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Icom IC-751 (IC 751 IC751) Bad Cap Problems
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This info may be helpful to ICOM-751 owners with units approaching five
years of many operating hours; two UNRELATED problems occurred, both due
to a similar 10uf electrolytic opening up. The first was the input cap.
to the 5-volt regulater chip which supplies PLL board. Proper by-passing
is required, probably to prevent internal oscillations in the chip.
Replaced it with a tantulum. (Also arbitrarily replaced electrolytic on
the output side of the 5-volt reg. chip). The symptoms were 3 or 4 minutes
delay before unit would function. Also the screw holding the reg-chip to
its heat-sink was a little loose...probably due to COLD-FLOW characteristics
of aluminum. This can also occur in pheof aluminum. This can also occur in
phenolic P-C boards where the groundfoil is supposed to be bolted tightly to
the chassis. It would probably be a
good idea to use small split-ring lock-washers (along with flat-washers) in
these cases to maintain tension over time.
The second malfunction was failure of the display digits etc to appear
until a warm-up time of perhaps 10 minutes or more (after which you must
power-down up to generate a new reset-pulse). It also turned out to
be an open 10uf electrolytic on the -5volt line which supplies IC-1 IC-2
on the Display Unit Board. This board attaches to the front assembly near
the display digits...parallel to the floor. The front assembly DOES unscrew
with 4 screws and move forward an inch or two, but this is actually not
necessary. The board has a 3-position connector attaching, but only the two
outside ones have wires....red is 13.8v outside ones have wires....red is
13.8v dc input, and purple is -5v output.
The purple (-5v) wire, but rather is
on the opposite side of the inductor (L1) which does connect to the
purple wire. A -5v zener-to-ground appears here, and the culprit 10uf is
directly in parallel across the zener. I didn't have a 10uf tantulum handy
but a 39uf did the job. I also arbitrarily shunted C-17 (easily visible)
with a 33uf tantulum, and C-25 which is another 10uf (easily visible) was
shunted with a 2.2uf tantulum. (Tack-soldered on foil-side).
I didn't have a 10uf tantulum handy but a 39uf did the job. I also arbitrarily
shunted C-17 (easily visible) with a 33uf tantulum, and C-25 which is another
10uf (easily visible) was shunted with a 2.2uf tantulum.
(Tack-soldered on foil-side).
With such a trend developing...it would appear that if you are experiencing
other kinds of malfunctions, open electrolytics might be under suspicion....
it should not be necessary to remove them in most cases...just tack-solder
a tantulum in parallel on the foil-side.
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