Midway Pacman to Jamma to Scart. Possible sync issue

[Shun]

Hi,

Hopefully someone can help me.

I have an original MS. pac-man board that I am trying to test out.
I have it hooked up to Jamma with a homemade adapter and the Jamma connection is connected to a scart tv.

Unfortunately I get a very quick vertical scroll and it is very hard to see what is happening on screen, however I think the game is booting up fine as I can see the colours change when the attract mode displays the pink maze.

I know that the Jamma to scart connection works ok as I have had my Neo Geo 64 running on it fine.
The pinout for ms. pacman/pacman says that it needs a negative sync, which I have connected to the sync on the jamma connection. Are these syncs compatible?

[Shun]

Ok I've hooked up a speaker to the board and it coins up fine and starts the game.
I've attached a picture of what I can see on screen. Like I said before I can make out the attract sequence by the colours on screen and I think it is most likely a sync issue. I have tried it on two different tvs now with scart and I get the same result.

Does anyone have any advice in getting the image fixed?

[matsadona]

I had the same problem with my Police trainer game when connected to a SCART TV through a simple cable with 150Ohm resistors. Couldn't get it to sync.

Eventually I tried with my Vogatec Supergun, and then it was a perfect picture. When investigated in detail I realized that the supergun was more or less the same as my cable, but also amplifying the sync signal. I.e. there is an OP-amp adding some extra power to the signal. Maybe you should try that.

[Shun]

Thanks matsadona. I am starting to think that there is either little or no signal getting to the TV.
I snipped the sync wire that I had made and there was no difference in the picture. I need to try and amplify the signal somehow to see if I can get a stable image.

I'm not sure what an OP-amp is. Is there a simple circuit that I could build to amplify the signal?

[matsadona]

op-amp = Operational Amplifier. So yes, basically it just amplifies the input signal (but there is more to it than that *).
A general transistor would probably do it as well.

I can look at my board tonight to verify the exact components.


* Here is a good document about op-amp details : http://www.ti.com/lit/an/sboa092a/sboa092a.pdf

[Shun]

I can look at my board tonight to verify the exact components.

That would be great thanks.  . I'll have a read of that document when I get home from work today.

[matsadona]

A simple circuit indeed. A standard BC transistor would probably do the same thing.

[Shun]

Thanks again matsadona. I couldn't find a C3140EZ. I did a quick google from some BC Transistors and there seem to be some stocked at the local Maplin Electronics store however I am unsure which one would be the one to get. Would you be able to have a quick look at the link below and suggest one?

http://www.maplin.co.uk/search?productsPerPage=24&text=NPN+TRANSISTOR+BC+182

[matsadona]

Well, according to their webshop they have it (perhaps not in stock): http://www.maplin.co.uk/p/ca3140ez-n35ch

[Shun]

Thanks matsadona. I'm going to pick one of these up today and have a go at wiring it up if I can figure out how 

[matsadona]

Would this make it easier for you?

By the way, I always use 150 Ohm resistors on the R, G, B signals. Some put potentiometers there in order to have it adjustable, but I have never felt the need for that.

And the resistor to SCART pin 16 I have are at 280 Ohm (if you have +12V).

[Shun]

Thanks again matsadona. This is great!
I was just about to create a post asking what voltage I should use, I was right in thinking it should be 5v  .
Would the resistor on the sync line be 150 ohms as well? I have a spare 10K pot lying around. Would that work?

I've been building a dedicated harness for the Pacman board as the Jamma one has way two many wires going everywhere. So I've not had a chance to wire up the CA3140, but I've finished it now and I'm getting the same screen as before so I'm ready to try the IC out

[matsadona]

Normally I use a 150 Ohm resistor on the sync-signal, when building Jamma-Scart adapters. But with the op.amp on the supergun there is a 460 Ohm resistor on the sync signal (as indicated in a picture posted previously).

So yes, a potentiometer will work. But with a maximum of 10.000 there will be a very small part of the range to get it right

[Shun]

OK, I've wired up the circuit and connected it to the scart connection, but I'm still getting the same issue. I tried with a 4.7K pot on the sync line and without it as well and it seems to make no difference. I think I need to find out if this board is actually outputting a sync signal. Is there a way of finding out what voltage sync signal is being produced? I'm hoping that I can do this with a multimeter but i'm not sure how.

[matsadona]

As far as I remember the arcade RGB signals are up to 5V (and scart is 0.7V, thus the need for the 150 Ohm resistors).
An oscilloscope would be preferable to see what the sync signal looks like.

[Shun]

I was thinking about getting an oscilloscope recently, but thought that they would be quite expensive. However I managed to find a Telequipment d67a 25 mhz oscilloscope for £25! I'm picking it up next week so I should hopefully be able to figure out what is wrong with the sync.

[Shun]

OK, I finally have an oscilloscope to test this packman PCB. I've read the manual and tested it and so I am now familiar wit the controls.
What I need to do know is figure out which points on the pac-man PCB I should test for the sync signal.

Can anyone help with this?

[Shun]

Just an update on this, I think there may have been an issue with grounding and the sync cable not making proper a proper connection.
Not really sure what caused the issue as I rewired everything and it is now working. I did notice that certain Scart cables can swap the sync cable from pin 20 to pin 19 and Vice Versa so if you have a similar issue, check the continuity of the sync pins.

Thanks matsadona for your help on this issue. I've updated my project thread, showing the board working.

[matsadona]

Great to see it working