JAMMA Genesis syncing issues

[airthrow]

Hi guys,

I got a model 2 Genesis working well in my arcade cab with a sync stripper circuit from an LM1881, but I've built two different model 1's that refuse to sync properly, with and without a sync stripping circuit (which is odd because the mameworld guide says that you don't even need one..)

I took the sync stripping circuit out of my working model 2 Genesis and tried putting it into 2 different model 1 systems, and neither would work.  I can get a rolling semi-recognizeable picture, but it's not useable.  I tried using both the composite video and composite sync as the input on the LM1881, neither worked.

So my questions would be:

1) Why does it work on the model 2 but no the model 1's?
and,
2) Is there any way to get the model 1's working?  They are 80% done already but useless to me right now.

I am getting my signals from where the A/V port meets the PCB, if that makes a difference.

[skforty]

The model 1 genny uses the CXA1145, make sure you have it hooked up correctly.  You dont NEED a sync stripper for gen2, or gen1...it has a dedicated csync out.

[airthrow]

Neither of the Genesis 1 systems I have will properly sync to my monitor with the composite sync.  I have tried it with and without a Sync seperator, with and without including vertical sync, these model ones just do not sync...  if they do display a picture it has black glitchy bars, this was the problem with my model 2 as well before I added the sync seperator and it had perfect video.

[skforty]

Its probably more about the sensitivity to voltage in on the sync line with your displays.  You should use several displays to test with.  I had similiar problems TG16 on 2 of my displays, but not the other.  You can try filtering out the voltage on sync, i had to do this for my NEC 39''and 2 of my arcade monitors for my duor  and tg16 mods, as well as my CDI220.  

Also, make sure you are going in on the right hsync for csync.  

But make no mistake, gen1,gen2, and snes don't need a sync seperator.  If it works with one, but not without for you, its probably due to something similiar about filtering the voltage out after running through the separator, or something completely different, but its not because they need to be separated.

They have dedicated cysnc lines with NO composite video on them, and almost EVERY arcade monitor ive worked on or used accepts CSYNC.

I have almost every system hooked up via RGB in my house all working on 10 different arcade monitors, 5 different sony PVMs, 3 different amiga monitors, 2 NEC presentations, 2 mitsubishi multisyncs, and my XRGB2+ on modern displays.  

You know where i live if you want to try and figure out the problem

[airthrow]

Are you Michael??  

If so, I definitely would like to come over and get your expertise on a few of these systems to figure it out sometime!  Just shoot me a text when you have a couple hours!

How do I seperate the voltage without the Sync seperator?  Id like to try this.  It would be great to figure out how to get these systems to sync properly without the sync seperator...

[l_oliveira]

The problem with the SEGA Mega Drive sync output is because the pin on the connector is routed straight from the VDP chip and it's not capable of driving low impedance sync input on some displays.  The solution is route the sync to use the CXA1145 sync buffer circuit, then add the 75 ohm resistor and the respective capacitor. (consult the CXA1145 datasheet for proper value for the capacitor. I'd say it's 220uf from the top of my mind but check please to be sure.)

[airthrow]

The problem with the SEGA Mega Drive sync output is because the pin on the connector is routed straight from the VDP chip and it's not capable of driving low impedance sync input on some displays.  The solution is route the sync to use the CXA1145 sync buffer circuit, then add the 75 ohm resistor and the respective capacitor. (consult the CXA1145 datasheet for proper value for the capacitor. I'd say it's 220uf from the top of my mind but check please to be sure.)

So I got the datasheet for the CXA1145M here:

http://pdf1.alldatasheet.com/datasheet-pdf/view/77344/SONY/CXA1145M.html

It says that pin 11 is composite sync out, all I have to do is use a 75 ohm resistor and then a 220uf capacitor as my sync output?  Would I not need an LM1881 then?

Also, since my sync on the Genesis model 2s were bad until I added the LM1881, do you think they were adding the neccessary amplification and fixing the sync problem?  It would be great if I could make these systems cheaper without needing the LM1881s, so far they have been what has worked for me though.

[l_oliveira]

LM1881 would have the same effect. What matters is that connecting a low impedance input will saturate the VDP sync output causing the sync waveform to distort, disturbing the image sync completely. So either way should fix your problem.  What's important is the the VDP pin does not go straight to the monitor, instead an amplifier need to be in the middle so the current draw doesn't cause the signal to degrade.