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Vintage Radio
By Ian Batty
The Philips model 198
transistor radio
Philips’ first Australian-made transistor set
Housed in an attractive leatherette case,
the model 198 was Philips’ first Australianmade transistor set. It was a 7-transistor
design and both it and the later model 199
offered excellent performance.
B
EGINNING IN Eindhoven in 1891
and founded by Gerard Philips and
his father Frederik, Philips became
one of the world’s largest technology
companies but today it concentrates on
lighting and healthcare. The company
began manufacturing in Australia in
1931 but produced only two models
before temporarily halting production
and then resuming in 1934.
Philips then quickly grew to become
one of Australia’s largest electronics
manufacturers, with radios sold under
84 Silicon Chip
the Briton, Mullard and Fleetwood
brand names. TV receiver production
subsequently started in 1956 and
continued until the 1980s. In addition, Philips manufactured valves, TV
picture tubes and transistors, including the famous OC44/45 and OC70/
71/72/74 series that many of us bought
to build our first transistor sets.
Design highlights
Described in Vintage Radio for April
2015, Australia’s first transistor radio,
the AWA 897P, was released in 1957.
This was followed just a year later by
Philips with their model 198. Like the
897P, this was another 7-transistor design and the case used by Philips was
modelled on a previous valve version,
the compact AC/battery model 196.
AWA’s engineers used three audio
stages in the 897P, based on four
transistors and three transformers. By
contrast, Philips opted for a design
that was to become standard, with
just three transistors and two transformers used for the audio amplifier.
Like AWA, Philips paid attention to
Australian conditions, by employing
a thermistor-stabilised output stage.
They also added adjustable output
stage bias, as described below.
The Philips 198 was more compact than the AWA 897P and it looks
somewhat like a small cosmetics case.
But don’t let its “domestic” appearance fool you – it really is a very good
radio. The accompanying photos show
the set’s controls which are, from left
to right: Volume, Off, Treble/On and
Bass/On along the top and, on the
front panel, a tuning control with
integral dial.
Philips 198 chassis details
Like Bush’s TR82C and AWA’s 897P,
the Philips 198 uses a pressed-andpunched metal chassis. Its successor
(the 199) is similar but with sufficient
differences to warrant a separate circuit diagram (the major differences are
noted in the text).
The 198 has five of its transistors
installed in chassis-mounted rubber
grommets, with the leads wired to adjacent solder tags. By contrast, the two
output transistors are held in heatsink
clips which are screw-mounted on the
underside of the chassis.
Unlike the AWA set, the chassis sits
horizontally inside the case, allowing
some access to the underside where
most of the components are mounted
on tagstrips. The IF transformers and
the LO coil, however, are mounted versiliconchip.com.au
Fig.1: the circuit details of the Philips 198. TR1 is the converter stage, while TR2 & TR3 are the IF amplifier stages. D2
is the detector and this feeds buffer stage TR4 which in turn drives an audio amplifier based on TR5-TR7.
tically, so that the chassis still needs
to be removed for any detailed work,
including alignment.
The case itself is made from leatherette-covered “composite” material
(cardboard), while the front dial at
top-right turns easily with a direct
drive. To the left of the dial is a large
speaker grille. Power is controlled by
pushbutton switches, so the volume
pot only controls the volume. Because
it doesn’t also function as an on/off
switch, the volume pot doesn’t have
to be turned down to or up from zero
each time the set is turned off or on,
thereby extending the pot’s life.
Circuit details
Fig.1 shows the circuit details of the
Philips 198. Transistor TR1 is the converter stage and this operates as an autodyne oscillator with collector-emitter
feedback. AGC is not applied to this
stage, so TR1 operates with fixed bias.
As shown, TR1’s bottom divider resistor (R2) and its bypass (C4) are connected between the “cold” (bottom)
end of the ferrite rod’s two windings
(tuned and base) and ground. This
means that the bottom of the antenna
windings are at base bias voltage,
thereby providing a handy test point.
By contrast, the later 199 model connects both antenna windings directly
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to ground, with the bottom of the base
bias circuit going to the top of the base
winding.
The base-emitter voltage is only
50mV, since converters must operate
close to Class B conditions to give the
non-linear “modulating” effect needed
for frequency conversion.
Because the tuning gang uses identical sections, padder capacitor C9 is
included to modify the capacitance
range of the oscillator section so that
the local oscillator tunes from about
990-2060kHz. The only unusual feature is that the oscillator coil’s secondary is held at the converter’s collector
voltage. While this eliminates any potential difference between primary and
secondary, it’s not common practice.
Converter TR1 feeds the first IF
stage (TR2) via IF transformer L3/L4
which has tuned and tapped primary
and secondary windings. TR2 is gaincontrolled by the AGC system and due
to the high feedback capacitance of
alloyed-junction transistors, this stage
is neutralised by feedback via C13 from
the second IF transformer’s untuned
(and untapped) secondary. By contrast,
in the 199 model, this feedback is derived from an overwind on the second
IF transformer’s primary. In addition,
the second IF transformer has a tuned
and tapped secondary.
The second IF stage is based on
TR3 and runs with fixed bias. It’s also
neutralised, via C19, and both the 198
and 199 models derive feedback from
the third IF transformer’s secondary.
The third IF transformer uses a
tuned, tapped primary and a lowimpedance, untuned secondary to feed
demodulator diode D2.
AGC circuit
Depending on the strength of the
incoming RF (and IF) signal, diode D2
provides a negative DC output to the
base of TR4, the first audio/AGC stage
transistor which is connected as an
emitter follower to buffer the detector.
Stronger IF signals will therefore
cause TR4’s emitter current to increase
but it does not amplify the resultant audio, merely passing it to the following
volume control potentiometer (R20).
However, the DC signal from TR4’s
emitter is then filtered by capacitor
C14, so that the resultant DC voltage is more or less proportional to
the IF signal strength. This DC voltage is applied to the emitter of TR2
and if this voltage increases, TR2’s
gain will tend to be reduced. As a
result, changes in signal strength
are counteracted and the set’s output
remains substantially constant for varying signal strengths.
June 2015 85
The model 198 is built on a metal chassis, with point-to-point wiring. Five of its transistors are installed in chassismounted rubber grommets while the two output transistors are held in heatsink clips which are screw-mounted on
the underside.
As well as AGC, the set also includes
an “overload diode”, better described
as an “AGC extender”. Simply controlling one stage (such as TR2) only
provides a limited range of control. In
this set, the stage gain is some 30dB
and thus simple AGC can only counteract about this range of input signal.
After that, the audio output begins to
rise noticeably or the second IF stage
goes into overload.
To prevent this, auxiliary AGC diode
D1 is connected between the primary
of the first IF transformer (L3) and the
collector supply to TR2 at the junction
of R7 and C16. This latter junction sits
at about -5.4V DC but is effectively at
signal ground due to C15. By contrast,
D1’s anode at L3’s primary sits at about
-6.2V DC and is at IF signal level. It’s
also connected (via L5) to converter
TR1’s collector.
With no signal, D1 has around 0.8V
of reverse bias. As TR2’s collector current falls with increasing signal, its
collector voltage (developed across R7)
rises. This pulls D1’s cathode towards
the supply voltage and (importantly)
reduces its reverse bias. As TR2’s
collector current falls further with
increasing signal strength, D1 eventually begins to conduct and damps the
IF signal at TR1’s collector, thereby
preventing it from increasing.
The result is that the model 198 has
86 Silicon Chip
effective AGC and provides consistent
audio output levels over a wide range
of signal strengths.
R32-C32, with the 100nF capacitor
increased in value for the 199.
Audio stages
The push-pull Class B output
stage is based on TR6 & TR7 and has
thermistor-compensated bias (R29).
This bias can be adjusted using R27.
The model 198 also has a shared 5Ω
emitter resistor (R31) but this was
removed for the 199. At 5.5mA, the
bias current is a little higher than in
most other sets but I found that I was
able to set it to almost zero with no
noticeable increase in crossover distortion. The alignment guide, by the way,
recommends running the set for three
minutes prior to checking or adjusting
the output stage bias.
In the model 198, audio stage feedback is applied from TR6’s collector
to TR5’s base via two paths. First,
there is a permanent feedback path via
R22-C27 (the 199 uses a single 220kΩ
feedback resistor and takes the feedback from the speaker terminal). And
second, Bass switch S1/S2 switches
C29 and R30 across the feedback path
to apply extra treble roll-off (the 199
uses slightly different values here). In
operation, this brings the upper -3dB
point down to just 2kHz (the model 199
also derives this “top-cut” feedback
from TR6’s collector).
S3/S4 (Treble) switches in C33 to
The audio stages begin unconventionally with the emitter follower/
buffer stage based on TR4. It’s more
usual to see a common-emitter stage
here but given the AGC design (which
feeds some current through NTC
thermistor R16 in order to operate), it
makes sense. In operation, TR4’s bias
is temperature-stabilised by R16, presumably to prevent TR4’s AGC action
from being disturbed by high or low
ambient temperatures.
Following TR4, the signal is fed to
the remaining audio stages via volume
control pot R20. Its circuit configuration is also unconventional: its “hot”
end connects to TR4’s emitter and
its “cold” end goes to TR5’s emitter,
which produces about 0.7V DC across
the pot. However, because TR5’s emitter is at AC (signal) ground, R20 works
just fine as a volume control. The
peculiarity is that there is standing
DC across the pot, which is usually a
recipe for noisy operation.
Audio driver TR5 is a conventional
transformer-coupled stage. No treble
roll-off is applied here but was added
in the 199 model. It is, however,
applied in the output stage using
Push-pull output stage
siliconchip.com.au
This is the under-chassis view of the model 198. The set was still in working order and no parts had to be replaced,
although the set did require alignment adjustments in order to optimise its performance.
provide extra bypassing for the demodulator (this is absent on the 199). Like
the Bass switch, it also turns the set on
via its second set of contacts.
By the way, the circuit shown here
is a redrawn version, since the original
circuit isn’t all that clear (at least as
found online). The original component
numbering has been preserved.
Restoration
The set shown here was quite tatty
when I took it out of storage. On the
outside, its leatherette-covered composite case and tuning dial were dirty
and the metalwork was tarnished and
corroded. In addition, the Philips
badge on top (just behind the switch
well) and the “All Transistor” badge
inside the escutcheon were both bent.
It’s a common problem and of the four
198/199 sets I have, this one was the
best preserved.
I attacked the leatherette case first
using a microfibre pad but soon noticed that some of the fawn colour was
transferring to the pad. I’d also had a
similar experience of colour lifting
with the Bush TR82C, so I’ll use these
microfibre pads with caution in future.
In the end, the case was cleaned
using spray cleaner, a toothbrush and
good old-fashioned elbow grease. I
then repeated the exercise on the tuning and volume control knobs. The
lettering on the switches was almost
absent but I decided to leave restoring
them for another time.
The metal strap covers, the switchbank end pieces and the Philips badge
all cleaned up nicely with Brasso but the
escutcheon trim was more problematic.
Rather than polish it out of existence, I
siliconchip.com.au
Philips 199 Transistor Radio – Differences
The 199 and 199AC look very similar to the model 198 but incorporate several
component and configuration changes, as follows:
(1) Bypass capacitors C11 & C14 changed from 40nF to 10nF;
(2) Neutralising capacitor C13 changed from 65pF to 50pF and connected to an overwind on IFT2’s primary rather than to its secondary;
(3) Neutralising capacitor C18 (15pF) becomes C19 (10pF);
(4) Audio input coupling capacitor C25 (2µF) becomes C26 (10µF);
(5) Treble-cut capacitor C25 (10nF) added between TR5’s collector and ground (model
199 only);
(6) Treble-cut capacitor C32 changed from 100nF to 220nF;
(7) Feedback capacitor C29 (Bass position) changed from 300pF to 200pF;
(8) Feedback resistor R22 reduced from 470kΩ to 220kΩ and its parallel capacitor C27
deleted;
(9) Feedback resistor R30 increased from 100kΩ to 220kΩ and resistor R31 replaced
with a link.
(10) As noted in the article, the 199 sets alter the connection from the ferrite rod to the
base of converter stage TR1.
(11) The 198 derives all of its audio feedback from TR6’s collector. In the model 199,
it’s permanently derived from the speaker, while Bass position feedback is still derived
from TR6’s collector and the Treble switch feedback has been omitted.
left it with the minimum of treatment.
Unfortunately, the strap stitching
had all but disappeared but that’s
also a job for another time. On the
other hand, the electronic circuitry all
looked good, with no battery corrosion
or other visible issues.
How good is it?
So just how well does the Philips
198 transistor radio perform? The
answer is “surprisingly well”.
First, the selectivity is quite narrow,
being just ± 2kHz at the -3 dB point and
±20kHz for 60dB down. It’s wider than
the AWA 897P’s with its six tuned IF
circuits, however.
The RF performance was so good it
had me rechecking my results. During alignment, I discovered that the
oscillator coil slug was jammed tight.
Another set had the same problem but
after some fiddling about with various
other adjustments, I managed to obtain
50mW output for a signal strength of
just 40µV/m at 600kHz and 42µV/m
at 1400kHz (although with only 16dB
and 15dB S/N ratios respectively).
In the absence of oscillator adjustment, the classic solution is to adjust
the antenna circuit by sliding the tuning coil along the ferrite rod. Sliding
June 2015 87
All controls except for the tuning control are mounted on the top of the case.
These include the volume control at left plus pushbutton switches for power
off, treble/power-on and bass/power-on. The lettering has mostly worn off the
pushbutton switches.
it towards the centre gives increased
inductance, while moving it towards
the end reduces the inductance. However, this adjustment on my set didn’t
offer much improvement.
I then decided on more drastic
measures in the form of a hot-air gun
applied to the oscillator coil, the intent
being to soften the wax covering sufficiently to loosen the slug. It did no
good; the slug still wouldn’t move. In
the end, rather than damage the slug
by over-zealous pressure, I left the set
with the best alignment possible.
In order to obtain the standard 20dB
S/N ratio, the signal strength needed
to be about 60µV/m at 600kHz and
65µV/m at 1400kHz. Compared to
the AWA 897 and to the even betterperforming Bush TR82C, this little
Philips set is an absolute gem. It’s
nearly as good as the most sensitive
set I’ve tested so far, the outstanding
Pye Jetliner.
It’s also interesting to compare it
to its model 196 valve predecessor.
On test, my 196 required a radiated
signal of just 25µV/m at 1400kHz for
a 50mW output and just 6µV at the
aerial terminal for the same output.
The Philips 198’s AGC control is
excellent, with the output rising by
just 16dB for an input signal increase
of some 66dB. It does go into overload
after some 40mV is fed directly into the
converter’s base. This is equivalent to
a field strength of about 500mV/metre,
so “overload” for this set means “sitting right under the tower”.
Audio performance
Like most small sets, the audio performance is adequate without being
outstanding. Its frequency response
from volume control to loudspeaker
with S3/S4 in the “Treble” position
is around 100Hz to 3.9kHz, with a
3dB peak at 250Hz. Switching S1/S2
to “Bass” position cuts the top end
response to around 2kHz.
By contrast, the overall response
when measured from the antenna
terminal to the loudspeaker is only
about 150Hz to 1kHz (S3/S4 in the
“Treble” position).
The audio stage goes into clipping at
about 160mW output, at which point
the total harmonic distortion (THD)
is around 6%. This increases to about
10% THD at 180mW. At 50mW, it’s
a bit under 1% and it maintains this
figure for an output of just 10mW. This
indicates that crossover distortion is
well-controlled.
If the battery voltage drops to 4.5V, it
clips at around 45mW. However, even
at this low supply voltage, the cross
over distortion is only just noticeable
at 35mW output.
Would I buy another?
So would I buy another model 198
or 199? Well, I did actually. It’s the
look-alike 199C which is fitted with
alloy-diffused OC170/169s in the RF/
IF end and OC74s in the output stage.
If you can get either the 198, the
199 or the 199C, you’ll have a fine little set that may appear unremarkable
compared to other designs. But once
you start using it, you’ll be reminded
of just how good it is. We’ll find out
just how good the 199C is in a forthcoming article.
Further reading
(1) For the 198 and the 199 circuits, see
Ian Malcolm’s Transistor Radio Page:
http://transistor.bigpondhosting.com/
circuits/philips198.jpg
(2) For information on the 199AC, including service data, see Kevin Chant’s
website: http://www.kevinchant.com/
uploads/7/1/0/8/7108231/199ac.pdf SC
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