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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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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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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 2630 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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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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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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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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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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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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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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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Aug Fri 30, 2019 10:52 pm 
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The link you had is broken but here is the working one. https://www.ebay.com/itm/143366789089


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Aug Sat 31, 2019 2:37 am 
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That kit looks nice but screw terminals may have been a better choice.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sat 21, 2020 3:16 am 
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There is some confusion in terminology. Each FM channel occupies 200KHZ of bandwidth. Of this 50khz is never used so as to create a guard band to protect adjacent channels. So you need 150khz of IF bandwidth to recover 100% of the audio. Any thing less than that and the audio wave form is distorted. The figure you sight is the recovered audio band width not the IF bandwidth. It is not necessary to have a audio band width up to 53 KHZ to recover the R and L signals. Most detectors roll off just above 38KHZ. The upper side band of the L-R sub-carrier is redundant information and tends creates noise if the range is extended up that high. In addition there are other services and channels just above 53 KHZ superimposed on the audio. These include SCA which is a second audio channel and RDS which is digital data usually used for displaying program content. These these spill into the subcarrier and create what I like to call Hawaiian noises. Very faint but annoying on quiet passages.

The LM1310 and the later LM1800 were developed for car radios in the seventies and as such not much was done with noise from weak signals, SCA RDS signals. A car is a much noisier environment than a living room so it's tolerable and goes unnoticed. A second generation of PLL decoders were developed for home units which had much better noise rejection and dynamic range. This was done with sample and hold techniques used in the UPC and it's other clones during the late 80's and early 90's. They added a adjustable separation control which the early chips lacked. They have better channel separation and quieting with weak signals.

The latest decoders are part of chips that perform RF front end IF and decoding all in one chip the idea being cram as much circuitry into a little space as possible. Most of these have poor separation but who cares on a mini book shelf system where you cant hear it anyway.

Whats most important with the old vacuum tube sets is hooking the decoder before the de-emphasis network. Some times components from the de-emphasis network have to be removed. In the early days there was no multiplexing so the placement of a MPX jack if it even existed was an educated guess. Many old tube detectors overdrive the newer decoder chips requiring a pad in between them.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sat 21, 2020 2:48 pm 
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Location: Utica, NY 13502 (USA)
Robert - I once had a boss, who was a GE design engineer, tell me that an FM signal has sidebands that, theoretically, extend way beyond the 200 kHz allocated channel and that, for lowest possible distortion, the detector must capture all of them (or as many as practical). I didn't think an FM carrier could even generate sidebands other than its own instantaneous frequency. Was he just blowing smoke?

Dave


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sat 21, 2020 3:42 pm 
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Dave Doughty wrote:
Robert - I once had a boss, who was a GE design engineer, tell me that an FM signal has sidebands that, theoretically, extend way beyond the 200 kHz allocated channel and that, for lowest possible distortion, the detector must capture all of them (or as many as practical). I didn't think an FM carrier could even generate sidebands other than its own instantaneous frequency. Was he just blowing smoke?

Dave


It was originally thought that FM would have a very narrow bandwidth, but the bandwidth of an FM modulated signal is theoretically infinite. In practice it's restricted to a finite bandwidth, although the more bandwidth you give it, the lower the distortion.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sat 21, 2020 3:50 pm 
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Look up "Bessel Functions"----truly messy math.

The practical answer is fundamentally the same for both AM and FM---you have to have enough bandwidth so that the end-user sound quality is "good enough". This is often empirical.

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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sat 21, 2020 4:30 pm 
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Location: South Central Montana
Robert,
Thank you for your concise explanation.

T.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sat 21, 2020 5:22 pm 
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So, without dealing with truly messy math, while +75 and -75 kHz (150 kHz total) is the carrier modulation limit of FM broadcasting, the actual IF bandwidth of a receiver needs to be much wider than 150 kHz in order to capture most of the sidebands without having audio distortion. Is that a correct assumption?

Dave


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sat 21, 2020 10:12 pm 
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That is correct -- the sidebands extend very far from the center frequency, as described by the Modulation Index.

-David


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Tue 24, 2020 3:06 pm 
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The bandwidth of the IF has to reject adjacent channels. Typically bandwidth is defined as the frequencies at which the signal drops buy 3 db either side of the center of the channel. So the 3 db points occur for a 10.7 mhz IF occur at 9.95 mhz and 11.45mhz. Some manufacturers tweak these depending their particular circuit. A raw FM signal has side bands that extend beyond this but they are filtered out at the transmitter and are not needed to recover the information transmitted.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Tue 24, 2020 5:01 pm 
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robertjt wrote:
A raw FM signal has side bands that extend beyond this but they are filtered out at the transmitter and are not needed to recover the information transmitted.

Where? The exciter? The final RF tank circuit? I don't think so.

-David


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Tue 24, 2020 5:08 pm 
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pixellany wrote:
The practical answer is fundamentally the same for both AM and FM---you have to have enough bandwidth so that the end-user sound quality is "good enough". This is often empirical.

"Good enough" has to be plenty good. FM stations are required by the FCC to annually demonstrate and document harmonic distortion of 1% or less from 50Hz to 15kHz. AM stations 5% from 50Hz to 10kHz.

-David


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