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Old July 29th 03, 03:14 PM
Cass Lewart
 
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Default Still superhet radios?

I recently took apart my AM/FM walkman and found no IF transformers in
it. Do these small radios still use superheterodyne type circuits with
a fixed IF frequency (455 kHz for AM, 10.7 MHz for FM)? I would love to
put my hands on a schematic but could not locate any.

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Old August 4th 03, 07:08 AM
KA Turner
 
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Default

Cheap but usable especially if you are fishing and well, you don't have your
cherished boom box.
Ken
"Michael Black" wrote in message
...
(Cass Lewart) wrote in message

...
I recently took apart my AM/FM walkman and found no IF transformers in
it. Do these small radios still use superheterodyne type circuits with
a fixed IF frequency (455 kHz for AM, 10.7 MHz for FM)? I would love to
put my hands on a schematic but could not locate any.


I doubt this is the newsgroup to discuss this.

You need to be more specific about what you saw inside the radio.

Except for cheap radios (and that too may have shifted in very recent
years), AM radios don't rely on IF transformers for selectivity. They
use ceramic filters, and for the common AM radio IFs, they'd be plastic
in green or orange squares (or sometimes rectangles).

Likewise, just about any FM broadcast radio (even cheap ones) use ceramic
filters, which at 10.7MHz would be three legged ceramic packages looking
rather like ceramic capacitors, though usually they are marked with the
IF frequency.

Though, depending on the design, there often is one IF transformer
in the picture, but once the ceramic filters are in place, they
are not a necessity.

Now, there seems to be a new wave of very cheap broadcast radios.
If you've seen these two-button FM only radios, I can get them here
for $1.99, they use an IC intended for mass consumer items. It's
a superhet design, but converts down to 70KHz or so, where they
can use active filters for the IF filtering. Likely big limitations
on performance, but likely also acceptable for a lot of uses. I
have no idea if they put these into "walkmans". The one I opened
up uses a TDA7088, though in my case the prefix was missing. You
can find the datasheet around the internet.

I hear, but haven't seen, that some cheap radios use a TRF IC.
A descendent of the thirty year old ZN414, but the present version
is the MK484. Good agc action, which helps make up for the single
tuned circuit at the front end.

Michael






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