(Topic ID: 244151)

LAH magnet board blowing fuses and FETs


By arolden

19 days ago



Topic Stats

  • 12 posts
  • 5 Pinsiders participating
  • Latest reply 6 days ago by arolden
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#1 19 days ago

Hi all. The magnet board in LAH is giving me nightmares. I'm hoping someone can give me another perspective on troubleshooting this as I'm not sure where to go from here.

I have read over a lot of magnet board threads on Pinside and have read Clay's guide, and everyone suggests rebuilding the magnet board as a first step, so I did. I installed three new IRL540N MOSFETs, as well as a new 74LS273 chip. I added the suggested capacitor across the clock and ground circuits on the chip. I also installed three new fuses under the playfield.

The problem is that, whenever the magnets are activated (i.e. multiball), two of the FETs fail, causing the magnets to lock on, and blow their associated fuse under the playfield. I replaced the FETs, double checked my other work, installed new fuses, but the same thing happened again. The same two FETs keep shorting. Strangely, the third magnet works just fine. All of the magnets have the same resistance and there is nothing wrong with them visually.

I'm not really sure where to go from here. I have rebuilt the magnet board a few times, but something is causing the FETs to short and I'm not really sure what there is to look at further up the chain. Any ideas are appreciated!

#2 19 days ago

Is this an original or aftermarket magnet board?

Have you tried swapping the "good" magnet with one of the "bad" ones to see if the problem moves?

#3 18 days ago

It's an original board. Haven't tried moving the magnets - I'll do that and see what happens.

#4 18 days ago

The 3 fuses should be slo-blo fuses.

My magnets were getting hot in multiball even when I was supposed to be down to one ball. I found out that I had trough switch issues so multiball never seemed to end. The magnets should be off anytime there is only one ball in play.

I had 2 trough switches and an issue with the trough upkicker switch not registering. So check them all in switch edge test.

What game code are you using? I also seem to faintly recall older game code had an issue.

#5 18 days ago

Did you check the back-EMF diodes on the board? And is there a good connection to the solenoid power at pin 1 of the magnet board?

#6 18 days ago

Do yourself a favor just replace it

ebay.com link

I was in same boat

Kept blowing fuses etc

This was worth every penny

1 week later
#7 9 days ago

OK, I've done some further work on this. I replaced the two shorted FETs again, and also replaced their associated diodes. Game started and played fine, but when multiball started, one of the magnets locked on again, and blew fuse F5. Now F5 blows each time the game is turned on because the magnets are locked on. The same FETs and also shorted on the magnet board again. Fuses under the playfield are OK but this is probably only because F5 is blowing first.

Quoted from PinballManiac40:

The 3 fuses should be slo-blo fuses.
My magnets were getting hot in multiball even when I was supposed to be down to one ball. I found out that I had trough switch issues so multiball never seemed to end. The magnets should be off anytime there is only one ball in play.
I had 2 trough switches and an issue with the trough upkicker switch not registering. So check them all in switch edge test.
What game code are you using? I also seem to faintly recall older game code had an issue.

Yes, I'm using slow blow fuses. Trough switches are all OK. ROM version is 1.10. It looks like 1.12 is the latest ROM on IPDB. I will install that ROM and see if that makes a difference. Can you find where you read about the ROM being the issue?

Quoted from G-P-E:

Did you check the back-EMF diodes on the board? And is there a good connection to the solenoid power at pin 1 of the magnet board?

Yes, just replaced them with this last attempt. Pin 1 had cracked solder but I reflowed it when I first rebuilt the board.

Quoted from TheCnyPinGuy:

Do yourself a favor just replace it
ebay.com link
I was in same boat
Kept blowing fuses etc
This was worth every penny

Yes that is also an option. But I am stubborn and want to try and get to the bottom of this first!

#8 9 days ago
Quoted from frunch:

Is this an original or aftermarket magnet board?
Have you tried swapping the "good" magnet with one of the "bad" ones to see if the problem moves?

Just did this test. Q3 is the good FET circuit so I removed the output wire from the connector and replaced it with an output wire for another magnet. The other magnet started working fine. So, definitely not a magnet issue. All signs point towards the board.

#9 9 days ago

I installed new FETs on the magnet board and upgraded the CPU and display ROMs. The issue still persists; one of the magnets locks on at the start of multiball.

#10 9 days ago

In another desperate effort I replaced the integrated circuit with a new one, installed new FETs and diodes, and still have the same issue. Running out of ideas!

#11 7 days ago

I have been doing some further reading on this. I'll use Leon's test ROM and test the PIA that controls the magnets, just to eliminate the possibility that the fault is further up the chain.

#12 6 days ago

Another update. I installed Leon's test ROM and tested the 6821 PIA at 11b, whose outputs control the transistors on the magnet board. Each pin oscillated from 0-5 volts as it should. So that suggests to me that the PIA is working. Once again the finger is pointing towards the magnet board. At this stage I am totally out of ideas. Would appreciate some input from the really knowledgeable techs out there before I give up and buy a new magnet board. cfh I followed your guide to a T; any ideas as to what I'm doing wrong?

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