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Vintage Radio
By Ian Batty
Astor M2 Cry-baby
a radio, intercom and baby monitor all in one
You might think that baby monitors are a fairly recent innovation but
this battery-powered solid-state mantel radio from Astor incorporated
an intercom and baby monitor in a design from 1962 – 56 years ago!
This mantel radio looks similar to
the Astor M5 and M6 mantel sets that
I wrote up in the September 2016
issue (www.siliconchip.com.au/
Article/10149). Those mains-powered
sets had Class-A audio output stages.
At the time, I’d pondered the design
brief but supposed that mains sets
could easily support the power drain
penalty of Class-A.
I did wonder about a battery version
and assumed it would need to use the
more complex (and thus more costly)
Class-B design for battery economy.
The 1962 M2 does indeed do this. The
Class-B output stage gives an overall
drain of around 10mA, meaning that
the original Eveready 276-P carbonzinc battery would last for some 150
hours of use.
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As well as the M5/M6’s radio function, a remote speaker connected to the
M2 allows baby monitoring, playing radio programs at the remote speaker, or
conventional “half-duplex” intercom
operation. Being able to use the extension speaker for the radio program
would be a useful feature even today.
We usually think of loudspeakers
as output transducers but (like many
electromechanical devices), we can
capitalise on their reciprocal nature
and use them as microphones. This
is what the M2 does with its remote
speaker, using it either as a speaker or
with a suitable step-up transformer, as
a microphone.
I was offered this set for review by a
fellow member of the Historical Radio
Society of Australia (HRSA), whom I
Celebrating 30 Years
have credited at the end of this article.
Appearance and controls
The cabinet of the Astor M2 is very
similar to that of the previously-reviewed M5 and has the same handspan dial for tuning and a 4-inch
speaker on the left-hand side of the
front panel.
The set retailed for £37.16s with the
case and external speaker presented in
a variety of different colours such as
cherry red, yellow, brown etc.
The major difference is a 5-position
function switch on the right-hand side
of the panel, while the volume controlcum-power switch is on the left-hand
side. The monitor speaker is in a black
circular housing with no styling similarity to the radio.
siliconchip.com.au
Fig.1: the complete circuit for the Astor Cry-baby. Note the complex wiring for the 4-pole function switch. The
loudspeaker was connected via a step-up transformer (#74) when it was being used as a baby monitor.
Position 1 of the function switch
simply parallels the monitor speaker
with the set’s internal speaker, so it
operates as an extension to play the
received program. Position 2 switches
off the monitor speaker.
Position 3 adds remote (baby) monitoring to the radio function but with
the remote input at full gain while the
radio program is subject to the volume
control setting. This would be ideal
for baby monitoring; you’d be alerted
to anything happening in the nursery
while listening to the radio or you
could turn the radio down while still
monitoring your infant.
Position 4 is a simple intercom
working from remote speaker to the
set while position 5 reverses the conversation, going from set to monitor.
Just as an aside, if you have one of
these sets, make sure that the remote
speaker is reasonably remote before
switching to position 3: insufficient
separation will result in very loud
acoustic feedback.
siliconchip.com.au
Construction
Like the Astor M5 & M6, the M2 uses
a single-sided phenolic PCB mounted
behind the plastic front panel and anchored by the volume pot and function
switch’s shafts, and by two screws;
there is no metal chassis.
The only unusual component is the
3.5mm external speaker jack mounted
in the rear of the case, and connected to
pins on the circuit board by fly leads.
Like the M5/6, tuning knob removal requires gently prising off the gold
dress cap in the centre of the dial, undoing the three small screws and securing ring that hold the tuning knob
on, then (for circuit board removal),
undoing two screws in the tuning boss
and sliding it off the gang’s shaft.
The set provides for external Aerial and Earth connections to improve
reception in fringe areas. As with the
M5 model, these “hide” under the set
and connect via the two bottom case
screws.
While such connections are always
Celebrating 30 Years
welcome, you’d need to be a long way
from the station before this sensitive
set needed external assistance.
Fig.1 shows the full circuit and as
with other Astor radios, each component has a simple “hash” number. All
the transistors are PNP germanium
types while the two diodes are also
germanium.
However, the battery supply is unconventional, with positive Earth and
the circuit has been drawn to show
conventional flow from positive to
negative, “up the page”.
A comparison with the circuit of
the Astor M5 featured in the September 2016 issue shows that it is quite
similar to that of the M2 model, with
the exception of the M2’s Class-B audio output amplifier; more correctly
termed “Class-AB” because the output transistors do have a small bias to
provide quiescent current.
The self-oscillating converter #78, a
2N412, uses collector-emitter feedback
with the incoming RF signal applied
April 2018 85
The large rotary switch for the function control is on the left hand side of the PCB. The two output transistors are fitted
with flag heatsinks which have been soldered to the frame of the output transformer. This photo was taken after replacing
numerous electrolytic capacitors.
to the base of the converter (from the
ferrite antenna via 10nF capacitor #3).
Like almost all such converters, no
AGC is applied to this stage.
The tuning gang uses a cut plate oscillator section, so there is no padder
capacitor.
The converter feeds through oscillator coil #70’s primary to the primary
of first IF transformer #71. Its tuned,
tapped primary couples to the untuned, untapped secondary.
The first IF amplifier (#80), a 2N410,
gets its bias via a 150kW resistor (#47)
connected to the +9V supply. It is
neutralised via an 8.2pF capacitor.
Its output goes to the second IF transformer’s tuned, tapped primary and
its untuned, untapped secondary
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feeds the second IF amplifier, another
2N410 (#81).
Astor recommended that both IF
transistors are picked from the same
gain group.
The 1st IF amplifier has AGC applied from the 1N295 detector diode (#82) via a 4.7kW resistor (#46).
This directly controls the amplifier’s
gain, and brings AGC extension diode
#79, another 1N295, into action with
stronger signals.
It’s the conventional “Mullard” design, allowing the set to respond to
varying signal strengths with a nearconstant output level.
The 2nd IF amplifier is neutralised
via a 27pF capacitor (#15). Its value is
some three times that of #7, necessary
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since #15 is fed from the IFT’s secondary rather than the primary as with the
1st IF amplifier’s #7.
As well as providing the AGC signal,
diode #82 provides the demodulated
audio signal which is filtered by two
10nF capacitors (#18 and #19) and a
220W resistor (#48).
The recovered audio signal is fed
to the 10kW volume control via a 2µF
capacitor and a section of the 4-pole
function switch wiring when set the
“radio” position.
Audio stages
The audio section of the M2 radio
comprises three stages, with the first
two 2N406 audio transistors, #83 and
#84, operating in a high-gain, direct-
siliconchip.com.au
The Aegis branded extension speaker is clearly not the original Astor-branded speaker which was made by Rola. It
was supplied with 23m of 2-core flex and it could be used as a baby monitor, extension speaker or as an intercom.
coupled configuration and with DC
feedback applied from #84’s emitter
to #83’s base, with the actual voltage picked off from the 330W/680W
(#56/#57) voltage divider. Some local
negative feedback is applied across
driver transistor #84 via 100kW resistor #59.
Both the emitter circuits are bypassed for audio, with transistor #83
having a 10W resistor (#54) allowing
overall AC feedback to be applied
from the speaker output via a 15kW
resistor (#66).
Transistor #84 feeds the primary of
driver transformer #75 and its centretapped secondary feeds the two output
transistors, #85 and #86. These two
OC74s drive the output transformer
in push-pull fashion.
They are fitted with flag heatsinks
soldered to the frame of the output
transformer, giving a large thermal
mass to help keep transistor junctions
at a constant temperature.
The bias for the output transistors
is derived from a voltage divider comprising resistors #62 (10kW) and #65
(560W) combined with a negative temperature coefficient (NTC) thermistor
(#64, 220W) to give temperature compensation.
It’s the usual arrangement whereby
the thermistor reduces bias at higher
temperatures to prevent excessive current in the output stage.
This bias network cannot compensate for falling battery voltage and nor
can the bias be optimised for individual
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transistors. This is borne out by the
small amount of crossover distortion
present even at full battery voltage.
Local feedback is provided by 10nF
capacitors #28 and #29 from the collectors to the bases of the output transistors, with further treble cut provided
by 10nF capacitor #30.
Monitoring and intercom
Position 3 of the function switch
sees a 470W resistor (#49) in series
between the volume pot’s wiper and
audio input, with #88/b also connecting a 470W resistor (#45) to the audio
input via the 2µF capacitor to the base
of transistor #83.
This allows audio from the external
speaker (operating as a microphone,
stepped up by 2kW:15W matching
transformer #74) to be passively mixed
with the audio coming via the volume
control.
Thus, while it’s possible to adjust
the level of the radio program, audio
from the remote speaker is conveyed
at maximum gain.
Position 4 of the function switch removes the radio program but connects
to the matching transformer’s secondary and conveys its signal to the volume control, while #88/b shorts out
resistor #49 to deliver the full signal
to the audio amplifier. The set is now
a conventional intercom in the “listen” position.
Signal direction, from the external
speaker to the internal speaker, is controlled by #88/c conveying the ampliCelebrating 30 Years
fier’s output to the internal speaker,
and #88/d connecting the external
speaker to the input of matching transformer #74.
Position 5 selects “talk” operation.
Switch #88/c connects the internal
speaker to matching transformer #74
to allow the internal speaker to act as a
microphone, while #88/d sends audio
output to the external speaker.
Fixing it up
As presented to me, the M2 needed
only a light clean and polish to make
it sparkle but electrically it was dead.
However, my Local Oscillator test
brought out that “swishing” sound
from my bench radio, so it looked like
an audio problem.
Homing in on and replacing electrolytic coupling capacitors #20 and
#23 brought immediate results. For
good measure, I replaced bypass caps
#9, #26 and #31, and got improved
performance with emitter bypasses
#25 and #27.
Poor quality manufacture? Well, the
set had probably been sitting unused
for some decades and it’s too much
to expect the chemically-formed dielectric to persist for so long with no
refreshing.
I did try reforming the capacitors but
with no success. I’m also pessimistic
about the long-term stability of such
old components anyway.
If you're having difficulty getting
axial-lead electrolytic capacitors,
both low-voltage (transistor radios,
April 2018 87
valve cathode bypasses) and highvoltage (valve power supplies) types
are available from local surplus stores
and online.
A tip though: chatting with one local store revealed that axial electrolytics are getting harder to find. I found a
range of axial electrolytics at Rockby
Electronics in Melbourne. You might
like to check out their online catalog.
How good is it?
The Astor M2 is right up there with
the best of the alloyed-junction germanium designs. Its sensitivity is aided
by the high-gain, three-stage audio
section.
For 50mW output, it needed around
45µV/m at 600kHz and 35µV/m at
1400kHz, but at signal-to-noise (S/N)
ratios of only -7dB and -10dB, respectively. For the more usual S/N value
of -20dB, the equivalent signals were
90µV/m at 600kHz and 65µV/m at
1400kHz.
At the antenna terminal, it needed
only 8.5µV at 600kHz and 12µV at
1400kHz, for S/N ratios of -9dB and
-10dB. For the usual -20dB ratios:
16µV and 21µV, respectively.
IF bandwidth is ±1.5kHz at -3dB
down and ±24kHz at -60dB down.
AGC allows some 6dB rise in audio
for a 50dB signal increase. I did finally
get it to overload at around 200mV/m;
an exceptional performance.
Audio response from antenna to
speaker was 50Hz to 1500Hz, with a
peak at about 80Hz. From volume control to speaker, it’s 65Hz to 7800Hz.
From the monitoring speaker input,
it’s around 135Hz to 8500Hz, with a
7dB peak at 5kHz. This may be due
to input transformer resonance but it
would help compensate for the monitor speaker’s expected weak high-frequency response.
At 50mW, audio distortion was commendably low, at only 0.8% but it rose
to 1.5% at 10mW, with discernible
crossover distortion. This confirms
the limitations of non-adjustable bias
circuits.
It went into clipping at 400mW,
with 10% distortion at 500mW. At a
low battery voltage of 4.5V, it clips at
80mW, with around 5% THD at 50mW,
noticeably crossover distortion.
As a radio, it’s great. And for its
special features, it’s equally so. The
remote speaker input gives 50mW
out with only 250µV of audio input
at full volume.
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Women’s Weekly, 18th July, 1962: https://trove.nla.gov.au/aww/read/222697
This is certainly adequate to pick up
sounds from the monitor speaker, even
with “house-length” runs of common
speaker cable. In fact, the radio was
supplied with 75 feet (23m) of 2-core
flex for this purpose.
Whether used as a portable radio,
nursery monitor or as an ordinary intercom, the audio section’s high sensitivity and generous audio output mean
that it easily fills the bill.
Would I Buy One?
That’s tempting but I’d like to get
an M2 with the original Astor speaker. While the substitute Aegis monitor works just fine, there’s no substitute for the genuine article (actually
made by Rola).
Celebrating 30 Years
If you happen to have either the
complete radio-and-speaker kit that
you’d like to move on, or even just the
speaker, please drop Graham or me a
line via Silicon Chip ([02]9939-3295
or silicon<at>siliconchip.com.au).
Acknowledgement:
Special thanks to Associate Professor (retired) Graham Parslow of the
HRSA, for this interesting example of
fine Australian engineering and manufacture.
Further reading
You will find the circuit and service info on Kevin Chant’s excellent site at www.kevinchant.com/
uploads/7/1/0/8/7108231/m2a.pdf SC
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