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H.H. Scott TYPE 348 FM Receiver
The H.H. Scott 348 Stereomaster
was the last tuner to be manufactured by Scott that featured
a nuvistor front end. A compact tube that compared in size to
a transistor, it saw use in a variety of industrial circuits
where its compact size, ruggedness and low threshold noise made
it a preferred choice in demanding applications.
The nuvistor saw little use in audio applications.
McIntosh used a pair in the front end of its MX series of tuners,
and Scott used the tube for two years (1963-65) in its premier
line of tuners, including the 4312, 312,
344, and the 348. Low cross modulation in excess of -65db yielded
front end performance that tuners equipped with a 6BS8-6U8 front
end could not match. The tube equivalent to the 4312, the 4310
has a maximum sensitivity of 1.9uv and a maximum channel separation
of 30db.
The 4312 series of tuners match the 4310's
sensitivity but add an extra 5db to the attainable channel separation.
The 4312 series upped the detector bandwidth from 2 MHz to 3
MHz, this attributed to the use of transistors.
TYPE
4312's successor
The 348 resembles the 4312, but it offers
a control layout that differs substantially. Separate volume
controls have been replaced by a single control and a balance
control. This is of small importance to the person who intends
to use the tuner's fixed output, bypassing the tone, filter,
and level controls that are in the low impedance output's signal
path.
Also omitted from the 348 front panel
is the stereo threshold control, which permitted the listener
to select the point where the tuner switches from stereo to
monaural. Scott may have chosen to omit this control in favor
of a circuit called automatic stereo that gradually switched
to mono as the sub channel's noise exceeded an acceptable threshold.
The stereo indicator light varies in intensity to reflect the
degree of channel separation, but only becomes sufficiently
bright to illuminate the amber lamp when a strong stereo signal
is present.
Although automatic stereo made unnecessary
the sub channel filter circuit since it supplied progressive
filtering that varied depending on the quality of the multiplex
signal, the sub channel filter was selectable on all Scott tuners
that featured automatic stereo until late 1967, with the 312
series retaining the function until it was discontinued.
A Dynaural muting circuit is included on the 348. This is the
last front panel muting control that Scott was to offer. Dynaural
"squelch" dates back to Scott's first tuner, the 310
Broadcast Monitor, made in 1954. The 348's Dynaural circuit
performs in much the same manner as its predecessors. It mutes
the signal when the FM tuner cannot discriminate between signal
and noise. Scott offered the following advice on how to set
the Dynaural control:
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set the control, first turn it to "0". Tune in
between two stations so that the interstation noise is quite
audible. Now turn the
squelch control clockwise
until the noise disappears. Turn the control just a little
beyond this point and leave it there. As you tune across
the dial, you will find that stations pop in and out from
a background of complete silence. |
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Although it is possible to adjust the
muting threshold on any Scott tuner that provides a mute circuit,
the adjustment point is on the inside of the chassis, inaccessible
to the FM radio enthusiast. Listeners who wish to listen to
a distant station will probably have to defeat the muting circuit
to receive it, while others may find that the threshold point
is set too low or too high for their reception area. This is
a common problem with fixed muting circuits.
A tuner's basic
function is...
A tuner converts the radio signal from
the antenna to an audio signal of sufficient strength to be
audible when amplified. A frequency modulated signal received
by the tuner's first RF amplifier. This amplified signal is
mixed with a locally generated RF signal that is produced by
an oscillator. The combined frequency of the incoming RF and
the oscillator is 10.7 MHz, regardless of the frequency being
tuned, and is referred to as the intermediate frequency, or
IF. The technique of frequency summing so described was developed
by Edwin Armstrong in 1919 and was first applied to amplitude
modulated radios. Called the superheterodyne effect, it made
radio sets affordable and easy to use. Armstrong applied the
same system to FM, when he developed that signal carrier system
in the late 1920's.
The combined intermediate frequency is
amplified, limited and detected in the IF stage of the tuner.
Amplification of the signal is in the order of several hundred
times, while limiting is employed to distinguish the signal
from noise and natural static. The AF signal is then detected,
and separated from the 10.7 MHz carrier. With a monophonic tuner,
the detected signal is amplified. Stereo tuners detect the multiplex
signal before amplification.
Look
at the parts:
The Scott 348 tuner section saw two revisions
during the lifespan of the unit. The first revision replaced
the nuvistors in the front end with 3 FETs and a NPN. The second
revision replaced the seven transistors in the IF stage with
four integrated circuits that Scott developed in partnership
with Fairchild's semiconductor division. Scott was the first
consumer electronics manufacturer to employ integrated circuits.
Both revisions to the 348 took place in step with the changes
that were being made to Scott's primary broadcast monitor, the
312.
Heavy
silver front end
The 348 nuvistor front end is similar to the 310's. All of
Scott's vacuum tube tuners were built around a 310 front end.
The 312B's solid state front end is the final version of the
310, and was used in all 312 tuners
until the series was discontinued in 1970.
The 310 style silver 'backplate' was replaced with the lineal
descendant of the 344B front end. The design placed all of the
front end's circuitry into a silver box. with a scaled down
tuning capacitor. The 400 series tuners that replaced the 312's
used a mature version of the 344 front end, first found in the
344C. It supplied a useable sensitivity of 1.9uv to the 342C
and when mated with the 433 IF, enabled the 431 to achieve a
stellar 1.7uv minimum sensitivity across the band, with performance
close to 1.5uv in the lower portion of the band.
Although a front end is not supposed
to be aligned, there are occasions when it benefits from some
adjustment. Over time, the functionality of a tuned circuit
is bound to be compromised, as components drift from their rated
values. In an AF circuit, this "drift" can exacerbate
circuit noise, but may be hard to detect. By contrast, tuned
circuit "drift" can shift a tuner sufficiently off
band to smear the intermediate frequency in a manner that no
adjustment to the IF stage can compensate for. The first victim
of drift is stereo performance. I'm not talking here about lack
of frequency response, or a shrunken image, but rather, a scrapy
noise that is a good 3db louder than the music signal at maximum
amplitude.
The 312 front end has adjustment
points that are easily reached. Three variable capacitors (antenna,
RF, and oscillator) are placed in parallel with each of the
three stages of the tuning capacitor. All must be adjusted to
supply the strongest possible 'peak' signal at an intermediate
frequency of 10.7 MHz.
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Intermediate Frequencies
The IF stage amplifies the RF signal
that is produced by the front end. It does so in cascaded
stages that amplify and sharpen the RF The more stages
that an IF. has, the more robust the signal that it presents
to the detector will be.
The detector extracts a phase modulated
audio signal added to the RF signal during transmission,
and blocks the RF signal carrier. This results in a monaural
AF signal that is then amplified to drive a loudspeaker.
The TYPE 348 employs a ratio detector
preceded by a cascaded IF that employs four IF traps,
and seven transistors. It is identical to the TYPE 312B
IF.
The TYPE 4312 utilizes a more advanced
IF. circuit. It terminates in a Foster-Seeley discriminator
in the place of the ratio detector. Scott's use of the
Foster-Seeley circuit dates back to the TYPE 310 tuner
that was entered into production in 1954.
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FM Stereo: Multiplex...
FM stereo tuners became available to the public in 1961, along
with a host of multiplex adapters designed to produce a stereo
signal, with success or not depending on the circuit used. Scott
was the first company to have multiplex encoders and decoders
ready for use by broadcasters. Scott manufactured a range of
RF products specifically for the broadcast industry. The TYPE
830 multiplex generator set a standard for performance that
matched the 310 series, and it became the standard for multiplex
generation. The 4310, 4312, and 433
tuners are examples of Scott items that were made for the broadcast
industry, but were also made available to the public at large.
They are just a small selection of the range of items that Scott's
Instrument Division produced.
Scott employed time switching at the outset, selecting it as
the best among several methods that they had explored. If the
4310 and 310E multiplex stages are
statements of the art in vacuum tube tuners, then the 4312,
a solid state equivalent tuner to the 4310, must have been their
statement of the art for solid state, for its circuit became
a performance setter until the 342C
ushered in Scott's next (and final) generation of multiplex
adapters.
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The 348 multiplex adapter,
the MX 11, is the third revision of the 4312 adapter,
and it became standard equipment on all of Scott's
RF units, including the 342. Depending on the
IF stage and front end that precede it, the MX
11 meets or exceeds 35db of channel separation.
A phase modulated system, time switching uses
the same method to detect stereo in audio frequencies
that a detector employs to extract an audio signal
from 10.7 MHz carrier that has audio phase-modulated
upon it.
In multiplex, ultrasonic
frequencies contain a signal that is composed
of the difference between the left and right channels.
Time switching applies
phase distinct difference information in turn
to each channel. This results in two distinct
left and right channels.
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Listening
Applications
Scott supplied tuners for use as monitors
and receivers in rebroadcasting networks that distributed FM
signals to geographically distant listening markets. Radio shacks
often had a Scott tuner installed to supply a demodulated signal
to the transmitter for retransmission on a different frequency.
Scott tuners routinely provided a full amplitude (and fully
limited) AF signal across distances of up to 150 km (over 90
miles).
The factors that make Scott tuners effective
in a broadcast setting contribute to their performance in the
home. Properly aligned, a first line tuner such as the 348 approaches
the performance litmus of Scott's 310E, 4310, or 4312, which
utilize a Foster-Seeley discriminator. With the release of the
six crystal IF 342C tuner, Scott raised the bar again. Performance
enhancements continued with the introduction of the 12 crystal
IF 312D tuner in 1969. The twelve stage IF was also applied
in the 4900 Tuner that was designed in 1969, and the 431 and
433 tuners in 1970. The 4900 was manufactured by hand on a unit
basis, and is so rare that most Scott experts know nothing about
it. The 4900 eventually morphed into the 590 by 1972. Like its
predecessor, the 590 was a special order item, and few were
made.
The nuvistor equipped 348 is also a rarity;
its tenure in the model's two year lifespan amounted to several
months, as the receiver kept pace with developments in the 312
series.
Any 312 tuner that is properly aligned
will outperform all but a small number of modern tuners. The
348 offers no exception. Its ability to separate CBC Radio 2
(94.1 MHz) and PBS (94.5 MHz) matched that of the 344. Both
are barely capable of rendering a clear stereo signal from the
120 km distant Buffalo PBS station that is 400 kHz upband from
the local Radio 2. Better stereo reception has been obtained
from Scott tuners that employ a Foster-Seeley circuit.
The TYPE 348 retailed for $500.00 when
it was introduced in 1965. The model appears on the eBay with
some regularity, where it fetches anywhere from $20.00 for a
beat up relic to $50.00 for one that is in mint condition. Like
any other old RF unit, it will need some maintenance before
it performs to its specifications. Oh, one other thing: Alignment
is an art, a dying art.
Still, if you are curious, considering
the price, you have nothing to lose by trying to align one of
these beasts yourself. All you need to know is...
ed.
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