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XT/CGA laptop dissection

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Reply 60 of 100, by Jinxter

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I want to make or buy a CGA (RGBi) to SCART to use on my Schneider Euro PC.
Anybody that sell these? If Not where can i buy the components needed? Any kit available?

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Reply 61 of 100, by keropi

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Jinxter wrote:

I want to make or buy a CGA (RGBi) to SCART to use on my Schneider Euro PC.
Anybody that sell these? If Not where can i buy the components needed? Any kit available?

I don't know if someone sells any kits/premade adapters but it's not that hard to make one for yourself , the parts are easy/common and cheap to buy.
Here is how I built mine: Re: Acer 500+ XT system , using this schematic, parts are written on it:

th_k-cga-to-scart_zpspvagjbgi.png

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Reply 64 of 100, by Jinxter

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bjt wrote:

This is the only place I know of selling them ready-made.
This should display brown correctly too.
http://bit-c64.com/

Why do i need the BIT-C-128? The Gonbes GBS-8200 says is supports CGA input?

Also: Could the BIT-C-128 bu connected to a SCART connector?

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Reply 65 of 100, by bjt

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When the Gonbes specs say it supports "CGA", they actually mean 15kHz analogue video.
PC CGA is 15kHz digital video, hence the need for a digital to analogue converter like the one linked and the circuits posted elsewhere.

Reply 67 of 100, by Jinxter

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bjt wrote:

When the Gonbes specs say it supports "CGA", they actually mean 15kHz analogue video.
PC CGA is 15kHz digital video, hence the need for a digital to analogue converter like the one linked and the circuits posted elsewhere.

Thanks for Your answer
But what about the BIT-C-128? It is called a DAC si i guess i converts the RGBi signal to an analog RGB (+HV) signal....
If so, can this signal be used as input to SCART? Any thoughts on this?
Could the RGB signal from the DAC og to RGB (15,11,7) in on the SCART. And the Grey (H/H+V) output from the DAC og to Pin 20 (composite) on the SCART?

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Reply 68 of 100, by bjt

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Yes, it should work, as long as the device can handle 60Hz. SCART spec is 0.7V P-P for colour and 1V P-P for sync, so some resistors may be needed depending on the output voltages of the DAC.

Reply 69 of 100, by MobyGamer

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One warning about the BIT-C-128 -- I have one and have verified that my unit outputs about 20% less green than the other signals, turning everything purple. I don't know if this is my specific unit, or if the design of the BIT-C-128 is this way on purpose; the author didn't respond to the last email I sent him. But, he communicated very well up until that point, and shipped quickly, so for the price I suppose it's worth a shot.

Reply 70 of 100, by Great Hierophant

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MobyGamer wrote:

One warning about the BIT-C-128 -- I have one and have verified that my unit outputs about 20% less green than the other signals, turning everything purple. I don't know if this is my specific unit, or if the design of the BIT-C-128 is this way on purpose; the author didn't respond to the last email I sent him. But, he communicated very well up until that point, and shipped quickly, so for the price I suppose it's worth a shot.

Are you sure it isn't the monitor that has an issue? I would check the board's resistors to see if there is an inordinate value on the green before the brown-fix. Maybe the wrong value got used. As much as it would please me to hear that John Carlsen screwed up a third time, best eliminate all other possible causes first.

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Reply 71 of 100, by Jinxter

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No i am not sure.
Have only tested on one monitor. Will try more.

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Reply 72 of 100, by MobyGamer

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Great Hierophant wrote:

Are you sure it isn't the monitor that has an issue? I would check the board's resistors to see if there is an inordinate value on the green before the brown-fix. Maybe the wrong value got used. As much as it would please me to hear that John Carlsen screwed up a third time, best eliminate all other possible causes first.

So far, he has been very responsive to my latest communication. When the troubleshooting is all over, I'll update this thread.

Reply 73 of 100, by Jinxter

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bjt wrote:

This is the only place I know of selling them ready-made.
This should display brown correctly too.
http://bit-c64.com/

Has anybody here tried the C-128 bit on a scart?

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Reply 74 of 100, by MobyGamer

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MobyGamer wrote:
Great Hierophant wrote:

Are you sure it isn't the monitor that has an issue? I would check the board's resistors to see if there is an inordinate value on the green before the brown-fix. Maybe the wrong value got used. As much as it would please me to hear that John Carlsen screwed up a third time, best eliminate all other possible causes first.

So far, he has been very responsive to my latest communication. When the troubleshooting is all over, I'll update this thread.

I forgot to update the thread, sorry. In my discussions with the designer, he designed his board by measuring the post-conversion voltage values of his actual Commodore 1084S-D monitor. It's my theory that his monitor was faulty/damaged/non-calibrated when he did his measurements.

I am not intentionally trying to paint him in a negative light, only reporting my experience. I tested two boards, including one certified by him as 100% functional before he sent it to me, and got identical wrong green levels with multiple capture devices and monitors. I even took the entire setup, Gonbes and all, to a vintage computer gathering so that its output could be compared to a real C128 hooked up to a 1084S-D, and no amount of Gonbes POT adjustment could make it look right. It is my personal opinion that the bit-c128 device produces incorrect green output and is not suitable for a CGA replacement.

Reply 75 of 100, by Great Hierophant

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MobyGamer wrote:
MobyGamer wrote:
Great Hierophant wrote:

Are you sure it isn't the monitor that has an issue? I would check the board's resistors to see if there is an inordinate value on the green before the brown-fix. Maybe the wrong value got used. As much as it would please me to hear that John Carlsen screwed up a third time, best eliminate all other possible causes first.

So far, he has been very responsive to my latest communication. When the troubleshooting is all over, I'll update this thread.

I forgot to update the thread, sorry. In my discussions with the designer, he designed his board by measuring the post-conversion voltage values of his actual Commodore 1084S-D monitor. It's my theory that his monitor was faulty/damaged/non-calibrated when he did his measurements.

I am not intentionally trying to paint him in a negative light, only reporting my experience.

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MobyGamer wrote:

I tested two boards, including one certified by him as 100% functional before he sent it to me, and got identical wrong green levels with multiple capture devices and monitors. I even took the entire setup, Gonbes and all, to a vintage computer gathering so that its output could be compared to a real C128 hooked up to a 1084S-D, and no amount of Gonbes POT adjustment could make it look right. It is my personal opinion that the bit-c128 device produces incorrect green output and is not suitable for a CGA replacement.

Or for a C128 monitor either, they must use the same colors. Unfortunately the bit-c128 board has surface mount resistors, so adjusting values is not as simple as turning a pot. But once it turns 4-bit digital RGBI into 3-signal analog RGB, you can't adjust the signals to get a CGA brown without muting every other color that uses a green component.

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Reply 76 of 100, by 386_junkie

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This thread is amazing... i've never seen an XT laptop before, it even has a "proper" mechanical clicky keyboard! Definitely don't make them like this anymore.

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the screen looks like a monster GameBoy 😁

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Great Hierophant wrote:

Or for a C128 monitor either, they must use the same colors. Unfortunately the bit-c128 board has surface mount resistors, so adjusting values is not as simple as turning a pot. But once it turns 4-bit digital RGBI into 3-signal analog RGB, you can't adjust the signals to get a CGA brown without muting every other color that uses a green component.

If you are trying to adjust the strength of the signal varying some of the inputs but not others it is certainly possible, well... physically at least, to remove said discrete components, the resistors, and replace them with four equivalent value pots. I've done similar stuff before... unless the next stage requires particular signal conditioning, just take out the resistors and put an equivalent valued pots (x4 of them) in their place.

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Reply 77 of 100, by Jinxter

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I finally made a CGA to SCART adapter. I followed this schematic.

The picture are fine. Especially on the CRT. On the LCD the screen is "crawling" and the colours ar not so bright. The photos does not show this. The photos show more vidid colours on the LCD, but this was not true when i looked at the screens.
I am very surprised how good the pictures are. I will recomend using this method for others.

PS! There is en error on the Schematics. Wher it says 4-G on the scart it should be 11-G. Also where it says 7 in the input side. Do not connect this to 7 in the CGA but to 7V+. Other descriptions says 5+ - but that did not work on my LCD or CRT. Also memember to connect 5V- to ground.

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Reply 78 of 100, by Jinxter

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I made av video of the project.

https://www.youtube.com/watch?v=ymulGiO4NrI

Check out my YouTube channel: Retro Erik https://www.youtube.com/channel/UCSUf7nCpwTgFbjGmcTYGZEQ
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