So you guys have a boombox. The tuner captures mono FM stations just fine but no matter what you do, it will not capture Stereo FM. You suspect that the issue is with the FM demodulator so you..........
(1) Adjusted the stereo pot to no effect
(2) Replaced the MPX decoder chip with no effect.... or
(3) the MPX decoder chip is obsolete and no longer available.... or
(4) the MPX decoding function is accomplished with discrete circuitry which is too complicated for you to comprehend.
But having FM without Stereo sucks, and you really really really want STEREO FM.
=============
Here is a solution for you guys.
It's easy in that it involves a single MPX decoder chip but unlike many decoder chips out there, this one uses a minimum of external components, has no adjustments save a single adjustable VR which you simply center at the point where FM lamp is illuminated.
HERE IS WHAT YOU DO:
Use the National Semiconductor LM1800 PLL (phase locked loop) FM Stereo Demodulator chip.
Bad news is that it's obsolete and no longer available from fresh stock. So an option is to go with the UA758 chip, which is pin for pin compatible.
Bad news is that the UA758 chip is obsoleted too, no longer available.
FORTUNATELY, NTE does manufacture a generic replacement for those chips. Use part no NTE743, which is still availabe...... for now.
Here is the NTE datasheet on this chip: http://www.nteinc.com/specs/700to799/pdf/nte743.pdf
Here is the current stock listing remaining for this chip. ONLY the 743 is applicable: http://dilp.netcomponents.com/cgi-bin/nteinc.asp?partnumber1=NTE743&partnumber2
The NS LM1800 datasheet is out there on the web. Just google it, if you want to review it.
==============
Here is how you do it:
Build the circuit below. Anybody handy with a soldering iron and some basic electronic skills should be able to build this on a small chip board.
(1) Inject a mono audio signal into pin 1.
(2) Pins 5 and 6 are your stereo L/R outputs.
(3) The stereo panel lamp is grounded via pin 7 and sourced from V+. If using an LED, an appropriate resistor should be installed in series. Depending upon normal operating voltage, the resistor value could change. Use VR if in doubt to adjust for best results.
(4) Best to use a frequency counter to adjust the oscillator-adjust pot. To do so, attach frequency counter to pin 11, and adjust the POT until you get 19khz +/- as little as possible. There is an alternate method which is not as accurate since the oscillator will naturally center at 19khz with strong stations but not so with weak stations where the accuracy of the frequency counter is really best. But anyhow, the simple alternate method is to merely center the pot between the two pot positions where the LED just begins to light, adjusted from the extremes in both direction. Only note when lamp lights, not when it extinguishes.
(5) That's it! All the resources you need to do this is here in this post. The rest is up to you!
(1) Adjusted the stereo pot to no effect
(2) Replaced the MPX decoder chip with no effect.... or
(3) the MPX decoder chip is obsolete and no longer available.... or
(4) the MPX decoding function is accomplished with discrete circuitry which is too complicated for you to comprehend.
But having FM without Stereo sucks, and you really really really want STEREO FM.
=============
Here is a solution for you guys.
It's easy in that it involves a single MPX decoder chip but unlike many decoder chips out there, this one uses a minimum of external components, has no adjustments save a single adjustable VR which you simply center at the point where FM lamp is illuminated.
HERE IS WHAT YOU DO:
Use the National Semiconductor LM1800 PLL (phase locked loop) FM Stereo Demodulator chip.
Bad news is that it's obsolete and no longer available from fresh stock. So an option is to go with the UA758 chip, which is pin for pin compatible.
Bad news is that the UA758 chip is obsoleted too, no longer available.
FORTUNATELY, NTE does manufacture a generic replacement for those chips. Use part no NTE743, which is still availabe...... for now.
Here is the NTE datasheet on this chip: http://www.nteinc.com/specs/700to799/pdf/nte743.pdf
Here is the current stock listing remaining for this chip. ONLY the 743 is applicable: http://dilp.netcomponents.com/cgi-bin/nteinc.asp?partnumber1=NTE743&partnumber2
The NS LM1800 datasheet is out there on the web. Just google it, if you want to review it.
==============
Here is how you do it:
Build the circuit below. Anybody handy with a soldering iron and some basic electronic skills should be able to build this on a small chip board.
(1) Inject a mono audio signal into pin 1.
(2) Pins 5 and 6 are your stereo L/R outputs.
(3) The stereo panel lamp is grounded via pin 7 and sourced from V+. If using an LED, an appropriate resistor should be installed in series. Depending upon normal operating voltage, the resistor value could change. Use VR if in doubt to adjust for best results.
(4) Best to use a frequency counter to adjust the oscillator-adjust pot. To do so, attach frequency counter to pin 11, and adjust the POT until you get 19khz +/- as little as possible. There is an alternate method which is not as accurate since the oscillator will naturally center at 19khz with strong stations but not so with weak stations where the accuracy of the frequency counter is really best. But anyhow, the simple alternate method is to merely center the pot between the two pot positions where the LED just begins to light, adjusted from the extremes in both direction. Only note when lamp lights, not when it extinguishes.
(5) That's it! All the resources you need to do this is here in this post. The rest is up to you!
And why are we up at three in the am thinking of stuff like this? 
I miss my BASSment and the testing toys whithin... 
:super: :super: :superduper: 
Great solution, Norm. Sounds like a small stock of those chips would come in handy someday...
:superduper:
:grim: 
