Duotron is a replay machine by design. There is no way to convert a special award to an extra ball. Moreover, it does not even have an extra ball feature that could potentially be hacked for this purpose.

On most replay machines set to free play, you rely on the non-zero replay counter to give you the satisfaction of a game well played (and the score, of course). Is your replay counter not counting up as a result of a special hit? Or free play doesn't work when the credit counter is non-zero? If so, search the forum for setting free play…

https://pinside.com/pinball/forum/topic/setting-for-free-play

Your fix looks good... it looks like, on Duotron, they might have used logic more appropriate to the "half-moon" credit unit than the drum credit unit that you have. It seems like a design error, possibly Duotron was designed to use the half moon, but they switched back to the drum credit unit late in the design or in production? The half moon didn't appear until Super Soccer, 6 pin designs later.

My half moon pins (Abra and Spirit of 76) do not have the 15th Pos switch protecting the add replay function, my drum pins (Volley and Bank-A-Ball) do.

It would be interesting to see the logic in Magnotron and the pins between Duotron and Super Soccer (Far Out, Free Fall, Royal Pair, Out of Sight, and Atlantis) to see if any others have this issue. I don't have any of those schematics.

Whoa, Atlantis early production used half moon also, then switched back to drum later in the production run. It really sounds like that half moon credit unit was a problem (and if you ask around, you'll find people that say it still is). The half moon only lasted about 15 pin designs or so....

So, are we in agreement that this is a design error? All credit unit switches are accounted for and the schematic is accurate?

If it is a design error, these Duotrons (and, possibly Magnotrons) have been out there for 45 years or so mashing their credit unit switch stacks occasionally when someone coins up too much or a really good player beats it like a drum.

The question is - how far do you want to go to fix it vs how much of a problem it really is in your home environment...

@JudeRussell - I’m looking at the credit unit (of a 76 Volley) in the 15 position. It doesn’t look like subsequent pulses of the add_replay relay would do any damage. The lower leaf of the opened “15th pos” switch is pinned between the 15th replay post mounted on the gear and a structural member on the switch stack, stopping any further rotation of the credit drum (see pic). Additional pulses result in relatively little movement or stresses on any component. Do you have that silver structural “leaf” in your switch stack?

IMG_1729 (resized).jpg

There's a bakelite board in my Bank-A-Ball, looks like that was upgraded to metal in the post half-moon era.

You should definitely make the game harder if you're living in the 15 replay region. Up the award levels, higher pitch, conservative posts, take those toilet paper rolls out from underneath the front legs...

You might also want to consider this tip:

https://pinside.com/pinball/forum/topic/em-tech-tip-convert-gottlieb-to-reset-credit-unit-per-game