Multiplex adapter tune-up

View previous topic View next topic Go down

Multiplex adapter tune-up

Post by Wildcat445 on Wed Jul 20, 2016 7:42 pm

I recently bought three Magnavox multiplex adapters, and found that one of them did not work right.  I knew nothing about one, had never tinkered with one before.  There is not much to one, so I got out the schematic and started digging.  I thought that perhaps somebody would be curious how these things could be serviced, so here goes.

I don't have pictures ready to post, but if anyone is interested, I can get some taken and posted.

There are several different versions of multiplex adapters for Magnavox stereos equipped for their use.  They are accessories for the 1959 thru 1961 models not so equipped from the factory.  I won't go into detail how Magnavox knew which FM stereo system was going to be approved in 1961, but they apparently did.  There are two basic types, two-tube and three-tube models.  I have all two-tube adapters, so we will concentrate on those.  All the three tube models I have seen have been later production.  As yet unconfirmed, my suspicion is that the three-tube models, with their 6EU7 audio amp, was designed to overcome instances of low gain in some model stereos.  As far as I know, both types are interchangeable, with gain considerations in mind.

The first thing I do is to test the tubes.  A 12AT7 is used as an audio amp and is typically Sylvania sourced.  A 6EA8 is used as a decoder amp and is typically RCA sourced.  These tubes are lowly stressed, so I have never had to replace one yet.  They each have tube shields on them, but I can't tell any difference in performance with them on or off.  My guess is that these are shielded to prevent interference with other components in the stereo cabinet.  Especially in the Concert Grand, components are fairly tightly packed into the cabinet.  

The next process is to replace the filter cap in the can.  There is no room under the chassis to place a terminal strip and an e-cap, so I re-stuff the can.  I cut the can in half with my Dremel tool, withdraw the insides, the replace them with a Nichicon PZ series 33uf@450 volt e-cap.  I glom the can back together with bright HVAC tape.  This stuff is super-sticky, withstands chassis heat well, and makes the repair nearly invisible.  Next up for replacement is a .22uf @ 600 volt coupling cap, typically found to be a junky old Good-All.  Then there are two electrolytic coupling caps, a 4uf@ 50 volts and a 2uf @ 50 Volts.  I replace these with 4.7Uf @ 50 volt non-polarized caps.  I use the non-polarized caps because I have an adequate supply, and I have found that electrolytics are unnecessary in that application.  So far, these have worked well.  The rest of the caps under the chassis are ceramic, so I typically leave those alone and move on.

Typically, the above tune-up procedures give me a working multiplex adapter.  There are two gain controls, one for each channel that occasionally suffer from being dirty.  I give those a good squirt of contact cleaner and WD-40.  ( I like WD-40).  These gain controls are set at such a position as to give the "FM MPX" position on the function switch the same volume as "FM AFC" and the regular "FM" positions.  For reference, please keep in mind that when these stereos were built, FM stereo was in its infancy.  Some stations sounded horrible on radios not equipped for FM stereo.  The "FM" position was designed to operate best on mono FM stations.  "FM AFC" of course is to lock in stations that tend to drift and make noise.  The multiplex position gives stereo FM reception, with a multiplex adapter connected.

The alignment procedures in Sams are complete and straightforward, so I'll not go into detail on those. Two methods of alignment are included. You can use the audio signal from the tuner or you can use a signal generator and a scope. I have used both methods and they work well. It depends upon the particular issue I'm having which procedure I use. I got one adapter that had been messed with previously, and was all out of whack. I had to break out my scope to fix it. You align for maximum signal and minimum squeal.

Regular, shielded audio cables can be used to connect the multiplex adapter to the tuner. The chassis can be cleaned and polished to whatever extent is desired. I typically don't spend too much time polishing. These things are typically buried deep into the cabinet and are not seen except when the cabinet back is off. I have found that an MPX is good for not only decoding the stereo FM signal, but it also improves fidelity from the tuner. These things are accessories that work well to decode and demodulate the digital stereo signal in use commonly today.


Last edited by Wildcat445 on Wed Jul 20, 2016 8:34 pm; edited 1 time in total

Wildcat445
Member
Member

Number of posts : 5168
Registration date : 2011-09-19

Back to top Go down

Re: Multiplex adapter tune-up

Post by Dr. Radio on Wed Jul 20, 2016 8:33 pm

Hey there Mr. Cat!

You never cease to amaze me with your historical and technical knowledge of Magnavox receivers. You should really put together some webpages for those seeking info. Just like there are Zenith, Crosley RCA sites, I'm sure many would appreciate the "magnificent" product info.

Glad to read you use Nichicon caps. I swear by Nichicon and Panasonic electrolytic capacitors and will not use anything else for restorations I plan on keeping and using or gifting.

avatar
Dr. Radio
Member
Member

Number of posts : 937
Age : 37
Registration date : 2012-09-17

Back to top Go down

Re: Multiplex adapter tune-up

Post by Bill Cahill on Wed Jul 20, 2016 9:14 pm

One thing I've learned. If that single filter electrolytic opens, the multiplex dies.

_________________
Please also join us here;  http://videokarma.org/index.php
avatar
Bill Cahill
Admin
Admin

Number of posts : 4538
Age : 66
Registration date : 2008-03-12

Back to top Go down

Re: Multiplex adapter tune-up

Post by Wildcat445 on Wed Jul 20, 2016 10:11 pm

True. If those electrolytic coupling caps get leaky, and they most always do, it dies, too.

Doc, I must admit that I am not always so strict about using Nichicon as I should be. I like the PZ series because the bodies of the 33 and 47 uf models are only 8mm in diameter, making them perfect for restuffing capacitor cans. This must typically be done on Magnavox, particularly on tuners, MPX adapters, etc., because there is little room under the chassis for placing external filter caps. I typically use caps from wherever is the cheapest for most applications.

Thanks for your kind words. I'm not an expert on Magnavox by any stretch of the imagination. I just make lots of mistakes and don't mind making a fool of myself sharing those experiences online! Very Happy

Wildcat445
Member
Member

Number of posts : 5168
Registration date : 2011-09-19

Back to top Go down

Re: Multiplex adapter tune-up

Post by Wildcat445 on Wed Jul 20, 2016 10:37 pm

Another couple of points.

Very few people are interested in the ticky-tack little details, trivia if you will, that I work really hard to research. I am in the habit of doing that, remnants from my auto restoration days. I liked to load my Buicks up will every conceivable factory option I could put my hands on. These had to be carefully researched for placement, authenticity, etc., so they would not only work correctly, but be of the correct era and for the correct year of car, so I would not get either myself or my customer gigged with a mandatory deduction in a car show. I'm in the habit of paying attention to the details that nobody else gives two hoots for. Most people only want to hotrod this old stuff to make it something it's not, nor was ever intended to be. I want it to work like it did when it was new, as much as possible. There are little improvements, in material and components that can be used to improve safety, reliability and performance without detracting from the basic "personality" of the item.

I put a MPX adapter in my Concert Grand without really getting into how it worked, because it worked okay, or so I thought. After digging into these, I find I need to go thru the one in the CG, because it is not as good as it could be. That will work out okay, because one of the ones I just fixed has the "short" cable, which will eliminate a rat's nest of cabling in the back of the CG. The little 616 I am refinishing needs the long cable, so the swap can be made.

Wildcat445
Member
Member

Number of posts : 5168
Registration date : 2011-09-19

Back to top Go down

Re: Multiplex adapter tune-up

Post by Sponsored content


Sponsored content


Back to top Go down

View previous topic View next topic Back to top

- Similar topics

 
Permissions in this forum:
You cannot reply to topics in this forum