PC Engine CoreGrafx RGB Mod problems

[Ganbare Goemon]

Hi,

I performed a RGB Mod on my CoreGrafx.
Now the problem is, when there is a black or gray screen, I get some some strange noise.
Please have a look at the video here:
https://streamable.com/61442a

The cable I have is from retrogamingcables.co.uk. I guess this should not be the source of error.

What could it be?

Thanks

[Keith Courage]

What RGB amp did you use?

[Ganbare Goemon]

I use the THS7374 RGB Amp with low pass filter off and using the c-sync pin
I bought 3 of them:
https://i.ebayimg.com/images/g/ifQAAOSwVRNfOWRf/s-l1600.jpg
They worked without problems in my N64 and SNES but I get that noise on the CoreGrafx.
In order to rule out if it is the Amp I tried a old THS7314 amp I still had and there it is the same problem.
Also I have recapped the CoreGrafx. The problem was before and after recapping.
Now I am lost.

[Ganbare Goemon]

Okay, I was able to fix that myself.
I was not familiar with that DIN connectors and forgot to connect the shielding properly.
But there is a question left.
The RGB Amp board I used were for N64. The PC Engine boards I saw on the net all have 330uf caps installed.
Should I add them to my mod too?

[Keith Courage]

I'd need to see the N64 board to know. Most PC engine RGB amps do have 330UF caps in line. You can also put the caps in the scart cable end instead.

Also, did you make sure you cut the trace for composite on the motherboard for the coregrafx so you are not feeding C-sync plus composite video into the same line? IF both are connected to the same pin it can damage the system.

[Ganbare Goemon]

The amp board looks like on the attachment.
I havent cut any line yet.
I did the wiring like on that picture.

RGB and csync go into the RGB amp board and from there to the DIN connector.
I am not sure if ther eis any composite video still involved.

[Keith Courage]

Let me see what is wired up to the DIN connector.

[Ganbare Goemon]

I did the wiring according to this page since I had two of their cables:
https://www.retrogamingcables.co.uk/pc-engine/PC-ENGINE-PCE-DUE-turbografx-16-nec-core-grafx-PACKAPUNCH-PRO-rgb-scart-video-av-cable

[Keith Courage]

So you did you wire a line from the amp for sync to pin 3 on the din connector? If so that can damage the system if you do not cut the trace on the motherboard for composite.

[Ganbare Goemon]

Yes, c-Sync goes to Pin 3.
I wasn't aware that that could case damage. I haven't used the system since I have opened the question here so there should be no damage done yet. 
Can you please advice me on were to cut that line?

[Keith Courage]

https://www.otakus-store.net/en/content/25-RGB-modding-kits-installation-guides-tutorials-for-PC-Engine-Core-Grafx-Duo-Duo-r

Scroll down to where it shows the Coregrafx. It shows where the trace should be cut if you plan to use c-sync for the sync line.
Some people just leave composite as the sync signal and don't bother with c-sync.

[Ganbare Goemon]

Thanks, that will help.
Now just out of curiosity, while reading the page I also saw the modding guide for the white pc engine.
Why are they using mono sound when they could get stereo from the back connector?

[Keith Courage]

I have no idea why they didn't use stereo audio for the PC engine mod. I mainly just sent you that link for where to cut the composite video trace.

[Ganbare Goemon]

I modded the system successfully.
But there was still a bit of noise visible.
It was barely visible but it was there.
I was a bit baffled because I thought I did it all right.
Then I tried another power supply and all the noise was gone.
Is it possible that power supplies can cause that?
I got the power supply from https://retrogamesupply.com/
Could it be that this one is faulty?

[Keith Courage]

Bad quality Power supplies can do that. Usually video interference from a generic AC adapter is more noticeable using composite video.