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. And 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 reistors 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 QVExxx.86, and are not easy to find. A commonly-sold replacement is the OPB84, but only about 20% of these will survive the extended clock test. Most of the rest will work for a little while, so these parts can be real timewasters. Check with a supplier as to whether you are getting good parts or not.
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 damage to the board.