[HBR] HBR -- Part 10

[Walt Hutchens]

I've been WAY too busy for the last 2+ weeks. However I've managed to
sneak in a few minutes of radio, now and then.

Well, and about 8 hours of work getting the 160M fullwave horizontal
loop back up.  When I put that antenna up the first time, maybe six
years ago, it was over a mostly empty field.  Since then we've let it
grow up in pines and they're now getting close to the antenna height
of ~15' in places.  A decision will have to be made in another year or
two.

Ice storms get it about ever second winter.  It's strong enough to
carry a small ice load and I can de-ice by running 24 volts through
it, but when branches break off above it and fall, THAT takes it down
and I usually can't get out and fix it until the deer have run through
the wire and made a mess of it.

It's #26 wire so it doesn't much hurt the deer.

One of the remaining sources of RF stage feedback was over the side of
the shield between the antenna and mixer coils. Although there was no
true line of sight, it was evidently close enough, because holding a
tinplate shield above the existing shield, made things slightly
better. I had made the coils 3-1/2" tall to allow more clearance
between the APC cap in the coil top and the coil itself but that was a
bit much for a 4" shield.

Other HBR builders knew about this issue: I note that on the Deluxe
HBR in the 17th (I think?) edition of Bill Orr's handbook, has the
APC's oriented so the stators are as far as possible away from each
other.

I rewound the antenna coil on a 3" form, oriented the cap stator away
from the mixer coil, and at the same time added a tap to allow
experimenting with cathode drive of the RF stage. And I rewired the
stage that way.

As wired this stage had the highest frequency UHF parasitic
oscillation I've ever encountered: 1/4 wave was about 6-8" as shown by
the fact that a plastic handle 4" screwdriver touching the (RF
grounded) grid could be 'tuned' by sliding a finger down the handle to
kill the oscillation and my 250 MCS counter displayed nothing useful.
A series resistor of 100 ohms in the grid-to-ground bypass fixed that.

The cathode-drive RF amp now works fine and the set is rock-stable
under all conditions.  Unfortunately it has nothing close to enough
gain.  100 uV signals are just about audible.  If the AF gain is
turned way up.

"I have some good news and I have some bad news ..."

The low gain issue is distributed fairly evenly among the stages: The
cathode-drive RF stage is low gain by design, ditto the mixer. However
these circuits offer significant benefits, IF the rest of the set can
make up for them.

The IF stages should do it, but they too have low gain. DC conditions
are okay (although the tubes I'm using don't meet the specs for new
tubes), so it's something more subtle.

The IFTs were wound on forms used as plate coils for test instrument
crystal oscillators: The tank of a precision crystal oscillator should
be low-Q so the crystal (rather than the tank) controls the frequency.
I'm betting the cores I used aren't really suitable for use at 1700
kcs. Should have thought of this when I was making the coils,
shouldn't I?

I have some higher Q cores that will fit (from 30 MCS IFTs), but I
think the coils will have to be rewound to use them. I think the Q's
can be further improved by changing the way they're wound, but
experimentation will be needed.

I will have a new batch of 19JN8's in the next day or so, so that'll
help some.

I'll increase the audio gain slightly and the detector might be
improved a bit, too, but the focus will be those three IFTs.

One thing that DID get done and is working properly is dial lighting.
Radio Shack 12V 25mA bulbs silicone glued into holes in an aluminum
angle at each end of the dial do the job. Four in series with a 3000
ohm resistor work fine on the 120 VAC line and don't generate enough
heat to matter.

Still haven't wired the BFO or calibration oscillators. And
unfortunately, time will be continue to be very short for the next
several months.

I think there's much to be said for building a well-established
design, using parts from a catalog -- or at least, NOS from eBay. And
W6TC's success in designing a series of receivers that could be built
without 'issues' by practically anyone with adequate soldering skills
is increasingly admired here.

Walt
KJ4KV