Black Knight Board Problem - Drop Target Coil

JODY

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One coil was burned so I replaced it. It controls the back right drop targets. The drop targets still would not work.

I traced it to a blown TIP120 transistor as well as its pre-driver transistor and resistor on the driver board. I replaced all three. Used a TIP102 for the TIP120.

At power up, the TIP102 blew immediately. Smoked and blew a hole in one side. The resistor and pre-driver transistor still check OK. I powered it off immediately.

Even if it was locked on due to a bad chip upstream would it blow immediately at power on? If not, do this point to a possible short somewhere on the playfield or something else?
 
Short some how replace the coil if it doesn't blow your golden if it does your polarity on your diod my be incorrect
 
The coil diode is probably backwards. It might be blown now too. Something is definitely shorted. I would take a look at the coil diode. You may have installed the coil the same way but the diode may be reversed compared to what was there originally.

If that's not the problem I would suspect a short in the wiring harness. In this case I would trace the wire right from the driver board out to the coil.
 
The new coil had a diode. It was installed the same way but I didn't think to check to make sure the doide was in properly. I'll check it tonight and post details.
 
The new coil had a diode. It was installed the same way but I didn't think to check to make sure the doide was in properly. I'll check it tonight and post details.

Not familiar with Sterns but do your other coils have diodes on them? If not cut the diode off, ohm the coil out and if it's under 3 ohms it's toast. I just went through this on a Williams game. I didn;t know they had moved the diodes to the driver board (new solenoid came with diode on it) and as soon as I turned it on it'd blow the tip102. Eventually I found that the coil had been damaged by my mistake and was the cause of now a repeated tip102 failure. Before you go any further ohm that coil out.
 
In comparing the coil with others in the machine it looks like the coil wires are reversed. Thus the diode is reversed.

Thus it appears I need to reverse the coil wires, check the coil diode (and probably replace), put in a new TIP102, and retry, or is there something else I should do to be more cautious?
 
Did as stated in my last message. Flipped the game on and the TIP102 blew again. Blew again right when the power switch was flipped on. The targets are in the up position so the coil shouldn't even fire. Also, normally they don't go up until start game time. It appears there is some sort of short somewhere going to this transister. Other than checking the wire harness and connections and ohming out the coil, anyone have other things to try / check?
 
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Did as stated in my last message. Flipped the game on and the TIP102 blew again. Blew again right when the power switch was flipped on. The targets are in the up position so the coil shouldn't even fire. Also, normally they don't go up until start game time. It appears there is some sort of short somewhere going to this transister. Other than checking the wire harness and connections and ohming out the coil, anyone have other things to try / check?

I would pull the connector going out to the coils and check the resistance between the pin going from that transistor out to a coil and the positive solenoid voltage coming out of the power supply. You'll want to do this with the game OFF. That will tell you what kind of load that transistor is going to drive when it turns on. If you're seeing close to zero ohms you know you've got a problem somewhere on the playfield side. If you see a sane value you know the problem is isolated to the driver board.

It's possible that the wire going out to the coil got hot enough to melt into an adjacent wire. I can't say that I've seen this on a pin but I've seen it happen on other things.

In either case you've got a problem causing the transistor to energize when it shouldn't be. I would suspect a shorted PIA output if you have tested or replaced everything else in the drive circuit. That may have been the problem in the first place.
 
I checked the, 7408 between the PIA and the transistors and it checked good but did not check the PIA. Had read it could be an issue. If it is bad, socketing it would be easiest for future replacement but I was wondering about how hard it is to socket a chip with that many pins. Socket without breaking something that is.

This Black Knight is turning out to be a learning experience for me. Especially since I just got into pinball a couple months ago.

Whoever worked on it in the past didn't make it easy either. I noticed after I pulled the board that someone had removed the battery holder from the main board and replaced it with an external one. Original holder was for 3 batteries. They put a 4 cell holder on it with 4 cells in it! I removed the 4th cell and put in a diode. Hope it didn't damage anything in the battery backup.

The coil had been replaced backwards. When I replaced it I didn't pay attention and put the new one in the same way, backwards. Fixed that after some responses on here.

Two connectors on the power board have been removed and wires soldered directly to the power board pinds. Think I best double check all those wires to make sure they are where they are supposed to be after finding all these other issues.
 
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I took the TIP102 off the driver board. Got detailed chip & pin details for that transistor and rechecked the 7408 chip that controls it. The chip still tested good using the diode settings on a meter.

Got a logic tester. Powered up the machine. Connected the red logic tester wire to pin 1 on the board interconnect and black to ground (aluminum behind boards that the boards screw to). Tested the PIA pin that drives the circuit. IC5 (PIA) pin 6 was LOW as it should be. Tested the pins on the 7408 (IC2) that drive it. Pin 10 (from PIA) was LOW, Pin 9 (blanking signal) was HIGH. Pin 8 (output from 7408) was HIGH. 7408 appears bad assuming I did the test right.

I need to replace the 7408 and the TIP102.

Still need to do more checking on the coil circuit with the machine off. Don't think I had the tests right. Probably have the wrong wires or something.

Question....I'm confused on what would normally happen with a bad 7408 resulting in a circuit to a coil always on. Some sites note the coil would lock on and to power off the machine ASAP. Others note that it could burn up a transistor in short order. Could my problems be simply a bad 7408 or must there still be a playfield problem?

I have not found any melted wires so far under the playfield. Did find another disconnected wire (to a different drop target) and a lose wire to the ball intake (just wrapped around but not soldered). Resoldered those.
 
If pin 10 was low on the 7408, then the output should also be low. That's definitely causing the coil to lock on.... which then will fry the coil, which usually results in the TIP102 getting fried.

Replace the 7408 either way, as it's bad, and I'd normally do the TIP102 again anyway just to make sure.
 
Question....I'm confused on what would normally happen with a bad 7408 resulting in a circuit to a coil always on. Some sites note the coil would lock on and to power off the machine ASAP. Others note that it could burn up a transistor in short order. Could my problems be simply a bad 7408 or must there still be a playfield problem?

Good work. You've basically answered your own question. The 7408 is a quad 2-input and gate. You tested one of the gates and it failed. The truth table in the 7408 datasheet confirms that. You could still have a playifield wiring problem but you've found why your transistor is energizing when you turn the game on.

I have not found any melted wires so far under the playfield. Did find another disconnected wire (to a different drop target) and a lose wire to the ball intake (just wrapped around but not soldered). Resoldered those.

The test I mentioned above will tell you the resistance that the transistor will be driving. This should tell you if you have a problem with the harness/coil.

I won't be surprised if you do have a problem with the playfield wiring because the transistor shouldn't blow up instantly if it locks on. I would expect the coil to energize, lock on and start melting then the transistor either failing or desoldering itself. If the transistor is blowing chunks (pun intended) I would suspect it's driving a dead short.
 
The transistor basically blew chunks. Cracked and blew a hole in it. I'm going to try everything noted here and that I can think of prior to trying again.

I took out the coil. Still looks new. The sleeve moved freely so it did not appear to get too hot. The resistance tested too low. .4 ohms vs. 4 on other coils. Took half of the diode off and it then tested fine. Diode was shorted. Replaced the diode. Coil now tests fine and is connected properly.

Only other thing is the testing of the wiring harness. Haven't found the right pins yet because so far I get nothing. No short, no resistance. Off to do more reading of schematics.
 
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