My Omni-D is alive and well again! Many thanks to all who responded
to my request. I want to record the fix here for those who may
search these archives in the future.
Conclusion: The presence of a nearby lightning strike damaged diodes
D1 and D2 on the SWR board (the forward power-sensing part of the ALC
circuitry). So it wasn't the LED that had problems, but in fact
the ALC circuit quit altogether. Replacing both diodes with 1N4148's
(the original component) fixed things nicely.
Detailed instructions for posteriety:
Symptom: Following a nearby lightning strike, the ALC circuit was no
longer operating. This is evidenced by a failure of the ALC LED to
light under any circumstances, and the rig would draw too much
current and cause the power supply to kick off.
Repair Steps:
1. Remove the top cover of the OMNI
2. Remove the top cover of the Bandpass Filters shielded "box" in
the back corner near the SO-238 connector.
3. Remove the SWR board (the one without the toroids) and make a
note of the orientation as you remove it. It is quite possible
to put it in *backwards* after repair, and that won't help things at
all. It's a tight squeeze getting the board out and my fingers won't
reach to the bottom of the board to pry it loose. I used a small
screwdriver to gently pry the lower portion of the board by inserting
the blade between the bakelite socket and the board, and gently
"twisting" the blade through 90 degrees. Whatever you do, be gentle.
4. Unsolder one leg of D1 (SWR reflected power) and D2 (ALC forward
power) and use a DVDM or diode checker to confirm that diodes are bad.
5. Replace 'em both with 1N4148. Since they are both connected to
the antenna port via T1, both are probably toast as a result of
the spike on the antenna.
6. Reinstall the board and be sure that piece of insulating
cardboard is covering the bare wires that run between the bakelite
sockets and the PC board. Use electrical tape to tack it in place
so it won't fall as you try to insert the board.
7. Replace the cover(s).
8. Insert a coax switch between your antenna and rig and disconnect
the antenna when not operating so as to prevent this from happening
again :-)
73,
Barrett / KE4R
KE4R@arrl.net
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