This is only a preview of the February 2015 issue of Silicon Chip. You can view 36 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "6-Digit Retro Nixie Clock Mk.2, Pt.1":
Items relevant to "What’s In A Spark? – Measuring The Energy":
Items relevant to "Spark Energy Meter For Ignition Checks, Pt.1":
Purchase a printed copy of this issue for $10.00. |
Vintage Radio
By Ian Batty
Trapped in Germanium Valley: The
Philco T7 Transistor Portable Radio
. . . early to market & early to retire
Released in 1956, the T7 was Philco’s first portable
transistor radio. It used the company’s proprietary
SBT germanium transistors and compared
favourably with other sets of the era but was soon
overtaken by sets based on silicon transistors.
T
HE PHILADELPHIA Storage Battery Company was registered in
1906 and began releasing products under the Philco brand name in 1919. As
noted in my article on Philco’s Safari
portable TV set in the January 2014 issue, the company was an early adopter
of transistor technology, releasing their
proprietary Surface Barrier Transistors
(SBTs) in 1953.
At that time, alloy-junction transistors (such as the OC45) were restricted
to a maximum frequency of about
15MHz. The limiting factor was how
thin the base region could reliably be
made. As a result, Arthur Varela of
Philco reasoned that electrical etch94 Silicon Chip
ing would be more controllable than
the somewhat random process of
high-temperature alloying and so he
invented the surface-barrier transistor
(SBT) process.
In this, the base slice was held vertically and chemically etched away by
very fine sprays to form emitter and
base “wells” on opposite sides of the
slice. Then, electrochemical deposition “plated” the emitter and collector
regions onto the base slice, creating a
fully-working transistor. The actual
junctions worked just fine but with
somewhat lower barrier potentials
than for alloy-junction devices.
Surface-barrier transistors offered
high-frequency operation to at least
30MHz. Adding diffusion to the process
(MADT – microalloy diffused-base
transistor) pushed frequencies to some
200MHz.
Philco devices (and their licensed
equivalents from US Sprague and
English Semiconductors Ltd) are
easily recognised by their distinctive
TO24 and TO25 “bullet” cases. These
are shown in the photos on the facing
page.
Philco’s 1955 release of “the world’s
first all-transistor car radio” and their
“fully transistorised portable phonograph” (released the same year) should
have ensured that Philco remained a
major consumer electronics manufacturer. However, Philips, working in
parallel, were developing their alloydiffused technology, eventually yielding the landmark AF186 transistor.
This could be used as an RF amplifier
at frequencies up to 860MHz.
Outstanding as this was, germanium’s days were numbered with
the rapid development of high-performance silicon technologies: mesa
and planar.
With their ability to form an impervious surface layer of silicon dioxide
(glass), silicon devices allowed cheap
plastic encapsulation. Mass-production
lithography also allowed many tens of
devices to be made on one silicon die,
resulting in skyrocketing volumes and
nose-diving costs of production.
Philco missed this new silicon
technology wave. Surface-barrier and
other pre-lithographic technologies
suffered from “one at a time” production techniques and their associated
high costs of manufacture. The failure
to move to silicon meant that Philco’s
lead in manufacturing had faded by the
early 1960s to re-supply for existing
equipment. Their specialist solid-state
siliconchip.com.au
ground-based and aircraft computers,
along with ground station equipment
for the space exploration programs of
Project Mercury, could not save Philco.
The company eventually filed for
bankruptcy in 1961 and was bought
out by the Ford Motor Company.
Philco’s T7 radio
While I dislike overblown descriptions, I just have to use the word “stunning” for this design. Its stark tuning
dial with its arrowhead design just
stands out against the white cabinet.
And why not add some gilt trim to
complete the effect? Put it among any
number of contemporaries and even
the casual viewer’s eye will linger
over this one.
My only reservation is the tuningdial thumbwheel. It’s well-designed
but the red lettering doesn’t stand out
against the black background as well
as I’d like. Given the stark black-onwhite of the cabinet, I’d have used
white dial markings both for legibility
and for aesthetics. Nevertheless, the
T7 is an eye-catching piece of 1950s
industrial design, even when fitted
inside its leather case.
Internally, most of the parts are
mounted on a PCB and this is secured
to a metal chassis that also holds the
speaker. The chassis is secured by
screws to the inside of the plastic case,
while the PCB is secured by twisted
metal tabs. This means that the PCB is
best left in place unless removing it is
absolutely necessary, since there is a
risk of breaking off these tabs.
Design details
The “first” of anything always interests me. That’s because the engineers
have created a solution to a problem
that’s sometimes well-understood but
more often only vaguely perceived.
In this case, the obvious aim was to
make an all-transistor radio that could
be carried around and used anywhere.
But how many transistors should be
used in the design?
Rival company Regency, cutting
costs savagely, began with eight transistors and finished with a mere four
in their landmark TR-1 design (see
SILICON CHIP, April 2013). The result
was a handsome “coat-pocket” set that
performed well enough in quiet living
rooms in the city.
But at a football game or in the country? It was “a toy that didn’t come at a
toy price”, as one wag put it.
siliconchip.com.au
Fig.1: the Philco T7-124 schematic with suggested test points and voltages. The
changes made for the T7-126 are shown in brackets. The set is a 7-transistor
superhet design with TR1 functioning as the converter, TR2 & TR3 as IF
amplifier stages, TR4 as the detector and TR5-TR7 as an audio amplifier.
By contrast, industrial giant Raytheon, with a massive market presence in the industrial, military and
domestic arenas and a reputation to
uphold, went for a “picnic portable”.
Designated the 8-TP-1, it boasted eight
transistors and a performance that
equalled similar-sized valve portables.
Philco, eager to carve out market share, went one less. Their T7
transistor radio not only challenged
Raytheon’s “big set performance” but
also targeted the personal portable/
coat-pocket niche that was also being
viewed by compatriot Zenith and by
foreign start-up Sony.
Circuit details
Fig.1 shows the circuit details of
the Philco T7. It’s basically a 7-transistor design using PNP transistors
TR1-TR7 and a 455kHz IF strip. Note
that although this set uses PNP transistors, the battery supply is negative
to ground rather than the positive
to ground as in most contemporary
Australian sets.
The following description is for the
first “124” series, with the later “126”
series modifications noted on the circuit diagram. Note that many online
circuit copies do not show decimal
points clearly (R22 is a 3.3Ω resistor,
This close-up view shows the TO24
cases used for the converter & IF
transistors (TR1-TR4) in the Philco T7.
The audio-stage transistors (TR5-TR7)
used the larger TO25 cases.
while C10 is a 0.1µF capacitor).
TR1 functions as a fairly conventional combined mixer-oscillator stage
(ie, as a converter). Like Raytheon’s
8TR/7TR chassis, Philco applied AGC
to the converter stage to give the
most effective gain control possible.
However, in Raytheon’s set, the AGC
controlled only the mixer stage and so
there was no drift from the separate
local oscillator.
So how did Philco fix this problem
February 2015 95
that although Philco’s surface-barrier
transistors work somewhat differently from the more-familiar OC44/45
alloy-junction types, their high collector-base capacitances still require
neutralisation in both IF stages. They
also have lower base-emitter voltages than the OC44/45 types.
AGC circuitry
Virtually all the parts in the Philco T7 are mounted on a single PCB. Note that
the first IF transformer (Z1) actually has its windings in two separate cans,
while the second and third IF transformers are each in a single can. The unit
just needed alignment adjustments to get it going again.
The PCB is mounted behind a chassis plate which also carries the loudspeaker
and the volume control at bottom right. The dial fits over the tuning gang shaft
at centre right and features red markings on a black background.
with just one transistor in the converter stage?
Easy – use a diode attenuator (D1)
in the antenna circuit. This technique
was similar to that employed in some
later sets which had an attenuator
diode in the primary of the first IF
transformer. Philco’s circuit had the
advantage of reducing signals before
the mixer, effectively preventing overload on strong of signals. The converter
itself operates with fixed bias, as do
almost all single-transistor designs.
IF stages
Two IF amplifier stages (TR2 & TR3)
follow, with conventional transformer
coupling. The first IF (Z1) has tuned
(and tapped) primary and secondary
96 Silicon Chip
windings. In reality, my set has two
separate cans for these windings,
with the associated 1.7pF capacitor
providing top coupling between the
two tuned circuits.
As with the Pye Jetliner, capacitive
coupling is an effective (if unusual)
means of coupling two single-tuned
transformers. This gives more compact
IF transformers and eliminates the
need to turn the set over to adjust a
“bottom” ferrite core.
The second and third stages (Z2 &
Z3) use tuned primaries and un-tuned
low-impedance secondaries.
In common with most other designs,
AGC is applied to the base of the first IF
amplifier (TR2) while the second stage
runs at full gain with fixed bias. Note
The demodulator (or detector), like
that in the Raytheon 8RT1 chassis,
consists of a transistor (TR4) operating
just at cut-off in class-B (R12 & R13 set
the bias). Compared to a diode demodulator, this class-B version provides
some audio gain plus DC amplification
for the AGC circuit.
As shown, TR4’s collector current
passes through R16 and the primary of
audio interstage transformer T2, with
the resulting audio signal then fed to
T2’s secondary. Bypass capacitor C12
filters the audio component across
R24, leaving only the DC component
to derive an AGC voltage (a simplified
version of this circuit, with positive
earthing, is shown in Fig.2).
This AGC voltage is applied directly
to the anode of AGC diode D1. Its
cathode is fixed at -0.96V by a voltage
divider based on resistors R3 & R2, so
that it is “just out” of conduction with
no applied AGC.
In operation, the stronger the received RF signal, the greater TR4’s
collector current and the higher the
AGC voltage across R24. This pulls
the AGC voltage towards the positive
supply rail, thereby making D1’s anode
positive with respect to its cathode.
As a result, D1 begins conducting and
“damps” the signal at the converter
base, thus reducing the signal reaching
the converter.
Audio circuitry
The audio signal from transformer
T2 is fed to a conventional 2-stage
transistor power amplifier (TR5-TR7).
TR5 operates as a class-A driver stage
and this drives the primary of audio
transformer T3 which acts as a phase
splitter. T3’s centre-tapped secondary
output then drives transistors TR6 &
TR7 which are configured as a class-B
push-pull output stage.
One neat design trick pulled by
Philco is that TR5 is biased by the voltage drop across R21 which also serves
as the decoupling resistor for the lowpower stages. Talk about squeezing the
last drop of juice out of a component!
siliconchip.com.au
Silicon Chip
Binders
REAL
VALUE
AT
$14.95
*
PLUS P
&P
The T7 radio was protected by an attractive leather case with a carrying strap.
It’s necessary to open the front flap in order to tune the radio and adjust the
volume control.
Volume control is achieved by rheostat-connected potentiometer R23.
This is connected directly across transformer T2’s secondary so that it acts
as a variable shunt. It’s less elegant
than a true potentiometer but effective
nonetheless.
The output stage operates with fixed
bias, as set by divider resistors R18
and R20. In addition, there’s a small
amount of local negative feedback via
shared emitter resistor R19. This resistor also helps balance any differences
in gain that might exist between output
transistors TR6 & TR7.
The Philco T7 also uses feedback
for the audio driver/output stages.
That’s done using feedback resistor
R17 which couples a small signal from
one of the speaker terminals back to
the emitter of driver transistor TR5.
This also implies an audio amplifier
voltage gain of about 40 (the ratio of
R17 to R22). Finally, the audio from
the output transformer (T4) is fed to
a 15Ω 2.5-inch (64mm) loudspeaker
(LS1) via a headphone jack.
Getting it going
Cosmetically, my T7 set was in tiptop condition when I acquired it. But
electrically? – it was very quiet and
that’s always a worry with any set that
has five or more transistors.
Well, a quick tweak of the IFs
couldn’t possibly hurt, could it? At
this point, you may be starting to
cringe. Some collectors are firmly
of the view that if a set has been left
alone, you shouldn’t expect “demon
drift” in the IFs to have degraded the
set’s performance. It’s often a wise to
leave these settings alone, especially
with complex equipment such as FM
radios and (especially) TV sets.
But I was rewarded by tweaking the
Are your copies of SILICON
CHIP getting damaged
or dog-eared just lying
around in a cupboard or
on a shelf? Can you quickly find a particular issue
that you need to refer to?
Keep your copies safe,
secure and always
available with these
handy binders
These binders will protect your
copies of S ILICON CHIP. They
feature heavy-board covers & are
made from a dis
tinctive 2-tone
green vinyl. They hold 12 issues &
will look great on your bookshelf.
H 80mm internal width
H SILICON CHIP logo printed in
gold-coloured lettering on spine
& cover
Silicon Chip Publications
PO Box 139
Collaroy Beach 2097
Fig.2: this diagram shows a simplified version of the AGC circuit used in
the Philco T7. The AGC voltage is derived from the primary of the audio
interstage transformer and is applied directly to the anode of AGC diode
D1 and to the base of the first IF amplifier (TR2) via a 4.7kΩ resistor. D1’s
cathode is fixed at -0.96V by a voltage divider based on resistors R3 & R2,
so that it is “just out” of conduction with no applied AGC.
siliconchip.com.au
Order online from www.
siliconchip.com.au/Shop/4
or call (02) 9939 3295 and
quote your credit card number or mail the handy order
form in this issue. *See
website for overseas prices.
February 2015 97
Fig.3: the SBT manufacturing process. The base slice was held vertically
and chemically etched away by very fine sprays to form emitter and base
“wells” on opposite sides of the slice. Electrochemical deposition was
then used to “plate” the emitter and collector regions onto the base slice,
creating a fully-working transistor.
IFs on this simple set. Starting with
barely any reception, each adjustment
of the IF coils brought in more and
more signal. The set then really came
alive when I adjusted the oscillator
coil slug (T1) and the two associated
trimmers.
A light spray of contact cleaner on
the volume control and this set was
done and dusted. In my opinion, it
was now working just as well as it was
when it has handed over to its original
owner, some 57 years ago.
By the way, the original T7-124
was introduced in 1956 for the 1957
model year. By contrast, the one I
have is a slightly later T7-126 model.
The Philco-manufactured tuning gang
bears a stamping with a “748” code,
implying the 48th week of 1957 for
this set.
How good is it?
So how good is it compared to the
7-transistor Raytheon T2500 (SILICON
CHIP, June 2013)? Well, leaving aside
the T2500’s higher audio output and
better overall response (two speakers
and a larger cabinet), it’s a good match.
The Philco T7 manages a sensitivity of around 220µV/m at 600kHz and
170µV/m at 1400kHz. Its 1400kHz
performance is a bit noisy at this level
An unusual hour-glass shaped tuning
dial is a feature of the T7. The tuning
thumbwheel has red lettering on a
black background but white lettering
would have been easier to read.
though, with a signal-to-noise ratio of
about 17dB. A 20dB signal-to-noise
ratio requires an RF signal level of
about 200µV/m.
These figures are pretty similar
to Raytheon’s T2500. However, the
Raytheon set makes up for its single
IF amplifier stage with an extra (third)
audio stage, making it a bit noisy at
minimum volume. Philco’s approach
of two IF amplifier stages and only
two audio stages pays off and the T7
is quiet at minimum volume.
The selectivity is around ±19kHz at
-60dB while the AGC is excellent, with
References
(1) Thanks to Ernst Erb for his Radio Museum site at: www.radiomuseum.org This site
has service data for two T7 models (the 124 & the 126) and for the T7X (model 128).
(2) Surface-barrier transistors are described at:
http://www.rfcafe.com/references/radio-news/amazing-surface-barrier-transistoraugust-1957-radio-tv-news.htm
http://en.wikipedia.org/wiki/Surface-barrier_transistor
http://www.google.com/patents/US2843809
98 Silicon Chip
only a 6dB increase in audio output
over the range from 200µV/m to around
40mV/m (the effective AGC range is
some 46dB). It does go into overload
soon after but 40mV/m is indeed a very
strong signal. The extensive AGC range
justified the “diode attenuator” approach and foreshadowed the Mullard
implementation by some four years.
Audio performance is a mixed bag,
with the set going into clipping at an
output of 80mW. Given the low supply
rail of only 3V and the fact that many
6V sets only manage around 250mW,
this is still quite respectable.
The audio response (starting at
400Hz) between the volume control
and the speaker terminals rises by about
2dB at 3kHz and then remains fairly
flat until dropping by -3dB below the
400Hz reference at 37kHz (some highfrequency roll-off would have been
nice). The low-frequency response following the volume control goes down
to around 110Hz. Unfortunately, it only
manages about 500-1800Hz from the
antenna to the speaker.
The THD (total harmonic distortion)
is reasonably low, the figures being
2.2% at 10mW, 3% at 50mW and
8% at 100mW. With a “flat battery”
supply of 1.5V, an oscilloscope trace
shows visible crossover distortion at
10mW output and the THD increases
to around 5%. The set manages a maximum output of around 20mW and 9%
THD with the low battery.
Would I buy another?
So would I buy another one? Well, it
is tempting – one for the lounge room
and one for the workshop display
shelf. As I write this, there’s one being
advertised on-line for just under $300
but I really do need to stop somewhere.
Different version
The original T7-124 and T17-126 are
quite similar, as Fig.1 shows. Note that
I’ve omitted the link connections (L1L9) that connect to the PCB for clarity.
So what are the differences? First,
AGC diode D1 and interstage transformer T2 have been removed from the
T7X (model 128). Output transformer
T4 has also been removed. Instead, the
TX7 employs a single-ended push-pull
“output-transformerless” circuit that
couples directly to the loudspeaker,
with the return connection going to a
1.5V tap on the 3V battery. This output
design will be described in an upcomSC
ing article on Philips portables.
siliconchip.com.au
|