(Topic ID: 116159)

Tips for replacing a transistor on a SAM driver board


By DnDPins

4 years ago



Topic Stats

  • 19 posts
  • 10 Pinsiders participating
  • Latest reply 4 years ago by Atomicboy
  • Topic is favorited by 4 Pinsiders

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#1 4 years ago

I have a LOTR LE (uses the SAM board as Whitestar was not available) with a failed pop bumper which does not fire in test mode. I checked the ohms on the coil and it seems good so probably a bad transistor. 2 questions:
1) Testing - Chas from Stern support suggested shorting the transistor to ground to determine that the transistor is the problem. Do you guys test this way? What is the safest way to test this? What about measuring in circuit?
2) This will be my first component replacement on a board. It seems pretty easy since it is a through component. Any things to be particularly careful for?

Thanks,
Dave

#2 4 years ago

Bump hoping for some help.

#3 4 years ago

Both boards use the same through hole driver transistors, so that is not an issue.

Chas probably wanted you to try shorting it to see if it fired, but given that you don't feel confident in this, that is not where I would start.

Disconnect the connector for this coil from the board.

1. measure the transistor against other ones, you should see one pin that doesn't give a value and is open, if the transistor is bad. If it appears within spec as for others, it's good.

2. I would inspect the wiring. There are circuits with multiple coils in series. Wires are pigtailed from one to the other. I haven’t looked up the schematic for this, but check the prior coil that feeds to the one in question for the same colour wire. It the pigtailed wire from the prior one has an issue (broken off), the prior coil will work but the next one will not, or any after.

3. Check the small coil wires that go into the coil and warp around and are soldered to the coil lugs. Sometimes these break free and are just touching, enough to give a DMM reading, but not enough to carry the current for a fire.

4. measure the coil again, with the connector pulled, and make sure you get resistance. If you get anything, you’re likely ok, as it’s not a locked on situation, just a no fire.

5. Your prob should lie in the above. If not, you may have logic issues, and likely other things not working, but at this point that is very unlikely.

#4 4 years ago

Regarding replacing the transistor, I just did one last night. I don't do much board work, but like to DIY when I can. Some advice based on my experience...

Make sure your soldering tips are free of oxidation (flux helps here)

Don't force things when removing the old transistor. I used a Hakko 808 desolderer and still had trouble with one leg. It's very tempting to start wiggling and pulling. That's a great way to lift a pad. I added more solder to the leg to build it up. Tried the 808 again and this time sucked up all of the solder. Transistor pulled right out.

I inserted the new transistor and bent the legs in opposite directions to hold it in place. Applied flux and soldered it in, then clipped the excess legs. It was the easiest part of the job. Cleaned up the flux with alcohol (get it all) and checked for solder splash and shorts. Lightly rubbed around the solder points with a fiberglass pen to clean those up.

#5 4 years ago
Quoted from DnDPins:

I checked the ohms on the coil and it seems good so probably a bad transistor. 2 questions:

When you measured the pop coil ohms, what did it read?

Were all 3 pop coils the same?

I had a dead pop bumper in a BDK, and when I checked the coil, I got 6-7ohms, but the two working coils read 10-11ohms.

When I de-soldered one of the wires and measured again, the bad coil was dead open.

RM

#6 4 years ago

Don't shotgun the transistor at this point though, go through the other things I mentioned first. I have rarely had to replace anything other than flipper transistors on Sterns for drivers, having said that, I've replaced many flipper ones as they under valued the need here.

#7 4 years ago

Thanks for the replies guys. I am OK with my soldering skills and I know that working on a board requires another level of skill. I will be ordering a temperature controlled soldering station and a magnifier and set up a desk for board work and have old Bally boards to practice with.

My first experience working on a board was the Wheel board on my WoF. It needed a new surface mount transistor...now that was a real pain. My biggest problem was that I removed the pad while desoldering. I was able to buy a new board and at least I have the original as a spare.

Hopefully, my issue will not require a new transistor. I will try the things mentioned above and post tonight.

Thanks again guys,
Dave

#8 4 years ago

What's wrong with shorting a Tx to ground? I've never done it on a SAM board but if Stern support says it OK then why not.

Simple test, game on, coin door closed - alligator clip a wire to ground strap inside the backbox and touch the metal tab on the Tx for a split second.

If the coil fires you know the coil is good, the voltage is good and wire connections are good. But then this means it is a board issue, probably the Tx.

If it does not fire it is either voltage, wiring or coil. Touch the tabs of the other pops. On Whitestar LOTR they are Q9, Q10, Q11

#9 4 years ago

Grounding the transistor is a standard test, if you own a machine you should know how to do this. If your going to call chas or pat for tech advice, you should listen to what they say in order to fix your issue.

#10 4 years ago
Quoted from Taxman:

What's wrong with shorting a Tx to ground?

It seemed safe enough to me...admittedly, my knowledge of electronics needs to be improved and I am always looking to learn new ways of troubleshooting and fixing my machines. Really I was really looking to see if that is a test that many others do as well. I have spoke to Chas and Pat many times and have always gotten excellent support, I was just looking for a little extra comfort before doing it.

In order to perform the grounding test, with the machine on and the coin door is closed I:
1) Clip a wire to the ground strap
2) Touch the other end of the same wire to the top tab of the appropriate transistor for a split second

Thanks to Taxman and inhomearcades for you feedback on this!

Dave

#11 4 years ago

Yes. The ground test is safe. I have had to do it a lot on old Williams system 6 and 7 boards.

#12 4 years ago

It's a good idea to add a 500 ohm or 1K resistor inline with the jumper. If the coil is shorted (I understand it's not in your case), diode reversed/shorted or a short in the wiring you just jumpered high voltage directly to ground. Also if you make a mistake on the jumpering this gives you a little more leeway before bad things happen.

#13 4 years ago

Also if it is a 20NE10L transistor gone bad replace with the IRL540.

#14 4 years ago

I'm amazed people are going this route. Most situations where a coil has stopped working comes down to a simple broken wire somewhere. All the things I outlined should be the first simple checks before short the transistor (which does have some risk if you screw it up).

#15 4 years ago
Quoted from Atomicboy:

I'm amazed people are going this route. Most situations where a coil has stopped working comes down to a simple broken wire somewhere. All the things I outlined should be the first simple checks before short the transistor (which does have some risk if you screw it up).

I agree. Test firing the coil by grounding the transistor is fine if you do it correctly, but do it wrong, and you can really mess things up. I'd at least test for power at the coil first.

#16 4 years ago
Quoted from Atomicboy:

I'm amazed people are going this route. Most situations where a coil has stopped working comes down to a simple broken wire somewhere. All the things I outlined should be the first simple checks before short the transistor (which does have some risk if you screw it up).

I agree. I was not suggesting the jumper as a first choice, just clarifying the safest way to do it.

#18 4 years ago

Atomicboy is the winner! Each coil measured 10.3 ohms and all wires looked fine. With the playfield up I went into test mode and jiggled the small wire and the pop fired. I did notice there was arching on this pop only and after I jiggled a little more, the small wire broke off. This will be an easy fix within my current skill set.

Thanks everyone!
Dave

#19 4 years ago

Always check the easy, quick stuff first.

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