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SERVICE
ALBA
MANUAL
AM/FM SUPERHET STEREOGRAMS.
Models
7701
-
7802-7903
ALBA 7701
GENERAL SPECIFICATION
Models 7701-7802-7903 employ the same chassis, but differ in
respect of the types of cabinet and loudspeaker used.
They are
designed for normal or stereophonic record reproduction, in addition
to the reception of A.M. broadcasts in the Long and Medium waves
and of frequency-modulated signals in the range 87-100 mc/s (Band
II). Waveband and Radio/Gram selection is by means of interlocked
push-button switching.
The output from the two identical audio sections is used in parallel
for radio reception and when playing 'monaural' recordings. Ganged
Tone Controls are used; and the knobs of the two independently
adjustable Volume Controls are friction-fitted, so that after balance
between the two channels has been achieved, the adjustment of one
knob controls the volume from both. Audio output is approximately
4 watts per channel.
Valves:
VI (ECC85)F.M. R.F. amplifier and mixer.
V2 (ECH81)—A.M. mixer/osc.; F.M. 1st I.F. amplifier.
V3 (EBF89)—A.M./F.M. 2nd I.F. amp.; A.M. detector & A.V.C.
V4 (EB91)—F.M. Ratio Detector double-diode.
V5 (ECC83)—1st audio amplifier (one half per channel).
V6
)
^ | (EL84)—audio output (one per channel).
V8 (EZ81)—H.T. rectifier.
CIRCUIT DESCRIPTION
F.M.
R.F. & Osc.:
Signals from the F.M. aerial socket are coupled by the aerial
transformer to the cathode of the first triode of the ECC85 (VI),
which acts as R.F. Amplifier, permeability tuned by LI, with C3
as R.F. trimmer. The R.F. output from this valve is coupled via a
bridge circuit to the second triode of the ECC85, which acts as a
mixer/oscillator with a permeability tuned osc. transformer L2 and
Cll as trimmer.
A.M.
R.F. & Osc.:
Signals from the M.W. or L.W. aerial windings on the ferrox-rod
are fed to the grid of the mixer section of the ECH81 (V2) via the
band-change switch, the secondary winding of the F.M. Tuner
I.F.T. (Tl), and C18 (100 pf). R.F. tuning is by means of C17
section of the gang, with tracking trimmer Cl6. The fixed tuning
condenser C15 is in circuit on L.W. only.
The triode portion of the ECH81 functions as oscillator. Osc. coils
L6 and L5 are selected by the band-change switch for M.W. and
L.W. respectively. Oscillator tuning is by means of C28 section of
the gang with trimmer C27.
I.F. & Detection:
The I.F. is 470 kc/s. L9/10 and L14/15 are the A.M. I.F. trans¬
formers, shunted by the 200 pf capacitors C22, C23, C35, C37.
The effect of the F.M. I.F. transformers in series with them is
negligible at 470 kc/s, and they in turn have negligible effect at the
F.M. I.F. of 10.7 mc/s.
Detection is carried out by one diode of the EBF89 (V3), the other
being strapped to cathode. A portion of the detected signal is applied
as A.V.C. to V3 and the mixer section of V2, via R16, decoupled by
C30, and R6.
The H.T. feed to the F.M. Tuner valve VI is disconnected when
the receiver is set to A.M.
I.F. & Detection:
The modulated I.F, appearing at the mixer anode is coupled to the
heptode section of the ECH81, which now acts as 1st I.F. amplifier,
by the I.F. output transformer Tl on the Tuner. H.T. feed to the
anode of the A.M. oscillator is disconnected. L7/8 and LI 1/12/13
are the 2nd F.M. and Ratio Detector transformers.
A separate
double-diode (V4, EB91) is used for detection.
Detected A.F.
voltages appear across C.38.
R14 and C39 form the de-emphasis
network.
continued over
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