pc engine core grafx with,rf mod .removed it =now no video out ,c vid/sync

[bigsanta]

I bought a second hand core grafx 1 off the online tat seller ebay.It had a PAL I rf mod installed and the original din socket is removed.The mod is just a RF modulator and  also a pcb with a few ICs attached ,identical to the ones below.

The problem is,once i desoldered the mod and tested the c sync output via my pluggable external expansion port mod there's only a very faint image showing through the 99% black image on my 1080 monitor ,the same thing occurs when i soldered wires to the expansion port solder points (c sync and Vss).I also tried with it in the briefcase ,using the c video output of the briefcase,again the same problem.

Audio works ok,this is the 3 rd engine i've had and have rgb modded them no problem ,but i never tested this core grafx with the rf mod inplace, so maybe it had this video problem to begin with  ? There's one thing that's on my mind though.Above the oscillator and between where the original din was ,there's a place marked for where a polarised cap would be but all that's   left are solder filled holes,is there meant to be a cap?

[Midori]

This seems a bit unclear to me :-)

Are you using your 1080 monitor to display a RGB image using either c-sync or composite video a sync source or are you trying to display only composite video?

If you are trying to display an RGB image and you get a stable image but weak colours then the sync should be ok, it's the colours that needs to be taken care of. Are you using an amp for the RGB signals?

If you are trying to display composite video image then I would try to trace the composite videos trace on the pcb and se if the missing cap is related. If you have another Core Grafx to use as reference that would be great. I actually have one at home... I could try to snag a pic if you show me what needs to be seen.

[bigsanta]

At that moment i was just checking that it could output a video signal ,so i used the 1084s d1's comp video input jack ,just for quickness.But what i've done now is also check both the comp vid then comp sync(just to make sure my memory wasn't forgetting something) on my divers 2000 dreamcast's video input  and it showed the same black screen with  very faint game graphics ,they're 99% invisible to the eye,like someone has turned the contrast and brightness all the way down.

So i thought the console was knackered,and i couldn't use my other console as it's the white engine and it's in bits (modifications work).So what i did was connect my detachable RGB mod (90% complete) to the expansion port and guess what ,it outputs a perfect picture through the d sub of my 1084s(even without the amp it's a good but a bit dark picture as expected).But still leaving me scratching my head as to why it won't display just a composite picture(yellow connector) through the briefcase's composite video output or just through the core grafx,as why one (rgb) but not the other.

I've checked all grounds ,all solder connections on the plastic type plate on the underside of the pcb and they all seem ok ,then i've checked continuity when the metal plate makes contact with the base of the pcb,again ,it shows a perfect common connection .  

edit: there's one point which i'm not sure about,and that's if you can check the ground connections on the solder side of the joypad's mini din socket,check to see if they are not part of the common ground loop like mine is.

About this point on the core grafx's mother board is where the graphic for a polarised cap is.

[Midori]

Ah, so you were feeding composite sync into a composite video input. Then you get a very dark picture indeed :-)

Although it is strange that your composite video output does the same thing.

And the cap was on the RF modulator, now I see. I would remove the RF modulator first, and then try to restore the composite video.

The joypad din connectors ground points are connected to the common ground yes, it is connected to it through the shielding. Why it ain't connected on the PCB though is a mystery :-)

[bigsanta]

Ah, so you were feeding composite sync into a composite video input. Then you get a very dark picture indeed :-)

Although it is strange that your composite video output does the same thing.

And the cap was on the RF modulator, now I see. I would remove the RF modulator first, and then try to restore the composite video.

The joypad din connectors ground points are connected to the common ground yes, it is connected to it through the shielding. Why it ain't connected on the PCB though is a mystery :-)

No ,the (missing )capacitor is on the core grafx's pcb ,directly under that point  (on the photo ,which isn't my picture)

Here we go,now is that spot meant to be cap (less) ?

[Midori]

Aha, I missunderstood you. Yes, there should be a capacitor there. I screwed mine together a while ago and am not too keen on opening it again...

But here is a pic of it and it shows that there should be a cap there.



Value I don't know... 220uF perhaps should do the trick? Or if you can find a cap on your board that looks the same size you could read from that.

[bigsanta]

That's the only big boy cap on the pcb,so i can't compare the value !