(Topic ID: 231517)

Could a transistor test as good, but fail under load?


By KellyCorcoran

6 months ago



Topic Stats

  • 8 posts
  • 5 Pinsiders participating
  • Latest reply 6 months ago by KellyCorcoran
  • Topic is favorited by 1 Pinsider

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#1 6 months ago

So, the saga of the Rollergames magnet. My lovely tech tale.

Symptom: Magnet is locked on.
Parts replaced: Magnet (it was crispy) and driver board under playfield.
Maintenance: CPU was thoroughly inspected for dirt or other shorts. Q78 and Q79 tested as good according to my meter.
Experiments:
-Disconnected 1J19 on the CPU board, which disables special solenoids. Ran a wire directly from the high power driver board to Q79 tab. Magnet still locked on, confirming the problem is not in the wiring between CPU and magnet.
-Touched the wire to Q77 (lower jet bumper) instead. Magnet did not lock on.
-Hooked up a light bulb through the disconnected CPU driver. Ran the coil test and left it on Deep Freeze Magnet. Bulb pulsed on and off exactly as the magnet should. Bulb did not lock on.

So my only logical explanation is that Q79 and/or Q77 -tests- as good, and is capable of driving a low current LED, but as soon as you hook it up to the 50V magnet, it loses all confidence and locks on. Is this possible, or are there any better ideas than replacing a seemingly good driver+predriver transistor?

#2 6 months ago

I have a bunch of TIP102s I bought on eBay that test perfectly. When I install them on a board they short or explode. The theory is they are a lower rated part that has been remarked.

Buying a genuine part from a reputable seller fixed the issue.

#3 6 months ago

Thank China for that crap

#4 6 months ago
Quoted from Cheddar:

I have a bunch of TIP102s I bought on eBay that test perfectly. When I install them on a board they short or explode. The theory is they are a lower rated part that has been remarked.
Buying a genuine part from a reputable seller fixed the issue.

last time i bought transistors from China was a couple years ago and they where TIP36C. They would last about 30 seconds in an addams family high power magnet board before shorting. One exploded exposing the die inside and it was tiny compared to a legit part.

A lot of stuff is OK to get from China, but power transistors are not safe. Arrow, Future Electronics, or Online Components usually have the best prices on transistors.

#5 6 months ago

Interesting, so not unheard of then. However, the transistor giving the problem looks to be a stock original on my System 11C board, and has apparently worked fine up until recently. Could age or past abuse cause it to behave like that?

Either way, I might still replace it with a TIP102 to see if it improves the situation. I'll do it tomorrow morning in case there are any more ideas in the meantime.

#6 6 months ago
Quoted from Gornkleschnitzer:

So, the saga of the Rollergames magnet. My lovely tech tale.
Symptom: Magnet is locked on.

So my only logical explanation is that Q79 and/or Q77 -tests- as good, and is capable of driving a low current LED, but as soon as you hook it up to the 50V magnet, it loses all confidence and locks on. Is this possible, or are there any better ideas than replacing a seemingly good driver+predriver transistor?

What could have happened is the transistor overheated from being locked on by the magnet. The overheating caused the reverse breakdown leakage to increase. Notes on leakage, more than likely your DMM doesn't source enough current during measurements to detect the leakage. Finally, the leakage will increase with voltage. Anyhow, I would just replace it.

Quoted from barakandl:

last time i bought transistors from China was a couple years ago and they where TIP36C. They would last about 30 seconds in an addams family high power magnet board before shorting. One exploded exposing the die inside and it was tiny compared to a legit part.
A lot of stuff is OK to get from China, but power transistors are not safe. Arrow, Future Electronics, or Online Components usually have the best prices on transistors.

Most of the time the fakes are a cheaper (lower rated) part in the same package but they sand off the markings and re-label it as a more expensive part.

#7 6 months ago
Quoted from Turtle:

What could have happened is the transistor overheated from being locked on by the magnet. The overheating caused the reverse breakdown leakage to increase. Notes on leakage, more than likely your DMM doesn't source enough current during measurements to detect the leakage. Finally, the leakage will increase with voltage. Anyhow, I would just replace it.

Wow, so my suspicions might indeed be true. Thanks! I'll let you all know how it goes.

#8 6 months ago

"DON'T FLIP!!"

Well, thanks guys! Replacing the "good" driver and pre-driver finally fixed my problem, and now the magnet works good as new.

TL;DR for future readers: The answer to this thread's question is YES. It is possible.

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