[MZ-700] Modding a SCART>HDMI to output RGBS on modern TV

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Joined: Thu Jan 25, 2018 9:31 pm

[MZ-700] Modding a SCART>HDMI to output RGBS on modern TV

Post by hlide »

I have bought this converter because I was told they work pretty good with RGB output for AMIGA. I also read an article about the synchronization issue with Atari when using a SCART on modern TVs and to address it by modding this converter so I decided to buy another one as I was expected to get a similar issue with MZ-700 and MZ-800.

Here is the converter:
61upuF6RuVL.jpg (72.24 KiB) Viewed 4032 times
This converter accepts RGBS source and composite (CVBS) source but not S-video source. In our case, only RGBS matters. Beware, there are a lot of SCART>HDMI converters out that don't accept RGBS source through SCART. At least this model does.

Here is my work environment:

You can see I have an endoscope plugged into an embedded PC which displays on my modern TV. The circuit of the converter is quite small so this camera is a handful device.

The idea is to use this converter to provide fast switching with the voltage that goes well so that the TV receives well in RGBS.

The idea is all the more attractive as I could make myself a SCART cable for the MZ-700 (or even for the MZ-800) which will have the same problem of not being able to provide the right signal for fast switching in RGBS. If this converter does the job instead, I could use this converter for both the Atari 520 STE and the MZ-700 not caring about fast and slow switching signals (the latter can be ignored by forcing the 4/3 TV).

But there is a problem. The circuit of the converter has obviously changed since the last update of the tutorial I link here. The link states that a trace must be cut between the fast switching pin and the first 75 Ohm resistor connected to the ground in parallel. Just after this resistor, there is a ZD1 diode where we must add a wire connected to a source of 5V with a serial 100 Ohm resistor. Now, this diode is no longer!

So I tried to follow the trace: it goes through the same 75 Ohm resistor connected to the ground in parallel. So I guess I just have to cut it. However, on the other branch in parallel, there is no more diode and the trace drives straightly to a pin of the big processing chip.

The circuit with the 'FAST SWITCH' trace drawn in red:

The voltage applied to FAST SWITCH must range between 0,3V and 3V so the SCART should use RGBS signals and not composite ones.

Now, I have two solutions:

1) Steal a 5V pad and turn it into a 2.15V to feed the big chip:
On my circuit, there is nothing after 74 Ohm resistance. We can try a voltage division that looks like this:
voltage-divider-schematic.png (1.91 KiB) Viewed 4032 times
where VIN = 5V, R1 = 100 Ohm and R2 = 75 Ohm.

As a result, we have: VOUT = 5V * (75 / (100 + 75)) = 2.15V.

Which gives us:

Last edited by hlide on Sun Feb 04, 2018 1:04 pm, edited 4 times in total.
Posts: 519
Joined: Thu Jan 25, 2018 9:31 pm

Re: [MZ-700] Modding a SCART>HDMI to output RGBS

Post by hlide »

2) Steal a voltage from a regulator:

There are several regulators
But 1.8V U4 is the best choice for soldering and voltage range matching.

I chose the second solution.

I noticed that there were two pads just above the 75 Ohm resistor and a trace that connected the two pads. If I want to be able to go back to the origin, I just need to connect these two pads and remove the wire:

- cut the track between pin 16 of the SCART (fast switching) and the connection point between two branches, one of which has a 75 Ohm resistance connected to ground.
- connect the 1.8 V output of the U4 controller to the connection point of the two branches with a wire.

Let's see the result!
Note: my converter is outputting 800x600 and my TV is currently in a special mode which allows showing one source pixel into one destination pixel in a centered window so the monitor screen may look tiny on my TV. The display and text are really much crisper than the one I got with composite or RF signal.
Posts: 519
Joined: Thu Jan 25, 2018 9:31 pm

Re: [MZ-700] Modding a SCART>HDMI to output RGBS on modern TV

Post by hlide »

If I try to plug the SCART cable to my TV instead of the converter, TV looks for the best signal and chooses a CVBS signal in the end. Here is what I got:
And closer:

I also tried with my first SCART>HDMI converter which isn't modded and got the similar result.

To compare to the output given by the converter with the proper modding:
Jo Even
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Re: [MZ-700] Modding a SCART>HDMI to output RGBS on modern TV

Post by Jo Even »

Great write-up :) Thanks for your patience with the attachment-issues.
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