Monitor losing sync: JPAC or CRT_emudriver?

[big10p]

Hi

I have an intermittent problem on one of my cabs. Sometimes, the monitor image becomes completely garbled due to loss of sync. The sync light on the JPAC goes out. This generally happens a few minutes after booting. Sometimes it'll happen when my front end is being displayed (interlaced mode), but I get sync back when launching a game. Sometimes it's the other way around where the front end displays fine, but games are garbled.

I suspect a faulty JPAC but wondered if it's possible my CRT_emudriver installation is faulty, somehow. I'm using win7 64 with CRT_emudriver 2.0. Anyone else experienced this problem? I haven't included logs as most of the time it works fine, so can't be a setup issue.

Cheers.

[brad808]

You can bypass the sync filters on the j-pac while keeping the video amp. Remove all the jumpers from the front then solder a small blob between pins 13-14 on the back of the jpac. If you do this it may solve your issue, if not you can eliminate the jpac as the culprit.

[big10p]

Thanks for the info, brad808. Just to be clear, will doing this mean the JPAC will just directly pass-through any sync signal it gets? Bit worried about passing the 31khz signal to my arcade monitor at boot up.

Can I ask why you made this mod to your JPAC? Were you having the same sync issue as me?

[brad808]

Thanks for the info, brad808. Just to be clear, will doing this mean the JPAC will just directly pass-through any sync signal it gets? Bit worried about passing the 31khz signal to my arcade monitor at boot up.

Can I ask why you made this mod to your JPAC? Were you having the same sync issue as me?

Yes it'll pass everything through. Don't be worried about sending a boot signal for the few seconds before windows drivers kick in, or install atom bios to card.

Yes I had problems with J-pac sync circuit, I've had 5 now and all exhibit the same behavior so now I do this mod before I even install them. The sync circuit can't swap between 15-24-15 repeatedly for multisync monitors and needs to power down in order to regain sync. This is a big problem while using groovymame with auto sync monitors.

Also I've had in the past many times j-pac "randomly" drops sync of a signal it is displaying perfectly fine before. I wrote andy about this and he mentioned some people have had issues with the sync circuit not being able to properly identify certain modelines based on the sync pulse width being too narrow.

It was a problem that plagued my systems for a couple of years before I discovered this will solve it and not a single problem since.



If you want to test the jpac as well next time it happens, unplug the usb and plug it back in, see if it regains sync.

[big10p]

I do remember a while back when this happened, I unplugged the VGA cable and plugged it back in, then the JPAC got sync again. Thanks again.

[Calamity]

@brad808, thanks a lot for this mod. I'll be trying it as soon as I get my new solderer.

[brad808]

@brad808, thanks a lot for this mod. I'll be trying it as soon as I get my new solderer.

Glad to help. If someone doesn't have a soldering iron they could likely accomplish this by simply modifying a VGA cable. Soldering is certainly going to be much easier and will take a fraction of the time. With soldering you also have the added benefit of easily being able to reverse the mod should you ever want to in the future. Should accomplish the same goal though. 6 one, half dozen the other.

[keropi]

that sync ciurcuit sure is strange on the JPAC... I have 2 of them on POLO28" monitors and I want to cut-off completely anything non-15khz... it doesn't work though no matter where I install the jumpers, I always get for example the garbled boot screens until CRT_EmuDriver loads... 15K only or 15K with "31K mode" is the same garbled screen :\

[Zipper]

This is really interesting. I've had a couple of j-pacs that lose sync and unplugging / plugging the vga was the only way to fix it.
I mailed Andy about it, and he suggested removing the jumpers... but that didn't help (just a messed up signal).

If blobbing the syncs together works then i'll dig out the old jpac. Thanks very much!

Interestingly, I do have a jpac that is absolutely solid without any mods... so i think there's is a tolerance issue on some of the jpacs.

Oh - Edit  - i found this video I made for Andy back in Jan, and I assume you are all seeing the same thing:

[brad808]

Yes the problem with just removing the jumpers is you lose comp sync. That's why you need to solder in order to regain it.

[big10p]

This is really interesting. I've had a couple of j-pacs that lose sync and unplugging / plugging the vga was the only way to fix it.
I mailed Andy about it, and he suggested removing the jumpers... but that didn't help (just a messed up signal).

If blobbing the syncs together works then i'll dig out the old jpac. Thanks very much!

Interestingly, I do have a jpac that is absolutely solid without any mods... so i think there's is a tolerance issue on some of the jpacs.

Oh - Edit  - i found this video I made for Andy back in Jan, and I assume you are all seeing the same thing:

I only get this issue with one J-PAC, too. My other cab - which has had a couple of J-PACs in it - is rock solid. Both cabs have the same monitor in them. I haven't had time to try out this mod yet, but will definitely give it a go!

[big10p]

Only just got round to doing this mod, as well as flashing the ATOM-15 BIOS (to be on the safe side). So far, so good - all seems to be working well. As my issue was intermittent, I will need to test for a few weeks. Will report back then...

[Murph74]

I know it's ancient, but the "blob" fix just saved/fixed my cabinet!  Thank you for posting this!

You can bypass the sync filters on the j-pac while keeping the video amp. Remove all the jumpers from the front then solder a small blob between pins 13-14 on the back of the jpac. If you do this it may solve your issue, if not you can eliminate the jpac as the culprit.

[big10p]

Yep. Great fix. Had no problems since. Do it to all my J-PACs now. Thanks again to brad808! 

[brad808]

No problem guys. Also do it to all my jpacs

[krick]

I have an old JPAC that I purchased back in February of 2004.  It's one of the ones with the PS/2 keyboard connector & passthrough.  It's been pretty reliable so far.  I wonder if the manufacturing quality has gone down on newer models.

[Jonny G]

This is really interesting. I've had a couple of j-pacs that lose sync and unplugging / plugging the vga was the only way to fix it.
I mailed Andy about it, and he suggested removing the jumpers... but that didn't help (just a messed up signal).

Thanks for this tip Zipper!

I ran a jumper wire between the 2 VGA pins on the JPac just for ease of testing and was getting a stable pic, just very dark and overly sharp. Removing the jumpers has got it back to normal. Just hope it can get sync after a few repeated reboots now. Thanks again.