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ALC problems HF radios.
Hi All,
I posted a query a few days ago re problems with no ALC on TX on 80m in 2 Yaesu radios - thanks to all who replied, while they were of limited use, it did get my brain thinking about the real peoblem The root cause has been found in one of them (A Yaesu FT707) and was caused by the "golden screwdriver" of a previous owner. Setting up power output level as per the manual corrected the problem - when power output was taken back to 110 watts, the ALC started working!. The ALC did work on 40m and above, I suspect the sensitivity of the directional coupler used to monitor ALC was less at the lower frequency. The mechanism is thus - power output is controlled by adjusting the level the ALC cuts in - if you crank it up to max power then the ALC amplifier cant generate enough voltage to limit transmitter drive - you wind up with lots more power out, but effectively no protection of the output transistors - the alc circuit also controls the thermal operation point of the cooling fan, and also provides high VSWR protection. The moral of this story - a fault aint necessarily complex, just stupid!. Hope this helps sme other people. Still have the FT747 to look at, but suspect the same sort of problem - complicated by not having a service manual for this one, so will have to poke around till I find the relevant adjustment points. 73 de VK3BFA Andrew |
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