If the Credit button doesn't activate the Credit relay,
Inspect and diagnose this circuit with Alligator clip jumper wires
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https://pinside.com/pinball/forum/topic/gottlieb-big-shot-repair#post-6305153
http://www.planetimming.com/Pinball/troubleshooting/EM%20Troubleshooting.pdf
http://www.pinrepair.com/em/index3.htm#features
Example of a pinsider actually doing this https://pinside.com/pinball/forum/topic/red-baron-tech-question#post-5858156

Are the credit unit contacts making??Game might think it doesn't have any credits.

Here are the 6 alligator jumper tests to help troubleshoot the credit relay circuit.
#1 in the backbox, find the 18 pin jones plug and connect one jumper end to it’s yellow wire.
On the 22 pin jones plug find the brown-red wire (should be the end pin) and touch the other jumper end to it. Credit relay should pull in.
The next 5 tests will be in the cabinet.
#2 on the coin door jones plug find the yellow wire and connect one jumper end to it. This jumper end will stay connected to the yellow wire thru out these 5 tests.
On either the ball count unit or the coin unit, find the gray-yellow wire connected to a blade switch connection. Touch other end of the jumper to the gray-yellow wire. Credit relay should pull in.
#3 on the coin door jones plug, find the white-orange wire and touch the other jumper end to it.
Credit relay should pull in.
#4 on the coin door jones plug, find the blue-orange wire and connect the other jumper end to it. Press replay button to test.
#5 on the score motor cam 1 switch D , find the black-white wire connection and connect the other jumper end to it. Press replay button to test.
#6 on the coin door jones plug, find the white wire and connect the other jumper end to it.
Press replay button to test.
At what # test does the credit relay fail to pull in?

This alligator jumper template can be used for many other Bally machines as well that has a replay button starting problem. The differences are: in step #1 the brown-red wire and the yellow wire are the correct wires, but they might be in different jones plug locations.
Step #5 the black-white wire is correct, but might be connected to a different switch on the stack, like 1E instead of 1D.

It’s acting like there isn’t any credits on the replay unit.
On the backbox jones plugs:
On the 18 pin jones plug find the white-orange wire # 57-8.
On the 20 pin jones plug find the gray-yellow wire # 93-2 it should be on the end of the plug.
Exact wire locations can be found on the schematic jones plug chart.
Clean and check jones plug connections.
Clean and check replay unit switch with the same colors.
The resistor on the coin door jones plug is 8.2k ohms at 1/2 watt. Factory installed.