Magnavox Imperial tune-up

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Magnavox Imperial tune-up

Post by Wildcat445 on Wed Nov 11, 2015 7:45 pm

It is time for a good tune-up on my Magnavox Imperial stereo. I have the following areas of concern:

*It is time for a good cleaning. It has been two years since I had the back off.
*I purchased a Multiplex adapter for it, so I wanted to install that.
*It developed a bug in the Channel 2 treble amp/speaker. Then, finally.......
*I wanted to check codes to finally try and determine a build date. It was sold to me as a 1961, the tubes were coded for the 23rd week of 1959, but I can't find documentation for that model in 1959.

I have taken some pictures if anyone has any interest in those.

I removed the two dozen screws holding the back panel on, disconnected and secured the wires for external speakers and antennae. There was not much dust, but I found a spider web, so the quest was worthwhile. I had tentatively decided that the problem with the Channel 2 treble speaker was a dirty function switch. When I would turn the instrument on, in the "Tape" position, frequently there would be no sound from the Channel 2 horn. If I moved the function switch to "AM" the speaker would come on. It would occasionally quit again, and moving the function switch to a different position would restore operation. To clean the function switch, it is first necessary to remove the tuner and the record changer. This is not a horrible job, just time consuming and tedious. I cleaned the function switch and all the other controls while I had the tuner out. I cleaned everything up well and got it ready to put back into the cabinet.

The Multiplex adapter, model number 70-02-00 is mounted in the cabinet in an area above the amplifier, behind the tuner, and under the changer. It needs to be mounted as high as possible to clean the amplifier. The unfortunate part of this installation is that the MPX adapter takes a lot of heat from the output tubes, and the Magnavox wire lead dress is as horrible as always. I had gone thru the MPX adapter last summer. This particular adapter is not the prettiest one I have seen. The metal was dark, so I had sanded it lightly with 1000 grit sandpaper than polished it with Simichrome and my Dremel tool. It is not as nice as the one I put in the Concert Grand, but "it is what it is" whatever that means. I got the wiring all routed around the hot tubes, but had a problem with the Molex connectors. The first time I fired up the instrument, I had no power to the tuner. Closer investigation revealed a broken clip on the Molex connector on the MPX end of the harness. I removed the MPX from the cabinet, removed the bottom cover, then pushed the Molex connecter together and secured it with a plastic tie. This is not "factory", but I know of no other way to repair it without replacing the Molex connector. I used wire ties in various locations in an effort to hold the wires up off the amplifier tubes. I started the instrument up again, and had a dead Channel 2 horn. Closer investigation revealed a Molex connector on the amplifier that a wire had pushed up out of, causing a poor connection. I carefully squeezed the connectors until the plug would seat and the connections checked good with an ohmmeter. This restored the Channel 2 horn operation. It is altogether possible that part of the dead Channel 2 horn was, in part, caused by this connection. In any case, all appears to be well at this point. While I was in there, I ran all the tubes thru the tube tester just because. I did not find anything amiss.

The code on the amplifier is 246073, and the code on the tuner is 246062. This makes the build date of the amp and tuner the 24th week of 1960. This Imperial stereo would then be a 1960 model. Why tube codes are so far off that, I have no idea. The tubes were made almost a year before the stereo was built. It is not always easy nor scientific dating this stuff. I had claimed 1960 as the correct year for this instrument, since it has an eye tube. The 1ST233L is not listed in the 1960 catalog. It is in the 1961 catalog, but has a tuning meter rather than the eye tube. With the MPX adapter, this instrument now sports a 26 tube complement. I'll get the pictures on when I get them on my computer.

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Re: Magnavox Imperial tune-up

Post by 75X11 on Wed Nov 11, 2015 7:52 pm

We're always interested in pictures and a good report.
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Re: Magnavox Imperial tune-up

Post by 75X11 on Wed Nov 11, 2015 7:59 pm

I'll bet they bought their tubes in lots, anticipating their need. If they were made with spec labeling they probably had the production for a given length of time.
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Re: Magnavox Imperial tune-up

Post by Wildcat445 on Fri Nov 13, 2015 12:35 am

Another possibility on the discrepancy on tube codes might be that the ones in my instrument were replacements. This stereo is a high hour unit. There were lots of indicators. Several of the tubes had been replaced.

There is a factory document, "KO 597B FM Stereo Adapter Kit Installation Guide" to aid in the placement and connection of the Multiplex adapter.

I was under the impression that I had brought the correct audio cables with me to connect the adapter to the amplifier. I could not find them anywhere. Finding audio cables in this day and age is tantamount to trying to find a power steering belt for a Packard. I found an "audio/video" cable at Walmart that should work. It has yellow, red, and white cables in the assembly. The yellow cable goes to the input on the MPX adapter from the "Sub" terminal on the rear apron of the tuner. The red cable connects Channel 2 from the tuner to the MPX adapter and the white cable connects Channel 1 in like manner. The shortest (and the only) cable I could get was six feet long. I need less than two feet. So there is another loop of wiring to go with the other two loops of wiring that are too long. I got home too late to test the MPX operation, so that will have to wait for another day. Nothing fried the last time I had it working without audio cables, so I don't expect any when I test operation.

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Re: Magnavox Imperial tune-up

Post by Wildcat445 on Sat Nov 14, 2015 6:55 pm

I now have stereo FM in my old Imperial stereo. The upside to having stereo FM is that fidelity is somewhat better. The MPX adapter gives about a half step increase in signal to the amplifier. The FM drifting tendencies in the Magnavox tube tuners is somewhat alleviated with a MPX adapter. The downside of the MPX adapter is that if you do not have a strong signal, enough to nearly completely close the eye tube, FM stereo reception is not good. What one would do in that case is to merely switch back to mono FM. The cabling for the MPX adapter to the tuner interferes with the speaker connections on the back panel. One has to be careful installing the back so as to not disturb any of the connections.

I am still in the market for a couple MPX adapters. The 7000 series if you please.

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Re: Magnavox Imperial tune-up

Post by 75X11 on Sat Nov 14, 2015 11:14 pm

I usually use an amplified TV/FM antenna with my receiver to get a good "lock" on the station.
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Re: Magnavox Imperial tune-up

Post by Wildcat445 on Sun Nov 15, 2015 12:31 am

Here we have plenty of strong FM signals. Still, it is advantageous with Magnavox tuners to use some sort of external FM antenna, as non-directional as possible. Consider that we have to deal with a digital HD signal in most cases. The tube Magnavox tuners have issues demodulating the digital signal. While I had the back off, I also adjusted the output of both treble amplifiers. I had them turned up about 3/4 of the way, now they are a little over 1/4 of the way up. This action drives the bass amplifiers harder, rendering more robust bass reproduction.

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Re: Magnavox Imperial tune-up

Post by Wildcat445 on Sun Nov 15, 2015 9:10 pm

Another adjustment that is not often discussed concerning Magnavox instruments is the balance control. Most tube Magnavox instruments do not have a balance control with a knob, per se. Concert Grand amplifiers have no balance control whatsoever. The Imperial that I own has a balance control, accessible thru the hollow shaft of the treble control. The balance control requires a thin-blade, long-shaft screwdriver to access. I use a Snap-on screwdriver that I bought to adjust the fuel mixture on Bosch K-Jetronic fuel injection systems in the 1980's. It is fairly critical that the balance be carefully adjusted. Weak output on one channel can have a detrimental effect on bass response. The balance is best set utilizing a monaural signal. I used AM radio as a source, and gave Channel 2 slightly more output than before. Balance controls are typically found on the 9300 series amplifier chassis, the hollow bass control shaft and the hollow shaft on the treble control. My opinion is that Magnavox would have better served its customers had it provided a knob controlled balance control and omitted the silly timbre control, but that is just me.

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Re: Magnavox Imperial tune-up

Post by 75X11 on Sun Nov 15, 2015 10:36 pm

Maybe they thought consumers weren't up to spatial leveling adjustments.
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