Hello beacon of Pinball wisdom!

I find myself in a position where I get to repair a DM I sold recently. It was working fine when it left, now the guy told me a capacitor has blown up and a coil is strangely locking on. Oh, the wonders of pinball technology.

The guy also told me that once the coil was buzzing away it took 10 minutes before the capacitor blew. Why he didn't think turning the game off was a smart idea is above me.

Anyway, I can't seem to find any specific things wrong with the driverboard at the moment. I must have read this passage 10 times now, but it doesn't really makes sense to me anymore.

Exploding +20 volt Capacitor.
There are cases when the +20 volt capacitor (Driver board C11 on WPC-S and prior, C10 on WPC-95) can just explode. This happens when a shorted flipper coil diode or shorted transistor on the Fliptronics board causes the 70 volt coil power to feedback into the 20 volt flashlamp circuitry. Because of reverse voltage, this blows the 20 volt capacitor. Also installing one of the ribbon cable connectors in the backbox on the header pins (top row of header pins to bottom row of housing) can do the same thing. And lastly, if connector J124 is mistakenly plugged into the driver board connector J128 (they are keyed alike!), this can cause capacitor C11 to explode.

First check the ribbon cable header pins to make sure they are attached correctly. Then check the flippers. If when the flippers are activated, one of the flashlamps dimly lights, there may be a bad flipper transistor on the Fliptronics board.

There is a preventive measure which can be taken for this. Install a blocking diode on the driver board ceramic 10 watt resistor R224 (or R9 on WPC-95). To do this, first remove the lower leg of resistor resistor R224 (the leg just above TP7). Connect the anode (non-banded end) of a 1N4004 (or 1N4007) diode to the resistor's leg. Then solder the cathode (banded side) of the diode back into the driver board (where one leg of R224 was removed). This will prevent the problem.

So here's me thinking that somehow this locked on coil (#2, bottom popper) is making the capacitor blow, so fixing that problem should take the capacitor problem out as well. Problem is that the coil is not solidly locking on. It it very weakly pulled up and buzzes, and it does this when other high powered coils are firing, but other times it is solidly locked on. In the test it solidly fires when it should, and buzzes when it shouldn't.

I am planning to switch out the ribbon cable from the CPU to see if that fixes something. Voltages seem ok, aside from the 20V.

Anything else I should definitely check?

It's not a bumper/jet, it's a VUK popper, I know which driver transistors belong to what coil, the problem is they all measure alike (in circuit, measured on the board). The transistor of that vuk does not deviate from the other transistor pairs.

So I tested and fixed the board:

- Transistors/diodes all measure good.
- Coil measured good, just had to apply a new coil sleeve to it.
- Replaced capacitor, added a blocking diode so it never should blow again.
- Replaced the ribbon cable from CPU to driver board

Put it in, applied power, and nothing blew. Coil was working again.

Next issue: apparantly the 50V rail is now 93V. The same voltage is present on the flasher lamps as well. All flashers that aren't LED's are blown. It would seem that this is the problem of the blowing C11, but the blocking diode prevents it from doing that.

Well it's fixed! Get ready to be educated...

First I noticed coil voltage to be present on the wire ramp exiting the bottom popper VUK.
2017-02-18 17.39.54 (resized).jpg

Strange, so I took another look at the coil. It was previously removed because I had to replace the sleeve, but I didn't take a good look. But what is this? Apparantly, the coil has a winding sticking out, which has worn against the popper bracket, and it made contact causing the coil voltage to be present on the bracket.
2017-02-18 17.55.47 (resized).jpg

Then there's this POS 20V flasherlamp connection making contact with the bracket...
2017-02-18 17.38.30 (resized).jpg

So, coil is worn and putting voltage on bracket, flasherlamp makes contact with said bracket and makes sure all of the flashers blow out AND the C11 capacitor blowing as a nice side-present. It did not do that a second time because the diode fix stopped that from happening.

This also explains why the coil may have been buzzing weakly, since it was powered from the coil voltage, through the bracket, and then to the much lower flasher voltage. Only a couple windings where powered though.

*edit* Please note that this wasn't reverse voltage as quoted in my first post, this is applying 70V+ to a capacitor rated for 25V. These things never handle over-voltage or reverse voltage very well.

I hope this information is of use to anyone in the future, I can go on fixing blown out flasher bulbs.

Have a good day!

For the interested: this is the diode fix they are talking about:
2017-02-18 15.41.09 (resized).jpg