AM stereo mod, using a Sony SRF-42 walkman, for your home stereo system tuner or receiver

Using Chris Cuff's AM stereo mod kits in a tube AM tuner
Modify a Sony SRF-42 to make it a synchronous AM detector
See bottom of this web page for a short description of the Motorola C-QUAM™ AM stereo system
This is a short description on how to modify your home stereo receiver or tuner to receive AM stereo. It assumes you have knowledge of analog audio and radio frequency electronics, and practical details of using soldering irons and such. Read all of this web page before starting work.

The Sony SRF-42 is a portable AM stereo walkman radio. Radio Shack used to sell them, but you'll probably have to find it on ebay or such. The SRF42 uses an onboard IF filter of 450KHz, which may be slightly different than that used in many AM receivers. Ideally, your stereo receiver or tuner uses an IF of 450KHz, in which case you can tap this IF signal and feed it into the input of the IF section of the SRF42. Either way, you will need to disable the SRF42 AM front end, jumper short out the AM local oscillator LC circuit. See the accompanying picture.

The SRF 42 comes apart easily, remove 3 screws from the back (two under the batteries). You don't need to pry off the on/off switch knob, don't try. Radio circuit board will stay with the back. Actually, the back will make a handy package to hold things when you mount it inside your stereo receiver.

Do all these mods in a "non-destructive" process, that is, if things don't turn out satisfactory, you can undo everything neatly and restore everything back the way it was.


The following is what to do IF your receiver's IF frequency is 450 KHz:

Again looking at the picture, see the red coax (drawn in red) center conductor feeding a node connecting R26 and a ceramic filter. The node on the other side of R26 connects to IC1. The coax ground ties to a local RF ground, the center pin of the ceramic filter is a good point. The impedance of the coax is not a critical number, any thin coax will do. At the other end of the coax, the ground, thru a cap of 0.1uF, ties to your AM receiver's RF ground (a shield can on a coil associated with the AM section will do fine. The cap is to block any DC paths and ground loops that might cause noise on the audio. The center conductor, also thru a cap, this one about 0.05UF (values not critical) connects to a point around the output of the receiver's AM mixer stage, upstream of the receiver's IF amp stage. You will need to poke around with this connection to find a good point to tap. Insert batteries and headphones into the SRF42 and, with the AM receiver's tuning, find a moderately strong AM station, one in stereo would be even better. You may not even need to hardwire this connection, you may pick up enough signal just having an inch or two of insulated wire intermingled with the AM mixer and IF strip circuits.

Leave the receiver's IF strip connected to the receiver's front end, so its automatic gain control feedback circuits will still control the gain of the front end.


Do this if the IF frequency is *NOT* 450KHz, but 455HKz or some such nearby number (we will skip over the SRF42's IF filter, and make use of your receiver's IF strip filtering):

If the receiver uses a differing IF frequency, like 455KHz, or 460KHz, the approach would be different: tap off the receiver's IF strip just before its audio demodulator (usually a diode) and thru a DC blocking capacitor, inject it into the SRF42's IC1, pin 23. You'll probably need to cut the trace connecting the SRF42's own 460KHz IF filter to this pin, to avoid messing up the sidebands. You will need good surface mount circuit board handling skills to do this, though. Use coax shielded wire to carry the signal to this point, else you may pick up a strong nearby in town AM station everywhere on the dial. Be sure to use a DC blocking capacitor on the IF's ground, to avoid ground loops and differing DC potentials. You might need to insert some signal attenuation to avoid possible signal overloads into the SRF42's IC1, with a resistive voltage divider or series resistor in line of the above coax. You may find even after this, with the two IF amplifier strips in series, very good reception of distant signals.

Leave the receiver's IF strip connected to the receiver's front end, so its automatic gain control feedback circuits will still control the gain of the front end.

You will probably need to adjust the SRF42's L5, labeled "3.6MHz" to get the stereo to decode properly. Otherwise, you'll get living mono. Mark this adjustment before you adjust it, so you can restore it back if ever desired. This inductor is located on the topside of the board, so you'll need to remove the board from the plastic case to get at it.


Do this following for both of the above methods:

Assuming satisfactory success in receiving AM stereo stations with the SRF-42 fed from the receiver's AM front end, you would need to disconnect the old AM section's audio from the receiver's source selector switch, and patch in the stereo audio from the SRF42. Easiest thing to do here is to take the headphone output of the SRF42 and connect it to the switch. You may need to add 1K series resistors to add some impedance, so there will be less "bleedthru" when the switch is selecting another source. You can use the SRF42 volume control to set the level to be compatible with the other audio sources.

If the fidelity of the AM stereo isn't that good as compared to that of the SRF42 when you first got it and before mods, the receiver's AM antenna front end circuit may be picking up more of one sideband than the other. this will cause some distortion. Try loading this LC circuit with a resistor around 10K. This will cut gain, but should help even out the asymmetry of the sidebands.

If the audio seems too hissy, try attaching a pair of caps to low pass the audio some, at the points indicated in the picture. These points are on the back side of the volume control pot. Other end of the caps connect to the silvered outermost circuit board trace. Try 0.05uF. Both caps should be the same value, though.


You'll need to build a 3 volt power supply, using an LM317 regulator should be enough. I found that I could feed it with more voltage, a few more volts, though I wouldn't try more than 5 volts (do this at your own risk, YMMV). The negative battery terminal may be connected to the headphone ground, so you just need to supply the positive terminal.
A 16 bit 11KHz WAV file, using this mod, in AM stereo at this link: 960 KABL (Oakland, CA)
Stereo indication circuit

The SRF 42 doesn't provide a "stereo signal being received" indication, just a strong signal indication. A pilot tone at 25Hz is transmitted on the quadature channel, it's about 5 to 10% of full signal strength. This pilot just serves to indicate stereo; it isn't needed to decode the stereo signal (unlike FM stereo, where the pilot IS necessary to decode that stereo). If a true stereo signal indication is desired, one needs to build a 25Hz tone detector. I used a 567 tone decoder chip. To get the signal strength high enough for this chip to sense, I also built a low-pass op-amp circuit designed to pass frequencies below 50Hz. This would help improve the 567's performance, as it won't be hit so hard with regular difference audio.

The stereo difference channel can be found on the SRF42 circuit board on the large chip near the negative battery terminal connection. It's on pin 2 of that chip, right next to the edge of the board. A transistor's base is connected to this pin, to act as a buffer and gain stage. This transistor in turn feeds the low pass op-amp.

The output of the 567 chip can be coupled into the receiver's FM stereo indicator, in an OR gate logic fashion. You may see the indicator flicker from time to time, as reception varies. You probably don't want to use this indication to automatically switch from stereo to mono, otherwise it will get too choppy. Best to leave that selection manual. I used the receiver's factory AM section's output as the mono source, and the SRF42's output as the stereo selection.


Modifications as done with an Akai AA-R32 receiver:

Or you can use one of Chris Cuff's AM stereo mod kits to modify your tuner or receiver. I used one in place of the above SRF42 mod in my Akai AA-R32 set and it works very well. And it is much smaller.

Modifying the AM section of a tube stereo receiver. One of
Chris Cuff's AM stereo mod kits can be used in the AM section of a tube tuner. To get the IF signal from the tube tuner's AM IF strip, create a "gimmick" capacitor by lightly wrapping a piece of insulated wire around the line from the last AM IF transformer to the tuner's AM detector tube. All you need is a picofarad or so of capacitance. And this won't detune the IF transformer. Chris' board has enough sensitivity to operate on this small amount of signal. Use coax between the tube IF and the board. The center conductor of the coax would be the above mentioned insulated wire. Leave the old AM detector circuit in place so the AGC function continues to work. You will need to install a 470K resistor, to load the tube diode detector (so automatic gain control (AGC, aka AVC) still works), in place of the volume control if you disconnect the old volume control from the old AM detector..

It will take a second or two to achieve stereo lock after you finish tuning in an AM stereo station.


Synchronous AM detection with modified SRF-42 board
Unfortunately, there's no AM stereo signals in my town anymore. But I can still make use of the SRF42 board as a synchronous AM detector. IC3, the CXA1758N has separate IF inputs. One feeds the sync oscillator, and the other feeds the detectors. To get the L-R sync detector to detect the mono info, we can insert a 90 degree phase angle delay circuit between these two inputs. The old in phase detector becomes a quad detector, and should detect almost no audio. Adjust the new phase shifter for lowest detected audio here. The envelope detector will still detect the audio like it did before. It's not clear where exactly the envelope detector actually resides, if it's in the "BM" block feeding pin 7. The old in phase detector is likely also in that block, and possibly the "mute" signal switches from one to the other. When the sync osc loses lock, the old quad detector mutes off. Be aware that, when this mod is complete, the reception will mute off if there is no signal being received. Pin 7 continues to output signal. According to the description of AM stereo reception below, the difference between the envelope detector and the in phase sync detector would be used to correct errors in the quad sync detector by a division process. This should trash the old quad sync detector now the in phase sync detector. This doesn't happen here, as this CXA1758N chip has nothing feeding into its "div in" pin (#2). Being in a portable walkman, the extra distortion caused by not correcting the L-R signal might not have been noticeable.
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Short Description of the C-QUAM™ Motorola AM Stereo System
Click
here for John Byrns' article on AM stereo.

The Motorola C-QUAM AM stereo system is based on a quadrature modulation (QUAM) of the AM radio station's carrier. But that's not the entire story. As most all AM radios use envelope detectors, the quadrature scheme needs to be modified (C-QUAM) some to make the L+R (mono) signal (which would be the "in phase" modulation) be detected correctly by an envelope detector when there is also the difference (L-R) "quadrature phase" modulation present. Mathematically speaking, the mono (L+R) signal is "multiplied" by a sine wave at the radio station carrier frequency. To be sure to get both sidebands AND a carrier, we add "1" to L+R. Else we get double sideband suppressed carrier, which an envelope detector can't handle. Add the difference (L-R) in quadrature (multiply by a cosine wave at the radio station frequency), and an envelope detector will detect (for a QUAM signal):

Ö((1+L+R)2+(L-R)2)
which isn't the same as the mono (L+R) signal desired in mono receivers. Only if the (L-R) part is zero does it work correctly. That's why envelope detectors work for mono AM radio stations. C-QUAM modifies this to make the envelope detection of a stereo signal come out right. If "t" (theta) is the instantaneous angle between what the sine (L+R) axis is and what the envelope detector yields when it detects the above QUAM signal, we can modify the signal that gets fed to the "in phase" sine multiplier as (1+L+R)cos(t). And modify the signal fed to the "quadrature" cosine multiplier as (L-R)cos(t). An envelope detector now will yield 1+L+R, which is correct sounding mono.

Now, we will have three detectors operating in the receiver: an envelope detector, and a synchronous system with an "in phase" and "quadrature" detectors. The envelope detector yields the mono (L+R) signal. We will use this to compare with the "in phase" detector output to determine what "cos(t)" is, so we can then divide the "quadrature" output ((L-R)cos(t) to get just (L-R). After that, we combine the (L+R) and (L-R) signals to get L and R to feed into the stereo amp and speakers.