
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Smile

Quote:
Originally Posted by Zetr0 View Post
Sounds to me that either the Fast Blanking voltage is either too high or too low.

I suspect you have one of "Those" display panels that is some what very temperamental on the input voltage that selects RGB.

Pin 16 is of the SCART is the decider here, you need to know the following

1. What voltage is being supplied (I suspect 5vcc but CHECK with AmigaKit)
2. What resistor is being applies (I suspect something like 330Ohms - check)
3. What load resistor is in the LCD panel (and this is the bit that will require a bit of effort)

From what I had read here, I suspect that this LCD TV Panel has a higher than 75ohm pull down (on pin 16) - if this is so, then its possible the input voltage is too low < 1volt pp.

While unlikely it is possible that there is NO pull down on the input of pin 16 which would mean the voltage could be to high > 3v pp.

I seriously suspect the former and you will have to investigate more.

Firstly I only encourage you to contact AmigaKit where you purchased the cable to seek their advice, for me to attempt a work-around, I would drop the input resistor 100 Ohms and see what happened.
That narrow it down thanks Zetro
Tbh with Amigakit I think they would be very hard pushed finding the correct value resistor without having the tv to hand and searching the internet I could'nt find anything on the tv myself so it looks like it has to be trial and error. Would it be advisable to place a variable resistor on line 16 until I find the correct value?
I just wish Amigakit had'nt hot glued the connectors as now it will have to be a hack job.
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Old 08 October 2014, 15:05   #7
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Quote:
Originally Posted by Ox. View Post
Would it be advisable to place a variable resistor on line 16 until I find the correct value?
Yes it would be, but it might be easier to try putting resistors of different values in parallel with the existing resistor until you find one which works.
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Old 08 October 2014, 16:18   #8
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Isopropanol (isopropyl alcohol) removes hot glue pretty easily. Of course if the whole connector is totally filled it's still going to take some doing

The variable resistor trick is a good idea. Remember though that if it's in parallel with the existing resistor, you can't get a higher resistance value than what is already there (and in series you can't go lower).
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Old 08 October 2014, 17:40   #9
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Remember - modern LCD are at best 3.3V devices - weird brand can be less robust (poorer design) than good brands. Placing to high voltage (higher than Vcc) on some SCART input pins can damage permanently TV (TV should have protection on all inputs but... cost reduction, worse design, lack of testing and...).
All measurements on SCART pins should be performed on load - nominal load for pin 8 is 10kOhm, for pin 16 is 75 Ohm.
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Old 08 October 2014, 19:10   #10
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Ian Steadman thought the most likely cable faults to affect LCDTv's were :

1) FAST BLANKING : Some users have said a 220 ohm (instead of the 75 ohm he originally recommend) should be used on SCART pin 16.

2) COMPOSITE_SYNC : voltages, the Amiga voltage range for that pin is TTL which is not the same range as the SCART voltage for that pin. Adding a 330 ohm resistor on SCART pin 20 will fix this.

3) No AC coupling. Adding 220uF capacitors in series on the RGB lines (SCART pins 7, 11 and 15) may help

http://www.ianstedman.co.uk/Amiga/am...iga_scart.html
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Old 09 October 2014, 02:09   #11
Ox.
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Thanks for the replys everyone, I'm now forearmed to make a stab at getting this cable working with my display.
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Old 13 October 2014, 13:22   #12
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I had exactly the same problem, in the end I just got a different LCD TV. They're so cheap now it's not worth the effort of troubleshooting, and if the TV doesn't like the Amiga's output there's not much you can do about it.
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