New replacement pop bumper driver board for Gottlieb System 80, 80A, 80B. One shot circuit built around HC4538. Robust IRL540 MOSFET coil driver. On board test button. 5V and switch LED indicators. $20 + $3 shipping

Available in the pinside store, http://nvram.weebly.com, and eBay.

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Expect to see more surface mounts parts used going forward. I love through hole but automating some assembly steps is going to help me keep items in stock.

100ms. 100k resistor 1uF cap.
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i guess a trim pot could be added if people want to play with the pulse length. 100ms seems good.

The paper math from the data sheet says ~40ms for gottlieb's original design. Actual pulse width timed from an original and a different repro found in a game was both close to 100ms. The electrolytic capacitor GTB used has a large tolerance range is my guess for the reason.

Gottlieb original 74121
Tw(ns) = K * Rx(k ohm) * Cx (pico F)
0.7 * 12 * 4700000
= 39,480,000ns or about 40ms

I am satisfied with the pulse width at 100ms but I updated the layout to have a 10k resistor + 100k 3362p type trim pot wired up like rheostat. That will give a pulse width range from about 10ms to a little over 100ms.

The pop bumper driver board now has a trim pot to adjust the pulse length of the one shot. They ship plug and play ready set to about 50ms, but can be adjusted between roughly 10ms to 110ms. I will update the picture soon, but looks similar.

Other new items are the Bally SDB with no high voltage (for LED displays) at a reduced cost and the WPC89 MPU.