Hi Matt
welcome to pinside. Are the pictures in Your post-1 "before" and "after" ? If Yes: You did a great job.
The problem with "Double Bonus" - Unfortunatel ipdb does not have the schematics to Knockout - https://www.ipdb.org/machine.cgi?id=1392
I did a search here in pinside and found topics showing parts of the schematics
- here a good part from the beginning: https://pinside.com/pinball/forum/topic/bally-knockout-will-not-start#post-4218165 (readable to 13,14,15). Here https://pinside.com/pinball/forum/topic/1975-bally-knockout-relay-acting-like-stuck-switch-switch-not-stuck#post-5572298 the third picture is interesting - the guy encircled orange the "coil of Double Bonus Relay" - when the relay gets activated it pulls in and closes its Self-Hold-Switch (in the schematics called "Double Bonus Relay (switch)" AND switch on Bonus-Zero-Relay (closed when this relay is non-pulling) helps staying pulled-in. I take from this: "Double-Bonus-Relay stays pulling until the Bonus is counted down reaching Zero (which makes the Bonus-Zero-Relay to pull-in).
So You should inspect these two switches (Self-Hold-Switch*** on the Double-Bonus-Relay --- Switch on Bonus-Zero-Relay) (((and also check "Does the Bonus-Zero-Relay faulty pull-in))).
Self-Hold-Switch*** - such switches are easy to be found - a short wire runs from "solderlug on coil on relay" to "(Normally Open) Switch on the relay".

Well, other topics / posts I found (having snippets of the schematics):
https://pinside.com/pinball/forum/topic/bally-knockout-startup-#post-5093884 and https://pinside.com/pinball/forum/topic/bally-knockout-coin-door-help#post-616097 . Greetings Rolf

Hi Matt
I am not much of a help - I do not know the pin - ipdb has no schematics, an the pics in ipdb are not so good - here https://www.ipdb.org/showpic.pl?id=1392&picno=57514&zoom=1 I try to figure out "how to get the Double Bonus activated" - I do not know ...
See my first JPG - the Center-Lane-Unit "my green A" is in a certain position and the Top Center Lane Relay is pulling and so "my green B" is closed (((I think the ball rolls through that curved way - am I right ?))) --- the Double-Bonus-Relay "my green C" actuates and so closes its Self-Hold-Switch (my green D, a short wire runs from one side of the coil on the relay to switch mounted on the relay) and as the Bonus-Zero-Relay is not actuated, pulling: "my green E switch on the Bonus-Zero-Relay" is closed.
"My green C, D, E" is a conventional "Self-Hold-Circuitry" on a Relay - one of the switches (D, E) no good or the wiring is no good (?).

Where is Your Double-Bonus-Relay monted ? In the Cabinet or in the Backbox or on the Playfield ? Where the Bonus-Zero-Relay ? Can You make the fault happen with the playfield up ? opened backbox ? A friend helping, actuating stuff and YOU watch the Bonus-Zero-Relay ?

Do You have the schematics ? See the second JPG --- not to read it - just to get an idea what I have put together from sources mentioned in post-2 --- see a lot of the schematics is missing, also (see my first JPG) what I have is hard to read --- do You have the schematics ? Greetings Rolf

0Knockout-Work-04 (resized).jpg

0Knockout-example (resized).jpg

Hi Matt
I was feeling anxious, tense the last couple of days - awaiting medical examination, my heart problems - O.K., result of the examination - my heart should function the next 2 to 3 years - maybe surgery at a time - but for the next months I decided not to worry.
To Your Bally Knockout pin (and my snippet of schematics) - thanks for post-6. You SEE the Double-Bonus-Relay engage (when the Center-Lane-Unit is in "right" position (pos-1, -6, -8) - then faulty the relay disengage.
See the snippet - "my green A" - I guess the Unit does not step (within the time of engage - disengage) - but I assume the "my green B, Top-Center-Lane-Relay" engages - then (is entitled to) disengages. Please do this test, please wear rubber gloves - current can kill, ruber gloves hinder current flowing through Your body: First do what You did - watch the Double-Bonus-Relay engage, then disengage - NOW press the armature of the Top-Center-Lane-Relay - keep on pressing - question: As long in time You press - as long the Double-Bonus-Relay stays engaged ?

I would like to use an Jumper-Wire - actually TWO Jumper-Wires - to install them: Toggle-off the pin, unplug the main power cord. One Jumper-Wire is clipped on on a solderlug somewhere - clipped on wire-YELLOW (abundant in the pin, example "Switch on Lock-Relay") - take the other end through the open Coin-Door and let the other end hang loose - not touching metal.
Take the second Jumper-Wire, clip-on at "Self-Hold-Switch on Double-Bonus-Relay, see the short wire running from 'Coil on the relay to switch on the relay' --- yes this switch BUT clip-on on the other blade, solder-lug" - take the other end of the Jumper-Wire through the open Coin-Door an let hang free - not touching metal.
Then plug-in, toggle-on, start a new game - then (wear rubber gloves !) put together permanent the ends of the two Jumper-Wires --- then make "Double-Bonus" - question: Does the Double-Bonus-Relay stays pulling ? Well this is just a test - You can not play this - as the Double-Bonus-Relay by now stays pulling until You take apart the two Jumper-Wires.

I do have Jumper-Wires with "built-in Toggle-Switch" - but for once doing such a test You can do as written above. Greetings Rolf

0Knockout-Work-05 (resized).jpg