So I've had a problem with my right kicker (sling) locking on with my Comet. I've posted about it in the club thread and decided to start a new one for my problem so I don't clog the club up.

As soon as I turn the machine on, the right kicker locks on and stays on. So far I have
-replaced the driver transistor Q77
-replaced the pre driver Q78
-replaced the IC chip U7
-inspected and removed leaf switches to make sure they are not making contact
-tested the diode at the solenoid
-looked for pinched wires
-replaced the solenoid with a known working one (the knocker)

Nothing is working. When I intitally pulled the leaf switches from the playfield and just let them hang there it worked for a few minutes. Then there was a hum and it locked on again. After that I completely took the switches off the circuit and it still locked on. Soldered them back on and still locks on.
Right now I am getting a reading of 3-5 volts from the collector of Q77 when I turn the game on and I am getting 20 volts from the other transistors that run the pop bumpers and other kicker and none of them lock on.
Any suggestions?

#2 5 years ago

DEC

GRUMPY

Pinside member

10y 95,050 8,699

OP
Quoted from Parkshow30:
Any suggestions?

Have you tested SR-21?

#3 5 years ago

JJP

Parkshow30

Pinside member

Macomb, MI

6y 65,300 826 7 22

Quoted from GRUMPY:
Have you tested SR-21?

I have not. I'm pretty new to this stuff and not sure how to test it. Is there a guide or a video somewhere? Thanks

#4 5 years ago

BOY

vid1900

Pinside member

Sana'a

12y 164K 40,202 18

The diode on the coil is probably fried again.

Sometimes I've tested the diode, and it tests good, so then I only replace the driver and pre-driver transistors, all is well, start a game and a minute later, the coil is locked on again.

So now, the flaky diode has killed the driver transistor again.

The moment the game is powered up, the coil locks on, even before the MPU boots.

I'll check the driver chip 7402 and make sure it's not cooked.

The fix usually is to, all at once, replace the Diode on the coil 4004, the Pre-driver and Driver transistor.

A bad (or missing) diode will kill your transistor instantly.

Sometimes a flaky diode will test good, but the first vibration from the coil being triggered will make it momentarily go open, killing the transistor.

Make sure the new diode's band faces the red wires.

#5 5 years ago

JJP

Parkshow30

Pinside member

Macomb, MI

6y 65,300 826 7 22

I will test the transistors again.
In the image below, what does the arrow on top of R19 and R20 mean?

IMG_3054 (resized).PNG

#6 5 years ago

JJP

Parkshow30

Pinside member

Macomb, MI

6y 65,300 826 7 22

Transistors test good. Same exact readings for the other transistors that I know are working

#7 5 years ago

BOY

vid1900

Pinside member

Sana'a

12y 164K 40,202 18

In a busy schematic, up arrows are often shortcuts to power, down arrows are often shortcuts to ground

#8 5 years ago

BOY

vid1900

Pinside member

Sana'a

12y 164K 40,202 18

OP
Quoted from Parkshow30:
Transistors test good. Same exact readings for the other transistors that I know are working

I hear ya, but our meters can't always test a transistor, they are only 90% right.

All at the same time, replace the coil diode 4004, the pre-driver 4401 and tip102 transistors. Do NOT apply power until all 3 items are replaced, or one will kill the other and you will keep feeding the cycle.

Put the board back in with new transistors (even if they old test good), put new diode on coil, band facing red wires(even if old diode tests good).

#9 5 years ago

DEC

GRUMPY

Pinside member

10y 95,050 8,699

OP
Quoted from Parkshow30:
I'm pretty new to this stuff and not sure how to test it.

One easy way to test it is to start by removing 1J-18 and 1J-19 so you don't melt a coil. Now test the switch inputs at 1J18 header pins on the cpu board. Each pin that is used as a switch input should be at 5 volts when the power is on. If it is not at 5 volts SR-21 maybe bad.

Quoted from vid1900:
All at the same time, replace the coil diode 4004, the pre-driver 4401 and tip102 transistors. Do NOT apply power until all 3 items are replaced, or one will kill the other and you will keep feeding the cycle.

I too recommend you do this when repairing boards as it saves time, wear and tear on the board and your sanity. Remember when a part test bad its bad, when a part test good it still maybe bad. (@ 10% of the time)

#10 5 years ago

JJP

Parkshow30

Pinside member

Macomb, MI

6y 65,300 826 7 22

Vid thanks a lot!
Replaced the transistor pre driver and diode and everything is working. Played a couple games and everything works as it should.

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