2 slot board no video

[malignantpoodle]

Have a large 2 slot board with no video or errors. Board has power (red LED is on) and dip #8 is not thrown. I did try to toggle dip # 8 to see if it was sticking but no results. Screen is powered on but there is no video output. Board is very clean save for a couple of tiny traces which are questionable, yet are in a place that would produce an error rather than no video at all.

I have a couple of parts boards so I could replace a component if need be. Any ideas on where I should start with this one?

 

[HeavyMachineGoob]

Make sure the sync line is okay?

 

[distropia]

Erase the backup ram and take a look at the Sync line with the logic probe.

 

[malignantpoodle]

Sync line is fine upon visual inspection. Don't have a logic probe but can get one, although I do not know the values I'm looking for.

Don't have any way to erase backup data if machine doesn't even boot. It does have the original battery and I'm pretty certain it's dead. Board has been sitting without charge for a long time.

I did replace the bios to rule that out.

 

[Fortune]

When you do probe it, it should give you a logic 1 with negative signal pulse reading.
On my probe, that's the HI indicator lit up, a pulsing LED, and a high pitched pulsing beep.

 

[malignantpoodle]

When you do probe it, it should give you a logic 1 with negative signal pulse reading.
On my probe, that's the HI indicator lit up, a pulsing LED, and a high pitched pulsing beep.

Thanks I'll try that.

 

[Xian Xi]

How are you viewing this video exactly? Supergun? Cab?

So you don't even get a crosshatch screen?

 

[malignantpoodle]

Through a cab, just connecting it in via harness.

Not even a crosshatch. Nothing at all. You can tell the screen is on and has power but nothing displays.

 

[Xian Xi]

All the voltage levels are fine? And other boards work with no problems?

 

[malignantpoodle]

Yep, the cab is tip top and is fully functional. I plug the problem board in and have zero on video. Not even static or interference or anything.

 

[Xian Xi]

Put a game in and see if it plays blind at least.

 

[malignantpoodle]

It is not playing blind. No sound from headphones or speakers.

 

[Tyranix95]

When was the last time the problem board worked?

 

[Xian Xi]

It is not playing blind. No sound from headphones or speakers.

Probe the 68k and see if there is any activity. Usually if nothing is being displayed it means that the clock isn't working at all. Check pin 15 and see if it at least pulses.

 

[malignantpoodle]

Ok will do. I have a probe on order and will check that when it's here.

If the 68k is dead, can I just piggyback one on? If so, which pin do I clip on the original to disable it?

When was the last time the problem board worked?

Not sure, but several years at least.

 

[Asure]

Ok will do. I have a probe on order and will check that when it's here.
If the 68k is dead, can I just piggyback one on? If so, which pin do I clip on the original to disable it?

You can try bypassing VCC pin (14 & 49), but if the 68000 has damage inside, and output/input is wrong even when powered off, piggyback won't work.

If you still want to do this cut/ desolder VCC pins (14 & 49) temporary, then piggyback the new 68000 on top with VCC pins bent outwards, solder two short wires to VCC on the board.
Still, not really recommended, depending on the damage the old one has, it may even damage the new one if it's short-circuited in some wierd way..

Here's the pinout:

 

[malignantpoodle]

Thanks a bunch, that's a big help.

I just had the thought as well that I could just clip the old chip off leaving only a tiny bit of the pins left which are soldered into the board and then connect there.

If I end up replacing this, I suppose I can't use any old 68k chip. The board chip is marked TMP68HC000N-12. Am I stuck with using just that? Reason I ask is it's very difficult to desolder that chip on these boards.

 

[Tyranix95]

Give this a read while you're waiting for your probe.

http://forum.arcadeotaku.com/viewtopic.php?f=26&t=21614

http://wiki.arcadeotaku.com/w/Game_Board_Repair_Guide

 

[BIG BEAR]

It reads like you have a bad 24mhz crystal..
BB

 

[Asure]

The 68000 can be replaced with any 12-mhz rated model in the same package. (64-pin, DIP)

But, read as much as possible. I would not clip anything off before i was sure it was indeed broken. The 'shotgun' repair method has a low success rate, and usually you end up buying a lot of parts, which you never needed, when you find out later it was a badly soldered resistor.

 

[malignantpoodle]

It reads like you have a bad 24mhz crystal..
BB

You know, that exact thought crossed my mind. I might just replace that since it's easy enough and I have the parts.

I would not clip anything off before i was sure it was indeed broken.

Certainly. Not going to replace the CPU unless I can verify that it's dead.

 

[malignantpoodle]

I replaced the 24mhz crystal. No change.

Still waiting on probe. I do have a spare 12mhz 68k laying around. If the probe reveals the cpu to be dead I'll replace that.