DIY Solid State Multiplex Adaptor Plans?
 

Hello Everyone, I have a late 1950s early 1960s vintage Magnavox Console stereo which has a mono tuner with provisions for a stereo multiplexer adaptor and I know that the multiplexer adaptors for these things are kind of getting harder to find and when you do find them they usually want over $50+ for them on that auction website and anyways I was wondering if there was some sort of FM Stere Multiplexer kit or plans that one could build one from scratch using solid state components that would just hook into the multiplexer RCA jacks on this Magnavox Console's Tuner to make it capable of doing FM Stereo so that I wouldn't have to sink a ton of money into buying a unit off of that auction site. I would like to eventually see if I could flip this console if you know what I mean, and I would like to see if I can get it so that it can have FM Stereo Capabilities so that it would be more desirable and fetch more money.

Thanks for your help,

Levi

I'm guessing nobody knows of any DIY Solid State Multiplex Adaptor plans or kits... :sigh:

There are some out there. Try the tuner forums on audio asylum or Audiokarma. I was lucky and aquired the remnants after someone else gutted a tube Magnavox for the amp, I got the record changer, tuner, and Multiplex adapter out of it. Its curently in my Motorola SK77 console.

Ah, well I so far haven't been so lucky none have shown up on that auction site yet and when they do show up there they usually want a lot of money for them, more than I can afford to pay right now, since I don't currently have a job right now, and I figure if there's some plans out there for a DIY Multiplex Adaptor out there that I could make out of a few dollars worth of parts from Radio Shack or something I might as well give that a shot.

I searched and found three different IC's and schematics and a full-blown FM receiver. Simple and easy!

Get yourself a copy of either an GE tube manual from the 60s (post 1962 - the 1962 manual doesn't have it - they have FM multiplex adapters in the sample circuits in the back.

GE's 1964 manual has one with a 6GH8 as the tube, and uses common Miller 19KHz, 67KHz, and 38KHz coils. I'd scan it, but the manual is still under copyright.

You can tap filament and B+ voltage from your existing unit. This was popular in the 1960s - and there are even project plans in Radio Electronics and Popular Electronics magazine, available at American Radio History....

I google Stereo Multiplex Adaptor schematic.

This came up.


https://www.google.com/search?q=Soli...ptor+schematic

Plenty of schematics in the search.

Well the reason why I want a solid state one is because I don't have access to the parts to make a tube powered multiplex adaptor and at least with a solid state one, all I would have to do is get a simple PC Board Kit from Radio Shack and then get all of the components I need for making the multiplex adaptor and then follow the schematic and put it together and then get the right type of Molex Plug like what I need to hook it up to the amplifier for power and then it should be all good to go.

All of them of which were tube powered and were from pre-built units that were designed by companies like Harmon-Kardon or some of the other companies like that which I would need to have some iron for those designs and I don't have any, and I also don't have tube sockets and I don't have experience building tube circuits, just repairing them. Solid state circuits I could build if I had easy enough instructions to follow, but tube circuits, forget about it, too many wires to have to keep track of and too many capacitors and resistors to have to wire up.

The problem with finding a solid state adapter is that by 1967, all FM Stereo receivers were solid state, and mono FM was a rarity.

Every adapter I've ever run across is tube based.

You could try a FM MPX chip, such as the TDA7040T, a SMT 8 pin device with only about 10 external components, no coils or oddball parts necessary. Powers up with a 3V supply, and output impedance is pretty low, about 5K ohms. The whole adapter could be put in a matchbox.

in on pin 8, out on 5/6, pilot (stereo lamp signal) on pin 7, oscillator and low pass filter (both RC networks) on pins 2/3, and power/ground on pins 4 and 1.

http://pdf.datasheetcatalog.com/data...ps/TDA7040.pdf

That would probably be more along the lines of what I'm looking for, but it would of been nice to of found an original Magnavox Multiplex Adaptor for this radio but like I said not many of them come up on ebay anymore and when they do come up they ususally want close $100 for them (the cheapest one I saw on there was about $39.95 plus $20 for shipping which brought the price up to roughly $60 which is way out of my price range).

OEM MPX adapters were rare back in the day - one had to have a stereo amplifier to listen to stereo, and a new stereo with stereo FM was just a little more than the MPX adapter was. That, and the mortality of the coils and transformers was quite high, so most were scrapped, or shelved.

What's weird is that I see more Pilot or Heathkit multiplex adaptors out there on the 'bay than I do the magnavox ones, which is weird.

I built one to decode TV-MTS for an analog design project in school.
I was able to alter the pilot frequency from 19kHz to 15.7 kHz

About as easy as the Philips chip circuit Brian posted above, it used an NTE743 or a Motorola MC1310 IIRC.

MCM electronics used to sell this kit http://www.communica.co.za/catalog/Details/P0215800312
I bought a bunch of them years ago,probably hiding in my basement somewhere.

Well It seems that my best option would be to either buy a Pilot or Heathkit multiplex adaptor (they go for way less than the OEM Magnavox ones do when they do come up on ebay) or just build one from a kit or just leave it as is and if the person that ends up buying the set doesn't like that it has a monorual tuner then they can go out and sink 100 bucks into a multiplex adaptor for the radio on this old Magnavox console themselves... :scratch2: :yes:

As I mentioned before, the two Magnavoxes I had, never came through with a Multiplex board. They way it connected up, was with a terminal board next to the cutout. I didn't seem to have a plug, only the audio cables.
Motorola had a transistorized multiplex board, ever though the rest of the tuner was mainly tube. :scratch2:

i think the power was supplied by the same plug used to connect the tuner to the amp/power supply chassis, you plugged in the adapter, then plugged the tuner power cable into the mpx decoder. the switching to defeat the mono bridge connection between the two channels is automatic in my Motorola, contacts built into the audio jacks. Some Motorola MPX units were tube, but some had like 1 tube and transistors as well.

so in your guys' opinion would I be better off sinking $20 into a Pilot multiplex adaptor for this Magnavox console, or should I just pass on those?
I noticed that the Pilot branded multiplex adaptors used straight 120v AC through a power cord to power them and then used the usual RCA style connectors (like my Maggie uses). :scratch2:

$20 is very cheap for a vintage stand alone MPX adapter. I have ones from Heathkit, Knight, and Fisher. Also obtained a Magnavox adapter out of a parted out console, that I plan to adapt it to my Motorola SK-77, Ive been running it with the Heathkit adapter.

Well I did a search on ebay yesterday and the cheapest standalone multiplex adaptor was around $85 after that the prices went into the stratosphere with some going for as much as $200 Although there was a Heathkit unit that was an auction item that the bid, as of when I looked at it was only $6, although someone found a buy it now listing for a OEM Magnavox multiplex adaptor on ebay for $25 plus shipping, either way would be a good option for me to go.