RadioBanter - 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