The XC-series converters (long)
Michael Hopkins mnhopkins at JUNO.COM
Mon Jul 5 12:23:14 EDT 1999
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The big two-tone green boxes (9 X 5.5 X 4.75 inches) Heath made
for the Mohawk receiver found favor with, strangely, the Black Box
Collins crowd back in the late 1950s. But they don't match black
crinkle very well so the later, appearance conscious Collins
hoarder often sells them off -- buy them.
These 6 and 2M receive converters have a nominal 22-26 mc
output, but the circuitry can feed up to the top of a National
NC-300, 30.5-34.5 range with the right crystal. But no lower, so
the Heaths represent a watershed. Prior to the coming of the
Mohawk and some other ham band only receivers, most hams and almost
all VHFers used general coverage receivers. A rule of thumb was
to make the first down conversion in a VHF converter between 10 and
20 per cent of the received frequency, so 14 -18 mc was right for
2M and, as bonus, semi direct readout resulted. A 144.5 signal
came out at 14.5 mc and so on.
For Six Meters, somewhere around 40M would be the predicted
output, but 6M was a stepchild to 2M in converter design so the
average old Tecraft or Tapetone converter will have a 20M output
for 6M too. But Heath was a leader in the exodus to high IFs --
you couldn't even get your XC-2 or XC-6 down to 20M.
The boxes are far from twins. The 2M tail did not wag the
design of the XC-6, which uses a pair of 6AK5s in conventional
grounded cathode to magnify 50 mc sigs. It converts with a 12AT7
but it does something that is seldom seen on a VHF converter. It
allows use of a RF gain control on the first RF amp. That degrades
noise figure and is simply not done above 6M. But the atmospheric
noise at 6M is so high there is little reason to go high tech.
Heath lists the noise figure of the HC-6 as 8 dB, a huge number
compared to other converters of the day, but probably what most
could use then and now.
The insides are very well shielded and an interstage transformer
straddles one of the partitions. Each tube has shielding at the
socket, too, and the roomy chassis allows proper separation between
coils. They don't even recommend alignment with a sweep generator
and 'scope and, save the construction, the XC-6 could be anyone's
HB.
The XC-2 is a different story. A 6BS8 twin triode like the one
in a cheap Ameco converter is there, sure, but not in the front
end. A 6AB4 UHF triode in grounded grid drives the 6BS8 in an
essentially garden variety Wallman cascode arrangement despite the
manual's hype. But the makings of a quiet front end are there and
Heath claims an impressive 4.5 dB. A stage of IF amp is fitted as
part of the 6EA8 that triode converts the 144 sigs to 22 mc, and
another 12AT7, like the XC-6, does the oscillator/multiplier chore.
The two meter version has internal silver plating and a special
shield box for each tube. It feeds on 150 volts instead of the 210
the Mohawk supplies, but no VR tube is fitted. Perhaps the
engineers felt the load was constant and no VR was needed. But the
use of 150 volts shows some thought as RF amps perform best at one
voltage. The XC-6 just uses the 210 as sent.
A big plus of the XC-series for the collector is that the
crystals don't show and are not easily removable so they cannot be
mislayed. Open the box up and look for a 28 mc rock in the XC-6
or a 61 mc in the XC-2 if the boxes fed the usual Mohawk. Higher
crystals mean a Collins or a National.
TNX to collectors Harmon and Morehouse for access to Heath manuals.
de ab5L, michael in dallas, MNHopkins at JUNO.com
Student of Tecraft, ICM and Six Meters' Golden age: 1957-58 Box
226841, Dallas, TX 75222
Banned for life from QRP-L mailer for various truths.
Sponsored by the City of Tempe
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