Author Topic: I might have learned something  (Read 106 times)

electra225

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I might have learned something
« on: October 04, 2017, 05:06:08 PM »
I tend to dive into projects head-on and let the consequences fall as they may. 

My wife's daily driver, a model 2ST653 Magnavox console stereo, only has mono FM.  I was advised some time ago that the correct MPX adapter for this stereo is a three tube unit, a 70-01-00 or similar.  I had often wondered what the difference was between the two-tube units and the three-tube multiplex adapters.  I figured both types were interchangeable and nobody seemed to disagree, so I considered it gospel.  That may not be as accurate as I once thought.

This little stereo is one of the bottom-line tube stereos from 1963 when Magnavox was pushing the new Astro-Sonic solid state models.  It has an 8803-00 amplifier and the 78-08-00 tuner.  The tuner is somewhat different than tuner in earlier models.  They reinforced the connections for the antenna, and moved the connection for outputs from the convenient rear apron of the chassis to a spot on top of the chassis.  Anytime you need to make connections to the outputs, the tuner chassis must be removed for access.  Then it is a pain to get the tuner lined up properly so the bezel fits properly and lets the knobs move freely.  The connections for external speakers and external antenna are mounted to the bottom of the cabinet and do not interfere with removing the back like before.  They did away with that awful two-string dial drive setup on the earlier small tuners in favor of a more conventional one-string setup.  The 8803 amp is identical to the 8802 in most respects.  The big difference is the Molex connector.  Not the same as on the 7002 MPX adapter.

So I now wonder if the difference between the two-tube and the three tube MPX adapters is that Molex connector.  Perhaps the system is wired differently on the 1963 models, so they had to make a new MPX adapter to work with it.  I do not have a three-tube adapter in front of me to make the comparison.  Does anyone have both types that could be compared?  Or, does anyone have a three-tube 7001 adapter for sale?
If it ain't broke, call me.  I can break it....

Motorola Minion

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Re: I might have learned something
« Reply #1 on: October 10, 2017, 09:07:10 PM »
I have a three-tube MPX adapter in a 1ST659, in the 1962 catalog "Normandy Provincial".

I'm checking for a picture in my file. The console (everything apart but restored) is buried in my attic as I work on a room for it
Tubes - Magical - Tubes

Dave

electra225

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Re: I might have learned something
« Reply #2 on: October 11, 2017, 03:10:35 PM »
Thanks, Dave.  All I can say for sure is that the 7002 adapter Molex connector is longer and wider than what is on that 8803 amp chassis.  Not very scientific, I know.   :-[
If it ain't broke, call me.  I can break it....

Motorola Minion

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Re: I might have learned something
« Reply #3 on: October 13, 2017, 12:07:55 PM »
Greg, 
To get the investigation started, compare the two sams photofacts in downloads under tuner and preamp schematic. A 7001-20 appears to be an earlier version of the MPX adapter chassis.
They are different in that the three tube unit has an dual triode audio output amp, while the two tube version has an input signal amp before the cathode follower, a 12AT7 triode used on both types.
Instead of a 6AB4 and 12AT7 in addition to the 6EU7, the two tube version uses a 6EA8 for 19KC amp and doubler, like Zenith and RCA do. ]

The differences are numerous but the good news is the 12-pin Molex plugs appear to be identical on both. How interesting it would be to swap out and compare :-\

I will get a better shot of the interconnect diagram in my cabinet when I can but the actual molex plugs are the same. The MPX plugs in between the amp and tuner.

I made a drawing when I pulled it apart. Not as complex as my Motorola SK112 with the FM adapter and verbaphonic.
That beast was a challenge to my aging memory, so I used electrician's Brady number tags to match all the Molex, RCA and spade lugs.

Fisher, RCA and Zenith consoles are a lot simpler in this regard but damn, aren't they are all fun?
Tubes - Magical - Tubes

Dave

electra225

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Re: I might have learned something
« Reply #4 on: October 13, 2017, 04:48:52 PM »
My understanding about the compatibility of the two types is that of your investigation.  I would have bet money that the 7002 adapter I had would work.  The Molex connector is definitely different on that little stereo.  It is more square and has three "teeth" on one side to index the plug.  Never saw one like it.
If it ain't broke, call me.  I can break it....