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Fix Your Onan Clock/Timer Module

Also, a caution about using PVC pipe for insulator purposes

Fig. 1: Identify the tab holes on the bottom of the generator modules. Crawford Chicago Chief Engineer Art Reis offers a tip for engineers who use Onan generators. He encountered a situation in which the generator’s exercise clock/timer failed, and he was put off by the $500 price tag for a replacement. Further, his service folks told him the clock/timer module could not be repaired — the wrong thing to tell a broadcast engineer.

Art got the module open and was relieved that it was not potted. He found a 0.22 F, 5-volt capacitor (Digi-Key SKU #2832813-ND).

After you order the capacitor, it’s time to disassemble the module. Remove the smoked plastic cover from the front of the readout. You’ll find two screws on the opposite corners of the black plastic readout bezel. Remove these screws completely using a medium-sized flathead screwdriver.

The white plastic housing, seen in Fig. 1, is in two parts that snap together. The arrows in Fig. 1 identify two of the three holes where plastic tabs are located. Insert a greenie, as seen in Fig. 2. The greenie will push aside the tabs, which hold the two pieces together.

Gently separate the two halves, using the larger flathead screwdriver as a wedge, but being careful not to snap off the three plastic tabs holding the two pieces together.

Inside the plastic box is the small circuit board, with about 15 parts on it, shown in Fig. 3. In one corner is a small IC with four pins. This is the bridge rectifier for the circuit. Next to the bridge rectifier is a 100-microfarad electrolytic capacitor. Also next to the bridge rectifier is the 0.22 F capacitor, designated C-3. The arrow in Fig. 3 identifies this capacitor. Unsolder, remove and replace this capacitor and reassemble — the module should work.

Reinstall the module to the control panel, and put it back online. Remember that both the “1” and “U-2” terminals connect to ground. The hot +24 VAC line goes to U-1. The command to turn the generator on connects to U-4, which is brought to ground via U-1 when the clock activates it.

Thanks, Art, for a money-saving tip — especially for engineers or contract engineers maintaining multiple sites.

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Fig 2: Use a greenie screwdriver to unlock the tabs and pull the two module halves apart. Art offers another useful tip for those with laptop computers with the rectangular mouse pads below the keyboard. He informs us that moving the cursor around is only the secondary purpose of this mouse pad on many computers. The primary purpose is to drive you nuts by causing functions to come up that you don’t want to happen, like erasing your work.

Art’s cure was to grab an anti-static bag such as the kind used to store integrated circuits. Cut the plastic bag in the shape of a patch that will fit your mouse pad, taping it over the pad. The sensitivity of the pad is reduced, and those incidents of out-of-control operation are reduced, if not eliminated.

* * *

I’ve received great comments concerning the use of PVC pipe for insulator purposes, as we discussed in an earlier column.

Ralph Hartwell says be sure the RF voltages are not high enough to cause the PVC to begin decomposing. Otherwise you might have a serious problem. Ralph sends a link to his personal website as evidence; visit He catalogues his experience and provides a number of pictures describing a PVC fire.

Lawrence Behr, whose company manufactures phasors, ATUs, matching networks and combiners, likes the idea of using PVC as a replacement for porcelain insulators, but says engineers need to be careful. If the PVC arcs and burns — and it does burn — it will generate hydrochloric acid and lots of black carbon, as you’ll see in Ralph’s pictures.

Fig. 3: Replace the errant capacitor, a 0.22 FARAD capacitor.
The acid will corrode all copper and silver in the ATU — or wherever the PVC is used — and the carbon will coat all insulating surfaces, creating further arc paths. The damage will probably be beyond economic repair. So, yes, this kind of temporary fix is fine, as long as it’s temporary and you realize the risks involved. It’s best to replace the cracked porcelain insulators with the proper replacements.

Interested in furthering your education? LBA offers a variety of online courses at

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Author John Bisset has spent 44 years in the broadcasting industry and is still learning. He is SBE Certified and is a past recipient of the SBE’s Educator of the Year Award.