HR-10B project.

[wa1vwl]

Over the years, I've owned three HR-10's, Heathkit's "basic" ham bands
receiver. A modification article appeared in the November 1972 issue of 73
magazine which I've done on 2 of them. The mod adds a voltage regulator for
the LO and BFO, and AF derived AGC on CW/SSB. It really works great and I
recommend it to anyone who intends to use the HR-10 on the air on a regular
basis. The mod requires the addition of a 12AX7 tube which means you'll have
to punch a hole in the chassis. Of course if you want to maintain the
receiver in pristine condition because you're a collector, mods are out of
the question. Personally, I feel that making modifications, when properly
executed, is a good old Ham Radio tradition. The key word here is
"properly". I've seen some real butchery over the years. Those familiar with
the old Saturday Night Live skits featuring John Belushi as "Samurai..."
something or other will know what I mean when I say "Samurai Technician" has
left his imprint on many a piece of Ham gear. He uses a Samurai sword to
whack and hack at the circuitry and equipment enclosure. I wouldn't drill
holes in the front panel or cover, but the chassis is another story. This is
after all an HR-10 and they're not rare or expensive to acquire.

Besides the lack of voltage regulation and AGC on CW/SSB, there is another
problem which I don't think has been covered before: 40 meter feedthrough on
80m meters. When using a 80/40 meter trap inverted vee and listening on 80
meters, some low-level 40 meter signals can be heard. I've confirmed this
using an HP-606B signal generator. I can discern 10 microvolts signals and
100 microvolts is perfectly readable. That could be a problem when trying to
hear weak signals on 80 meters during the evening hours. The problem goes
away when using a single-band 80 meter dipole. It also disappears when using
either a preselector,  or antenna tuner. Why this is happening is quite
simple: images. Twice the 1.682 MHz IF frequency is 3.364 MHz. Add 3.5 MHz
and you get 6.864 MHZ. If you add twice the IF to 4.0 MHz, you get 7.364
MHz. The solution was to incorporate a band-pass filter in the antenna lead
of the 80 meter RF input coil. I used a circuit from QST February 1989,
P-43. It's a simple top-coupled resonator. Two coils are wound with 29 turns
of #24 on a T-50-2 cores, tapped at 4 turns from ground for 50 Ohms. Each
coil is resonated with 330 pf. A 33 pf cap takes care of the coupling. With
this filter, images are no-longer a problem and I can tune the 80 meter band
with complete confidence that what I'm hearing really is on 80 meters. One
more note about the 73 magazine article. I found that using a 330 Ohm, 5
Watt resistor from the solid-state rectifiers to filter capacitor C57 drops
the B+ down to about where it was with the 6X4 rectifier tube. I was going
to leave the 6X4 in but I also have the HRA-10-1 calibrator with a 6BA6, so
I needed to reduce heater current to keep the power transformer from getting
too hot.



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