Circuit
Review
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H.H.
Scott Type 310C FM Tuner
The 4310 is the most elaborate example of the 310, but perhaps
mostly for its multiplex adaptor which, equipped with vacuum
tube diodes, was an early ultimate statement about the art of
time switching design. With solid state diodes, the 310E might
be seen as a step down, but call that a small step, as its adaptor
is also a one-off that may be evolutionary to the 4310, since
it was designed in 1963, more than a year after the 4310. But
the 4310 is an interesting tuner to look at in an examination
of the 310 series of tuners. Circuit-wise, it is identical to
a late 310C in its front end and IF stages.
The only substantial difference between the front ends of a
310 and the 4310 is found in the elaborate switching network
that Scott supplied to permit switching between 72 and 300 ohms,
balanced and unbalanced. As Scott tuner expert Daev
Rohr recently noted, the front end of Scott tuners see 72
ohms, regardless of the hookup arrangement that had been supplied
at the back of the tuner. The 300 ohm terminal feeds an internal
72 ohm line, and the best way to connect a Scott tuner is with
a 75 ohm line with proper orientation to ground. Attaching the
bare wires will do it best.
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An examination of the antenna matching network shows
that the simplest path to the first inductor that the
antenna sees is via the 72 ohm, unbalanced path. The 300
ohm network is shorted to ground when the system is switched
to 72 ohms.
The antenna connects to the low tap of the aforementioned
inductor. The high tap is connected to the regenerative
circuit that connects the plate to grid through an r-c
network, and feeds the positive signal that energizes
the inductor, which along with a capacitively tunable
circuit comprises the antenna circuit. For the tuner to
work, the correct amount or regeneration must appear at
the grid of the tube to reinforce the signal to thousands
of times larger than its initial strength. A 310D could
supply a useful result in mono to 1.9 uv, but most signals
are much larger than this, most being in the 60 to 100
microvolt range at the antenna terminal.
The capacitively tunable circuit mentioned above, includes,
importantly, the tuning capacitor, which is controlled
by the.. etc... This tunable circuit is triple ganged,
supply three identical tuning capacitors to circuits that
make it possible for the front end to distinguish and
amplify the exact signal that has been selected, and then
convert the selected frequency to one that is fixed, regardless
of the initially selected frequency. This is called superheterodyning,
and the concept was developed by the inventor of generation,
Edwin Armstrong.
The only remaining important piece of information there
is about V201 is that it is grid-connected to the AGC
buss, which is a negative feedback that among other things,
curbs regeneration. AGC stands for automatic gain control.
The larger the wave that develops across the grid of the
6BN6, the more AGC suppression. It is a rectified signal
by the time it hits the grid of V201, and it can be switched
to partial suppression to make it possible to obtain optimal
stereo performance in poor signal areas. The best way
to understand the function of AGC is that it regulates
the amplitude of signals across the band, and it curbs
superregeneration at the grid of V201, the antenna circuit.
Reducing the AGC signal makes it possible to receive weak
signals with better retention of the sideband component
of the wave that is critical for both reception of weak
signals, and for stereo reception, which uses the upper
part of the frequency channel centered at 38 kHz. Full
AGC suppression can make a weak stereo signal mono, while
partial suppression will permit more upper band information
to be retrieved.
AGC is not AFC. This needs to be repeated often. All
tuners have AGC, second rate, drift-prone tuners can do
wonderfully well with AFC, or automatic frequency control.
After having tuned a station, the AFC switch will 'hold'
the frequency. It is actually possible to detune a signal
and have AFC hold the initial frequency. Switch it out
and the signal goes to interstation noise. Cute parlor
trick.
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V201 has done the sturdy job of supplying nonjudgmental amplification
to the antenna signal. The task of supplying a strong rf signal
falls to V202, a grounded-grid rf amplifier that is married
to the capacitively tuned circuit. V202 is cathode connected
to V201, but the circuit is decoupled, and on the cathode side
of V202 is found the regenerative components. The third capacitively
coupled part of the tunable circuit is the superheterodyne oscillator.
It is calibrated to produce a signal that, when mixed with the
output of V202, will always supply 10.7 MHz. The local oscillator
is very similar to the rf circuit. The only pentode used in
the front end mixes the rf and local oscillator signals and
this output feeds the first intermediate frequency stage, IF1,
which can be considered the terminal point of the front end.
The IF stage that follows contains two stages that closely
duplicate the function of IF1, and the Foster-Seeley limiter/detector.
IF1 differs from IF2 and IF3 in that it utilizes an r-c circuit
on the tail of its plate to supply the first stage of limiting
that is supplied by the circuit to terminate IF1 in a manner
that would permit it to be used exclusively in connection to
the limiter/detector, as a complete IF, in other words. The
4310 IF examined here shares its initial topology with all other
Scott tuners, and differs only in the bandwidth of the detector.
The story of the circuit is probably found in the patent for
the 6BN6, a gated beam detector tube that when used in conjunction
with a double-sized, balanced secondary detector, delivered
2 MHz bandwidth at a time when 500 kHz was considered to be
very good.
Scott first used this circuit in the 310. Although the circuit
is much the same as later 310 tuners, the format had yet to
evolve, and Scott placed the tubes facing downward. With the
addition of a detector tap, it can be used with a multiplex
adaptor. The 310B is supplied with a detector tap, along with
the 311, and all other Scotts with the exception of those models
with their own adapter. The first 310 seems largely inspired
by a 1948 Browning FM tuner that must have been considered the
reference of its time. It uses what looks to be a similar cascade
front end and the Foster-Seeley circuit. With a modernization
here and there, the 310 is born. The Browning sat its front
end on a silver plate, which Scott began to do, and had the
very best IF transformers available, which Scott never used.
Look at a Marantz 10B to get an idea of the fit and finish of
the Browning Drake.
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IF2, the start point of the IF stage, applies AGC to the first
grid of its 6AU6 tube, but through a significant value of resistor,
in this case 470k. That means the primary AGC function takes
place elsewhere. This small signal is a local feedback that
acts to suppress the signal that it gets from IF1, but it also
acts as a near-intimate tie with the grid signal of V201. Only
resistance stands between these circuit points, and it is a
far closer electrical connection than exists with the grid input
of the 6BN6, or V5, which is the unrectified, coupled output
from the secondary of the limiter.
I mentioned above how it would be possible to hotrod a front
end to the limiter and get something out of it. Although it
stands as the single most effective way to assess a front end
- you can safely jumper the connection to judge the result of
bypassing IF2 and IF3 - you wouldn't want to use the tuner in
this manner. On the transmission line that the IF stage is,
each transformer is one further opportunity to lock on to the
intermediate frequency and amplify it. The more stages, the
finer the result. That's why you pay more to get more.
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Series coupling along the B+ transmission line connects
the plates of the 6AU6 on a line that employs 220k resistors
in series along the line. A further plate resistor varies
the plate voltage that IF2-3, and the limiter see. Inductive
coupling wasn't employed by any tuner manufacturer at
the time. In fact, Scott introduced inductive coupling
in its most modest IF of all time, the board that went
into the 341 receiver in 1968. The early 312D
has them, as does the final model of that type.
Local time correction is applied to the second grid of
the 6AU6 in IF2-3. It is negative feedback that is supplied
from the bottom of the primary of the transformer that
is 'fed' by the plate of the tube.
The third, suppressor grid connects to yet another line,
this also connecting the bottom leg of the secondary of
the IF transformer. All of these latter components are
'hard wired' to ground. All of this description also applies
to the limiter circuit. It adds an r-c network to the
tail of its plate, and this signal feeds the AGC line
and also supplies a terminated signal to the detector.
The only other major difference is that the cathode supplies
of IF3 and the limiter have capacitance added in shunt
with the cathode resistor. The capacitor's stored charge
controls the amplitude of the cathode supply more evenly
across the frequency spectrum in exactly the same manner
as when the circuit is used in the cathode supply of an
audio circuit. It's important to remember that there is
no actual difference between rf and af, except for the
frequencies being dealt with. The entire description of
the function of a 310 tuner could also be well supplied
as an explanation for how an AM tuner works. That's because
up to this point, there isn't much difference between
the two. The 310 tuner is an amplitude tuner so far, not
at all concerned with the eventual way that the frequency
that is being amplified will be detected.
That's the next story. The detector. In the case of the
310 tuner, much of the magic is found in the Foster-Seeley
discriminator. This design made it possible to detect
a signal with triple the average bandwidth that a 'decent'
tuner might offer. Second-rate tuners could supply no
more than 500 kHz. The FCC was convinced that wideband
FM tuners would be the order of the day on April 19, 1961,
with Docket 13506. Death knell to the junk-makers. The
2 MHz that the Foster-Seeley circuit offered was the ultimate
for stereo reception.
The high composite output off the 310 tuner (and any
other) will employ a pair of diodes in opposition on each
of the high and low terminals of the detector transformer.
The output of these signals are accumulated through a
balancing network and it is this output, the discrimination
of the time differential between a normal wave and the
modulated wave that a signal is detected. The high frequency
carrier, 10.7 MHz, has been suppressed and the resultant
tiny signal is then available to be amplified and equalized
for monophonic sound, or it will be further worked on
by the next device in circuit under consideration. 310
owners can now attach their 335 multiplex adaptors.
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It is a major task to understand the 4310 multiplex adaptor
because it has a lot of extra plumbing associated with the multiple
diversity circuit. It permitted several diversity-capable tuners
to be hooked together. The tuner set to be the master unit would
then select from among all the other 4310's hooked up, which
supplied the cleanest stereo signal. That was the aim, so the
control interface was built across the adaptor, taking the output
from the bridge detectors and making some sort of analog-computer
judgment about which signal was best. Elaborate relays were
used to select the signal that would then feed the audio amplifier
and equalization circuits that when applied with intelligence,
produce flat frequency response. This is done by de-emphasis,
which is a way of saying tailored negative feedback, but it
is expressed as a measure of time. In the USA and Canada, the
standard is 75 microseconds, in Europe it is 50.
Multiplex began with the notion that the bandwidth allocated
to FM stations was not only sufficient for the low-fi background
music channels that were a design of the system and kicked in
around 60 kHz. That left a space for multiplex at the aforementioned
38 kHz. This value was chosen in concert with 19 kHz, a frequency
that would be used to supply a signal that stereo was being
transmitted. The 38 kHz oscillator is synchronized through a
simple doubling. In order for stereo to be received, the 19
kHz signal had to be present to produce an oscillation out of
the 38 kHz circuit that would instruct the time switching bridge
when to consider the composite signal to be left channel information,
or right.
Time division multiplex is based on very simple math: Consider
that FM is comprised of a monophonic sum signal that we hear
on a mono radio, and a difference signal that we hear only in
stereo. Time switching derives the signals as follows: (L+R)+(L-R)=2L,
and, (L+R)-(L-R)=2R. The entire purpose of the adaptor is to
produce the result of L and R as separate signals with adequate
provision against things like crosstalk. Too much crosstalk
and you have mono. The stereo signal is developed across a diode
bridge that is usually solid state, but in the case of the 4310
(and the 370), vacuum tube diodes were used instead. This signal
that the multiplex adaptor has been developing is based on the
high output of the detector. The low output always serves a
purpose, either as a meter drive, or in the age of stereo FM,
as a benchmark signal to mix with the composite output signals.
The low signal is injected into each channel to suppress the
common mode of the signal, at some expense of amplitude. This
signal is amplified and equalized and if the conditions are
right, and you don't walk in front of the antenna, you have
nice stereo.
ed.
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