HW8


BAD HW-8 OUTPUT CORES

Do you have an HW-8 which has low output on 80 and/or 40 meters? 
When all else fails, cast an evil eye on the ferrite cores in the
output networks on those bands.  This was presented in more detail in
my article in the QRP Quarterly in the October 1992 issue and in QST
(Hints & Kinks) in April 1993, but the bottom line is that the
permeability of the cores can shift, increasing the inductance and
throwing off the tuning network.  The cores used are type FT37-63 and
FT50A-63 for 80 meters, and a pair of FT37-63's for 40 meters.  (The
other bands use powdered irons, which are more stable and do not
exhibit this problem.)  

The fix is simple--replace the suspect cores with fresh ones of type
63, or type 67, which is a replacement material.  Wind the same
number of turns as the original coils, although you may have to add
or remove a few due to variations in the new cores.  Warning--do not
substitute other materials, even if you scale the number of turns to
get the proper inductance, or pull turns off the original cores to
restore the inductance.  I tried both, and neither worked properly.

This is not an isolated occurrence--there are now 8 HW-8s in the
world which work properly after the owners took my advice and
replaced the cores.

By the way, the HW-8 output network is a high impedance circuit, and
can develop surprisingly high voltages.  The highest is seen on 80
meters, and can exceed 400 volts peak to peak.  You can confirm this
with a high impedance scope or by touching one end of a neon bulb to
the rotor of the output tuning capacitor.



--Mike WA8MCQ, GQRP 4792, QRP ARCI 2706, MQRP 453, NEQRP 33