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Vintage Radio
By Ian Batty
Stromberg-Carlson’s
78T11/79T11 transistor set
radio manufacture, though at a much
slower rate. They also produced telephones and telephone switchboards
for the Australian Army.
With the advent of television in the
mid-1950s, Stromberg-Carlson also
tried to establish itself in that market
but failed to make inroads. The 19581959 78T11/79T11 transistor radio
sets described here were among their
last Australian products.
Main features
The 78T11 was Stromberg-Carlson’s first
Australian-made transistor set. It was a 7transistor design built onto a metal chassis
with point-to-point wiring and it offered
excellent performance.
S
TROMBERG-CARLSON’S US parent commenced operation in 1894,
when Alexander Graham Bell’s patent
for the telephone expired. At the time,
Stromberg and Carlson worked for the
Bell Telephone Company (later AT&T)
and they each invested $500 to begin
manufacturing equipment, primarily
subscriber sets (“home” and business
telephones) for sale to independent
companies. Their home base was in
Chicago and Stromberg-Carlson quickly established a reputation for reliable
equipment and stable prices.
siliconchip.com.au
Stromberg-Carlson Australia bore
little resemblance to its American parent. The company began by importing
receivers from the United States in
1927, before commencing local manufacture in 1928. Their radios mostly
used local components.
Stromberg-Carlson made components both for their own radio
receivers and for sets made by other
companies. Their brands included
Strom
b erg-Carlson, Audiola and
Crosley. Between 1939 and 1945
Stromberg-Carlson continued with
The 78T11/79T11s are both large
sets and fall into a category that I think
of as “picnic portables”. The 78T11
was Stromberg-Carlson’s first transistor set and was released in 1958, a year
after Australia’s very first transistor
radio, AWA’s model 891. In terms of
styling, the 78T11 resembles both
Sony’s early TR72 and Raytheon’s 8TP
which also had top-mounted controls.
Unlike the Sony’s simple dial, the
78T11 offers a slow-motion tuning
drive, albeit using a rather “agricultural” spindle that (when it works)
drives the tuning knob’s rim via a
rubber grommet.
The back of the case flips open to
reveal the circuitry. Like many sets of
its day, it uses a pressed-and-stamped
steel chassis, with the low-power transistors mounted through the chassis
in rubber grommets. By contrast, the
two output transistors are mounted
in heatsink flags which are screwed
to the chassis.
The various connections are made
using a combination of tagstrips and
point-to-point wiring. The components used were something of a mixed
bag – the IF coils are the slim rectangular Philips types, the capacitors are
a mix of UCC and Philips electrolytics
and the bypasses are AEE “microcaps”,
mostly the brown variety. The lowpower transistors are all in the familiar
black-painted “bullet” outline, so it’s
safe to assume they’re from Philips.
The same goes for the demodulator
diode (D1).
A large tuning-gang with identiJuly 2015 83
Fig.1: the circuit is a fairly-conventional 7-transistor superhet design. TR1 is
the converter stage, TR2 & TR3 are IF amplifiers, D1 is the demodulator and
TR4-TR7 the audio amplifier stage.
cal sections is mounted at one end
of the chassis, adjacent to a 5-inch
Rola loudspeaker. As with the audio transformers used in the set, it’s
about the same size as those used in
various valve portables of the era. In
fact, judging by the parts used, it appears that the application circuits in
Philips’ “Miniwatt” handbook of 1957
were used as a guide by StrombergCarlson’s designers.
The bias adjustment for the output
stage is a real oddity. It’s a slider-type
wirewound resistor with a 10W rating
and I suspect that the principal criterion for its use was availability rather
than its power rating.
A date stamp on the audio driver
transformer (9 May 1958) places this
particular set near the beginning of the
production run.
Circuit description
The “Transistor Seven”, as the set
was called, was issued in two versions: the 78T11 portable and the very
similar 79T11 with switching for an
external car radio aerial. This article
describes the 78T11 and any component differences between the two are
noted in the text.
Both the 78T11 and 79T11 use
OC-series transistors throughout,
beginning with an OC44 converter
(TR1) – see Fig.1. This converter uses
collector-emitter feedback to give
minimal local oscillator radiation.
A 440pF padder capacitor (490pF in
84 Silicon Chip
the 79T11) across one section of the
gang sets the local oscillator range to
the standard 990-2060kHz range for
broadcast-band reception.
Since the OC44 is configured as
a self-excited converter, no AGC is
applied. The output from the converter feeds the 1st IF transformer
(L3) via a double-tuned IF transformer
with tapped primary and secondary
windings.
The 78T11 has a permanentlyconnected aerial socket which goes
directly to the base of TR1. By contrast,
the 79T11, which is purpose-built as a
car/portable set, has an antenna change
over switch. This selects either a fullymatched antenna coil that’s coupled
to a car radio antenna (for car use) or
an internal ferrite rod for portable use.
Each antenna coil (car and portable)
has its own trimmer. The car antenna
coil uses capacitive and inductive coupling to give maximum signal pick-up,
Fig.2: a changeover switch in the
79T11 enabled it to select between
its internal antenna & an external
car radio antenna. This circuit
replaced the shaded area in Fig.1.
necessary because of the short antenna
that car sets usually connect to.
The first IF amplifier’s OC45 (TR2)
has AGC applied via its base bias network. In addition, neutralisation is ap-
The controls for the Stromberg-Carlson 78T11/79T11 are mounted on the top
of the case, with the volume control at left and the tuning control at right. The
tuning wheel’s rim is driven via a rubber grommet attached to a small knob.
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plied from a tap on the primary of the
second IF transformer (the 79T11 takes
its neutralisation from the second IF
transformer’s secondary). Even though
the 78T11’s first IF has a double-tuned
primary and secondary, the first IF
transistor (TR2) gets its signal from a
tertiary winding on this IF transformer
(L3). By contrast, the 79T11 uses the
more conventional tapping on the first
IF’s secondary.
As shown on Fig.1, the 78T11’s
second IF transformer (L4) is doubletuned, with tapped primary and
secondary windings. Once again, the
79T11 uses a different arrangement –
its second IF uses a tuned and tapped
primary, while its secondary is untuned and untapped.
IF amplifier TR3 operates with fixed
bias and has neutralisation applied
from a tap on the third IF transformer’s
primary (the 79T11 takes its neutralisation from the third IF’s secondary).
This third IF transformer (L5) is
double-tuned and has tapped primary
and secondary windings. The 79T11
differs yet again in that its secondary
is un-tuned and untapped.
Diode D1 demodulates the IF signal
and also supplies the AGC. As shown,
the AGC line feeds back to TR2’s base
via R6 and the tertiary winding in L3.
TR2’s bias is set by R12 & R13 and
this also applies a small forward bias
voltage to D1, thereby increasing its
sensitivity.
The demodulated output from D1
is positive-going and the AGC action
results in strong signals reducing the
bias on TR2. This in turn reduces its
gain and keeps the audio output fairly
constant with varying signal strengths.
The circuit is built on a metal chassis, with tagstrips and point-to-point wiring.
Transistors TR1-TR5 are mounted through the chassis in rubber grommets,
while output transistors TR6 & TR7 are secured in place using flag heatsinks.
One unusual design aspect is that
the output stage transistors (TR6 &
TR7) have no emitter resistor(s). It’s
more common to see either two lowvalue emitter resistors of about 10Ω
or a single shared emitter resistor of
similar value. These normally help
reduce output stage distortion and
provide some extra temperature compensation but have been omitted from
this design.
Finally, it’s worth noting that
subsequent releases (designated the
78T12, 70T11 and Wayfarer) used
alloy-diffused OC170/169s in the RF/
IF stages and OC74s in the output. In
addition, the Wayfarer featured an in-
Audio output stage
The audio section is a conventional
4-transistor design based on preamplifier stage TR4, driver stage TR5 and
Class B push-pull output stage TR6 &
TR7. As shown, the drive from TR5 is
coupled to the output stage via centretapped transformer L6 which acts as
a phase splitter. The push-pull output
stage then drives the loudspeaker via
centre-tapped transformer L7.
Feedback is applied from the
speaker back to the base of driver stage
TR5 via R26 & C26 in parallel. In common with other Australian designs,
the output stage bias is adjustable,
in this case via trimpot RV2, and is
temperature-compensated using R24,
a 130Ω thermistor.
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built Hoffman solar battery and could
also be slipped into a cradle for use as
a car radio.
Restoration
The first job in restoring the set involved a good clean-up. As with other
sets of the era, the Stromberg 78T11
uses a leatherette case and its heavy
texture had me reaching for soap and
a toothbrush.
After some patient effort, it cleaned
up quite nicely. The “Transistor 7”
badge on the top of the case had corroded at the edges but was left in place.
This set is nearly 60 years old, after all.
The knobs all had small edge cracks
around their skirts but rather than use
a windscreen repair kit to make the
cracks invisible, they were again left
as they were; it’s all part of the set’s
patina and commensurate with its age.
Testing
The electrolytic capacitors were all
replaced, as was transistor TR2 which
had excessive collector-base leakage.
Having cleaned the set up, it was
time to see if it worked. It’s always
a good idea to increase the voltage
slowly while monitoring the current
when testing transistor sets, just as
it is with valve sets. Admittedly,
transistor sets are less likely to have
disastrously leaky electrolytics (the
reason for caution in valve sets) but
it’s possible for output stage faults to
cause massive current flow, resulting
in further damage.
In this set, increasing the supply
voltage slowly up to 6V resulted in a
current drain of around 10mA. AnyJuly 2015 85
This view inside the case shows the top of the chassis. Note the large ferrite-rod antenna, the valve-era tuning gang and
the 5-inch (127mm) loudspeaker. All the parts are readily accessible and only the electrolytic capacitors and a single
transistor required replacement. A few alignment adjustments then restored the set to full working order.
where from around 5-15mA is pretty
normal, so this indicated that the output stage was probably OK. However,
there was no sound from the set apart
from a brief “click” when the power
supply was connected.
It was time for some troubleshooting. First, I injected a 455kHz signal
from a signal generator into the aerial
coil but there was still no audio output.
I then cranked the signal generator
up to some tens of millivolts (mV)
and this time got a barely audible,
distorted tone.
This indicated that the front end
Removing The Knobs
As with the Bush TR82’s tuning
knob, the 78T11’s tuning and volume
knobs must be removed carefully. In
this case, I was able to remove the
knobs by applying steady finger pressure but you may prefer to use several
lengths of string under the knobs to
spread the load. The Vintage Radio
column in the September 2013 issue
shows the method.
Metallic levers (such as screwdrivers) are a recipe for disaster. Don’t
even think of using them.
86 Silicon Chip
could be OK, so I tried injecting an
audio signal into the volume control
(VR1). I found that I needed to feed in
over 100mV to get anything through
the audio stage and it was the same
when I fed the signal directly to TR4’s
base.
Replacing coupling capacitors C20 &
C22, along with new bypass capacitors
for C21 & C24, solved the problem, with
the required signal level for an audible output now reduced to just 5mV.
What’s more I could now receive ABC
Melbourne (774kHz) and Radio National (621kHz) at reasonable volume.
Injecting 455kHz into the aerial
terminal then allowed me to tweak up
the IF strip. I also adjusted the oscillator coil for maximum sensitivity and
this resulted in a sensitivity of just a
few microvolts at the aerial terminal.
Unfortunately, when I cranked up
the signal, the output first increased
but then flattened off and decreased!
I checked TR2’s emitter voltage and
found that it fell from around 0.8V to
only about 0.6V at full signal, whereas
it should have fallen to almost 0V due
to AGC action.
The culprit turned out to be excessive collector-base leakage in TR2. This
was acting as an internal bias circuit,
preventing the AGC circuit from correctly reducing the bias for strong signals. Replacing TR2 fixed that problem
and both TR1 & TR3 were also checked
to make sure they were OK.
Leakage is a known problem with
germanium transistors. A transistor
may work just fine in some circuits
but can cause problems in low-level
gain-controlled stages and output
stages. Alternatively, they can fail catastrophically due to excessive leakage
current. If you work on old equipment
(especially using germanium devices),
a leakage tester is vital to check that
the transistors are OK.
Capacitor replacement
Some (if not most) restorers regard
all old capacitors as suspect – paper
types will be probably leaky, while
electrolytics may also be leaky and/or
of low value. In their view, a complete
“recap” eliminates the possibility of
faulty capacitors and makes restoration more straightforward.
I generally prefer to take a more conservative approach but given that I’d
found all four audio-stage electrolytics
to be faulty, I went ahead and replaced
the remaining electrolytics as well.
This set also had an annoying lowsiliconchip.com.au
How Far Do You Go With Restoration?
Old valve radios present many wellknown problems for restorers. These
include leaky or shorted capacitors, high
or open-circuit resistors, dead or lowemission valves, open-circuit transformer
windings, battery corrosion and noisy
volume control pots. My own experience
with all kinds of radios shows that while
a set may appear to “work”, a thorough
examination often reveals defects that
detract from its intended performance.
Now add a novel type of deterioration for early transistor sets: leakage
in (mostly) germanium transistors and
capacitors that allow a set to work “pretty
well” but not up to its original specifica-
tion. Both the 78T11 and the Pye Jetliner
that I recently restored suffered AGC
faults due to leakage (in a transistor and
a capacitor, respectively).
Often, a restorer won’t bother to troubleshoot further if it works OK on local
stations. Indeed, it’s up to the individual
to decide just how far to go in the restoration process and whether they want the
set to perform to its maximum potential.
Some things to consider include: nostation current drain, distortion and
current drain at full output, sensitivity,
freedom from oscillation (or “howling”),
the AGC action and the audio frequency
response.
level “wip-wip-wip” oscillation on all
volume settings. An oscilloscope check
showed a trace much like the parasitic
oscillation that’s sometimes seen in
high-gain audio and HF/VHF RF power
amplifiers. The culprit was C17, the
main audio bypass capacitor. A faulty
AGC bypass capacitor (C9 in this set)
can cause audio oscillation. It certainly
did on the TR-1 set that I restored (see
SILICON CHIP, September 2012).
pressively, with a frequency response
from the volume pot onwards of
about 45Hz to 7kHz (-3dB points). By
contrast, the response from the aerial
terminal to the output is about 40Hz
to 2kHz. The distortion (THD) was
well-controlled: 1.7% at 10mW, 3.5%
at 50mW and 5.2% at the onset of clipping (160mW). At full output (about
200mW), the THD rises to some 13%.
Performance
The supply voltage for the set is
nominally 6V (4 x 1.5V cells). When
the supply is down to just 3V, the
maximum output is around 40mW for
a THD of 5%, falling to about 2.6%
at 10mW.
All in all, the Stromberg-Carlson
78T11 is a solid performer and is an
important example of early Australian
transistor radio design. If you have
one, get it out and restore it to full
working order.
Describing a set as being “very good
for its age” can be a cheap shot but this
set really is a good performer. In fact,
it matches the excellent Philips 198 –
it’s pretty much the same design but
with better audio response according
to my test results.
Getting down to actual figures,
at maximum gain, it needed field
strengths of 30µV/m and 35µV/m for
50mW output at 600kHz and 1400kHz
respectively – but with corresponding
signal-to-noise (S/N) ratios of just 7dB
and 5dB.
For a 20dB S/N ratio, the sensitivity
at 600kHz is about 100µV/m and at
1400kHz about 150µV/m. This set’s
AGC action has a very early onset, so
delayed AGC would have given an
even better figure than my test results.
As for selectivity, this measured
±1.5kHz at -3dB and ±11.5kHz at
-60dB. The AGC held the output to a
6dB increase for a signal increase of
34dB and the set needed some 40mV/m
in order to go into overload.
Distortion measurements
The audio stage also performs imsiliconchip.com.au
Supply voltage
Further Reading
For schematics, see Kevin Chant’s
website:
www.kevinchant.com/uploads/7/1/
0/8/7108231/78t11.pdf
www.kevinchant.com/uploads/7/1/
0/8/7108231/79t11.pdf
For Stromberg-Carlson’s Australian
history:
www.radiomuseum.org/dsp_
hersteller_detail.cfm?company_
id=7578
Many references also exist for the
US parent. Among them, see:
www.radiomuseum.org/dsp_
hersteller_detail.cfm?company_
SC
id=751
Ultra-LD Mk.4 Power Amplifier
Preview . . . continued from p82
negative output excursions via the
other pair of output transistors are
shown in blue and cyan.
During positive output excursions,
current flows from the positive supply
input connector to Q10 and Q11 (the
NPN output transistors) and then to the
output filter (L1, etc) and the positive
speaker lead, via paths that overlap almost completely. Return current from
the black speaker lead to the power
supply ground connection completes
the loop. The part of the loop where
the current paths diverge is the section
around the RLC output filter and this
is difficult to avoid.
The negative path through Q12 and
Q13 is shorter but otherwise similar;
again, the only real loop area is through
the output filter. In fact, since the positive and negative paths converge at
the top end of L1, the current in this
section of the loop is not half-wave
rectified (ie, it is effectively just the
output current) and so it’s far less of an
issue in terms of radiation and distortion as it lacks the sharp transitions of
the Class-B current.
Note that the 0.1Ω emitter resistors
for the power transistors are 3W SMD
types mounted directly under the respective positive and negative supply
fuses. Besides being far more compact
than the previously specified 5W wirewound resistors, the SMD types are
non-inductive and their positioning
gives much better field cancellation.
L1 will generate its own magnetic
field due to this current flow and this
is why its winding direction and the
number of turns are quite critical; if
orientated correctly, the field generated by the output current flowing
through L1 will at least partially cancel
with the field generated by current
flowing through the loop formed by the
PCB tracks that was explained above.
This does not consider current supplied to the output from any of the
on-board bypass capacitors, however
their paths have been designed to be
relatively tight loops as well.
Acknowledgement
Thanks to reader Alan Wilson for
suggesting many of the part substitutions that we are using in the new
design and prompting us to investigate
some of the other changes we were
considering for our next amplifier. SC
July 2015 87
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