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MPX decoder schematic?

R

RossW

Addicted Member
Does anyone have or know where I might find a schematic for a vacuum tube MPX decoder? I have an old tuner from the 50s that has an MPX output described in the manual as:

"This jack provides a high impedance output from the FM section of the tuner only. Multiplex transmissions of binaural programs are contemplated by the broadcasting industry at the present time. When the FM stations start transmitting this type of signal, equipment will be available for connection to the Criterion tuner for separation of the multiplexed signals."

Unfortunately the company stopped making hifi equipment before MPX became common and thus, no decoder. I know Fisher made a couple different stand-alone MPX units so a schematic of one of them would fit the bill nicely. Thanks.
 
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Building a tube MPX decoder can be fun, and is possible. But chances are that unless one is really lucky with the NOS parts sources one will have to wind their own coils for the 19KC and 38KC circuits.

When I've built such things as MPX decoders--and also encoders (used in FM Stereo transmission)--I've had good luck using the horizontal sweep frequency coils from condemned old TV sets, and altering the coils as needed to serve the purpose. Such measure can save a lotta time running lotsa fine wire into a coil.

Alignment can be tricky, and it is best to have a signal generator handy that will reach both operating frequencies. A good-quality o-scope is also a good item to have on hand.

But there are indeed outboard MPX decoders out there under numerous brand names. IIRC it was a Pilot brand decoder I found in the barn the other day while moving other stuff around. Never tried it to see if it works or not....

But such can often be found at estate sales or flea markets if one is patient. I've passed a number of them by in the past when visiting yard sales. Usually the people tending such sales have no idea what it is, and will let it go for peanuts--or zilch if the buyer is taking other stuff as well.
 
spaceman said:
Definitely bookmarked, thanks for posting it. :thmbsp: I have that Eico MX-99, & it's a nice little unit. I just had to enlarge the holes for the RCA jacks, they're too close for more modern interconnects.
Click to expand...

Wonderful service from these guys, I can attest.
 
Old1625 said:
But such can often be found at estate sales or flea markets if one is patient.
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Or even ebay. I paid $11.50 for an Ampex MPX stand alone unit. That price includes shipping. I've never seen anything with a tube in it at a garage sale around here.
 
Scuzzer said:
Or even ebay. I paid $11.50 for an Ampex MPX stand alone unit. That price includes shipping. I've never seen anything with a tube in it at a garage sale around here.
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Here in New England in the hilltowns it's a whole different story as regards garage, yard or estate sales. Also around these parts they are called "tag sales." No old-time New Englander--especially in rural areas--is apt to throw anything away. Thus the variety when an estate sale happens.

Good find on that Ampex unit, BTW. :thmbsp:
 
I would imagine you have more tubes per capita in that area than any other in the US. I've actually purchased a few "dug out of an old barn" units for cheap from that region.
 
I picked up a Heathkit MPX decoder for $5 in good working order from a local electronics parts place (Littleton, MA) a few years back. They're not all that hard to find; just don't look where everyone else is looking (e.g., eBAY)...
 
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