Author Topic: PC engine, RGB mod - AMP brightness switching problems (Read 741 times)

cheezio · « **on:** August 29, 2006, 04:20:33 AM »

Hi - I hope someone else has had this problem and can help me out.
I`m modding a white PC Engine from RF output into RGB. I`m taking the lead from this page: http://www.gamesx.com/rgbadd/rgbturbo.htm

I did the RGB mod to scart - all went very well just the colours were a bit dark as expected. Composite video was normal composite, no problems there.
Now I then moved on to making the RGB amp - got the parts, put it all together, plugged it into the telly and hey presto! RGB colours now look normal.

BUT

- after a minute or so, the picture starts to do this odd switching from the nice new brightness the amp gives - down to a slightly darker picture for a second, then back to being bright again for a second - etc. Not good.

Turning the unit off and on fixes the problem, for a few minutes or so, until the same problem re-occurs again.

I have checked the circuit voltage for the AMP circuit, and it's at 4.7 volts when the picture is nice and bright - but then dips to 3.8volts when the picture is dark.

Any ideas on how i fix this? is there just not enought current perhaps (I am tapping the 5v from the Vout from the expansion connector)? would extra capacitors help regulate the problem?

thanks for any help!!

FM-77 · « **Reply #1 on:** August 29, 2006, 04:43:52 AM »

I never made an amp for this mod and I get a great picture anyway. Just connect those transistors directly to the R, G and B lines - that should do the trick!

cheezio · « **Reply #2 on:** August 29, 2006, 04:53:45 AM »

I should also say that I`m not converting the RF to RGB - the RF is ripped out and I`m tapping RGB from the rear expansion of the console.

Thanks Seldane - i`ll try the AMP without all the capacitors. The way the brightness flicks up /down/ up / down so regularly makes me think the capacitors must getting to an odd state where they discharge/fillup/etc ??

I was amping the Sync, which I dont seem to need for SCART (as composite video seems to do fine) - so i`ll try decoupling that first to see if it was the power muncher.

I`m using a 9v 650mA original japanese psu via a stepdown. And the game all runs fine whilst the brightness was going up/down ....

cheezio · « **Reply #3 on:** August 29, 2006, 05:37:31 AM »

one other thing - on the scart connector I grounded lines where they ask to be (eg red GND, blue GND etc).

so I have the following:
1 = audio R
3 = audio L
4 = audio GND
5 = BLUE GND
7 = BLUE
9 = GREEN GND
11= GREEN
13 = RED GND
15 = RED
16 = +5volts (via 100ohm)
17 = video GND
19 = video/sync

on gamesx they dont mention grounding R/G/B - should I not ground them?

FM-77 · « **Reply #4 on:** August 29, 2006, 06:42:40 AM »

Do not ground R, G and B nor audio and composite video. You should not use composite sync if you're connecting it to a TV.

Connect these to the scart connector: (nothing else!)

2 AUDIO input RIGHT
6 AUDIO input LEFT
7 BLUE in
11 GREEN in
15 RED in
18 Blanking GROUND
16 Blanking (+5V)
20 COMPOSITE VIDEO in

Joe Redifer · « **Reply #5 on:** August 29, 2006, 07:27:30 AM »

The transistors directly in the RGB lines works fine. Get rid of any resistors that may be in the SCART cable. Open it up and make sure no resistors are in there. Remove them.

happy happy.

cheezio · « **Reply #6 on:** August 29, 2006, 08:56:38 AM »

thanks for the info. the scart lead has no resistors or anything nasty inside - just wires.

So just to confirm - the amp - instead of the gamesx version (capacitors/resisitors) - I just wire each of R+G+B into a transistor each, then add 5v to each transistor, and then take the output to the scart lead. Any images/diagrams would be great - just to make it super doopa clear

thanks!!!

FM-77 · « **Reply #7 on:** August 29, 2006, 12:34:38 PM »

That's exactly right.

Joe Redifer · « **Reply #8 on:** August 29, 2006, 06:24:59 PM »

What's weird when I turn my TurboGrafx on after being wired up like this, the image takes a second or two to kind of "dissolve in". I assume that's the transistors getting up to speed or something weird like that.

cheezio · « **Reply #9 on:** August 30, 2006, 01:18:10 AM »

hi - ok, out with the gamesx amp - and now just using transistors (2SC1815 types).

All seems good, now the colours are back to normal luminance (MR HELI without the AMP just looked awful, strong colours but all too dark, R-TYPE looked so odd too with dark colours).

1st time i tried it something odd was going on where the console kept rebooting itself, but retrying now i have been able to play through to level 4 r-type with no issues (so maybe a loose wire was shorting stuff).

thanks again for your help on this - I just wish I knew why the gamesx AMP wasn't working though... perhaps its only good for duo machines, that have more power?

Joe Redifer · « **Reply #10 on:** August 30, 2006, 05:36:38 AM »

I didn't have any luck with the GamesX amp either. But then again I was going to SCART, they weren't.

cheezio · « **Reply #11 on:** September 02, 2006, 10:15:57 AM »

hi - i just have one more question (thanks for bearing with me).

When using the console in RGB mode (not composite) - you can see something like very faint noise or something rolling in the background . Its most evident on blank screens like menus - very subtle but definately there. And rolling quite fast - just lines - no image that you can make out.

i have tried changing PSUs - no change.
the cable to tv is a proper sheilded one, so shouldnt be the problem.

Is this a normal thing - just the best the console can do? (as it wasnt meant to do RGB as such out of the box?)

Anyone else noticed this?

ta!!!

FM-77 · « **Reply #12 on:** September 02, 2006, 12:06:28 PM »

I've never experienced this. Sounds like a grounding problem.

Joe Redifer · « **Reply #13 on:** September 02, 2006, 12:35:03 PM »

**double post**

Joe Redifer · « **Reply #14 on:** September 02, 2006, 12:35:21 PM »

Are the lines diagonal, vertical, or horizontal?

Maybe the wire you used is too thin?