Turning your FM Mono tuner into FM Stereo

Renslipevol

Renslipevol

Active Member
I have been thinking about this for a while now. The reason is, it's just signal routing, in my opinion. I have an external, tube based multiplex demodulator to do the stereo signal crunching, but I find its terrible. It needs alignment and I really not interested in getting more test equipment that is a unitasker!

The plan is to install a UA758 IC onto the multiplex output from the tuner (on the inside) and have L and R, and AM outputs from the tuner.

I've figured i've entered the realm of tuner-nerd-ville. :yes::no:

Im wondering if anyone else has done this with older tube tuners? I would not normally put IC's into tube audio or tuners, but I figure its just signal routing. :scratch2:
 
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I would think you might need a separate filtered and regulated power supply for the IC, unless of course your tuner has both 6 and 12v heater lines in it... What does The IC use +- 12V? It will need to be super clean to get a decent noise free output. You'll also need to have two AF amps instead of a single one for the MPX output and change the output of the AM section to feed into both AF tubes.

If anyone else has done this, I'd be curious in doing this myself.

*EDIT* and don't forget about adding the separation, stereo threshold, and 19 / 38khz tank devices.

If you really like the tuner and want it compact, play with it. However I think I would just get the MPX unit recapped and aligned.
 
The voltage is 6-18vdc. I plan on building a voltage doubler off of the 6.3V heater circuit.

The literature says nothing about needing to add other components for seperation, threshold, and tank devices?

Here's a pinout

ua758.jpg
 
I did this myself years ago. I had an HH Scott mono tuner, and I added a board with an LM1800 PLL MPX decoder on it. I tried the voltage doubler. This was troublesome because, as I recall, I could not get a good audio ground because of the doubler, and I had lots of hum. I ended up adding a second transformer and supply, and that worked fine. No other issues, and the PLL circuit has only one adjustment pot, which you can really just adjust for the middle of the range that it receives stereo, so no test equipment is needed. The LM1800 is no longer available, so go with what you have.
 
I've been wanting to build a multiplex decoder to use with my mono tuners. This is great!
 
Those LM1800 and the UA758 are still out there in fleabay, the UA758 is half the price though. As I understand it, the UA758 is the 'up-to-date" version of the LM1800

Dr*Audio, now that you brought it up, there really isn't a -, but a 'ground' of some kind. I will have to put in transformer. Thanks for saving me the headache. And I was contemplating asking about this issue before I dove into it.
 
I'd go with a separate power supply. I wouldn't take the audio directly off the MPX chip. You'll need a 75usec de-emphisis, and some sort of Low pass filter/amp would be nice also. You'll have 19 and 38Khz residual. On the audio.
 
Hi PunkerX,

Got you an idea! Why not design a complete external FM Multiplex unit for your tuna. Doing it outboard would be a boon for those with bum MPX sections or those with mono only tuners. :tresbon:
 
Punker X said:
I'd go with a separate power supply. I wouldn't take the audio directly off the MPX chip. You'll need a 75usec de-emphisis, and some sort of Low pass filter/amp would be nice also. You'll have 19 and 38Khz residual. On the audio.
Click to expand...

Hmm...From my other diagram, and literature, pins 3 and 6 are the left and right de-emphasis.
Here is some more information if this helps. From what Im reading, I thought this was all I needed. Then again, I know almost nothing about radio. I will experiment and let everyone know what the scoop is.
ua758-3.jpg

ua758-2.jpg
 
If you go back to the applications schematic posted earlier, you will see the de-emphasis filters on pins 3 and 6. Just use those values and it should be ok.
 
I had a look at the datasheet. The output impedance is not specified, but they show a schematic. There are emitter follower buffers on the outputs, with 1.3K resistors in series, so the output impedance will be 1.3K. An additional output buffer isn't necessary unless you want to drive more than 6 feet of cable with it.
 
External MPX Adapter: Done

Well. I orderd some capacitors, and gathered a bunch of parts and I finally built it. Its Done, and it works better than I could have expected. No adjustments. :banana: Crystal clear FM Stereo from the MPX out of my Heathkit AJ-11. Here are some pics. I have the electronics to build two more of these. I just might do that. :music:
MPX.jpg


mpx2.jpg


mpx3.jpg
 
Nice job! I like those cases, too. I have used them at work and at home. It's easy to make a nice looking piece of gear with those.
 
dr*audio said:
Nice job! I like those cases, too. I have used them at work and at home. It's easy to make a nice looking piece of gear with those.
Click to expand...

Yes, They really are worth the $20. Easy to stuff, and so versatile. Next time, I will have to find the circuit board to fit that size box, then its no more mounting, they just slide into the builtin tracks...Im sure you know about that if you've used them before.

Very simple to put together actually...All in all, I spent about $60. And 2/3 of that was the case and the 12 volt switching power supply. Ohya, I cannot forget to thank all the other electronics that were recyled to make this project possible...
 
Renslipevol said:
Yes, They really are worth the $20. Easy to stuff, and so versatile. Next time, I will have to find the circuit board to fit that size box, then its no more mounting, they just slide into the builtin tracks...Im sure you know about that if you've used them before.

Very simple to put together actually...All in all, I spent about $60. And 2/3 of that was the case and the 12 volt switching power supply. Ohya, I cannot forget to thank all the other electronics that were recyled to make this project possible...
Click to expand...

All you have to do is score the breadboard with a hobby knife and break it on a surface with a flat edge, like a table. I use these breadboards:
http://www.mpja.com/prodinfo.asp?number=15912+PB
 
drknstrmyknight said:
Deva vu?
Click to expand...

That is weird. I've never seen that thread before, even when I did a search. I even have that EICO MX99. That MPX adapter is probably the reason that we both did a IC setup...

Dr.audio....I am wondering if I should have ganged up the negatives on the input/outputs. For some reason, now, when I turn up or down the volume on the amp, the woofers extend and retract.....very odd.
 
Renslipevol said:
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For some reason, now, when I turn up or down the volume on the amp, the woofers extend and retract.....very odd.
Click to expand...

That might indicate you have DC voltage present on the volume pot. You may want to use a multimeter to measure, and see if that is so. It may take a long time after turning the unit on for it to go away. Typically a resistor to ground, somewhere in the circuit after the coupling cap bleeds this DC away. It is usually on the preamp input.

Your board is what my first 3 looked like, then I had boards made - www.fmmpx.com
I also used a similar black enclosure for my last build. I recommend an external linear wall wart supply, which is readily available and cheap (under $8.) Switcher supplies have a lot of noise.
Bob
 
It has something to do with the MPX adapter I made. The amp does not cause the woofers to go in/out in AUX mode. Even with the tuner off it will do this. When the MPX adapter is switched off, it also happens. Hmmm....
 
Well, in the last couple of weeks of listening to 93.7, CKUA in Calgary, at least twice i've heard a really high pitch, could have even been 19Khz, but who knows.....
 
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