Fix Your Onan Clock/Timer Module
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.
Fig. 1: Identify the
tab holes on the bottom of the generator modules.
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).
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
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.
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.
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
Art, for a money-saving tip — especially for engineers or contract
engineers maintaining multiple sites.
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.
Fig 2: Use a greenie
screwdriver to unlock the tabs and pull the two module halves apart.
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.
received great comments concerning the use of PVC pipe for insulator
purposes, as we discussed in an earlier column.
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 W5JGV.com. He catalogues his experience and provides a
number of pictures describing a PVC fire.
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.
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
Interested in furthering your education? LBA offers a variety of
online courses at www.lbagroup.com/lbauniversity.
to Workbench. You’ll help your fellow engineers and qualify for SBE
recertification credit. Send Workbench tips to firstname.lastname@example.org.
Fax to (603) 472-4944.
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.