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A question about the 2N7002NXBKR Trench MOSFET #8

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soniccd123 opened this issue Jan 25, 2022 · 9 comments
Open

A question about the 2N7002NXBKR Trench MOSFET #8

soniccd123 opened this issue Jan 25, 2022 · 9 comments

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@soniccd123
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Hello friend,

Great project, looking forward to the next developments, would love to use some of it in one of my TVs someday

I have a question. For the last year and a half i've been working on a Panasonic TC-14RM15L as a test bed for some modifications with the objective of creating a adequate and safe way to add RGB inputs to non European CRT TVs. There are ways to do it in older TVs that have the Jungle IC separated from the TV MCU, but most modern CRTs do not have this configuration.

One area that I'm having trouble is the HBlanking on this set, without correct color signal atenuation, lots of regularly timed transversal lines are visible on the screen. These lines seem to be the beam returning to draw the next line. The problem is that some devices, like the Sega Dreamcast, in specific video modes, will display these lines even with the needed atenuation. Looking at you schematics I supose that I need to pull each color signal low every blank interval and maybe these lines will disapear. I'm excited to try your solution (from what i could understand in the schematics), but I need to know if the 2N7002NXBKR is strictly necessary or one could use more acessible alternatives like the 2N7002L.

Thank you very much, good work!
@tdaede
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tdaede commented Jan 25, 2022

Hi,

Yes, you can use a plain old 2N7002. The exact one I used is just a particular manufacturer's model that I chose at the time based on availability.

There are two RGB circuits in this repo - one is on td-deflect, and one is in td-rgbinput. The td-deflect one is older and works well, but uses an old stock IC. The td-rgbinput one is functionally similar but built with discrete components.

The td-rgbinput circuit has 4 separate functionalities - a DC restore circuit for AC coupled inputs, a blanking circiut, a gain control circuit, and a bias circuit. You might not need all these parts depending on where you're inserting the signal.

@soniccd123
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I'm gonna take a look and try to implement them to test, I'm connecting directly to the neckboard of the TV, more specifically, to the video output amplifier that drives the cathodes (TDA6107AJFN1), i'm connecting just the RGB signals to it and letting the TV's circuitry take charge of the Sync. Between the neckboard and the device I've added some simple circuitry to control the signals, but not nearly as elegant as your aproach. It works quite well, this TV actually became my main way to play old video games, but I think that its possible to make it even better adapting your designs to it.

Do you recommend any resources to learn more about TVs and analog electronics? While i've been playing around with this TV, i lack a lot of the knowledge to really understand deeply how all of these parts work

@tdaede
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tdaede commented Jan 25, 2022 via email

@soniccd123
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Thanks for the resources, it is quite hard indeed to find these things on internet

Was taking a look at the TV service manual, unfortunately the neck board doesn't seem to have a blanking input, it does have a AKB output from the video amplifier that drives the cathodes, but I suspect to me that the blanking is done on the MCU/Jungle chip before this step (TDA11115NDS, a chip which not datasheet could be found).

I took a look at the td-deflect board and saw that you used a M51387P as video amplifier which i found pretty interesting, not that I didn't like your discrete aproach, but the M51387P seems to be quite cheap where I live. This amplifier is suposed to be used to drive tube directly? In the case of my TV that uses a TDA6107AJFN1 to drive the tube, should the M51387P outputs drive this amplifier or the tube directly?

@tdaede
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tdaede commented Jan 26, 2022

The outputs should drive the amplifier.

I think that the AKB signal is actually an input to the amplifier, not an output.

@soniccd123
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Are you sure? I found the amplifier datasheet, from what I understood, AKB is connected to the amplifier pin 5 which is a "black-current measurement output" in the datasheet words. It would be very helpful if it was the blank signal input.

@tdaede
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tdaede commented Jan 27, 2022

No, not sure. Didn't realize a monitor like this would bother measuring that, but I suppose it could.

@soniccd123
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Hey, the M51387P finally arrived, i'm taking a look at the TV schematics and signals, having a hard time to find how to extract the blanking signal from sync. I see that it is very similar to H-sync, can i use it or do i need another solution?

@tdaede
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tdaede commented Jun 27, 2022

Yes, you can use H-sync to blank. However, clamping needs to not intersect with blanking - it needs to happen after blanking.

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@tdaede @soniccd123