Author Topic: Magnavox Imperial Restoration #1  (Read 1515 times)

AlexanderMartin

  • Sr. Member
  • ****
  • Posts: 341
    • View Profile
Magnavox Imperial Restoration #1
« on: August 27, 2016, 02:53:31 PM »
This is a unit a friend of a friend brought in for me to do the usual electronics shebang, new capacitors in the tuner/amp/multiplex, check the tubes, MFBA modification, and I'll be seeing if I can bypass the tape filter in the tuner for a true aux in. I got a couple questions pertaining this guy.
#1 this unit has the installed multiplex chassis, were there a variety of multiplex chassis for these side firing imperials, or is there just one? And if so, what's the chassis number called so I can order parts soon.
#2 and this is for mine, how do I take the stinkin sliding door off?
My console is on the left, his is on the right.

electra225

  • Hero Member
  • *****
  • Posts: 1641
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #1 on: August 27, 2016, 05:10:49 PM »
There were several different models of MPX adapter, but there was only two different adapters, two tube and three tube.  As far as I know, your Imperials should use the two tube version(s).  There are long and short cable versions, and those Imperials will work well with either, although the short cable version makes a cleaner installation.  Aside from the 30uf filter cap in the can, there are two electrolytic coupling caps that are typically found leaky.  There is not much to one, so not much to go wrong.

To remove the sliding door, look at the very top track, on the edge of the door.  You should see a little plastic finger sticking out from each upper corner.  Gingerly, carefully pull down on the little plastic finger.  This action should release the slider and let you bring the top of the door toward you.  That plastic is 60 years old, and may be brittle.  It might be unobtanium without using some ingenuity.  Good luck.

What is an "MFBA" modification?
If it ain't broke, call me.  I can break it....

AlexanderMartin

  • Sr. Member
  • ****
  • Posts: 341
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #2 on: August 27, 2016, 05:46:58 PM »
Appreciate the info Electra, if my memory is right this is a two tuber mounted to the side with the longer leads, kinda a wiring mess in there, that'll be fun to straighten out  ;D

Thanks for the tips on the door, I'll try that on mine.

The MFBA term is a term my friend david pulled out of his hat for my crossover mod, it stands for Measurably Flat Bi Amplifier. Just short terminology.

electra225

  • Hero Member
  • *****
  • Posts: 1641
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #3 on: August 27, 2016, 06:27:16 PM »
A "Magnavox mess" with wiring is legendary.  There is a manual in Downloads that show the installation of MPX adapters and how to tame the wiring.  Plastic saddle clamps that you can get at a hardware store beats those heavy paper clamps that Magnavox supplied from the factory. 

Would you care to share the details of your crossover mods?
If it ain't broke, call me.  I can break it....

AlexanderMartin

  • Sr. Member
  • ****
  • Posts: 341
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #4 on: August 27, 2016, 06:46:37 PM »
They're in a thread titled "182ba experimentation" I encourage you to read it.

electra225

  • Hero Member
  • *****
  • Posts: 1641
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #5 on: August 27, 2016, 07:42:47 PM »
Thank you.  I had forgotten where it was.  I'll refresh my memory.
If it ain't broke, call me.  I can break it....

AlexanderMartin

  • Sr. Member
  • ****
  • Posts: 341
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #6 on: August 28, 2016, 09:00:23 PM »
Well, the multiplex ended up being a three tuber. Odd. The wiring in there is disgusting, sticky and totally dirty. Took awhile cleaning it. Progress pictures coming soon, parts are being ordered tonight.

AlexanderMartin

  • Sr. Member
  • ****
  • Posts: 341
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #7 on: August 29, 2016, 10:32:49 PM »
Made a bit of progress, cleaned up the chassis, tubes, tested all the parts, gonna be ordering replacement parts tonight off Mouser. Amp chassis turned out quite well for my tastes

AlexanderMartin

  • Sr. Member
  • ****
  • Posts: 341
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #8 on: September 02, 2016, 12:44:16 AM »
Got my parts today! Made progress on the tuner and multiplex, all checking out good. All wax/electrolytic capacitors replaced. The amp is about 50% done as well. Just gotta finish the amplifier up and replace some open 220k plate resistors and I'll fire her up and see how she's doing after all these years. I'm also gonna install a CL-80 thermistor as the 6EU7's in these seem kinda touchy, as my imperial has one with a dead filament and one of my friend's had the same deal in the tuner, and those tubes aren't cheap. Took a little while when it came to installing the tuner's mains filters but as you can see it turned out pretty slick. No botch work around here.

AlexanderMartin

  • Sr. Member
  • ****
  • Posts: 341
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #9 on: September 03, 2016, 11:55:28 AM »
Well ladies and gentlemen, I've ran into a snag. I got my amplifier done, gotta do some buttoning up but it works and sounds excellent. Gotta love great sounding original tubes! However, I got a problem with the tuner, and that is everything that comes out of if is distorted. No matter what it is, tape, fm, am, fm multi, ect, at a certain level the audio gets distorted and harshly clips the power amplifier. My plans currently are of course to check all the audio tubes on the amplifier and button it up (change the filter caps and add the new network.) and see if it's still there. If it is, I'm at a loss. All the resistors and components check out good. The FM/AM get good reception and even the original eye tube is a good green color. Everything in the preamp circuitry tests good, and all the voltages check out good. I'm starting to suspect it might actually be one of the couplates in the tone circuitry isn't working properly, however it is on both channels and effects them evenly. I'm at a loss of words here guys, anyone ever deal with this problem and won? Otherwise I might have to get my friend another tuner chassis.

Motorola Minion

  • Hero Member
  • *****
  • Posts: 620
  • Southern Pennsylvania
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #10 on: September 06, 2016, 08:59:03 AM »
Having recently done work on 1960 and 1962 tuner-preamps similar to yours, do NOT underestimate how important the preamp stage is to anything resembling fidelity. One of mine uses a 6EU7, the other a 12AX7 but both chassis had resistors all over the tolerance map! Channel 1 had resistors drifted low, ch 2 drifted high.

Make sure that both plate resistors and cathode resistors are 10% or closer. These resistors determine your tube's operating point. Being primarily a high-mu triode and set up for voltage gain, even a mildly drifted resistor moves the operating point out of the happy spot.
Tubes - Magical - Tubes

Dave

AlexanderMartin

  • Sr. Member
  • ****
  • Posts: 341
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #11 on: September 11, 2016, 12:38:50 PM »
Alrighty, I think I found the problem. The 6EU7 on the 64-01-00 tuner has 275 volts on the plate  :o Definitely not a good thing. Could someone provide a readable schematic for this tuner? The one in the downloads is impossible to read.

AlexanderMartin

  • Sr. Member
  • ****
  • Posts: 341
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #12 on: September 12, 2016, 09:11:12 PM »
Code cracked, pro tip. When you hook these up and you're putting in the new filter, make sure to put the cathode bypass capacitors in. don't be a dummy like me and let the -10v from the cathode follower tuner out to screw up the amp's bias. I would still like the readable 64-01-00 schematic not just for myself but for anyone else that needs it.
Also was able to bypass the tape input, so now the unit has a 'true aux' input and sounds quite good. Will go into that later, it's quite the simple procedure involving a single wire,

AlexanderMartin

  • Sr. Member
  • ****
  • Posts: 341
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #13 on: September 23, 2016, 01:01:54 PM »
Ok, now that my college interval is over, here's more progress! I forgot to take an underside picture of the amp, if anyone wants that speak now or forever hold your breath. Otherwise, she's coming together nicely!

AlexanderMartin

  • Sr. Member
  • ****
  • Posts: 341
    • View Profile
Re: Magnavox Imperial Restoration #1
« Reply #14 on: September 24, 2016, 10:50:40 AM »
Some final pics my friends!
The owner stopped by today and was quite pleased. I had to tweak the crossover mod for the side firing, and it sounded quite good. The dual 15" woofers give a great low end response, convinced me to keep my Imperial side firing in all honesty. Over all I'm happy how it turned out. I did not do the cabinet or record changer, as the cabinet was quite severe and 'out of my league' if you will. The changer I recommended him to send to Chris Cuff of youtube fame. If any of you have a full functioning imperial micromatic record changer that the owner could swap I would happily swap contact info  ;)

Can't beat that grin, eh?
I recommended to him that when he gets the changer serviced he swap the eye tube out. I'd like to just throw it in but with time constraints plus the cost of straightening out the tuner/amp issues it was a bit out of the park. Not to mention those things are $25 a pop!