10 top monitor not steady: 'heart-beating'

[Nes4life]

Got an issue with the 10" top monitor from my Sega Mega-Tech. Originally sold to me as 'not working', I found it just wasn't plugged in (needed a crazy star shaped security bit with a central hole to open the top panel).

Anyway, after a long time playing with the pots (20mins) I got a semi-stable image but it still rolls slightly and it looks like it's heart-beating. It's an odd concept but take a look at the video just below.

I've discharged it (hooray!), removed the annode-cap, unplugged and removed it, and gently taken off the neck board. Are there any easy things I can do / check / replace before I end up booking the board in with a resident monitor expert?

Cheers all!

Direct link: http://youtu.be/rHxHgvHiDRc

Embedded:

Some pics of the board:

 

[virtvic]

One of the 'hold' pots knackered?

 

[Nes4life]

One of the 'hold' pots knackered?

You think I can just unsolder the hold post and replace? I can give it a go. The pulsing was the odd bit. No caps or fuses then?

 

[virtvic]

You could check the pot with a multi meter first.

 

[RGP]

Can you plug the feed for the top monitor into either the bottom monitor or another to rule out the pcb first?

The pulsing side could easily be power supply section (B+) as it could be the sync circuit or the pcb.

Rule out the simplest part first if you can.

 

[stevebm1]

I had a similar issue with mine,turned out to be a faulty remote board.I swapped it over and it was fine

 

[Nes4life]

Thanks guys! I'll try a few of the easys before I press on with desoldering anything.
I'll definitely try putting the feed into the bottom monitor. If that's ok I'll swap the remote board. If it's still broke then I'll inspect the sync circuit.

 

[guddler]

Another easy thing to check is that black slide-switch behind the power socket. OK, so unless someones been messing then its unlikely to be the problem but it switches between negative and positive sync so it could cause a picture that is impossible to stop rolling.

1st things 1st, plug the PCB into the bottom monitor.

 

[Nes4life]

Another easy thing to check is that black slide-switch behind the power socket

You know, I did slide it but it didn't seem to do anything! Would you expect an obvious visual change if you change it?

 

[guddler]

If the PCB and Monitor sync are working correctly then yes, I would expect you to see something. It probably wouldn't magically be correctly in sync, but in one position you should be able to get a picture to sync properly and in another position you wouldn't.

If you can't sync the picture properly whichever way you set that switch then it's pointing to an invalid input sync signal or a faulty sync circuit on the monitor. And which of those can be determined by moving the input down to the lower monitor.

 

[Nes4life]

Can't get a steady pic on the bottom monitor either... It's even worse on that monitor.
So faulty PCB? Sync circuit on that output?

 

[guddler]

Sounds like it. Is the picture from the big screen stable when displayed on the top one? Bear in mind you have been twiddling the sync on the top monitor (i assume) now so it will need adjustment.

Just to prove categorically that the top monitor is OK and it is indeed the secondary monitor output on the PCB. I don't think you've explicitly said that the main screen's picture is OK but it sounds from what you're saying that it is...

guddler2014-06-14 17:52:46

 

[Nes4life]

Is the picture from the big screen stable when displayed on the top one?

Yes - the top monitor is stable with the bottom's feed and the bottom is unstable with the top's feed... therefore it's PCB related - the top's sync is faulty on the PCB side.

I'm taking this PCB to Arc-aid in Cambridge on the 12th July so maybe someone can help me track this issue down - unless you guys know what I should check for on the Sega Mega-Tech board. The board already has a sound issue and an issue with 2 buttons on P1 triggering off at the same time. I've got a replacement board but that's even worse as it doesn't even fire up!

 

[guddler]

PCB sync issues are usually pretty trivial provided there are no custom chips involved. I (or someone else) would have to have a nose at the schematics but you normally find that you can trace the composite sync signal back to a logic chip that combines and maybe inverts the separate horizontal and vertical signals and to be honest, from the fact that you could get an almost stable picture but couldn't quite get it right then it seems to me that is probably what the problem is.

But like I say, that's provided the sync isn't coming out of some custom or other. Here I'm very much generalising on how "old school" video signals work without any knowledge of Sega Mega-Tech boards specifically.

 

[Nes4life]

Thanks guddler. I'll get the schematics and trace the sync signal back. Hopefully this'll be one issue down soon.

Do you know where or how I can make a Mega-tech to Jamma edge converter? I'm guessing I'll need to get both my boards hooked up to a test rig at some point in the future. Is there a standard kit you can solder wires to etc to map the pins?

 

[Purity]

http://wiki.arcadeotaku.com/w/Sega_Mega-Tech_Pinout

According to this, you'd just need to make up an CN6 connector and CN8 or CN9 connector to Jamma depending on what you want to look at. Should be fairly straight forward
Purity2014-06-15 23:09:11

 

[guddler]

I was going to comment "nice simple loom then

" but yeah, fair point, you'd not need to make it all up so not too bad at all.