PCE old Pal mod reversal.

[snow Crash]

Hello people of Pcenginefx! I got myself a pretty good condition original pc engine of e-bay wit the intention of doing an rgb mod on it and playing r-type. Once it arrived opened the casing and found it had been already modded to PAL RF.

I've checked the output over RF and it's pretty awful, white bleeds horizontally across the screen and the picture jumps and rolls quite often, so it's definitely got to go.



Looks like i'm gonna have to replace the missing resistors and capacitors to get this to work again. Could someone upload a very high rez scan/picture of the underside of a white pc engine board?

alternatively:

r137
r138
r141

and

c137 are the components that are missing. if anyone can supply info on these components it would be much appreciated!

[thesteve]

i would expect you have a crystal swapped to reduce the frame rate for pal
you need to compare the crystal value with a non modded unit.
your rgb mod should work once the frame rate is corrected

[snow Crash]

It looks like the original crystal is still installed on the pce board. There was another board attached to the rf modulator with a different oscillator so I think That should be fine. My big concern is the 3 surface mount resistors and capacitor that were removed when the mod was installed. I doubt the PCE will work with 4 components missing.

[thesteve]

the rgb skips all anolog vid processing, thus erelevent.
how ever the scan frequency was changed for pal, and will need to be corrected.
the processor clock had to change to change the scan rate.
find the right clock and it will solve your issue.

[mamejay]

I have tested the c137 and it gives me resistance and I am suspecting it's not a cap.  That is the only thing I cannot determine 100% from my pal modded board.  If anyone has info or a schematic this would make the reversal easy

[snow Crash]

mamejay, do you think you could take a photo of the pce board and post it here? Might be able to see what components are on yours but removed on mine  and vice-versa

[mamejay]

I will get a picture taken and also get all the values of the resisters.  We will need to work out what that C137 is as I am pretty sure its a CAP.

[mamejay]

OK.  I have done some testing and reversed my PCengin back to NTSC.
I ripped out a crystal from a working PCengine and popped that in.
I then replaced the resisters with the follow.
R137 - 6.7k
R138 - 6.7k
R141 - 13.2k
C137 - Bridged points.
Anyway its working fine.
I had a resister on C137 but i got a shacky picture at the top.  once I bridged the points it was fine and stable.
So now just need to source out a 21.4772MHz crystal oscillator and I will be set.
Let me know if you need a hand.

[snow Crash]

Thanks mamejay! That's exactly the info I was looking for! Luckily for me there was an override 17.734475 MHz crystal was on the daughter-board and the original crystal is intact in the pce itself.

Glad to hear your pce is working again. Were the resistors hard to find?

[mamejay]

We have a problem.
I did some more testing and the colours are a bit off.  I need to play around with the resistance values a bit more.  Looks like greens are not coming through 100%.
I will let you know the values once I work them out

[mamejay]

OK.  I have worked it all out now.
Here are the exact values.
R137 - 6.2k
R138 - 8.2k
R141 - 26.7k
C137 - 220uf capacitor
I used whatever resistors I had available.  6.7k for the first, 8.1k for the second and 26.8k for the last.
For C137 I just used a 220uf electrolytic capacitor.  I fired up the console without the cap and did a voltage check on C137 to see the polarity of the points.  Then you attach positive to positive and neg to neg.
Got nice colour now and the games are looking correct.

[snow Crash]

Great news! Do you know what part numbers some of the resistors are? the only ones i can find look different but should be the same value.

[thesteve]

any resistors of the right value should work.
you can get them SMD so they can solder right in

[mamejay]

I just used regular resisters and trimmed the legs and did some bending of them to make them as flush as possible.
Be careful as the track on the board can lift really easily

[snow Crash]

Thanks a lot, ordered some surface mount resistors from e-bay cheap enough, so hopefully all will be cool.