Step one with problems like this is to see if the switch associated with the coil is registering.
Place the game in switch test.
Close the switch.
Did it register?

Assuming it did, next up is testing for power at the coil.
Meter set to DC volts
Black lead on the ground braid you will find in the cabinet.
Red lead on both coil lugs, one at a time.
Got coil power?

Next up...test the path from the transistor to the coil.
Use a length of stripped wire.
Tuck one end under the ground braid in the head.
BRIEFLY touch the other end to the tab of the driver board transistor labeled Q11 (see: http://www.pinwiki.com/wiki/index.php?title=Transistor_Mapping_for_Bally_Games)
The coil should fire. If it does, this proves the path from the transistor to the coil is solid.

Test a bit. Report back.
--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The Place to go for Pinball Repair Info

Two things...

1) Start a game with the glass off. Are you able to manually trigger the coil by pressing on the switch? If so, try adjusting the switch so it is more responsive to the ball, by tightening the switch gap.

2) If the answer to #1 is no, I'm guessing the issue is the connection between the switch and the coil?

ok. Thanks for the answers.
There are a few more possibilities. The subject transistor is open, the 3081 predriver has failed, the .100 header on the lower right of the driver board isn't receiving the signals from the MPU correctly.

Test the transistor with your DMM set to diode test. Black on the center lead. Red on each flanking lead should read .5 to .7.

--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The new place for pinball repair info

He's talking about the Q11 transistor you tested connectivity for before, now you're testing the transistor itself.

I'm pretty sure he was referring to the SCR labeled Q11 on the SDB.

Edit: oops, beaten to the punch!

OK. That's a start. Now test the adjacent transistors the same way so we can compare readings with a known good transistor. This helps filter out DMM differences.
--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The new place for pinball repair info

Kudos for Mr. Hibler for giving excellent advice here---actually describing the detailed process of accurately troubleshooting it rather than just saying "Replace Q11 and all connectors" (which probably would do the trick).

No worries. We will get the game going again. On the playfield bottom, nearly between the flippers, you will find a 1 amp sloblo fuse. Remove it. Use your meter to test for continuity across the fuse. Let's see if it's blown.
--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The new place for pinball repair info

Are the attract mode lights starting and the displays alternating scores? That would mean that the MPU is at least booting.

Assuming yes, let's test coil power again. Remove the ball, raise the PF, and measure DC voltage at one of the coil lugs like the sling kicker...just like we did previously. Got power?
--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The Place to go for Pinball Repair Info

I've been keeping my mouth shut because I seem to preach this too much. Without question, this era of Bally machines suffers from degradation of the connectors. My opinion on this is that if the machine is having bizarre problems, especially if when moving and reseating connectors makes a difference, the connectors must be rebuilt before any serious diagnosis can be done. Typically I replace the pins and plugs on the MPU and the solenoid driver board and completely rebuild the power supply. Once this is done, most problems just disappear and then the game is pretty much bullet proof for years. Note I only replace the pins and plugs on the Light driver board if there are obvious problems. Re-pinning and rebuilding connectors is one of the most important pinball skills to learn.. Not hard, just time consuming. Replacing and rebuilding what I detailed above usually takes 6-8 hours.

Agreed.
Those .100 connectors on the regulator/driver board, after this many years, are generally worn out or suffer from cracked solder joints. To rehab these boards, I replace all connectors including the .156 headers, the 5V filter cap (12,000uf), the HV trim pot, update the grounds for the HV and 5V caps, and jump test point 1 to test point 3 which eliminates the brown wire jumper connection in the female housing.
See: http://www.pinwiki.com/wiki/index.php?title=Bally/Stern#Solenoid_Driver_Upgrades

If you've wiggled connectors and things got better/worse, it's time to repin both sides of those connectors, just as Freeplay40 suggests.

--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The Place to go for Pinball Repair Info

also never unplug the connectors while the power is on

Sounds like you have to at least reflow solder, if not replace headers, for the .100 SDB pins. From your notes you say you reflowed the .156 headers but not the .100 ones?

Specifically reflow J5 pin 14 on the SDB (or just replace the header) and repin the J5 connector. It sounds like you already reflowed the header, actually (.156) but from your list you didn't repin the J5 connector, correct?

snsschem_(resized).jpg

You say the coil fires when you first start playing a game, though, and then stops. So I'm guessing heat/vibration of gameplay may cause that connection go from good to shaky. I'd repin (just pin J5-14 if you want) to make sure that's not the cause.

PS you said shorting the transistor fires the coil every time, but I'm wondering if this is before or after significant gameplay.

I would at least replace the J5 header on the SDB before sending it off. I'm not sure what else makes sense if that one coil is the only one that's intermittent.

Play the game until the coil fails.
Connect a jumper to SDB TP6.
CAREFULLY tap the other end to the Base of the driver tranny. Did it fire?

Good call, it could be a loose joint on the transistor rather than the header, and playing shakes the connection.

Quit beating around the bush and replace all of your connectors. Swapping a board is not a good way to test for connector problems.

Here's your circuit isolated:

SnS_right_kicker_1_(resized).png

So here we see that the right slingshot ON wire goes to board A3 (the solenoid driver/regulator board), connector J5 pin 14; wire color 83

SnS_right_kicker_2_(resized).png

We know this because

SnS_right_kicker_3_(resized).png

Now back at the SDB/R A3 J5 Pin 14 goes to Q11. Electronically, the banded end of CR11 is the base of the transistor. That is where you will tap your jumper.

SnS_right_kicker_4_(resized).png

The CR will be labeled on the board as well as the tranny. "Some folks calls em transistors...I call em trannys...uhm..."

We got yer tranny right here

35547429001_2601341637001_005-PorterRidge_(resized).jpg

OK, you just proved the drive tranny is good and problem is upstream from there. Now we move to the transistor array chip (pre-driver) U3. In particular, pin 6.

Connect a jumper to Ground and carefully tap it to U3.6 Does solenoid fire?

I think you grounded the wrong leg of the chip. For U3, pin 1 is top left. That is what that little divot in the top of the chip signifies, pin 1. Then count down the left side to 6.

Ground that pin 6. Any action?

SnS_SDB_U3_pins_(resized).jpg

Ah-so! You have a Stern board in there.

You need to take one more measurement. A little tricky. While holding pin 6 to ground, measure the voltage at the tranny collector. If it measures 0VDC, Replace U3. If the collector voltage is 5VDC with pin 6 grounded, and no solenoid action, replace CR11 diode.

Let us know what you discover.

SnS_schem_detail_(resized).png

Props to cody for taking the time to make nice diagrams