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DB15 pass through with RGB adjustments for use with consolized MVS?

Kujako

Kujako

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I built out a MV1C system using a consolization kit I'd seen bouncing around on Tindie.
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It's not too bad, but has a couple short comings. The biggest for me is the lack of RGB adjustment potentiometers. On some of my scalers (RetroTink5x) the output image is very washed out. Since this kit uses DB15 for the RGB output (mimicking the pinout of the Analogue DAC) I'm thinking a DB15 to DB15 with RGB potentiometers would address the issue. But alas I can find no such PCB or device, closest is this picture of an old GBS setup. Anyone know of anything like the blue RGB adjustment board pictured here? Or, any other ideas...

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Here's an example of the output. Looks fine going through my Lotharek Medusa, but the OSSC and RetroTink 5x are washed out.
1702831504903.png
 
RGB signals look to be out of spec, the Vp-p is way too high. Are there some surface mount components on the kit, perhaps a video buffer/amp on the bottom of the PCB? If not, then replace these x3 220uF capacitors with 220 Ohm resistors and you should be good.


caps.jpg
 
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RGB said:
RGB signals look to be out of spec, the Vp-p is way too high. Are there some surface mount components on the kit, perhaps a video buffer/amp on the bottom of the PCB? If not, then replace these x3 220uF capacitors with 220 Ohm resistors and you should be good.


caps.jpg
Click to expand...

Nothing surface mount, will take a look at the region when I get a chance to check where those capacitors go.
 
@RGB took me awhile to disassemble everything, but I've confirmed that the RGB is running through the 220uF capacitors (green path shown).
IMG_1689.jpg


Nothing else going on with the RGB output other than a resistor on sync.
IMG_1690.jpg


So based on this, you think replacing the capacitors with resistors is a wise move? Makes sense to me, but trying to figure out why the board was designed as it is.
 
Kujako said:
So based on this, you think replacing the capacitors with resistors is a wise move? Makes sense to me, but trying to figure out why the board was designed as it is.
Click to expand...
These capacitors make no sense in this scenario. An inline resistor with the 75 Ohm pull-down resistor on the OSSC's end will form a voltage divider and bring the RGB voltage to acceptable levels. I suggested 220 Ohm, but I checked using the oscilloscope, and 330-360 Ohm is a better choice, it will keep the max Vp-p at around ~700 mV (checked by displaying a 100% white screen using the MVS 240p test suite). Ideally, what you want is a buffer (the popular THS7316 or THS7374), but that will require a custom PCB since the part is in an SMD package.
 
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RGB said:
These capacitors make no sense in this scenario. An inline resistor with the 75 Ohm pull-down resistor on the OSSC's end will form a voltage divider and bring the RGB voltage to acceptable levels. I suggested 220 Ohm, but I checked using the oscilloscope, and 330-360 Ohm is a better choice, it will keep the max Vp-p at around ~700 mV (checked by displaying a 100% white screen using the MVS 240p test suite). Ideally, what you want is a buffer (the popular THS7316 or THS7374), but that will require a custom PCB since the part is in an SMD package.
Click to expand...

Will dig through the resistor pile and see what I have. With my limited knowledge, I'd have put potentiometers in where the capacitors are. Still may do that in the long term, but I'll try some 220-360Ohm resistors and measure the output.
 
If you want to go the potentiometer road, then a 500 Ohm potentiometer will be the best choice. Wire it as a rheostat (one terminal is input, the second terminal + wiper is output).

1.jpg
 
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@RGB so went with 330 Ohm since that's what I had on hand, and it's definitely better but I think perhaps too dim. Will see about measuring things with an oscilloscope when I can.
Screen Shot 2023-12-19 at 6.27.35 PM.jpg

 
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RGB said:
If you want to go the potentiometer road, then a 500 Ohm potentiometer will be the best choice. Wire it as a rheostat (one terminal is input, the second terminal + wiper is output).

1.jpg
Click to expand...
Quick question since it's been awhile since I've done this. Should polarity be observed? Input negative output positive?
 
There's no polarity, just make sure that wiper (the middle leg of the pot) is connected to one of the legs that you want to have as the output.
 
RGB said:
There's no polarity, just make sure that wiper (the middle leg of the pot) is connected to one of the legs that you want to have as the output.
Click to expand...

That's what I thought, but my memory is fuzzy these days. Pots are in. What do you think about installing a proper filter/buffer? Thinking I could wire in one of [these] fairly easily.
IMG_1696 (1).jpeg
 
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Kujako said:
That's what I thought, but my memory is fuzzy these days. Pots are in. What do you think about installing a proper filter/buffer? Thinking I could wire in one of [these] fairly easily.
Click to expand...
Definitely worth installing.
 
RGB said:
Definitely worth installing.
Click to expand...

In your opinion, would it negate the need for the potenitomiters? My hesitancy is that I can see a path to hook up one or the other, but not both. I dislike having no means to tune the output for the display.
 
The PCB you linked to seems to be MVS dedicated, so no further tuning should be required.

It would require some changes if you want to keep the pots and add that board. The potentiometers mustn't be wired as variable resistors. One terminal should be input, the second terminal (wiper) should be connected to GND and the third terminal should be output. The resistors that form voltage dividers (R2-R7) on the buffer/amp PCB should be removed and output signals from the pots should be wired directly to input coupling capacitors (C1-C3) of the buffer/amp chip.
 
RGB said:
The PCB you linked to seems to be MVS dedicated, so no further tuning should be required.

It would require some changes if you want to keep the pots and add that board. The potentiometers mustn't be wired as variable resistors. One terminal should be input, the second terminal (wiper) should be connected to GND and the third terminal should be output. The resistors that form voltage dividers (R2-R7) on the buffer/amp PCB should be removed and output signals from the pots should be wired directly to input coupling capacitors (C1-C3) of the buffer/amp chip.
Click to expand...

About what I thought. Not impossible to do but probably not worth it. Will try just the filter/buffer at some point.
 
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