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coupling in the transformers. With Selec-
tivity Switch in Position 1, the second and
third I-F Transformers are expanded and
over-coupled. It is well in going through
tht alignment steps outlined above to check
the I-F curves on the oscillograph screen
with switch in Position 1 to see that the
curves expand symmetrically.
6.4-4 Adjustment of Crystal Phasing Contrekh—
This adjustment is best made by means of
a signal generator and a high resistance
sensitive DC voltmeter. Place Selectivity
Switch in Position 3. Connect the gener-
ator to the grid of the 6SA7 first det., and
the Voltmeter to Terminal C on last I-F
transformer (L-47, L-48). Tune the gen-
erator to about 7 k.c. off I-F resonance
and adjust the crystal phasing control C-75
for minimum response. If C-75 is adjusted
much from its position as in Section 6.4-3,
check alignment of first I.F. transformer.
6.4-5 Adjustment of Crystal Load Circuit:—
Make connections as listed in Table III (a).
(a) Place Selectivity switch in Position 3.
Rock the signal generator frequency
back and forth across the I-F resonant
frequency and adjust the crystal load
circuit trimmer L-34 or symme-
trical round-top curve.
(b) Place the Selectivity switch in Posi-
tion 4, Rock the signal generator
frequency and adjust trimmer C-81 for
symmetrical curve.
(c) Place the Selectivity switch in Posi-
tion 5. Adjust trimmer C-SO rocking
the signal generator as for (a) and
(b) above.
The above three adjustments are very
critical and must be made carefully to ob-
tain symmetrical curves.
6.4-6 Adjustment of Wave Trap: A wave trap
is connected across the No. 3 band, first
R.F. primary to increase the rejection of
I-F signal frequencies. With the range
switch on Position 3, apply a modulated
I-F signal to the antenna and ground
terminals. Adjust the wave trap trimmer
L-57 (See Fig. 3) for minimum output.
The wave trap should be adjusted before
the final R-F alignment on No. 3 band, or
the R.F. coil alignment may be affected.
6.4-7 R-F Alignment:— A signal generator cov-
ering a range from 70 to 550 kc and 1.5
to 30 megacycles, and an output volt-
meter, are required. It is desirable to con-
nect a speaker across the output terminals.
The output voltmeter should then be con-
nected across the speaker voice coil. The
output impedance is 2.5 ohms. Remove the
cover from over the R-F unit by loosening
the four knurled screws and lifting off.
Refer to Figures 3 and 4. Make connections
as per Table IV (a). Align the R.F. circuits
carefully, following the procedure as in-
dicated in Table IV (b). Always align for
maximum output. A correctly aligned re-
ceiver should have performance equivalent
to that described in Section 3. Replace cover
after alignment is completed.
6.4-8 Adjustment of Beat Frequency Oscil-
lator: Tune in a signal either R-F or I-F
to exact resonance with Power-Transmit-
Receive Switch at "Rec. Mod" (Fig. 1.).
Turn on beat frequency oscillator by turn-
ing switch to "Rec. CW". If zero beat does
not fall within the range of the BFO con-
trol, adjust BFO Trimmer L-22 (see Fig.
4) until zero beat occurs at the mid-point
setting of the BFO control.
6.5 Mechanical Construction:-
6.5-1 The receiver has been designed to be very
rugged so that it will stand up under
severe conditions of use, and yet have all
parts available for easy replacement. All
component parts such as transformers,
chokes, filter and by-pass capacitors, etc.,
are mounted with screws and nuts rather
than with rivets. All wiring other than
that involving high frequency circuits is
made up in the form of a laced cable so
that no loose leads are left floating which
might cause damage or change capacity to
various portions of the circuit. The tuning
condenser is mounted so as to be rigid
with respect to the tuning unit, and yet is
flexible with respect to the chassis. This
prevents distortion of the chassis from
having any appreciable effect on the sta-
bility of the oscillator.
6.5-2 The R-F unit which consists of the tun-
ing condenser, tuning unit, range switch,
and all of the R-F and oscillator coils and
trimmers, is mounted on a separate base
which bolts to the main base. The various
coils and trimmers on this base may he
easily replaced by means of a single nut
which screws on the individual mounting
bushings. However if a major repair is to
(Continued on page 14)
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