(technical) -Icom IC-M800


Colin Duncan:
Has anyone with an Icom IC-M800 experienced problems with the main dial memory selector switch. Mine will select upwards but only sometimes select downwards. This knob on the display head has a fibre optic connection to the transceiver where the selecting is actually done. I also have problems with the AT 120 tuner which struggles with tuning 4Mhz and 6Mhz - with a resultant poor SWR. Any advice would be appreciated.

Tim Prosser:
I'm not familiar with your rig, but you might see if the cable containing the fibre optics is crimped or bent such that one of the fibers might be damaged.  

If the control head is purely optical it probably uses a light beam interrupter concept, such that a light path might be partially or completely blocked by a bit of debris, possibly from the manufacturing process.  Opening it, inspecting it, and perhaps gently blowing the dust out of it (especially if nothing is obviously wrong) might improve the situation.

As for the AT-120 tuner, I don't know what kind of antenna you are trying to match, but a high reactance on 4 and 6 MHz might present a higher SWR than the tuner can handle.  Using an antenna tuned for the band of interest is always a better solution compared with forcing an antenna to resonate where it won't naturally.  

Best of luck & 73 de kt8k - Tim

Colin Duncan:
Many many thanks for your reply Tim. It's really great to have your input. The installation is on a yacht and the Icom people seem only to suggest getting a technician to look at it. I'm a DIY-er and have to try and be independent when out at sea!! I'm also trying to cover both marine frequencies and ham bands of course!
The fibre optic is in good order (and meticulously protected!) It's the irregularity of the selector switch which has me baffled. I've had a look at it and found a few dry joints where the fibre optic enters the display head. Correcting these seemed to help but alas no, I'm back to square one! The selector switch is not much bigger than a sugar cube and seems to be a sealed unit -might try and do some brain surgery on it I suppose if I can avoid destroying it.(Would you advise?) CAn't find any dry joints at the other end.
We did have problems with my transmissions suddenly dropping out of tune at times and I don't know whether all these problems are related. The AT 120 feeds into an 11 meter long backstay from the mast and seems to cope with most other frequencies, -repressing the tune knob just produces a quick click (most comforting!). I'm going to try bypassing the tuner and run the transceiver straight to a tuneable whip to see if this can be tuned and so eliminate the transceiver being at fault.
Once again -I really appreciate your assistance and any other ideas you might have, thank you. Cheers,Colin  


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