©1994 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 10/2/07
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FUNCTIONAL DESCRIPTION.
The R451 is a premium, com-
mercial- grade single-channel uhf fm
receiver. It features a GaAs FET rf
amplifier for very good sensitivity, an
8-pole crystal filter plus a ceramic fil-
ter for superior i-f selectivity, and a
hysteresis squelch circuit to lock onto
fading signals.
The R451 kit is available for the
420-470 MHz band, and wired units
are available for this band and also
the
commercial
and
government
bands directly above and below this
range.
CRYSTALS.
The channel crystal plugs into
sockets identified in component lo-
cation diagram as Y1. We can order
crystals for any frequency desired. If
you order your own, be sure to supply
these specs.
The receiver uses 32 pF parallel
resonant crystals in HC-25/u holders.
Crystals operate in the fundamental
mode at a frequency of (F-10.7)/27.
Frequency tolerance is .001%. We
recommend that any new crystals be
ordered directly from us to be sure
that they will perform properly over
the -30 to +60°C range for which the
unit was designed. This is especially
true for commercial receivers with the
temperature-compensated crystal os-
cillator (TCXO) option, since the crys-
tal must be matched exactly to the
compensation circuit in the receiver.
If you use an OV-1 crystal oven, spec-
ify a crystal with a 60°C breakpoint.
POWER CONNECTIONS.
The receiver operates on +13.6 Vdc
at about 150 mA peak with full audio.
Current drain with no audio is only
35-40 mA. A crystal oven adds about
450 mA peak current drain when cold
and only about 25 mA when warm. A
well regulated power supply should be
used.
Be sure that the power source does
not carry high voltage or reverse po-
larity transients on the line, since
semiconductors in the receiver can be
damaged. The positive power supply
lead should be connected to the re-
ceiver at terminal E3, and the negative
power lead should be connected to the
ground plane of the board through the
mounting hardware or the shield of
the coaxial cable. Be sure to observe
polarity!
SPEAKER.
An 8
Ω
loudspeaker should be con-
nected to E2 with ground return to
the ground plane through the mount-
ing hardware. Use of lower imped-
ance speaker or shorting of speaker
terminal can result in ic damage. The
receiver can also drive higher imped-
ances, like 1K to 10K input imped-
ances of COR boards, etc. There is no
need to load down the output to 8
Ω
.
Note that the audio output ic is de-
signed to be heat sunk to the pc board
through the many ground pins on the
ic. When running moderately low au-
dio levels as most applications re-
quire, it is no problem to use an ic
socket; so we have provided one for
your convenience. If you will be run-
ning high audio levels, check to see if
the ic is getting hot. If so, you should
remove the ic socket, and solder the
LM-380 ic directly to the board for
better heatsinking.
ANTENNA CONNECTIONS.
The antenna connection should be
made to the receiver with a phono
plug. If you want to extend the an-
tenna connection to a panel connec-
tor, we recommend using a short
length of RG-174/u coax and a good
phono plug with cable clamp (see
catalog). We do not recommend trying
to use direct coax soldered to board or
another type of connector. The
method designed into the board re-
sults in lowest loss practical. When
soldering the cable, keep the stripped
ends as short as possible.
ALIGNMENT.
Equipment needed for alignment is
an fet voltmeter, an rf signal genera-
tor, a regulated 13.6Vdc power supply
with a 0-200 mA meter internally or
externally connected in the supply
line.
The slug tuned coils in the receiver
should be adjusted with the proper
.062" square tuning tool to avoid
cracking the powdered iron slugs.
Variable capacitors should be ad-
justed with a plastic tool with a small
metal bit on the end. Tools are avail-
able for adjusting the rf coils (model
A28) and for variable capacitors
(model A2).
Unless already tuned once, vari-
able capacitors C1, C5-C8, and C14
should be set to midrange, as in-
dicated by the slot on the rotor being
aligned with the ends of the capacitor
(as illustrated for C6 in the parts loca-
tion diagram). The SQUELCH pot
should be set fully counterclockwise.
a. Install channel crystal in socket
Y1.
b. Connect speaker and +13.6
Vdc. You should hear white noise.
c. Connect dc voltmeter to TP3
(top lead of R18). Adjust first L4, then
L3 and L4 alternately for maximum
response. (Typical indication is about
+1.5 Vdc.)
d. Connect dc voltmeter to TP4
(top lead of R9). Adjust L5 and L6 al-
ternately for maximum response.
(Typical indication is about +1.5 Vdc.)
e. Connect stable signal gener-
ator to TP5 (left end of coil L11), using
coax clip lead. Set generator to exactly
10.7000 MHz. Use a frequency
counter or synthesized signal genera-
tor. Set level just high enough for full
quieting. (At 5-10 uV, you should no-
tice some quieting, but you need
something near full quieting for the
test.
f. Connect dc voltmeter to TP1 (top
lead of R16). Adjust discriminator
transformer L8 for +3.5Vdc. Note that
the voltage changes very rapidly with
tuning. Full swing of about 1 to 8V
occurs within a few kHz.
Note: There are two methods of
adjusting the mixer and front end.
One is to use an fet voltmeter with test
point TP2, which is the top lead of
CR3. The voltage at this point is pro-
portional to the amount of noise de-
tected in the squelch circuit; so it
gives an indication of the quieting of
the receiver. A signal peak, therefore,
is indicated by minimum noise volt-
age, not maximum.
The other method is to use a regu-
lar professional SINAD meter.
In either case, a weak to moderate
signal is required to observe any
change in noise. If the signal is too
strong, there will be no change in the
reading as tuning progresses; so keep
the signal generator turned down as
receiver sensitivity increases during
tuning.
If you use TP2 with a voltmeter,
the signal can be modulated or un-
modulated. If you use a SINAD meter,
the standard method is a 1000 Hz
tone with 3 kHz deviation.
g. Check that signal generator is
HAMTRONICS
R451 UHF FM RECEIVER:
INSTALLATION, OPERATION, & MAINTENANCE
