Installing a knocker in Stern’s Star Trek

I have had a Star Trek (Pro) and the parts for a knocker install, for well over two years. I decided it was time. I think this general plan would go for any metal-head game.

Get a real manual from an old SAM game. This is helpful in finding places to pick up the needed voltages. I used WPT, because I have one. Stern manuals are terrible after about 2008, but they were superb before that.

To tap voltages off existing connectors, get an IDC punch-down tool for .156″ pins. The trick to these is to run the wire into the connector just far enough. If it’s too far, it won’t punch down on the far side, no matter how hard you push.

Star Trek never had a high voltage board for its DMD, so there are some mounting posts in the head not being used. Maybe it’s different in the LE. I installed the same step-up board that I used in WPT and Simpsons, except it now requires +20V. I just put it on one post and held it down with a #8 nut.

The real pain point in installing the knocker is figuring out where to pick up the needed voltages. The 520-5254-00 step up board requires +50V, +20V, solenoid ground, solenoid input (from Q24) and solenoid output. (Older versions of this board omit the +20V but sometimes pull in at power on. Like a System 80 game!)

The cable dressing here is pretty terrible. I may have to re-do this due to poor crimping anyway, and if I do, I will trim all of the wires to the same length and dress the harness neatly.


+20V is available on J7-1, right next to another source that the game actually uses. In my Star Trek, it’s a fat orange wire. I suspect most SAM games have available +20V.


Solenoid ground is available from J10-2,4,5. On Star Trek, pins 2 and 4 are not populated. It appears the game generally uses black for this. I suspect most SAM games have available grounds.

+50V is not as easy to get, as there are no spare supplies. On WPT, the dedicated magnet supply wasn’t used, so there was an unused pin in the J10 connector. Star Trek, however, has a magnet. I tapped the magnet with a suitcase connector (Scotchlok). I had some trouble getting this to make contact. I either didn’t squeeze hard enough, or the second wire slipped out of position before I clamped down. In either case, next time, I’ll check continuity immediately before moving onto the next step.

I also crimped two wires into the +50V pin for the step-up board so that I’d have a +50V wire to feed the coil with. I probably won’t do that again. Another suitcase connector would have worked just as well. (In my WPT, I fed +50V to the coil, then daisy chained it to the step-up board, which is a better idea when putting the knocker in the head.)

I ran a long cable with three wires out of the head and into the cabinet. One wire picks up the Q24 output and gives it to the step-up board. One is +50V and one is the output from the step-up board to fire the solenoid.

I screwed the knocker in the bottom and firing at the cross member near the coin box. I used screws that are a little too long and pierced the bottom of the cabinet. The cabinet bottom is not great plywood. I may change this over to T-nuts in the future.

If I make any further changes, or I do this again, I want more connectors to make the harness a little easier to work with and debug. Having the connector near the solenoid makes cable management easier. Having connectors for the patches into the existing connectors would make removal easier.

Another possible improvement: Does the shaker motor offer a place to hook in for +50V?

Installing a knocker in Stern’s World Poker Tour pinball

I just installed a knocker in my World Poker Tour. I used Stern’s step-up board, the one that is used for the kickout from the bumpers, available from the usual suspects. I got most of my information from this page.

A few random notes:

I used a 520-5254-00, available from Pinball Life, and documented in the WPT manual.  Because the knocker drive transistor can only sink 20V, a step-up board is required.

Three connections to the game are required: +50V, solenoid ground, and signal. Okay, you could probably use +20V if you wanted a wimpy knock from a wimpy coil. You can source the signal from the connector under the playfield that is used to drive the coin counter that nobody installs. +50V and solenoid ground can be found on J10. On WPT, it appears that pin 8 is not used, but it is fused by F7. Great deal! Three pins on J10 offer solenoid ground.  I used pin 2, but 4 and 5 would have worked. (You can pick up ground anywhere, but it is probably better to pick up the officially blessed solenoid ground.)  I happen to have an IDC punch-down tool.  I’ve had it for fourteen years, and now I’ve used it.  I didn’t have to solder anything (other than the coil) because I picked up all of the connections from connectors that already existed, which makes for a nice clean install.

I installed a Williams knocker, in the head. Mine was apparently from a High Speed, but it’s the same mechanism Williams used until they gave up on knockers.  The geometry of the Stern head leaves a lot to be desired from a knocker perspective. I will probably install the next knocker in the cabinet, and even that can be trying as I have discovered previously.

I put the head in the upper-right mounted to the side of the cabinet.  This was a tight squeeze for the assembly, and it was hard to install and it will be hard to remove for service.  I mounted the knocker on a block of scrap wood, then screwed that to the cabinet.  I don’t like the adapter block, but because of the diagonal brace in the corners of the head, a the knocker wouldn’t have clearance if it screwed to the head directly, and there’s no room for it on the back of the head.

I set the Q24 option to “knocker” and the “knocker volume” to off.  I discovered the “knocker” test drives both the real knocker and the fake knocker and it’s loud as hell. There is no other test for Q24.

Next time I install one of these, should I ever get another Stern game, I’ll probably try the Gottlieb knocker.  It doesn’t require a separate strike plate.  Running on +50V, it’s probably not as anemic as in a Gottlieb game. Or, perhaps I’ll mount a Data East-style unit in the head, down below the computers, where there’s space.

A modern Williams knocker fires up and has no spring.  Bally, Data East/Sega, and old Stern Electronics knockers fire at the side of the cabinet.  The bracket used for Data East is the same as Williams, but the plunger is different.  Old Williams knockers, like on Firepower, and Gottlieb knockers fire at their own bracket, and are mounted in the cabinet. Stern Pinball has never used a knocker.

The sound of a Stern knocker is different than a Williams knocker.  It’s more of a thud than a crack.  I suspect the coil pulse is longer. I prefer the knocker on my Twilight Zone, but any mechanical knocker is better than the pop on a Stern game.

Adding a knocker to my Simpsons Pinball Party was similar, but I used a “Data East” knocker assembly (really, it appears to be a kickback assembly) and mounted it in the body. This gets even more of a “thud”, and it’s not nearly as loud, but not as satisfying.  Finding solenoid ground and +50V was more difficult and I did a bad job.  I should clean it up.


Update: there may be two versions of the 520-5254-00 board, one that takes a 20V input and one that doesn’t.  I haven’t used the 20V on yet, but that’s the one I want for future games. I asked about the difference to Stern’s technical support, and they responded:

The newer step up driver boards use 20V and 50V. The coil will fire using the 50V power. The 20V input is used to sense weather or not the voltage is present. This is important because if you have 50V without 20V, the coil will turn on until the fuse blows. I would recommend the new board!

I’m trying to confirm that the two boards have the same part number.  There are pictures of a five-pin version online with the same version number, but the parts sellers only picture the four-pin version.

With the four-pin version, sometimes my games knock on power-on, I assume due to the problem outlined above. (A little like the System 80 thunk, I’m sorry to say.)