Which Pop Bumper is it?
Was/is the diode on the coil measuring good?

On the SDB, it might be a faulty U3 transistor array or the 74L154 causing the transistor driving the Pop Bumper to lock on.

It can also be the 74L154 binary-decoder or CA3081 transistor-array ICs. I would start with socketing and replacing the CA3081 (these do fail), and then the decoder. These ICs are not expensive..

It can also be a simple short. Look at the leads for the solenoid and take a good look where the bottom of the play field meets the lock down bar receiver. Sometimes a wire just gets a little out of place.

Disconnect the J5 connector from the Solenoid Driver Board (SDB) which will disconnect that Pop Bumper so you no longer blow the playfield fuse.

Measure the voltages on the SDB at:
a) Pin 14 of U2 (74L154)
b) Pin 10 of U3 (CA-3081)
c) Pin 9 of U3 (CA-3081)
d) The banded side of diode CR12 (near transistor Q12)

Report back your measurements.

U3 is a transistor array. The voltage measured on pin 10 of U3 (base pin of a transistor within the array) is excessively high most likely because pin 10 has internally gone open circuit. This is resulting in pin 9 of U3 (collector pin of the same transistor within the array) to be pulled high which is always activating the Q12 transistor down stream and locking on the Pop Bumper solenoid.
Normally, when the solenoid is not being selected to activate, pin 10 will sit at its highest level of around 0.8 volts and pin 9 should be near zero volts which leaves transistor Q12 switched off.

Replace the CA-3081 device at U3.

If you're really really desperate to get it going while you wait for replacement parts, you can temporarily swap U3 with the CA-3081 at U4. U4 is not driving any solenoids with that faulty transistor within the array. Just keep your fingers crossed no other transistors within the U3 array are faulty if you do this...

Good luck!

Another thing you can do as long as the common emitter leg is not shorted is to snip off other offending legs of and wire in a 2n4401.

Looks like you've got an issue in the switch matrix.

Can you remove all balls from the game and make sure all drop targets are raised.

Go to switch test mode.
Does it report "0" in the Ball in play display?
When you activate the lower pop bumper switch with your finger does the game report 18 or 19 in the player displays?
And what number is displayed when you activate the left pop bumper switch with your finger?

Ok if you've got the manual, go to the switch test number page 17 and activate each switch one at a time starting from switch #40 working backwards to switch #01 (it's important you do it backwards).

Report back the details of which switches are reporting the wrong number in the switch test.

No. The chip at U3 on the SDB is completely unrelated to the switches.

You've got a short between switch return line "I2 and "I1" shown circled in red below.
Firstly, make sure the three coin slot switches on the coin door are in the open position and not stuck closed.
Check the capacitors circled in green on the switches shown below and make sure they aren't short circuited.
Then check the diodes circled in blue on those switches, again looking for any that are shorted.
Let us know what you find.

XandO_SwitchMatrix.jpg

I am resurrecting this thread because I am having a similar problem with a Medusa. The bottom right pop bumper locks on when the game is started and blows the playfield fuse. I have replaced the q14 transistor and resistor and installed a new coil on the pop bumper.

I followed Quench's advice and tested the following voltages on the SDB. Here are the measurements. They appear to be okay.

Measure the voltages on the SDB at:
a) Pin 14 of U2 3.68vdc
b) Pin 10 of U3 .92vdc
c) Pin 9 of U3 .55vdc
d) The banded side of diode CR14 1.28vdc

Thanks in advance for the advice.

Hi,
The measurements you've made on pin 14 of U2 and pins 9 and 10 of U3 drive the left slingshot.

For the lower right pop bumper you want to measure the voltages on pin 10 of U2 and pins 13 and 14 of U3.
.

Voltage here on the banded side of the CR14 diode indicates the driver transistor Q14 is being activated (locked on), possibly by a malfunctioning U3.

Medusa SDB Lower Right Pop Bumper circuit

Quench,

Thank you. I have should have known that - still learning.

How do you know what the correct voltages should be at the different pins and locations?

The part of the Solenoid Driver schematic I've highlighted above is essentially replicated for all solenoids.
See the below diagram for indicative voltages when the machine is powered on and idle.

The banded side of diode CR14 (right leg shown in the diagram below) where you measured 1.28V is connected to the "base" pin of transistor Q14. Consider transistors base pin like an input pin.
When the game is idle, the base pin of those Q driver transistors should have near zero volts meaning they're switched off resulting in their solenoids not being activated.

When those Q driver transistors have around 1.2V on the base pin, that switches them on and activates the respective solenoid.
None of the Q driver transistors driving momentary solenoids should have a constant 1.2V on the base pin.
Momentary solenoids are selected to activate for only a very brief time - typically 0.3 - 0.4 seconds.

Q driver transistors driving Continuous coils on the other hand can and do have constant 1.2V on the base pin when they switch on things like flipper enable relays, coin lockout coils, playfield gates, etc. The specific Q transistors wired for continuous coils are Q15, Q17, Q18 and Q19 which are listed on the top right of the SDB schematic.
.

Medusa_SDB_LowerRightPopBumper_Voltages.jpg

This is really helpful. Thanks!

I replaced U3 and everything worked fine for a while. Then the same pop bumper - bottom right decided to quit working. Q14 had failed. I replaced it and all was good for a while and then Q14 failed again.

Have you replaced the diode on the pop bumper solenoid? Does the coil measure good (around 11 ohms resistance)?
How about diode CR14 on the SDB, did you replace it when replacing U3?

It is a new coil with diode so it should be good. I did not replace the cr14 on the sdb. Will do that and report back.

Are these readings with the board fully plugged into the machine?
Q16 is listed as not having any solenoids hooked up to it.
What actual readings are you getting on each transistor?
What type of transistors did you replace with?

With this board installed in the machine, but *only* J3 plugged onto it, what voltages do you measure in the pre-drive circuits for those three transistors with respect to the voltages listed in post #21 above?

Quench Resurrecting this thread again.

I just replaced C23 and C26 on my SDB.
Now the SDB keeps blowing the transistor at Q9 (Bottom Pop Bumper) on a Bally $6MDM.
I just replaced Q9 with a brand new transistor TIP102.

Before I connect J5, I took a reading at pin 10 on U2 = 3.642
The banded side of diode CR9 = .012

Do you think I have an issue with the U2 IC Chip?
Let me know your thoughts?

No.
Your voltages on the SDB look correct.

Perform these two resistance measurements with the game OFF.
With J5 disconnected, what resistance reading do you get between ground and the metal tab of the Q9 transistor?
When you plug in the J5 connector, what resistance reading do you get between ground and the metal tab of the Q9 transistor?

If the coil became locked on straight after first activation in game, it says the diode on the pop bumper coil is faulty or come loose (i.e. the diode is open circuit at the coil). Inspect the diode to see if one leg has broken loose from the coil lug.

I’m out of power with the storm. I’ll check as soon as it’s back up.

One other point. I have extra SDB so I swapped it and I have no issues with the coil getting stuck or not working. I’m thinking it’s gotta be something on the other SDB.

Working with a cell and a flashlight.
With the power off. I get no reading on Top of Q9.

Once my house power comes back up I think I’ll blow another TIP if I plug in J5.

Let me know your thoughts. Thanks.

Including no resistance reading when you plug in the J5 connector? (without power).

Give me some background on when the coil started locking on. Did it first happen immediately when you powered on the game or was it at first activation in a game? When did it first happen after you replaced C23 and C26?

Sorry with power off, I get a reading on the top of TIP both sides of the legs of 3.39.

Originally when I powered up the game the coil would lock up.

After I replaced Q9 it seemed to work for a while then lock up of the pop bumper in play.

Next I replaced C23 and C26 and then when I power up the game and once the ball hits the pop bumper it seems to blow the TIP.

Let me know if you have any other questions. Thanks

This says the fault is at the coil.

Things to check:
The coil should measure around 11 ohms resistance across the two lugs. Confirm/report your resistance reading. It should match the other pop bumpers.
Closely inspect the diode on the coil when you get light back in the house. Make sure both legs are well connected.
If your coil measures within resistance spec and the diode is connected well, then the diode might be faulty (open circuit). The diode is responsible from clamping high coil voltages from damaging the respective driver transistor on the SDB.

My power is back on in the house.

Here's what I have on the coils and diodes:

Left Coil 10.5 Diode 10.8
Right Coil 10.5 Diode 10.6
Bottom Coil the one with the issue 10.4 Diode 10.6

I inspected the diode on the bottom coil but I don't see any issues.

What are the specs on that diode?

Let me know your thoughts.

A couple of tips:
*Examine the coil wiring. Make sure the power bus, 30 wire, is soldered to the banded side of the coil diode. Test the coil diode to be sure it's not shorted. If the transistor blows almost immediately on powerup it's usually caused by reverse wiring or a bad diode.
*If a coil locks on with powerup immediately power off.
Lockup that blows the fuse but not the transistor is usually caused by a bad drive transistor or a bad 3801 predriver chip.

It's a 1N4004 diode which can operate up to 400V. You can also use a 1N4007 which can operate up to 1000V and are usually easier to get/cheaper these days.
I would test/replace it.

Also as BigAl56 mentioned, make sure the yellow 43V wires are soldered on the coil lug where the banded side of the diode is.

That was it. Bad diode on the coil. When I removed the diode from the coil it tested shorted. Interesting on the coil it didn't.

I replaced it with a new 1N4004 diode I had on hand.

Thanks so much for all your help!

Quench, fighting similar problem. What / where is "U2" I find J 5 connector at Solenoid driver board but nothing marked "U2" .

Some boards are marked, but not all unfortunately. Love the fully-labeled boards! U2 is the 74154 chip (it has 24 pins) sitting a bit below the heat sink for the high voltage regulator:
PXL_20240116_144717485.jpg

Which game are you working on?
Which solenoid on your game are you having the problem with?
There are multiple games mixed in this thread and they require different locations on the solenoid driver board to be tested for different solenoid assignments.

So many nuggets for diagnosing SDB issues in this thread. Thx Quench

Thanks again-again quench. Bally Kings of Steel.

It ended up being a bad " memory coil" or transistor on a drop target bank. I suspect it cooked off q-10 on my Solenoid driver board. Q-10 reads 40 ohm to ground at all times and no other driver transistors show continuity.
Im waiting on coil and the transistors to go through the memory coil bank bank. it the mean time I pushed the offending Orange wire w GRN tracer out of molex connector at Solenoid driver board J5 , put my last 1 amp slo blo into playfield fuse holder and Im back in business w 1 drop target that doesnt drop "on memory" .

I used " A-3" solenoid driver locations that I found hidden on a large fold out diagram page from my manual to see what solenoids were protected by the playfield 1 amp fuse and started checking those circuits first.
I was measuring 40 ohm Resistance to GRND from outlet side of fuse holder and traced it to " jack J5, Pin 15 wire color 74" . No other solenoid driver allowed path to ground when not being called to drive a solenoid. Removing solenoid from circuit by disconnecting a molex connector and testing wire to board. circuit still still showed path to ground. to me this means bad driver. memory coils all read approx12 ohm resistance on both bias so I strongly suspect that I will find bad transistor once memory coils are removed and tested. Putting all this here in case it helps others like yalls previous postings help guide me. Thanks pinsiders! BIG shoutout to David at ALLTEK boards for guidance.

Throwing my hat in the ring with a locked on coil on a Star Trek. Apologies if I've missed something already covered in this thread.

Right thumper bumper, locks on immediately after power on. No fuse blowing if I shut it off (and then remove connector J5 11 from harness to play without it)

I believe the culprit is:
Q14 - Resistance J5 disconnected 1600ohm <--- This is different than the rest
But I don't quite know where to look after seeing that.

Full readings:

• R24 315 ohms
• • CR14 0.05v (banded side, on)
  • Diode setting
  • red banded .480
  • black banded .458
• • U3 13 0.85v (on)
  • Diode setting .711
• • U3 14 0.5v (on)
  • Diode setting .212
• R23 118ohms
• R66 1182ohms
• • Q14
  • Diode setting
  • black to base, red to c 0
  • black to base, red to b .528
  • black to base, red to e .506
  • Resistance J5 connected 515ohm
  • Resistance J5 disconnected 1600ohm <--- This is different than the rest
• U2 9 3.08v (on)
• Coil resistance 11ohms
• Coil diode tested ok after desoldering one side

Any thoughts? Thanks in advance.

Your measurements all point to a faulty Q14 transistor.
It likely failed because the diode on the right thumper bumper coil has gone open circuit / leg broken from a coil lug.
Inspect/repair/replace the diode on the coil and replace the transistor at Q14 with a TIP102 transistor.

Appreciate the reply. I actually did replace this Q14, swapped it with Q7 which I believe is unused for Star Trek. Is it the resistance measurements that indicate its faulty? From the bally repair guide floating around, it looks like the diode test values for it are ok? I believe there are still more unused on the SDB that I can swap.

Also I did check the diode on the coil and it appears fine when I desoldered a leg and tested it. I will go ahead and check that again/replace.

You measured zero volts at the base of the Q14 transistor (i.e. the banded side of diode CR14). That says the transistor was not being switched on yet the output was conducting and driving the coil.

You were spot on. Swapped again with another unused one, back to working order. Appreciate the explanation as well.

Ah shoot. Turned the game on tonight, same right thumper bumper coil locked on and heard some sizzling, Q14 was hot to the touch. Guessing it's fried again.

I took out the coil, still getting 11 ohms resistance and the diode tested good after desoldering a leg. Assuming there's something else going on here.

*Update
I checked Q14 and it still tested good. Put it back in, replaced CR14 and the diode on the coil. Seems to be back in working order. Will keep an eye on it, hoping there's not a random short somewhere.

Did the coil lock on immediately when you flicked the power switch on?

Post a picture of the coil showing the diode.
Post a picture of the solenoid driver board.