Improved AM stereo mod for your AM/FM stereo tuner, using SRF42

Improved AM stereo mod for your AM/FM stereo tuner, using SRF42

Post by Robert Bisset » Mon, 09 Sep 1996 04:00:00


> Found an improvement on this AM stereo mod I posted a few weeks ago.
> Namely, better fidelity and less distortion by raising the SRF42
> circuit supply from 3V to 5V.

> Also note you can get a SRF42 at radio shack, ask for catalog number 12-127
> (look at page 61 of the '97 catalog).  And be sure to look for astonished
> looks on the sales droids faces, when they see it there!

> AM STEREO modification of digital AM/FM stereo receivers, using a Sony
> SRF42 AM stereo walkman radio chassis

> You probably have a stereo FM/ mono AM receiver for your home
> stereo system.  And it probably is a PLL digitally tuned radio,
> maybe even tunes up to 1700KHz.  But still mono.  It's nearly
> impossible to find an AM stereo set, the only one you can hope to
> find is a portable, a Sony SRF42.  But it's analog tuned, but it is
> possible to use the innards of this Sony to demodulate AM stereo
> signals tuned by your PLL digital mono set's front end.  Idea
> is to disable the Sony's front end, and feed the IF (intermediate
> frequency) from the PLL digital circuit to the IF amp section of
> the Sony.  And take the left and right outputs and substitute
> those in your reciever for the mono AM signal.

> This is not a project for beginners, you'll need some experience
> with radio circuits and fine soldering.

> Before you tear into the SRF42, tear into your receiver, and look
> for the AM circuitry.  Probably a chip.  Helps if you have the
> service manual.  Look for a small squarish plastic part, probably
> colored orange.  Hopefully it has a marking like "450".  Other
> shapes these filters come in are a rounded brown 3 leaded thing,
> marked "450" (AM) or "107" (FM).  The IF frequency of the AM
> section needs to be 450 KHz, because that's the frequency used in
> the SRF42.  Other IF freqs used are 455KHz, which is too much
> of a mismatch for the below to work.

> Open up the Sony SRF42.  First, remove the volume control knob (pulls
> out).  No need to remove the tuning knob, and don't try to remove
> the on=off switch knob.  unscrew three screws, and the front comes
> off.  Unsolder the battery connections to then be able to remove
> the radio's circuit board.  You'll see a plastic gear that controls
> the tuning capacitors.  replace the volume control knob on the
> volume control shaft (so you can vary the output level later).
> Now for the interesting part: connect a jumper wire across the
> AM local oscillator coil, located next to tuning cap trimmer CT4.
> Jumper goes from ground to the pad connecting the coil to the
> tuning cap.  This disables the Sony's front end.

> Now connect a small cap (about 0.01 uF ceramic) to the SRF 42 at
> this point:  Look for pin 12 of IC1, and follow the pad to R26,
> the point you want to connect the cap to is the pad on the other
> side of R26 away from the IC1.  Keep the leads short on the cap.
> Connect a length of shielded wire to the free end of the cap, and
> the shield connects to a 0.1uF cap, and that connects to a local
> ground (look for the connection of the metal shield of a nearby
> IF transformer).  Purpose of this cap is to provide an RF ground,
> but to block DC and audio frequencies.

> **** Update, distortion improvement ****
> I found that the SRF42 offers beter fidelity and less distortion
> if operated at 5V DC.  Normal supply voltage is 3V.  The extra
> voltage appears to offer more dynamic range on the audio.  I don't
> know how close I'm pushing overvoltage, though, so do this at
> your own risk!

> At the other end of the shielded cable connect a 1K resistor to the
> center conductor.  Now you will need to do a bit of poking around
> in your reciever with the power on, and power up the SRF42 and listen
> with headphones (you should only hear some hiss, no stations).  Tune
> a local AM stereo station on your reciever (though your reciever only
> gets it in mono now).  Look around the above mentioned IF frequency
> modules you checked before you took the SRF42 apart.  Now tune the
> volume on the reciever down, and the SRF42 up some, and start poking
> around with the 1K resistor attached to the cable.  Oh yeah, connect
> the minus power terminal of the SRF42 to the reciever's ground.  Poke
> near modules marked "450" and you should soon hear the SRF42 playing
> the station you tuned the reciever to.  Try a number of locations
> and note which sounds better (less distortion, a reasonable amount
> of high audio freqs, and when you select a point, check a few other
> stations.  When I selected a point to tap, I connected a 1K resistor
> to the top of a 1K trimpot, the other end grounded, and the wiper
> feeding the cable.  Then I could tweak the level for a reasonable
> signal, decent signal to hiss, vs distortion if the signal gets too
> strong.  If the point you selected has a DC bias on it, use a 0.01
> cap in series with the 1K resistor to avoid loading the DC.

> To power the SRF42 "chassis", I used a 5V regulator chip.  See above
> note on overvoltaging hazards that may exist when running 3V circuits
> at 5V.  Though analog circuits tend to be more forgiving of overvoltage
> than digital stuff.  Proceed at your own risk on this.

> To cut some of the high frequency audio hiss and noise, I connected
> 0.015 uF caps to ground, and the other end of one to a pad connected
> to IC5 pin 2 and the other cap to pin 4.  A more convenient attachment
> points are located just under the volume control.  Not the pot
> connections themselves, but look for 2 pads closer to the board edge
> that lead to the IC5 pins 2 and 4.

> I just connected the headphone jack to the audio amp, and used the
> SRF42 volume control to set a reasonable and undistorted signal
> level.

Haven't you noticed that most all auto radios have am stereo? Both of
our cars have them and I have one for my bench radio that's at least
10 years old.  Works fine on the few am stereo stations here.

--
      *********

      *********

 
 
 

Improved AM stereo mod for your AM/FM stereo tuner, using SRF42

Post by Robert Cas » Tue, 10 Sep 1996 04:00:00


Found an improvement on this AM stereo mod I posted a few weeks ago.
Namely, better fidelity and less distortion by raising the SRF42
circuit supply from 3V to 5V.

Also note you can get a SRF42 at radio shack, ask for catalog number 12-127
(look at page 61 of the '97 catalog).  And be sure to look for astonished
looks on the sales droids faces, when they see it there!

AM STEREO modification of digital AM/FM stereo receivers, using a Sony
SRF42 AM stereo walkman radio chassis

You probably have a stereo FM/ mono AM receiver for your home
stereo system.  And it probably is a PLL digitally tuned radio,
maybe even tunes up to 1700KHz.  But still mono.  It's nearly
impossible to find an AM stereo set, the only one you can hope to
find is a portable, a Sony SRF42.  But it's analog tuned, but it is
possible to use the innards of this Sony to demodulate AM stereo
signals tuned by your PLL digital mono set's front end.  Idea
is to disable the Sony's front end, and feed the IF (intermediate
frequency) from the PLL digital circuit to the IF amp section of
the Sony.  And take the left and right outputs and substitute
those in your reciever for the mono AM signal.  

This is not a project for beginners, you'll need some experience
with radio circuits and fine soldering.  

Before you tear into the SRF42, tear into your receiver, and look
for the AM circuitry.  Probably a chip.  Helps if you have the
service manual.  Look for a small squarish plastic part, probably
colored orange.  Hopefully it has a marking like "450".  Other
shapes these filters come in are a rounded brown 3 leaded thing,
marked "450" (AM) or "107" (FM).  The IF frequency of the AM
section needs to be 450 KHz, because that's the frequency used in
the SRF42.  Other IF freqs used are 455KHz, which is too much
of a mismatch for the below to work.

Open up the Sony SRF42.  First, remove the volume control knob (pulls
out).  No need to remove the tuning knob, and don't try to remove
the on=off switch knob.  unscrew three screws, and the front comes
off.  Unsolder the battery connections to then be able to remove
the radio's circuit board.  You'll see a plastic gear that controls
the tuning capacitors.  replace the volume control knob on the
volume control shaft (so you can vary the output level later).
Now for the interesting part: connect a jumper wire across the
AM local oscillator coil, located next to tuning cap trimmer CT4.
Jumper goes from ground to the pad connecting the coil to the
tuning cap.  This disables the Sony's front end.

Now connect a small cap (about 0.01 uF ceramic) to the SRF 42 at
this point:  Look for pin 12 of IC1, and follow the pad to R26,
the point you want to connect the cap to is the pad on the other
side of R26 away from the IC1.  Keep the leads short on the cap.
Connect a length of shielded wire to the free end of the cap, and
the shield connects to a 0.1uF cap, and that connects to a local
ground (look for the connection of the metal shield of a nearby
IF transformer).  Purpose of this cap is to provide an RF ground,
but to block DC and audio frequencies.  

**** Update, distortion improvement ****
I found that the SRF42 offers beter fidelity and less distortion
if operated at 5V DC.  Normal supply voltage is 3V.  The extra
voltage appears to offer more dynamic range on the audio.  I don't
know how close I'm pushing overvoltage, though, so do this at
your own risk!  

At the other end of the shielded cable connect a 1K resistor to the
center conductor.  Now you will need to do a bit of poking around
in your reciever with the power on, and power up the SRF42 and listen
with headphones (you should only hear some hiss, no stations).  Tune
a local AM stereo station on your reciever (though your reciever only
gets it in mono now).  Look around the above mentioned IF frequency
modules you checked before you took the SRF42 apart.  Now tune the
volume on the reciever down, and the SRF42 up some, and start poking
around with the 1K resistor attached to the cable.  Oh yeah, connect
the minus power terminal of the SRF42 to the reciever's ground.  Poke
near modules marked "450" and you should soon hear the SRF42 playing
the station you tuned the reciever to.  Try a number of locations
and note which sounds better (less distortion, a reasonable amount
of high audio freqs, and when you select a point, check a few other
stations.  When I selected a point to tap, I connected a 1K resistor
to the top of a 1K trimpot, the other end grounded, and the wiper
feeding the cable.  Then I could tweak the level for a reasonable
signal, decent signal to hiss, vs distortion if the signal gets too
strong.  If the point you selected has a DC bias on it, use a 0.01
cap in series with the 1K resistor to avoid loading the DC.

To power the SRF42 "chassis", I used a 5V regulator chip.  See above
note on overvoltaging hazards that may exist when running 3V circuits
at 5V.  Though analog circuits tend to be more forgiving of overvoltage
than digital stuff.  Proceed at your own risk on this.  

To cut some of the high frequency audio hiss and noise, I connected
0.015 uF caps to ground, and the other end of one to a pad connected
to IC5 pin 2 and the other cap to pin 4.  A more convenient attachment
points are located just under the volume control.  Not the pot
connections themselves, but look for 2 pads closer to the board edge
that lead to the IC5 pins 2 and 4.  

I just connected the headphone jack to the audio amp, and used the
SRF42 volume control to set a reasonable and undistorted signal
level.  

 
 
 

Improved AM stereo mod for your AM/FM stereo tuner, using SRF42

Post by Greg J Szeker » Tue, 10 Sep 1996 04:00:00



......

Quote:>> Open up the Sony SRF42.  First, remove the volume control knob (pulls
>> out).  No need to rem> connections themselves, but look for 2 pads closer to the board edge
>> that lead to the IC5 pins 2 and 4.

>> I just connected the headphone jack to the audio amp, and used the
>> SRF42 volume control to set a reasonable and undistorted signal
>> level.

>Haven't you noticed that most all auto radios have am stereo? Both of
>our cars have them and I have one for my bench radio that's at least
>10 years old.  Works fine on the few am stereo stations here.

Daaah, what. I've never seen one yet. My truck dosn't
have one, none of the other cars I've seen have one, not
one aftermarket unit in Crutchfield has one.

greg

 
 
 

Improved AM stereo mod for your AM/FM stereo tuner, using SRF42

Post by Robert Bisset » Wed, 11 Sep 1996 04:00:00





> ......

> >> Open up the Sony SRF42.  First, remove the volume control knob (pulls
> >> out).  No need to rem> connections themselves, but look for 2 pads closer to the board edge
> >> that lead to the IC5 pins 2 and 4.

> >> I just connected the headphone jack to the audio amp, and used the
> >> SRF42 volume control to set a reasonable and undistorted signal
> >> level.

> >Haven't you noticed that most all auto radios have am stereo? Both of
> >our cars have them and I have one for my bench radio that's at least
> >10 years old.  Works fine on the few am stereo stations here.

> Daaah, what. I've never seen one yet. My truck dosn't
> have one, none of the other cars I've seen have one, not
> one aftermarket unit in Crutchfield has one.

> greg

Well, actually there are no marking on the panels of our sets that say
 AM Stereo until you tune in to a station that is transmitting am stereo
and then it lights up. The first car that I had it in was a 86 Plymouth
 Reliant and then a 88 Dodge. Look for one of those in the junk yards.
 The problem now is that there are only a few stations still using
am stereo, and in our area there were only two. Now one.
 It may be nobody is making the am stereo radios anymore, I can't say
for
sure, our newest car is a 91. Call a local am station and ask if they
know
of any am stereo stations in your area.   Good luck.

--
      *********

      *********

 
 
 

Improved AM stereo mod for your AM/FM stereo tuner, using SRF42

Post by Ed Eller » Wed, 11 Sep 1996 04:00:00


> Haven't you noticed that most all auto radios have am stereo? Both of
> our cars have them and I have one for my bench radio that's at least
> 10 years old.  Works fine on the few am stereo stations here.

Many (but not all) *factory* car radios these days have AM stereo, but
it's a very rare feature on aftermarket radios.
 
 
 

Improved AM stereo mod for your AM/FM stereo tuner, using SRF42

Post by Fredric J. Einste » Wed, 11 Sep 1996 04:00:00



>> >Haven't you noticed that most all auto radios have am stereo? Both of
>> >our cars have them and I have one for my bench radio that's at least
>> >10 years old.  Works fine on the few am stereo stations here.

Ford premium radios with cassette and/or CD players all have
AM stereo.  Also, while in Japan, I found that AM stereo is
really popular due to difficult FM propagation (due to
Japan's mountainous terrain).  Almost all stations in
Japan use AM stereo.  There's an excellent
AM Stereo/FM/Japan TV pocket radio made
by AIWA sold in Japan.  It tunes 76-108 MHz and
has selectable 9 kHz, 10 kHz AM band spacing.
I have one and it gives superb reception of AM
stereo.  They go for about $65.00 in Japan. It
would be a great idea if Grove or someone would
import a bunch of these things.  AM Stereo or not,
it's a great receiver!  AIWA does market a similar
unit here in the U.S. but it lacks AM Stereo,
the Japanese FM band, and TV band.
 
 
 

Improved AM stereo mod for your AM/FM stereo tuner, using SRF42

Post by Bill Newki » Thu, 19 Sep 1996 04:00:00



>> Haven't you noticed that most all auto radios have am stereo? Both of
>> our cars have them and I have one for my bench radio that's at least
>> 10 years old.  Works fine on the few am stereo stations here.

>Many (but not all) *factory* car radios these days have AM stereo, but
>it's a very rare feature on aftermarket radios.

i don't know if it's standard these days. got a new car last year and it
didn't have AM stereo offered as a factory option at all.

Bill Newkirk WB9IVR                  The Space Coast Amateur Technical Group
Melbourne, FL                       duty now for the future of amateur radio
Lombardi's 1st Law of Business:  
Companies succeed in spite of their best effort.  If they succeed at all.

 
 
 

Improved AM stereo mod for your AM/FM stereo tuner, using SRF42

Post by Tom Mors » Thu, 19 Sep 1996 04:00:00


> i don't know if it's standard these days. got a new car last year and it
> didn't have AM stereo offered as a factory option at all.

> Bill Newkirk WB9IVR                  The Space Coast Amateur Technical Group

For some reason they almost never mention that it exists. My sister bought a new
Mark VIII and it had AM stereo, but nothing on the brochure or owners manual
mentioned it.

Tom  KE6DIO