Author Topic: Magnavox Multiplex FM stereo adapter  (Read 782 times)

electra225

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Magnavox Multiplex FM stereo adapter
« on: January 22, 2017, 12:16:48 PM »
I have recently become interested in how these devices operate.  I would like to offer this discussion as information, based on my experience not scientific evidence, to those who may also be curious.  The electronics gurus among us may not be terribly impressed, but I find these little doodads rather ingenious in how they work and miraculous in what they do.


FM stereo decoders, herein known in Magnavox-speak as a Multiplex adapter (or simply MPX) are designed to seperate the stereo composite signal and feed such signal to the amplifier to provide stereo FM.  FM is broadcast on a wider band of frequencies than AM and the stereo composite signal is a part of that bandwidth. 


https://en.wikipedia.org/wiki/FM_broadcasting


The article I linked will provide technical information on FM broadcast to keep this thread from becoming a book.


Information and schematics on Magnavox Multiplex adapters are available on this forum at the top of the page, under Downloads then just go to Magnificent Magnavox and click on what you want.


There are two basic styles of MPX adapters for Magnavox tube stereo instruments.  A two-tube model and a three-tube model.  I have used the two styles interchangeably on instruments with push-pull audio amplification.  My guess as to why two versions were built would be that the single-ended amplifier instruments may not have had enough pre-amplification, so a 6EU7 audio amplifier was included in the three tube version.  Both types utilize audio output controls for each channel.  I don't know this to be fact, however, so if any of you have more definitive information, and you would care to share it, that would be great.


My concern here is how does one repair one of these things that appears to be on the fritz?  The only Magnavox instrument I owned that had an MPX when I got it is my Symphony.  It is a model 1ST655 which was not factory-equipped with an MPX.  That would have been model 1ST656, the same instrument, but equipped from the factory with MPX.  All my instruments expect the 2ST653 are too old to have been factory equipped with MPX.  The first MPX I bought for retrofitting my collection was from a member of this forum.  I was a bit apprehensive at first.  What if it did not work?  I decided that I needed to figure out what an MPX was and how it worked, at least enough to fix it if it broke.  The first one I bought worked very well, at least long enough for me to play with it a bit.  The second one I bought, off another forum, did not work very well.  It suffered from extremely weak output. 


There is not much to an MPX and so not much to go wrong.  Basically two tubes and their sockets, three electrolytic caps and one paper cap.  Occasionally one finds a drifted resistor.  The balance of the caps are ceramic.  There are three transformers that look like IF cans.  They kinda are, but not exactly.  That helps, huh?   ;)


If you look at the schematic, you will see a 30Uf electrolytic can cap mounted on top of the chassis at one end.  This MPX already gets rectified B+ from the 5U4 on the amplifier chassis, so why does it need a filter cap?  The B+ for the MPX branches off right after the first filter cap in the amplifier, so the one on the MPX serves as the second filter cap and also helps, I suspect, with hum suppression.  There is a metal cover for the bottom of the MPX, I suspect also to help shield the MPX from stray signals.  I seldom (okay, never) change this filter cap.  I have never seen one bad, and am of the school of thought "if it ain't broke, don't fix it".  There are different opinion about changing filter caps, so use your own judgment and preference on this.  Next is a .22uf paper caps across the line.  I change it just because it is a paper cap.  It is there to help with hum suppression.  Then there is the reason most MPX adapters display weak output sooner or later.  There are two 4Uf@50 volt electrolytic coupling capacitors.  I usually replace these with  non-polarized caps I bought for speaker crossovers by the bag from Radio Shack.  These two e-caps get leaky and weaken the audio output on one or both channels.  These MUST be changed from jump street if you expect dependable, long-term performance.  I test the tubes and clean the tube sockets.  I like to do a long time tube test, meaning putting the tube on the tester and covering it to get it good and hot.  Magnavox liked to mount the MPX in the hottest part of a typically poorly-ventilated cabinet.  The 6EA8 has proven to me to be sketchy in operation at times.  I like to make sure it is not gassy and has no shorts.  The 12AT7 should be well balanced between triode sections.  And, of course, not have gas or shorts.  I am wanting to experiment one of these days by replacing the 6EA8 with a 6GH8A and the 12AT7 with a 12AX7 for a bit more gain.  I like replacing the 6EA8 in tuners with the GH8A, but that is a topic for another day.  Then I give the two gain controls a good squirt of contact cleaner just because.


This brings us to those three IF can looking transformers.  To make it really simple, imagine those transformers to be the oscillator and second detector sections of an AM radio.  The first one is tuned to a band of frequencies, the second two to two different frequencies.  After reading the article I linked above, we see that a mono FM tuner uses the L+R signal so that all the music is heard in one speaker.  The MPX generates the L-R signal to seperate the two channels the way they were broadcast.  If these transformers are "misaligned" audio output may be weak and there may be additional noise and hash heard in the speaker.  These transformers may be adjusted by ear, just as an AM radio can under normal circumstances.  One can use a signal generator and a scope if they would care to, but I typically touch up this adjustment by ear.  I seldom find it needs adjustment, but it is fun to play with.  Adjust for maximum signal and minimum noise.  Do the ones closest to the speaker first and work backwards.  Typically, if the first one changes operation appreciably, I touch up the first two I adjusted again.  The 6EA8 creates the signal to seperate the audio channels.  The last two transformers, each tuned to different frequencies (see article) then send the audio signal to the amplifier.  Nothing to it, right?   :)


The original cables that connect the MPX to the tuner chassis were shielded.  I have used common old stereo patch cords and they work just fine.  Lead dress in the cabinet is important.  Try to keep the big power cable with the Molex connecter seperated from the audio cables.  The lead dress is so horrible in Magnavox instruments, I am in constant awe of how they work at all.  Such beautifully engineered and built equipment with no consideration for lead dress, and the resulting rat's nest in the cabinet.


These simple steps should give you a working Magnavox Multiplex stereo adapter.  The added benefit, to my way of thinking of an MPX is reduced fade, particularly in FM, and enhanced audio, again particularly in FM bass response.  Like most older mono FM tuners, our beloved Magnavox mono, stereo-compatible tuners suffer from the inability to demodulate and reproduce the Fm digital stereo composite.  The MPX appears to help remedy these conditions considerably.  These suggestions assume a properly working and calibrated FM tuner and a good antenna.  The bottom cover can be polished with Simichrome and elbow grease if it shows and if you care how it looks.  The ceramic caps in the MPX typically cause no problem, so I ignore them.  I test the resistors, but have never found one drifted enough to change.  Most original tubes I find are still good.  I suspect an MPX is not driven very hard, so is easy on components.  Heat is the big concern for me.  I have also discovered that it is not necessary to know exactly how an MPX works to repair one.  Only that you are able to figure out what to do if it does not work!   :)
If it ain't broke, call me.  I can break it....

19and41

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Re: Magnavox Multiplex FM stereo adapter
« Reply #1 on: January 22, 2017, 01:21:46 PM »
Good article!
"Any sufficiently advanced technology is indistinguishable from magic."
Arthur C. Clarke

voxACthirtee

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Re: Magnavox Multiplex FM stereo adapter
« Reply #2 on: January 22, 2017, 08:21:54 PM »
I've got a couple of units that i will be adding the MPX adapter to at some point so it i'm sure this will be elightening.

If you look at the schematic, you will see a 30Uf electrolytic can cap mounted on top of the chassis at one end.  This MPX already gets rectified B+ from the 5U4 on the amplifier chassis, so why does it need a filter cap?  The B+ for the MPX branches off right after the first filter cap in the amplifier, so the one on the MPX serves as the second filter cap and also helps, I suspect, with hum suppression. 

To offer an FYI/basic power supply answer to your question.....
Every time you have a resistor that just lowers voltage or current, that doesn't terminate in a tube somewhere, you need to add a cap.
The mpx schematic i have shows a 2 watt resistor. If you don't add a cap after that resistor you will get motorboating.

The "plate resistors" in a circuit won't need a cap as they set the plate voltage.
But you will will need a cap after the  blocking resistor, prior to  plate resistors .

Typical Flow chart in an AMP -
Rectifier - CAP (DC to Power tube plates)- Choke or Large resistor(5 watt) - CAP (DC to Power tube Screen Grids) - Medium Resistor(1 or 2 Watts) - CAP(DC to Driver/inverter tube) - Resistor(half watt) - CAP(DC to preamp/input tubes).......etc

electra225

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Re: Magnavox Multiplex FM stereo adapter
« Reply #3 on: January 22, 2017, 11:03:19 PM »
I hope this thread helps somebody else who may be wondering about repairing their MPX.  Or how to add one if they do not have one.  They are not common, but are not rare.  They frequently come up for sale on ebay.  Somebody on this forum may be kindly disposed to sell one.  They are typically available from $15 to $35 or so.  There are short cable and long cable versions available along with two and three tube versions.  I have never had one that did not work at all, or was totally kaput.  There should be sufficient cable length to connect the MPX to the rest of the electronics and run it out of the cabinet for test.  My thinking is that a totally kaput MPX would be diagnosed and repaired like a radio.  I have tinkered with the adjustments enough to be convinced that even "happy fingers at home" could not likely "adjust" one enough to render it totally mute.  It should cause squeal or other noise to go to the amplifier when switched to the "MPX Stereo" position on the function switch.  If it is completely dead, my belief is that there would have to be a power problem somewhere.  Either way, if you have capability to fix the rest of the instrument, you should not have a huge issue working with an MPX adapter.
If it ain't broke, call me.  I can break it....

bobstone

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Re: Magnavox Multiplex FM stereo adapter
« Reply #4 on: December 16, 2017, 03:37:33 PM »
I have a 1958 2sr295hr that I want to add the fm ad
aptor.
It has mpx connections on the tuner but no fm mpx
Selection on front of the unit.  Will the stereo adaptor
Work on the fm setting ?

Bob with a  Magnavox obsession

bobstone

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Re: Magnavox Multiplex FM stereo adapter
« Reply #5 on: December 26, 2017, 09:06:18 PM »
Just added the mpx adaptor and I think I can tell some separation. Not much room to work with on the contentinal but installed under the record changer.
Now the selector on front has no fm mpx but had terminals on back to connect cables to.
Those tubes sure sound sweet!