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Vintage Radio
By RODNEY CHAMPNESS, VK3UG
The Kriesler 41-29 “Trans-Mantel”
Developed during the early 1960s, the
Australian-made Kriesler 41-29 “TransMantel” was a 7-transistor receiver based
on PNP germanium transistors. It was an
excellent little set that could be used both as
a mantel receiver and as a portable.
T
HE TRANSISTOR RECEIVER era
effectively started in Australia
with the importation of a number of
radios from Japan in the mid-1950s.
These early sets were not brilliant
performers as I soon found out when
I bought a small Sony pocket receiver.
It proved to be extremely “hissy”, even
when tuned to a strong local station. In
fact, the signal had to be quite strong
for the set to even receive it.
Despite these shortcomings, I became hooked on this new technology
– a technology I didn’t understand at
the time but wanted to learn about. I
kept the little pocket set for a quite few
years but its limitations meant that it
had very little use and I mostly listened
to my old faithful valve radios.
Australian manufacturers started
making transistor receivers in the late
1950s. Initially, they assembled the
sets in the same way as their valve
sets, with point-to-point wiring, and in
some radios, the transistors were even
mounted in special sockets – just like
valves. This method of manufacture
was expensive, particularly when
Australian manufacturers had to compete with the Japanese manufacturing
techniques of PC board construction
and cheap employee wages.
However, Australian manufacturers
quickly realised that transistors should
be treated as just another component.
Mounting most of the receiver on
a PC board would also be cheaper,
with fewer wiring mistakes. These
techniques coupled with tariff barriers
helped Australian manufacturers stay
competitive until the barriers were
reduced in the early 1970s.
The transitional Kriesler
This is the fully-restored set, shown here with its carrying handle raised.
Its styling resembled the earlier valve mantel receivers.
98 Silicon Chip
Kriesler Australia was one of many
firms making both valve and transistor sets in the 1960s. Like most
manufacturers at that time, they built
their transistor receivers in a style that
suited valve technology.
Just why they did this isn’t clear. It’s
hard to be sure whether the manufacturers were ultra-conservative when
it came to designing their radios
(cabinets in particular) or whether
they felt that customers would not
accept the styling changes that were
possible with transistors (including
portability). In reality, it was probably
a combination of both scenarios.
The 41-29 is one of these transitional receivers, being called a “TransMantel”. It is in a case similar to the
Kriesler 11-90 valve mantel receiver,
although the dial mechanism is quite
different. It could be used either as a
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Fig.1: the Kriesler Model 41-29 employed a conventional superhet circuit based on seven PNP germanium transistors.
conventional portable (although the
cabinet may not be all that rugged) or
as a cordless mantel receiver.
Unlike some other sets, the battery
life was quite good, as a 286 battery
was used to provide power. The 286
was in reality two 276 type batteries in
the one case. In fact, a life of up to 1000
hours has been quoted for this battery
in some of the Kriesler receivers.
Main features
The Kriesler 41-29 is housed in
an attractive plastic case and its size
suited both portable or mantel-piece
operation. It has a large slide-rule dial
and is tuned by a relatively large knob
at the righthand end of the scale.
Apart from that, there are just two
other controls: a combined on-off/
tone control and a volume control at
bottom right. It all adds up to a rather
neat and functional layout.
The rear of the set is held in place
by two screws, which serve dual functions. When viewed from the back,
the lefthand screw is also the antenna
connection, while the righthand screw
serves as the earth connection.
It is necessary to remove these two
screws and the back to replace the battery. In fact, the instructions for this are
on a small piece of paper attached to
the underside of the receiver. Another
set of instructions, this time inside
the back, describe how the rest of the
receiver can be dismantled.
This same piece of paper also has a
layout diagram of the major parts on
the PC board, plus a rather small copy
of the circuit. This circuit is hard to
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This rear view shows the fully restored set, just before the back was refitted.
read because of its small size but it’s
much better than having no information at all about the circuitry.
Dismantling and cleaning
The two control knobs fitted to the
unit that was given to me to restore
were certainly not the originals. They
were both black and much bigger than
the originals that came with the set, so
they certainly looked out of place.
Fortunately, I had some old Kriesler
knobs stashed away and I found two
which looked similar to the original
knobs. The two black control knobs
and the tuning knob were then removed, followed by the four screws
that secured the chassis to the front of
the cabinet. That done, the front of the
cabinet came away in three separate
pieces (that’s how it was made).
The chassis was dusty but a few
minutes work with an old paintbrush
fixed that. The dial pulleys were then
oiled, as were other moving parts.
In this set, the volume and tone
control shafts are split down the centre and require a knob with a metal
insert (this insert anchors to the sides
of the knob). The Kriesler knobs that
I’d dug up (originally scrounged from
a Kriesler TV set) were suitable but to
accommodate the metal inserts, I had
to increase the width of the slots (the
June 2006 99
This rear view shows how the PC board could have
been hinged for easier service access.
Despite its age, no PC board
parts required replacement.
The alignment required
adjustment though, to get the
set to tune correctly.
remove as it could have been due to
the location of the “earth”, which is
also the mounting plate for a screw
holding the back of the set against the
metal chassis.
First, the single-strand wire from the
oscillator section of the tuning gang
to the PC board has to be desoldered,
after which the four screws holding
the board in place are removed. The
“earth” plate is then sprung outwards
so that the board can be removed. In
practice, the board can then be turned
over (so that the parts face upwards)
and a piece of cardboard or cloth
placed underneath it to prevent shorts
to the frame during testing.
This procedure could have been
simplified by mounting the board in
a slightly different position, so that it
could be directly removed without the
“earth” being in the way. In fact, with
a little more thought, the board could
have been hinged on the edge near the
centre of the receiver, which would
have made it a dream to service.
The old AWA P1 portable TV set
had a hinged board and it made the
set very easy to service. In fact, AWA
did such a good job of making the P1
accessible that the picture tube could
be replaced in 15 minutes.
Circuit details
original knobs obviously had a much
narrower insert). This was done by
carefully filing them with a needlenosed file.
That done, the inner retaining screw
for the tuning knob was cleaned and
repainted with gold spray paint, as
was one stud underneath the cabinet
front. In addition, the dial pointer was
resprayed with white paint, as it too
was looking a bit grubby.
The speaker had some fluff trapped
at the front of the cone, which meant
it had to be removed. This involves
first removing the PC board (more on
this later), after which it’s a matter of
undoing the nuts and bolts that hold
the speaker in place.
In the end, it was necessary to only
100 Silicon Chip
partially remove the speaker, after
which the fluff was easily brushed
out. The speaker was then correctly
refitted into position.
Next, the cabinet and tuning knob
were washed using soapy water, a nailbrush and a toothbrush. However, I did
take care to ensure that the paper stickers didn’t get wet. Any shallow scuff
marks in the case were then removed
using automotive cut and polish but
there were also some marks that were
too deep to get out – at least not without cutting well into the plastic case.
However, the remaining scratches are
not particularly obvious.
Removing the PC board
The PC board wasn’t as easy to
The circuit configuration is similar
to many other transistor receivers
of the era – see Fig.1. It’s a standard
7-transistor superhet circuit, with the
front-end using an OC170 transistor
as an autodyne converter.
Kriesler economised on the windings on the loopstick antenna by connecting the low-impedance section of
L2 to both the base of TR1 (via R1, C1
& R3) and to the antenna via a broadlytuned coil (L1). Coils used in the L1
position are usually tuned below the
broadcast band when used with a
7-10m long antenna. This coil boosts
the performance at the low-frequency
end of the dial, as the performance
here is usually inferior to that at the
high-frequency end.
The output from TR1 is fed via
455kHz IF (intermediate frequency)
transformers IFT1A and IFT1B to the
base of the first IF amplifier (TR2,
OC169). It then goes to IF amplifier
stage TR3 (OC169) and from there to
detector stage D2 (OA79) which also
provides the AGC voltage. Resistors
R17 and R18 forward bias D2 almost
to the point of conduction, thereby
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Despite its apparent simplicity, the Kriesler Model 41-29 Trans-Mantel is
not all that easy to disassemble for service.
greatly increasing its sensitivity and
reducing distortion.
As the signal strength increases, D2
applies an increasingly positive voltage to R10 and hence to the base of
PNP transistor TR2. This AGC voltage
in turn causes TR2 to draw less current
as it cuts off. As a result, the voltage at
the junction of IFT2 and R12 becomes
more negative (note: the circuit is positive earth).
Now let’s consider the action of
diode D1 (OA90), which is effectively
wired between the collector circuit of
TR1 and the collector circuit of TR2.
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Normally, D1 does not conduct as
TR1’s collector is at -7.1V and TR2’s is
-5.1V (ie, D1 is reverse biased). However, as TR2 shuts down due to AGC
action, its collector voltage progressively becomes more negative. When
this voltage goes below about -7.3V, D1
becomes forward biased and conducts,
thus causing the signal from TR1 to
largely bypass TR2 and go straight to
TR3 (via IFT2).
As a result, the gain of the set is
reduced on strong signals. At the same
time, the selectivity is also reduced, as
IFT1 is bypassed and only IFT2 and
IFT3 are effectively in circuit. This
loss of selectivity probably doesn’t
matter a great deal and actually has the
benefit of improving the audio quality
on strong signals.
Following the detector, the audio is
fed to volume control R21 and then to
the first audio amplifier (TR4, OC71).
This is then followed by audio stage
TR5 (OC75) which in turn drives two
class B audio transistors TR6 and TR7
(OC74s) via transformer T1. TR6 and
TR7 operate in push-pull configuration and drive the loudspeaker via
transformer T2. Negative feedback is
applied from the loudspeaker voice
coil to the emitter of TR5.
The output stage quiescent current
is regulated by R37, a negative temperature coefficient (NTC) thermistor.
Germanium transistors are particularly prone to thermal runaway as their
temperature increases. In fact, it can
be so bad in circuits like this that the
transistors can self-destruct.
To overcome this, the NTC thermistor decreases its resistance with
increased temperature, thereby reducing the forward bias on the bases of
transistors TR6 and TR7. This compensates for the tendency of the TR6
and TR7 to conduct more heavily with
increasing temperature.
In this circuit, the thermistor senses
the ambient temperature inside the
case but some later transistor equipment had the thermistor physically
connected to the output transistor
June 2006 101
the rest point for the pointer changed
which wasn’t good.
I then found that the plastic gear
shaft on the dial reduction drive had
expanded, so that when tuned to the
end of the dial, you could keep turning it even though the tuning gang was
now stationary. This was easily fixed
by drilling through the plastic gear
and the brass tuning capacitor shaft
and then locking the two together with
a wire pin.
Summary
The geared drive for the tuning capacitor is inside the compartment to the left.
This made difficult to drill a hole to lock the gear to the tuning capacitor shaft.
heatsinks to improve the response time
and provide more effective control.
Restoration
Initially, this set was completely
dead but this was quickly traced to
a faulty on/off switch. Not having a
replacement on hand, I decided to
simply bypass the switch until a suitable control became available. The set
then showed signs of working.
Next, I sat my Leader LSG11 signal
generator on the other side of the workshop, so that it could provide a weak
test signal for the set. This was tuned
to 455kHz and I could immediately
hear a beat tone from the radio, so it
was quite a sensitive set.
The cores of the IF amplifier transformers had all been sealed but their
alignment appeared to be pretty much
spot on. However, I found that the
set would only tune down to 600kHz
instead of the more normal 525kHz.
This was easily fixed. All I had to do
was adjust the oscillator coil so that it
tuned down to 525kHz with the gang
shut, then adjust the oscillator trimmer so that it tuned to 1620kHz with
the gang fully open. There seemed
to be little interaction between these
two adjustments so I then aligned
the antenna circuit by adjusting the
position of the loopstick coil for best
performance at about 600kHz and the
antenna trimmer for best performance
at around 1500kHz.
The set was now working rather well
and that was with no external antenna.
In fact, it worked so well that the addition of an external antenna almost
caused overload.
At this stage, I ran into a problem.
The dial scale has a “start point”
marked on it but I couldn’t get it right.
I tuned to either end of the dial and
Kriesler made many interesting
receivers over the years and the 41-29
Trans-Mantel was an excellent little set
with long battery life. The dial drive
system is a little more complicated
than needed and the PC board could
have been mounted in such a way that
it could have been hinged for easy
service. However, these are comparatively minor quibbles.
Finally, note that the circuit indicates that the on-off switch is part
of the volume control. In practice,
it’s part of the tone control instead,
so someone didn’t check the circuit
too well.
Errata
In Vintage Radio for June 2004, I stated that the suppressor grid on a valve
caught the electrons that bounced off
the plate. This isn’t strictly correct as
the electrons that hit the plate dislodge
other electrons, which are collected by
the suppressor. This is called “secondary emission”.
It has also been pointed out that the
screen grid reduces the capacitance
between the grid and the plate. This
should have added to my statement
that the screen grid screens the grid
SC
from the plate.
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