Weak pop bumper on classic stern

Seeking the advice of those who know more than me.

One of the pop bumpers on my stern magic is really weak both when activated by the switch, and when activated in the solenoid test.

However when I ground the transistor (by clipping a croc clip to the ground wire in the head box and tapping the other end to the metal tab on the top of the transistor) it fires with normal full power.

I think I have ruled out a switch issue as it occurs in both gameplay and in the solenoid test which is independent of the switch, and it can't be wiring as it works when the transistor is grounded.

Is this just a bad transistor, and should I have a go at swapping it out, or is this a symptom that you recognise as being caused by something else. Thanks in advance.

#2 3 years ago

PIP

Quench

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OP
Quoted from Grefla:
One of the pop bumpers on my stern magic is really weak

Which pop bumper is it?

OP
Quoted from Grefla:
Is this just a bad transistor,

Maybe, maybe not. Once we know which pop bumper we can get you to test the transistor in circuit. Grounding the metel tab of the transistor just confirms you have continuity from the transistir to the pop bumper coil. It doesn't actually test the transistor.

What model number is the transistor marked with? (just incase someone's previously changed it for the wrong type).
If you post clear high res pictures of the solenoid driver board it might help.

#3 3 years ago

???

Grefla

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Hi Quench, thanks for replying.

The top bumper.
I’ve checked voltage across the coils of all bumpers and all 3 are the same.

Transistor Q5 is the one that controls the solenoid in question.

Testing Q5 on diode mode
B-C = 495
B-E = 308
C-B = 881
E-B = 307

It tests the same as all of the others on the board, so maybe it’s not bad after all. I just can’t work out why the solenoid fires weakly in the solenoid test, but with full force when I ground the transistor tab.
EDED1347-1223-4B30-8650-9ABEFACE5BBE (resized).jpegF616EB36-39D9-4829-8F59-582E33520905 (resized).jpegFD9C1CD6-C58E-476E-B0F6-F1C57409742C (resized).jpeg

image (resized).jpg

#4 3 years ago

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Quench

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Ok, grab a jumper wire and connect one end to test point TP6 on the solenoid driver board. Very *briefly* touch the other end of the wire on the banded side of diode CR5 which is CR5's lower leg. Does the pop bumper activate weakly or strongly? This tests the Q5 transistor.

Next going upstream, remove the jumper wire from TP6 and now connect the wire to ground. Very *briefly* touch the other end of the wire on pin 13 of the U1 transistor array chip on the solenoid driver board. Pin 13 is the 4th pin from the bottom on the left side of the U1 chip. Does the pop bumper activate weakly or strongly? This tests the predrive transistor circuit.

#5 3 years ago

???

Grefla

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So I touched CR5 and it fired but I couldn’t tell if it fired strong or weak, so I did it again. It fired weakly and then locked on. Before I could turn the machine off the playfield fuse blew. I Replaced it then when I turned the machine on again it blew the fuse immediately. Tried again. The solenoid is locking on as soon as the machine is turned on.

This is different, whatever touching cr5 did, it’s now causing the solenoid to lock on.

I tested Q5 again and the numbers are all the same as before.

#6 3 years ago

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Quench

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OP
Quoted from Grefla:
I Replaced it

Does the top pop bumper have the correct coil or did someone install a wrong coil from a different manufacturer? It should be a "26-1200" coil.

Is the yellow 43VDC solenoid power wire soldered to the coil lug that has the banded side of the diode across the coil?

With the machine OFF, do a resistance measurement across the coil. It should read around 11 ohms.

The Q5 transistor is likely blown now since the coil is locking on at powerup. Retest Q5 in diode mode and compare the results against the readings you wrote in post #3 above.

#7 3 years ago

???

Grefla

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Yeah. Q5 has blown. I’ve taken the board out to test and all of the measurements are different with the board out.
Now.
B-c 535 Same as the others
B-e 515 compared with 545 in the others
C-b 1297 compared with “open” in the others
E-b 579 compared with 590 in the others.

The coil is original and the same as the other pop bumper coils in the machine. 11 ohms confirmed.

Looks like I can’t do much more tonight. Will have to order some TIP 102s and wait for them to arrive before I can do much else.

#8 3 years ago

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Quench

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What do you measure across the Q5 collector and emitter legs? - that's where the short will be.

There's a bunch of transistors not assigned to any coils: Q6, Q7, Q8, Q9, Q10, Q13, Q17 and Q18.
If you're desperate, swap say Q18 into Q5.
Do a diode check of CR5, replace if bad with a spare 1N4004 or1N4007 if you have. Consider replacing the 1N4004 diode on the coil too. If it's open circuit, that can cause the transistor to blow again.

The TP6 jumper test should not have blown the transistor. But it needed to be done quickly - if you had it connected longer than say half a second it would have blown the playfield fuse but not the transistor.

#9 3 years ago

???

Grefla

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c-e 646 compared with open in the others
e-c 308 compared with 495 in the others

CR5 tests the same as the others so that looks to be OK.

I have a new coil (with diode) which I bought today because I thought is was an issue with he coil -but when I change it the issue was still there so I just put the old one back in - now that it's always on, if changing the transistor doesn't fix it I'll change the coil too.

#10 3 years ago

???

Grefla

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Looks like someone has been fiddling with Q5 before. This is what the back looks like.

FA23D795-7430-41BC-ADFA-783AAD3939BD (resized).jpeg

#11 3 years ago

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Quench

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OP
Quoted from Grefla:
Looks like someone has been fiddling with Q5 before.

Indeed.

You have one of the Stern boards with the design flaw of the ground trace running directly from one transistor to the next. A sloppy replacement of a transistor where someone damages that trace takes out the ground connection to all the transistors downstream.

#12 3 years ago

???

Grefla

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That May be why grounding it with a jumper made the solenoid fire well, as it wasn’t relying on this ground trace. But when the transistor tried to ground itself it wasn’t working properly.

#13 3 years ago

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Quench

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Inspect the lifting ground trace where the emitter is soldered with a magnifying glass to see if the trace is fractured/cracked at that joint.

#14 3 years ago

???

Grefla

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Having taken out the transistor I think I’ve found the issue. There is a gap in the trace. Perhaps the solder covering was only giving a poor connection.

image (resized).jpg

1 week later

#15 3 years ago

???

Grefla

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