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Old 12th Aug 2017, 9:08 pm   #1
Andrew B
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Default Loading capacitor Yaesu FT401

Running my FT401 into a dummy load for testing purposes, I have found that the "LOAD" capacitor needs to be almost fully >engaged< (maximum capacitance ), whereas the instructions invariably give a recommended setting for between 1/3 and 1/2 of load capacitance. To me, a control that has to be fully rotated indicates some sort of problem, especially in a tank circuit. Has anyone got any suggestions?
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Old 12th Aug 2017, 10:30 pm   #2
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Default Re: Loading capacitor Yaesu FT401

Relay contacts OK?
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Old 13th Aug 2017, 9:36 am   #3
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Default Re: Loading capacitor Yaesu FT401

Antenna relay has been swapped out for a pair of "man" sized Omron relays, original relay only used to switch the bias and switch the Omron's, They make a very satisfying "CLUNK" on change over. 60mA PA bias and no shortage of PA current available, it seems to be a case of coaxing the RF out of the tank circuit thru the matching network.
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Old 13th Aug 2017, 10:07 am   #4
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Default Re: Loading capacitor Yaesu FT401

I take it your test load is impeding correctly and that all coax connections are sound?
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Old 13th Aug 2017, 10:20 am   #5
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Default Re: Loading capacitor Yaesu FT401

I see the spec is 50-100 Ohms at the antenna socket, have you tried it on a 75 Ohm load?

Is the capacitor position the same on all bands?

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Old 13th Aug 2017, 8:19 pm   #6
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Default Re: Loading capacitor Yaesu FT401

There is a 500pf in parallel with one gang of the load cap on some (lower bands)

Maybe that has gone bad or been disconnected?

Is it OK on 20m for example?

73 Fred
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Old 13th Aug 2017, 8:41 pm   #7
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Default Re: Loading capacitor Yaesu FT401

I'm going to give it all a try and record/report the results.
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Old 13th Aug 2017, 9:15 pm   #8
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Default Re: Loading capacitor Yaesu FT401

No sign of burned / HR switch contacts at all?
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Old 14th Aug 2017, 8:42 am   #9
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Default Re: Loading capacitor Yaesu FT401

Even a dirty band switch worth a check.
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Old 14th Aug 2017, 12:25 pm   #10
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Default Re: Loading capacitor Yaesu FT401

Check also the dummy load Resistance as I have 2 of and 1 is high and the other low on the Ohms range.
Looking at the circuit there are a few possible caps that may be out of spec.I would be tempted to check all those components in this area.
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Old 14th Aug 2017, 5:56 pm   #11
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Default Re: Loading capacitor Yaesu FT401

Thanks Hamish. I suspect there may be other issues at play in the driver/PA stage, so I'm going to isolate the PA and run a test or 2.The 50 Ohm load is a homebrew affair consisting of 20 5watt carbon resistors in a syrup tin full of cooking oil. I think I may have a "take-off" problem in either the driver or PA stage (or both), so I'll try seperating the PA stage and driving it up with an external RF source (an old CB rig).
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