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 Post subject: An FM Multiplex stereo decoder board that works!
PostPosted: May Fri 31, 2019 12:06 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 302
Location: Windsor, CT USA
I've been trying to align the multiplex section of my Sherwood S-8000 without success. After some research, I determined that my S-8000 has the same multiplex circuit as a Sherwood SM3X multiplex adapter. Unfortunately, I pretty much got nowhere. One slug of one of the coils was either plugged or frozen. I wasn't able to peak the frequency doubler coil. The result was no multiplex (but good FM otherwise).

So, I bought one of these: https://www.ebay.com/itm/143194185056

Anyway, after a simple hookup, it worked great! I have no financial (or other) relationship with this ebay seller. I just thought this would be of interest to anyone with a stereo tuner with no multiplex output, or a mono tuner where stereo was desired. I took the output directly from the 6AL5 detector. I installed a 50k trimmer pot to reduce the audio signal to the converter PCB. Power was derived by hooking in parallel to the 6.3VAC 6AL5 filaments (pin 4 and 5). I disconnected the existing multiplex output coax connections to the selector switch and substituted the output of the converter. The converter comes with it's own stereo pilot LED, which I put in place of the stereo light on the front panel. The PCB is so tiny that it fits behind and underneath the stereo/mono selector switch. I covered the back of the PCB with black electrical tape. The board is held loosely in place by the pressure of the connecting wires in that area. The board came ready to use; no adjustments were necessary. In the example I purchased, the IC was socketed. I left the original multiplex section in place. However, it no longer processes any signal.

I wish I had the skills and equipment necessary to repair the original multiplex circuit. However, since I intend to actually use this receiver, I wanted this problem resolved quickly. The board cost $70 including shipping. Not cheap, but very convenient!


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: May Fri 31, 2019 2:03 am 
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Joined: Sep Tue 30, 2014 6:08 am
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Location: Norfolk, VA
Looks like NEC's version of the venerable MC1310 FM MPX decoder - no tuned circuits needed. I converted a Magnavox and a Curtis Mathes to stereo with that IC back in the 80s. I used the ECG801, the MC1310 equivalent.

Attachment:
stereo-decoder-circuit.png
stereo-decoder-circuit.png [ 27.06 KiB | Viewed 469 times ]

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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Jun Sat 01, 2019 4:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 14543
Location: Fernandina Beach, FL
Findm-Keepm wrote:
Looks like NEC's version of the venerable MC1310 FM MPX decoder - no tuned circuits needed. I converted a Magnavox and a Curtis Mathes to stereo with that IC back in the 80s. I used the ECG801, the MC1310 equivalent.

Decades ago, I tried to build a multiplex decoder with an IC, but it used some coils/transformers. It never worked right.

This little guy looks like the ticket!

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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Jun Thu 06, 2019 3:04 pm 
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Location: Warner Robins, GA
mstamper wrote:
One slug of one of the coils was either plugged or frozen

I wish I had the skills and equipment necessary to repair the original multiplex circuit. However, since I intend to actually use this receiver, I wanted this problem resolved quickly. The board cost $70 including shipping. Not cheap, but very convenient!


I've sometimes used a little oil of some sorts to free up frozen coil slugs. Also if the slug is near the top of the coil form, some heat applied by a soldering iron might free it up.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Jun Thu 06, 2019 6:35 pm 
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Location: Mission Viejo, southern California
Interesting! How would it be connected to get stereo from a mono tuner?

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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Jun Thu 06, 2019 7:11 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 302
Location: Windsor, CT USA
"How would it be connected to get stereo from a mono tuner?" I connected it to pin 5 of the 6AL5 detector tube through a 100k trimpot. One side went to ground, the other side went to the detector, and the wiper went to the input of the pcb. I adjusted the trimpot for maximum volume with no distortion or overload. Power was supplied from the 6.3vac filament voltage on the 6AL5. IF section must be aligned properly to pick up the 19khz pilot.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Jul Thu 18, 2019 6:24 pm 
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Location: Beaver Falls, PA. USA
That is a PLL (phase locked loop) chip, and it works very well. To set the pot, tune in a station broadcasting in stereo, and adjust it until the LED lights; that's all there is to it.

On an older tuner, the input to the decoder must be ahead of the de-emphasis network. Some had a jack just for that purpose.

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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Jul Thu 18, 2019 7:22 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20058
Location: Warner Robins, GA
Also not all mono tuners will work. The tuner must have an IF bandwidth of 53KHz and be flat to 53KHz in order for the stereo signal to be properly demodulated

http://www.audiomisc.co.uk/HFN/BandwidthBlues/Fig1.gif


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