HAMTRONICS R451 Owner's manual

©1994 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised:
10
/
23
/
02
-
Page
1
-
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
filter 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 hold-
ers. Crystals operate in the funda-
mental 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 oscillator
(TCXO) option, since the crystal must
be matched exactly to the compen-
sation circuit in the receiver. If you
use an OV-1 crystal oven, specify 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
polarity transients on the line, since
semiconductors in the receiver can
be damaged. The positive power sup-
ply lead should be connected to the
receiver at terminal E3, and the
negative power lead should be con-
nected to the ground plane of the
board through the mounting hard-
ware or the shield of the coaxial ca-
ble. Be sure to observe polarity!
SPEAKER.
An 8Ωloudspeaker 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
impedances, like 1K to 10K input im-
pedances of COR boards, etc. There
is no need to load down the output to
8Ω.
Note that the audio output ic is
designed to be heat sunk to the pc
board through the many ground pins
on the ic. When running moderately
low audio levels as most applications
require, 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 try-
ing to use direct coax soldered to
board or another type of connector.
The method designed into the board
results 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 gen-
erator, a regulated 13.6Vdc power
supply with a 0-200 mA meter inter-
nally or externally connected in the
supply line.
The slug tuned coils in the re-
ceiver should be adjusted with the
proper .062" square tuning tool to
avoid cracking the powdered iron
slugs. Variable capacitors should be
adjusted with a plastic tool with a
small metal bit on the end. Tools are
available for adjusting the rf coils
(model A28) and for variable capaci-
tors (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 capaci-
tor (as illustrated for C6 in the parts
location 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 maxi-
mum 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), us-
ing coax clip lead. Set generator to
exactly 10.7000 MHz. Use a fre-
quency counter or synthesized signal
generator. Set level just high
enough for full quieting. (At 5-10 uV,
you should notice some quieting, but
you need something near full quiet-
ing for the test.
f. Connect dc voltmeter to TP1
(top lead of R16). Adjust discrim-
inator 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
proportional to the amount of noise
detected in the squelch circuit; so it
gives an indication of the quieting of
HAMTRONICSR451 UHF FM RECEIVER:
INSTALLATION, OPERATION, & MAINTENANCE

©1994 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised:
10
/
23
/
02
-
Page
2
-
the receiver. A signal peak, there-
fore, is indicated by minimum noise
voltage, not maximum.
The other method is to use a
regular professional SINAD meter.
In either case, a weak to mod-
erate 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 in-
creases during tuning.
If you use TP2 with a voltmeter,
the signal can be modulated or un-
modulated. If you use a SINAD me-
ter, the standard method is a 1000
Hz tone with 3 kHz deviation.
g. Check that signal generator is
still on 10.7000 MHz. With weak sig-
nal applied to Q2 gate-1 as before,
adjust L2 for minimum noise or dis-
tortion. This step is critical to get
lowest distortion in the crystal filter.
h. Remove signal generator so
the receiver hears just noise. Read-
just L8 slightly so that the voltage at
TP1 is +3.5V with just noise coming
through the i-f.
i. Connect signal generator to J1.
Adjust to exact channel frequency,
and turn output level up fairly high.
Adjust frequency trimmer capacitor
C16 to net the crystal to channel fre-
quency, indicated by +3.5Vdc at test
point TP1. If you can't find the signal
at all, tune your signal generator up
and down the band slightly. (Also
check that oscillator is peaked as per
step c.)
If your crystal has the wrong load
correlation or is slightly out of toler-
ance, you may be able to compensate
by changing the value of C15 so C16
can net the crystal on frequency.
The piston capacitor tuning range is
restricted to achieve best frequency
stability; so sometimes it may be
necessary to change the fixed ca-
pacitor. The proper adjustment re-
sults in +3.5Vdc, the same as preset
for the exact 10.700 MHz i-f fre-
quency earlier.
Maximum capacitance (lowest fre-
quency) occurs with the piston
screwed in all the way, and mini-
mum capacitance (highest frequency)
is with the piston all the way up. Be
careful not to completely remove the
piston. If the piston screw becomes a
little tight (squeaky), you can apply a
small amount of silicone oil to the
threads.
j. Connect fet dc voltmeter to TP2
(top lead of CR3). Set signal genera-
tor for relatively weak signal, one
which shows some change in the dc
voltage indication at TP2. Alternately
peak C14, C8, C7, C6, C5, and C2 un-
til no further improvement can be
made. On C8, you may get two peaks
over 180° rotation: a peak at the in-
jection frequency and a peak at the
signal frequency. The correct peak is
at the injection frequency, which oc-
curs at the setting of greater capaci-
tance (toward flat end of C8). Note
that the tuning of input variable ca-
pacitor C2 normally is very broad.
When properly tuned, sensitivity
should be about 0.2µV for 12 dB
SINAD.
SQUELCH CIRCUIT.
The squelch circuit has about 3 to
6 dB of hysteresis built in, so that
once the squelch opens, the signal
must drop 3 to 6 dB below the open-
ing threshold before squelching
again. This allows for some fading on
mobile stations and prevents squelch
pumping on heavy modulation. It also
prevents cycling due to slight de-
sense in repeater installations. Of
course, this requires setting the
threshold a little higher than if there
was no hysteresis so that it will close
with no signal. If you prefer the
older type squelch, you can simply
remove Q5 from the circuit; however,
this is not recommended for repeater
installations. If you want more or
less hysteresis, you can decrease or
increase the value of R25.
REPEATER USE.
E4 provides a "carrier operated
switch" output which may be con-
nected to a COR module to turn a
transmitter on and off. The output
level is about 7V unsquelched and 0V
squelched. There is a resistor in se-
ries with the output to limit current.
Refer to COR module instructions for
details.
If your repeater controller uses
discriminator audio, rather than the
speaker output, filtered discriminator
audio is available at E5. The level is
about 2V p-p. If you need audio
which is squelched, take it from the
input (right hand) terminal on the
VOLUME control.
AUDIO MUTING.
If the receiver is used as a part of
a transceiver, audio muting can be
accomplished without switching the
power or speaker lines. If the trans-
mitter is keyed by applying B+ to the
exciter, simply connect the keyed B+
through a 100K resistor to the junc-
tion of R25 and R27 on the receiver
board. The dc level will be sufficient
to trigger the squelch circuit in U2,
regardless of the rf signal level com-
ing into the receiver.
Of course, some means of discon-
necting the receiver from the an-
tenna must be provided, and we rec-
ommend our TRR Coax Relay Module
if the power level is under 25 Watts.
Otherwise, a larger coax relay will be
required.
DISCRIMINATOR METER.
If you wish to use a discriminator
meter and you are handy in de-
signing with op-amps, you can run a
sample of the dc voltage at DIS-
CRIMINATOR output terminal E5 to
one input of an op-amp and tie the
other input to a voltage divider pot
set to provide a reference voltage of
about +3.5Vdc.
S-METER.
There is no s-meter function, as
such, available in if amplifier ic's
made for professional receivers; how-
ever, a signal strength indication is
available at test point TP2. This volt-
age is a function of the noise level
detected in the squelch circuit. It is
about +3Vdc with no signal and 0Vdc
with full quieting. You can tap off
this point with a high-impedance cir-
cuit, such as an op-amp, to drive a
meter or a computerized repeater
controller.
SUBAUDIBLE TONE
DECODER.
To use our TD3 Subaudible Tone
Decoder or a similar module, connect
its audio input to DISCRIMINATOR
terminal E5. If you want to use it to
mute the audio (instead of inhibiting
a repeater transmitter as is normally
done), connect the mute output of
the TD-3 to the right-hand lug of the

©1994 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised:
10
/
23
/
02
-
Page
3
-
volume control.
MOUNTING.
Some form of support should be
provided under the pc board, gener-
ally mounting the board with spacers
to a chassis. 3/8-inch holes should
be provided in a front panel for the
bushings of the SQUELCH and VOL-
UME controls. After sliding bushings
through panel, washers and nuts can
be installed on the outside of the
panel. Be sure to provide support for
the board; do not rely on the controls
to support the board.
For repeater applications, the re-
ceiver should be mounted in an rf
tight box, such as our model A16.
The receiver board relies on the
mounting hardware to provide the dc
and speaker ground connections to
the ground plane on the board.
TROUBLESHOOTING.
The usual troubleshooting tech-
niques of checking dc voltages and
signal tracing work well in trou-
bleshooting the receiver. A dc volt-
age chart and a list of typical audio
levels are given to act as a guide to
troubleshooting. Although voltages
may vary widely from set to set and
under various operating and mea-
surement conditions, the indications
may be helpful when used in a logical
troubleshooting procedure.
The most common troubles in all
kits are interchanged components,
cold solder joints, and solder
splashes. Another common trouble is
blown transistors and ic's due to re-
verse polarity or power line tran-
sients. Remember if you encounter
problems during initial testing that it
is easy to install parts in the wrong
place. Don't take anything for
granted. Double check everything in
the event of trouble.
If the receiver is completely dead,
try a 10.700 MHz signal applied to
TP5 (left side of coil L11) with a coax
cable clip lead. You should be able to
hear the quieting effect of a 20-30 uV
carrier at 10.700 MHz. You can also
connect the 10.700 MHz clip lead
through a blocking capacitor to vari-
ous sections of the crystal filter to
see if there is a large loss of signal
across one of the filter sections.
Also, check the 10.245 MHz oscillator
with a scope or by listening with an
hf receiver or service monitor.
A signal generator on the chan-
nel frequency can be injected at
various points in the front end. If
the mixer is more sensitive than the
rf amplifier, the rf stage is suspect.
Check the dc voltages looking for a
damaged fet.
If audio is present at the
VOLUME control but not at the
speaker, the audio ic may have been
damaged by reverse polarity or a
transient on the B+ line. If no audio
is present on the volume control, the
squelch circuit may not be operating
properly. Check the dc voltages, and
look for noise in the 10 kHz region,
which should be applied to noise de-
tector CR2/CR3 with no input signal.
(Between pins 12 and 13 of U2 is an
op amp active filter tuned to 10 kHz.)
Typical Dc Voltages.
The dc levels in Table 1 were
measured with an 11 megohm fet vm
on a sample unit with 13.6 Vdc B+
applied. All voltages may vary con-
siderably without necessarily indicat-
ing trouble. The chart should be
used with a logical troubleshooting
plan. All voltages are positive with
respect to ground except as indi-
cated. Voltages are measured with
no signal applied but oscillator run-
ning properly and with squelch open
unless otherwise specified.
Typical Audio Levels.
Following are rough measure-
ments of audio circuits, using an os-
cilloscope. Measurements were
taken under two conditions. The
first is with no input signal, just
white noise so conditions can be re-
produced easily. The second is with
an input signal having a 1000 Hz tone
modulated ±3kHz.
TABLE 1. TYPICAL DC VOLTAGES.
Xstr E(S)B(G1)C(D) G2
Q1 0084
Q2 1.5 084
Q3 448-
Q4 1.5 08-
Q5 Squelched 000.7 -
Q5 Unsq 00.7 0-
Q6 1.5 08-
U1 16814
70713.6
U2 12345678
7.9 7.2 7.2 81.1 1.1 1.1 8
U2 910 11 12 13 14
4.4 3.5 1 To 8 2.5 2.5 .3 To .7
U2 .15 16 17 18
0(Sq), 7.2(Unsq)002.3
TABLE 2. TYPICAL AUDIO LEVELS (V p-p)
Measured At On Noise
On Tone
U2 pin 10 (discrim.): 4
2
E5 (discriminator out): 3
1.5
U2 pin 13 (noise ampl): 4
0.4
Top of volume control: 0.8
0.2
Across 8Ωspkr term: 12
10

©1994 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised:
10
/
23
/
02
-
Page
4
-
PARTS LIST FOR R451 RCVR
Ref # Value (marking)
C1 6 pf
C2 4.5 pf variable (white)
C3 .01 uf chip capacitor
C4 not assigned
C5-C8 4.5 pf variable (white)
C9 .01 uf disc (103)
C10 .001 uf (102, 1nM, or 1nK)
C11 4 pf
C12 6 pf
C13 5 pf
C14 4.5 pf variable (white)
C15 39 pf
C16 Piston trimmer, 2-11.2 pf
C17-C18 150 pf (151)
C19 .001 uf (102, 1nM, or 1nK)
C20 62 pf
C21 2 pf
C22 7 pf (TXCO option only -
see text)
C23 62 pf
C24 .01 uf disc (103)
C25 220 pf
C26 .001 uf (102, 1nM, or 1nK)
C27 18 pf
C28 0.5 pf
C29 33 pf
C30 .01 uf disc (103)
C31 .001 uf (102, 1nM, or 1nK)
C32 33 pf
C33 47 uf electrolytic
C34470 uf electrolytic
C35 4.7 uf electrolytic
C36 0.15 uf mylar (red)
C37 0.1 uf monolithic (104)
C38 0.15 uf mylar (red)
C39 .01 uf disc (103)
C40-C41 .001 uf (102, 1nM, or 1nK)
C42 .01 uf disc (103)
C43 0.47 uf electrolytic
C44 62 pf
C45 220 pf (221)
C46-C48 0.1 uF monolithic (104)
C49 120 pf
C50 0.47 uf electrolytic
C51 27 pf
C52 68 pf chip capacitor
C53-C54 .01 uf chip capacitor
C55 68 pf chip capacitor
C56 .01 uf disc (103)
C57 27 pf
C58 not used
CR1 not used
CR2-CR3 1N4148 (may not be
marked)
E1 Not assigned
FL1-FL4 Matched xtal filter set
FL5 Ceramic filter (blue)
J1 RCA jack
JMP-1&2 Jumper -see diagram
L1 ¾ turn loop #20 bus
L2 7A-691F IF transformer
L3-L4 6-1/2 turns (blue)
L5-L6 2-1/2 turns (red)
L7 0.33 uH rf choke (red-sil-
orn-orn)
L8 IF transformer p/n 831-5 or
YMC-15002 or T1003
L9 2¾ turns #20 bus
L10 3¾ turns #20 bus
L11 1¾ turns #20 bus
L12 3¾ turns #20 bus
L13 2¾ turns #20 bus
Q1-Q2 N.E.C. 3SK122 MOS FET
(static handling precau-
tions required.)
Q3-Q5 2N3563, 2N5770, or
2N3904
Q6 PN5179
R1* 86K chip resistor
R2 86K chip resistor
R3-R4 Not assigned
R5-R6 100K
R7 180 ohms
R8 1.2K
R9 270 ohms
R10-R11 Not assigned
R12 10K
R13 4.7K
R14 270 ohms
R15 not used
R16 330K
R17 1.2K
R18 270 ohms
R19 27K
R20 100K pot
R21 27K
R22 100K pot
R23 6.8K
R24 510K
R25 100K
R26 3.3K
R27 100K
R28 330K
R29 680 ohms
R30 3.3Ω(orn-orn-gold-gold)
R31 47K
R32 15K
RT1 Thermistor (TXCO option
only -see text)
U1 LM-380N Spkr Amplifier
U2 MC-3359P IF Amplifier
U3 78L08 +8Vdc Regulator
Y1 Channel xtal (see text)
Y2 10.245 MHz, 62 pf IF xtal
Z1-Z7 Ferrite beads
* = install under pcb

©1994 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised:
10
/
23
/
02
-
Page
5
-

©1994 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised:
10
/
23
/
02
-
Page
6
-