Twilight Zone Clock
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The Twilight Zone clock is a relatively troublesome toy on the machine.
The electronics suffer from two design problems:
- Excessive heat generated inside the clock
- Special optos needed to guarantee reliable operation
In addition, the clock optos can fail in the event of switch matrix shorts. These shorts damage the receivers, reducing their sensitivty and making them more likely to fail when warm. Also, the connectors between the boards can become unreliable.
The first problem with the clock is that it overheats. About half the heat comes from the incandescent bulbs, which generate 4.8W. The balance comes from the opto emitter circuits, which generate a total of about 4W of heat (8 * 40mA* 12V). The clock therefore becomes hot, and the opto emitter resistors get hot enough to burn the board. Plus, the clock case yellows more quickly.
Aftermarket clock boards address the heat issue due the the lamps, but the opto issue remains. There are some designs that address all issues, but these are not commonly available.
Testing the Clock
The WPC Test Menu includes a test specifically for the clock. It can run the clock forwards or backwards, fast or slow, and reports the stat of each of the eight optos. If any opto reports bad, the test will stop. You'll want to run the clock test for a couple of hours to be sure that you have knocked out all of the opto problems.
Replacing the Optos
Williams used a specially selected opto that has the necessary strength to drive the switch matrix. These parts are labeled QVE11233.0086, and are not easy to find. A commonly-sold replacement is the OPB84, but this part does not meet the needs of the circuit. Only about 20% of OPB804 will survive the extended clock test. Most of the rest will work for a little while before demonstrating inconsisent behavior, so these parts can be real timewasters. Check with a supplier as to whether you are getting correct parts or not. As of June 2011, most suppliers have stopped trying to offer incorrect parts as substitutes.
Replacing the optos is straightforwards, but the boards are often damaged and the traces are extra delicate. Also, only replace the resistors if they test bad. No point in risking further damage to the board.
Pinball Lizard - NLA
This was the orignial replacement clock board, a clone of the original design but with cooler LEDs instead of lamps. The board did not resolve the opto issue, so don't be surprised to find one of these showing inconsistent operation.
Rottendog - www.rottendogamusements.com
A clone of the original circuit, but laid out onto one board instead of two. Rottendog has a supply of optos that meet the original specs. These boards are still subject to the original opto design problems, but are unlikely to fail in home use.
The lamps are easily replaced with LEDs. The biggest problem is going to be flaky connectors and burned-up boards.