Air Parts Lock Haven has now replaced my meter movements and could
find nothing wrong with the rest of the gauge internals. They spent
a long time on the phone with some electrical systems specialist at
Beechcraft, and jointly concluded that 1) they don't know what's
wrong; 2) it's in the rest of the plane, not the gauges; 3) the most
likely culprit is a voltage spike; and 4) said voltage spike could
have arisen from any one of a number of sources, in principle from
anything in the plane that turns on or off. According to Lock Haven,
18-19 volts will do in the Rochester gauges.

So now I've got the gauge cluster back in the plane, without the
electric leads hooked up, yet, and next week I bring it to my radio
shop on a ferry permit to pursue this wild goose chase of seeing
where the spikes are coming from. They all tell me that the most
likely culprits are the alternator being switched on, the voltage
regulator or the overvoltage relay not reacting fast enough to shocks
to the system, an old battery losing its ability to absorb pulses, or
high-current motors such as the starter motor or the gear motor. All
of the aforementioned components except the gear motor have been
replaced within the last two hundred hours on this plane. The radio
shop says they're used to this sort of problem, but it sure sounds
like a lot of shop hours to me just to find the problem.

One other detail I'll mention: the Beech rep was asked specifically
about the POH-specified procedure of turning on the battery switch,
then starting the engine, then turning on the alternator field
switch. This remains Beech's official position, but his personal
opinion, and mine, is that it might well be better to turn on both
switches before starting the engine. On my last plane, lots of
electrical problems stopped happening when I started turning on both
halves of the split master switch before starting. I invite
differing views.

Craig MacCallum
Sierra N525SB
Montclair, NJ





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