help required with am radio diagram

B

Bruno

Guest
Hello,
Does anyone know the am radio from the Maxi-tronix company
(self assembly, for children I think). I was simply wondering since
the circuit diagram is provided but no explanation as to how it
works, if someone can help. I scanned in the circuit and put it here:

www.geocities.com/myelectronicsrevision/amradio.jpg


thanks.
 
Hi,

Don't know the particular radio but the first stage is a
reflex amplifier/detector and the second one is an audio
amplifier.

The signal is picked up by the ferrite rod antenna, the
high impedance winding of which is resonated by VC. The
lower impedance winding (better suited to match the input
impedance of the transistor) is closely coupled to the
first and then to the base of the 2N3904, which is biassed
on the 1-Meg resistor.

At this point the stage is acting as a simple RF
amplifier (note the 800uH choke) that feeds the detector
consisting of a 1N60 diode and a 0.05uF bypass capacitor.
The resulting audio signal is now fed back into the base of
the 2N3904 (hence the word reflex), through terminals 4 and
3 of the second winding, where it is amplified. You will
see now that the audio signal out to the next stage is
taken from the bottom of the 2.2K load only whereas with
the RF it came from the bottom of the choke.

The second stage is a simple audio amplifier feeding a
high impedance earpiece strapped across the 1K8 collector
load of the 9014 transistor. Bias is provided by the 1-Meg
and any stray RF that might cause feedback problems is
removed by the action of the 470pF capacitor.


Cheers - Joe
 
"Bruno" <Bruno@planetnet.com> schreef in bericht
news:c4f9rm$83i$1@news.epidc.co.kr...
Hello,
Does anyone know the am radio from the Maxi-tronix company
(self assembly, for children I think). I was simply wondering since
the circuit diagram is provided but no explanation as to how it
works, if someone can help. I scanned in the circuit and put it here:

www.geocities.com/myelectronicsrevision/amradio.jpg


thanks.
Click to expand...
Bruno,

It's a reflex receiver that is the2N3904 is used both as an hf and as an
audio amplifier. Shortly explained it goes:

The coils to the left on the ferrite rod are both antenne and tuning. Actual
tuning is done by (1,2). The hf signal is transformed to (3,4) and feed into
the base of the 2N3904. The ampilfied hf signal appears on th collector
(10). The hf is blocked by the 800uH coil but fed to the diode via
the0.005uF capacitor. So the audiosignal appears on (8). The hf coil (3,4)
is a shortcut for the audio signal so the audiosignal goes into the base of
the 2N3903 (9). Amplified to (10) it is blocked by the 0.005 cap but the
800uH coil is a shortcut for audio. the audio signal goes via the 1uF
capacitor into the base of the 9014 (14). The once more amplified signal can
be heard on the collector of the 9014 (15) by the earphone.

This type of receivers were very popular in the days of the germanium
transistors. They often had two small audio transformers. One to match the
output of the first transistor to the input of the second. The other was an
output transformer that matched the output of the second transistor to a
small speaker. Most of them were Japanese made and transformers were cheap
and reliable, transistors were expensive and sensitive.

petrus


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On Thu, 01 Apr 2004 08:18:41 +0000, petrus bitbyter wrote:

This type of receivers were very popular in the days of the germanium
transistors. They often had two small audio transformers. One to match the
output of the first transistor to the input of the second. The other was an
output transformer that matched the output of the second transistor to a
small speaker. Most of them were Japanese made and transformers were cheap
and reliable, transistors were expensive and sensitive.
Click to expand...
They where also known as "Boy's radios". They where imported as
toys to avoid expensive import tariffs on radios coming from Japan.
Something to do with if a radio has *two* transistors or less it was
classified as a toy.

http://www.transistor.org/FAQ/faq.html#13
----------
Certain radios will have the label "Boy's radio" on them. Tariffs on
Japanese radios were higher than those for Japanese toys. US tariff law
did not define a radio containing two transistors or less as a radio;
instead, it was defined as a toy. Japan took advantage of this by
producing countless variations of radios containing two transistors or
less. The performance of these radios was usually quite marginal, but
they sold well anyway due to their low prices. Most weren't aimed at
children, although some were. Check out a brochure for the Coronet radio,
which is extremely common. Nowhere is it mentioned that this is a
two-transistor circuit!
----------

--
Michael Turner
Email (ROT13)
zvxr.gheare1963@ivetva.arg
 
Thanks to all !!! I have a similar circuit for fm, I will post it in a new
thread. Thanks.
 
"Bruno" <Bruno@planetnet.com> wrote in message
news:c4hfn7$eql$1@news.epidc.co.kr...
Thanks to all !!! I have a similar circuit for fm, I will post it in a new
thread. Thanks.


Actually I think it is better to post it here as it is very similar.
Click to expand...
O.k., the circuit is here:
http://www.geocities.com/myelectronicsrevision/fmradio.gif
(if you don't see it, try typing the link manually in a new window)

The right half of the circuit seems to be the same, so I shall need
help to understand only the left side, which deals with the fm.
Also, I also wanted to ask how do antennai work, because the
other end is simply unconnected, so the current has nowhere to
go to?
 
"Bruno" (Bruno@planetnet.com) writes:
"Bruno" <Bruno@planetnet.com> wrote in message
news:c4hfn7$eql$1@news.epidc.co.kr...
Thanks to all !!! I have a similar circuit for fm, I will post it in a new
thread. Thanks.


Actually I think it is better to post it here as it is very similar.
O.k., the circuit is here:
http://www.geocities.com/myelectronicsrevision/fmradio.gif
(if you don't see it, try typing the link manually in a new window)

The right half of the circuit seems to be the same, so I shall need
help to understand only the left side, which deals with the fm.
Click to expand...
That section on the left is an oscillator, so really it must be
a superregerative detector.

The reason the section on the right looks the same as your previous circuit
is because it is. But in this case, it's just being used as an audio
amplifier chain. They've simply taken off the loopstick from the AM
circuit, and are feeding the audio signal from the left stage into
the "am radio" circuit. Remember that the first transistor on the right
serves as both a radio frequency amplifier, and an audio amplifier. Feed
an audio signal into there, and it just acts like an audio amplifier.

A regenerative detector is an amplifier with controlled positive feedback.
Feed the output into the input, and each time the signal goes through the
amplifier, it's amplified more.

This is not the same thing as the previous reflex circuit, where a stage
is being used to amplify two signals, audio and radio, at the same time.
A regenerative detector is amplifying the same signal a whole bunch of
times.

The problem with the regenerative detector is that if you let this
happen too much, then the stage starts oscillating, ie it generates
a signal. For receiving some types of signals, this is actually needed,
but one does not want it happening if you don't need it. The point at
which it goes from near infintie amplification to oscillation is very sharp,
and this tended to be a problem.

(Note, that another way of looking at the regenerative detector is
that it's an oscillator that's been tamed to it's just on the verge of
oscillation. When Howard Armstrong came upon regneration (he patented
the detector in 1914), he also came upon a means of generating signals
with vaccuum tubes.)

You can see Armstrong's patent of the regenerative receiver at
the US Patent Office website, http://www.uspto.gov/patft Patent #1,113,149.

A lot of people made use of regenerative receivers, because they were
much better than the "crystal radios" that many were using up to that
point. They provided a lot of gain without needing a lot of tubes. But
they could be finicky.

Armstrong went on to design the superheterodyne receiver, patent
#1,342,885 issued in 1920. This was a much more complicated design,
but overcome the faults of the simpler receivers, though it brought its
own faults.

Then he was having legal problems over the regenerative patent, because
someone was claiming they'd come up with it first. So in preparation for
going to court, he hooked up his regen, and noticed a quirk that he'd
noticed before. If the regenerative receiver was adjusted just right,
there was an even greater gain, and no oscillation. He pursued this,
and was issued patent #1,342,885 in 1922 for the superegenerative detector,
the one in your diagram.

I've gone on this history trip because the superregenerative detector
is just a souped up regenerative detector. By taking the regenerative
detector and turning it on and off at an ultrasonic rate, one gets the really
high gain without the squealing that comes when it goes into oscillation.
Much of the time, superregenerative detectors use the same active element
as the RF stage and the "modulating" stage. In other words, while that
stage is amplfying the incoming signal again and again up to near infinity,
it's also oscillating at an ultrasonic frequency. This "quenching" keeps
the stage from actually oscillating at the radio frequency, but taking the
fussiness out of it.

The fallout is that they tend to be very broad.

Michael
 
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