I'm helping a buddy of mine get his Black Knight LE fixed up, and the left side flippers stopped working. Right side flippers work fine, though. I did nearly everything I could think of to get the flippers working...here's what I've tried so far:

-New cab switch (for upper and lower left flippers)
-New header pins on 2J10 and new female connector w/ trifurcon connector pins
-New coil diodes for upper and lower flipper coils (triple-checked diode orientation to make sure I didn't mess it up, the diodes that were originally on the coils checked good w/ my meter but replaced them anyway)
-Cleaned/filed contacts on flipper relay on driver board
-Cleaned/adjusted EOS switches for both flippers
-Checked flipper coils (out of circuit) and got approximately the same readings on both: middle lug to non-banded side of diode lug: 23.5 ohms, middle lug to banded side of diode lug: 1.0 ohm, banded lug to non-banded lug: 24.5 ohms. I believe that means the flipper coils are ok.

Now here's what gets me: there is approximately 32 VDC at each of the left side coils, on all 3 lugs. The voltage to the right side coils (which again, are working) may be a volt higher, but it's very close to the voltage I'm getting on the left ones. I tried grounding the right side upper flipper coil very quickly using an alligator clip on the non-banded side of the flipper coil diode (the ground side) and it fired (I had touched the other end of the alligator clip to one of the cabinet side rails). Tried the same thing on the upper left flipper coil--nothing. It appears that the voltage is there, but even grounding the coil directly isn't firing it! I think I may need to try this test again, it just doesn't make sense to me that it wouldn't fire that way.

I feel that I've exhausted nearly every possibility, so I was hoping to find some help here. I'm at the point now that I think I need to double-check all my work from yesterday to make sure I didn't mess up something else while trying to fix the problem. I did check any board work I did with the 2J10 connector and flipper relay to make sure I had continuity for each trace from each solder joint to wherever it went to. After all the work we did on the game, the right flippers continue to work fine, and we still had about 32 VDC at each of the non-working flipper coils.

Like I said, I'm going to double-check all the work I did so far to make sure I didn't mess something up while servicing the game. Also, I presume the LE version of this game should not behave any differently than a non-LE version but figured I'd specify that it is an LE in case that makes any difference.

Any suggestions where to start looking next?

Thanks for any and all help!
Trevor

On BK, the coils are grounded through a relay on the driver board. The only transistor in the equation is the one that turns on the relay. That transistor must be working, as the relay is turning on.

Good suggestions! As far as the relay, I removed the cover (what a pain in the ass it was to remove that little cover!) to clean and file the contacts on the 4 switches the relay activates, then put the cover back on. While the cover was off, I checked for continuity for each switch while my buddy held the relays armature down. All 4 switches tested good. That obviously doesn't rule out the relay entirely-the contacts for the left flippers may not be closing properly when the relay is activated electrically. I'll have to figure out a way to test the continuity of the relay switches while the game activates the relay. Would it be worth trying to ground another coil through the left flipper ground at the 2J10 connector? That could at least tell me that the relay is working, creating a good ground for the left flippers. Right?

Newtoit: I did re-flow solder to the relay solder terminals on the driver board and tested all board traces to/from relay for continuity (they all checked out). I also put new male and female connectors at 2J10, where the relay switches ultimately lead to.

Thanks for this response! It really got me thinking: maybe I've got good/better ground now, but the wiring and connectors carrying the voltage to the coils is where the problem is! Even though my meter is reading about 32-33 volts, what if it's not capable of carrying enough current!

To test the idea, and to continue to study the flipper circuit, I opened up my Black Knight at home. I checked out the connector for the flipper coil power coming off the Power Supply board, I think at connector J3. Either way, I discovered that the header pins carrying the 38 volts DC for the flippers on *my* BK had a semi-burnt pin for the left flippers! I pulled the board, installed new header pins and a female connector in the backbox, put it back together and wouldn't you know it--my left flippers became stronger and more consistent! Heck, even the right flippers seem better now, too! The 38 volts going to the flippers comes from the same source, but goes out the left side and right side flippers on their own circuits. That's why the voltage readings for the right and left side were slightly different on my buddy's game, and mine too.

So I'm nearly convinced now that the problem on my buddy's game has to do with the connections bringing the voltage -to- the flippers. I'll replace the m&f connectors on the power supply board and check the connectors on whichever backbox-to-cabinet connector (I think it's one of the big 34-pin ones) is bringing the voltage to the flippers. If that doesn't do it, I'll have to go back over the work I did last time and see if the culprit is there somewhere.

Anyway, thanks for that response--it made the light bulb turn on in my head!

I'd like to follow up regarding the progress I made with the left flipper on my friend's bk. It turned out it was the flipper +38v connector on the power supply board! Male pins had already been replaced, but the female connector was still original. Replaced female connector and they came back to life! Of course I replaced virtually every switch in that circuit before I arrived at the solution, but at least his flippers should be good to go for a long, long time!

Glad it's of use! Definitely get a close look at the male and female connectors on the power supply board for the flipper power. You may even benefit by replacing the connectors anyway, even if it doesn't solve the problem.

Oh man, it's always something right? Did you ever come up with the source of your problem where the fuse is blowing when the ball drains?

Good catch, man! Tracking something like that down can be difficult. Glad you got to the bottom of it...With stronger flippers to boot!