Hi all, I am working on a friend's TOM that has been in storage for about ten years.

The issue:

The trap door is reading 65v all the time and it heats up. To me, this reads as the hi voltage used to surge the door up, yet it barely holds the door up if I manually pull it up. Seems to be the hold voltage, but it reads high?

The strange thing is that it works sometimes, stays up permanently sometimes, and won't pop up at all sometimes. The problem appears to be twofold: the coil has constant voltage and the bracket has been repaired at the bottom end.

What has been done before:

The previous tech guy put a new coil in and repaired the bracket. It looks like he gave up after that.

What I have done

1. I have read every post on the internet about the trap door. I understand that the positioning of the coil pin can cause malfunction. I have adjusted the bracket (via a fabricated metal stop with rubber stopper) to deal with this issue.

2. I have checked the transistors and diodes (on board) correlating with the high and low voltage. They test fine when compared to other transistors/diodes around them.

3. I have tested the new coil and the old coil for resistance. Both test fine.

4. I have followed all wires to the coils, searching for shorts etc. All looks good.

To me, the voltage is the major problem here. After I get that fixed, it will likely be a case of fidgeting with the bracket length to find the sweet spot.

I thought this machine was going to take me a couple hours to shop, but this trap door is a PAIN IN THE ASS! I guess that's pinball...

Thanks in advance for any help you might be able to offer.

So I retested the transistors and found that the low-voltage transistor did not return a reading when I tested the center pin to ground. The others in the same line did return a reading, so I am assuming the transistor is fried. I replaced the transistor (q48) and the problem remains. I then checked the diode (D19) and it was fine. I replaced it anyway.

It appears to me that hold voltage is being applied to the coil as it does not get hot enough to melt, and the trap door stays up if you manually lift it.

It would be wonderful if someone could give me some feedback on this issue. Aside from the transistor, what would be causing the hold voltage to remain on?

Hi, so I have replaced q48, D19 and the "helper" transistor, q47. I'm about to undertake replacing U4, the 74LS374 chip that controls the low voltage triggering as I'm still getting the low voltage constant to the coil. I followed the trail in the schematics, and this seems to be the logical step.

If anyone could chime in to let me know if I'm on the right track, that would be great.

Thanks,

Lance

Thanks for the reply!

I tried to look up how to test the chip, but couldn't quite understand how, so I just replaced it. Still the same issue. I'm thinking maybe the high voltage is maybe causing the issue as I've changed then entire low voltage line on the board. I'm reading 67V at the coil, but it still fires up as it should. However, it does stay up, which would be the hold voltage. I'm use to EOS switch to control the low end, so the pulse style of the trapdoor coil has me confused. Any other thoughts?

Thanks for the link and the info. So this means that the low (hold) voltage (possibly pulse voltage?) of the trap door coil somehow has a path to ground. I think I've changed everything on the board. So would replacing the coil be the only other issue? There is a new coil in there, but it was swapped by the previous tech. It seems to test well according to my meter...

Bump for the Saturday afternoon crowd.

Any have any recommendations?

Thanks for the reply!

I did replace the LS374 and both transistors in the low-voltage line.

You can touch the coil, but barely. It heats up in about 10 minutes to what I'd say is a very hot temp, but not hot enough to melt it.

The fact that the trap door still fires open but stays open makes me think it's the low voltage line that is the problem.

Is there a way to test the transistor test you're recommending without desoldering? I've pulled this board and messed with it so much that I'd rather not again.

On a side note, do you know which lug on the coil is the low voltage? I think it's the middle? I should really disconnect that to be sure that is the line. Should have done that first.