I was given a 285HV that was totally non-working in any of it's auto
modes or manual fractional power modes. I was able to determine by
substitution from another 285HV that the problem was with the sensor module.
I fixed the 285 by disassembling and cleaning the sensor module.
Internally it has metal wipers rotating along a circular track that
switch tiny resistors in and out of the circuit when you rotate the
sensor knob. The circular track had a thin layer of, presumably,
conductive grease on it. The grease had congealed over the years and
had turned into a non-conductive insulating film. I cleaned off the
congealed grease with naptha and reassembled it.
It now agrees to within 1/3 stop (and frequently less) in all modes with
another 285 I have. It's still nowhere near the guide number of 120
that it's supposed to be but neither is the other one. Both test at
guide number 80-90 at ISO 100 and "normal" angle setting. Anyhow, I
think it is now working as designed if not as advertised.
I was quite surprised to see how it works internally. The photo
sensitive diode is covered by a semi-circular rotating window that moves
with the sensor dial. The window has 4 positions where the first is
clear and the second through fourth are covered with increasing
densities of tiny black dots which progressively reduce the amount of
light striking the sensor. Kind of an el-cheapo ND filter.
Maybe this treatment will rejuvenate one of your old flash units. The
sensor is composed of 3 pieces held together by 3 screws. To
disassemble the sensor for cleaning remove the two philips screws from
the back side of the sensor. Remove the back 1/3 which is soldered to a
couple of wires. No need to unsolder. Remove the "axle" screw that is
now revealed in the center of the remaining front 2/3's and separate the
other two pieces. They are quite stubborn in resisting separation but
continue pulling and wiggling until they come apart.
Clean the circular tracks that will be revealed at center and perimeter
of the center piece. Caution: There is a small semi-circular window in
on piece and a (perhaps 6mm diameter) window on the back side of front
piece of the sensor. These can easily fall out. The semi-circular
window has a right and wrong orientation which is controlled by a notch
at one corner. Bend the wipers up a bit to assure a good spring contact
and then reassemble.
Chuck Norcutt
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