Monopoly
Stern, 2001This is a Stern Monopoly. I had a stuck coil on the drop target. I checked the associated transistor, which was shorted, and changed it.
So with everything ready, I tested the coil by grounding the negative. And bing, the coil is stuck again. I have not checked the transistor but I expect it to be shorted. So what next? What should I check?
Thanks.
There is a chain of transistors that piggy back on one another. Largest transistor at the end, smaller transistor in front if it, a transistor in a chip (along with other transistors), then the logic chip that starts it all.
Replace the largest transistor and then the next one in line.
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Jeremy Agema
Central WI
Hi Jeremy,
Thanks for the info. What I don't understand is why testing the coil by grounding it fried the whole thing again. Shouldn't it just bypass the electonic circuit? When I turned the game on, it was fine until I did the test. Also, the manual coil page shows the transistor to be Q7. There is nothing else in terms of hinting what else to look at, at least on this page.
Bedtime reading is going to be the electronic diagrams tonight.
Ludovic
Monopoly uses mosfets to drive coils. So there is no chain of transistors as it is the case on older machines.
The mosfet Q7 is driven from an IC (U1 - HCT273). If the mosfet is continuously energized this output is continuous high. This you can measure on pin 16. Just compare with the other outputs for coils (e.g. pin 15 for Q6, if this is a known to be good coil).
Quoted from lb1:So with everything ready, I tested the coil by grounding the negative. And bing, the coil is stuck again.
That is strange, could be that the new mosfet is still good.
Perhaps what happened is: the mosfet was shorted, the coil was continuously energized as a result. Perhaps the coil has been damaged?
Can you check by taking the coil out of the assembly (or smell) and check if something was melted?
See if everything operates smooth?
Measure the resistance of the coil.
Quoted from Richard_BoK:Monopoly uses mosfets to drive coils. So there is no chain of transistors as it is the case on older machines.
The mosfet Q7 is driven from an IC (U1 - HCT273). If the mosfet is continuously energized this output is continuous high. This you can measure on pin 16. Just compare with the other outputs for coils (e.g. pin 15 for Q6, if this is a known to be good coil).
Great info Richard, thanks for the correction.
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Jeremy Agema
Central WI
Hi Richard,
Thanks a lot for the info.
I don't think that the mosfet was continuously energized. There are two reasons for this. First, when I tested the old one in the board and outside of the board relative to its neighbors and the new one I had purchased, it looked shorted. That is, it had a very low voltage / resistance 0.05 relative to the 0.7 that I was expecting. Second, when I put the new one in and turned the machine on, the coil was not originally energized. It is only when I grounded the coil and it fired up for the first time that it became stuck. I have not remeasured the new mosfet that I had put in after this happened, but I suspect that it is fried now.
When I got the machine, there was no drop target assembly. I purchased a new one and put it in.
The first time I turned on the machine and played, within 10 seconds of the drop target being reset, smoke started coming out of under the playfield. I immediately turned off the machine. The coil had started melting. Luckily, there was absolutely no damage to anything but the coil.
That's when I looked closely at the mosfet and found out it was shorted. I bought two new coils, and two new mosfets - because I anticipated that it might take more than one attempt to fix this.
On this particular coil, the diode is not supposed to be on the coil. It is on a separate assembly, under the playfield. Don't ask me why. The new assembly I bought had the diode on the coil. On the new coil I bought, I removed the diode from the coil, but I did not check that the diode that is away from the coil under the playfield was ok. Could this be the problem? Could a bad diode mess of the mosfet?
I was looking at http://pinside.com/pinball/forum/topic/what-does-a-coil-diode-actually-do to see, and it could be the reason. However, I would have expected the problem not to show up the very first time the coil gets energized, without even going back to its initial rest state.
That's it for now.
Thanks everyone for your help.
The first step is to put in the new coil and turn on the game. Check to see if it locks on--do this as quickly as possible!! You don't want to fry your last coil. If it does lock on, turn off the machine and disconnect the coil. Now turn it back on, and try to see if the mosfet is locked on, or if the chip that is driving it is sending it a high signal all the time. Note: It may not energize the coil until you start a game. I am not familiar with this board set, just telling you how I'd start this.
Check the diode, also. Could be as simple as that.
As suggested, check the diode. If it was shorted it would most likely fry something on the board when you jumpered the transistor.
When jumpering across transistors to test a solenoid put a 500 Ohm or 1K resistor inline with the jumper. That will typically prevent blowing things (as long as you only briefly ground the circuit).
If you don't fix everything at once you will keep blowing one thing or another.
1. Test the output of U1 as Richard_Bok suggested (remove the coil fuse while doing this so you don't damage anything). Check with a scope or logic probe, I'm pretty sure that signal should be pulsed when the coil is energized (maybe Richard can jump in on that). In any case you can compare to another solenoid circuit.
2. Check the mosfet for shorts (see note below).
3. Check resistance on coil and test diode.
Mosfet's work differently than bipolar transistors and you really can't test them with a DMM. The one exception is that a short would show up, but I would not consider a .05 reading a short.
So after changing the diode, the coil and the transistor, everything's ok. I made the mistake of removing the diode from the coil thinking, from memory, that the diode was under the playfield somewhere else. It turns out that it is the drop target switch that has the diode under the playfield. So what happened is that I fried the brand new transistor because there was no diode. It is probably what happened to the first transistor.
Thanks again for the pointers.
Quoted from lb1:So after changing the diode, the coil and the transistor, everything's ok. I made the mistake of removing the diode from the coil thinking, from memory, that the diode was under the playfield somewhere else. It turns out that it is the drop target switch that has the diode under the playfield. So what happened is that I fried the brand new transistor because there was no diode. It is probably what happened to the first transistor.
Thanks again for the pointers.
Thanks for posting the fix!
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