Author Topic: Magnavox Stereo Theater 1MR418M restoration  (Read 8061 times)

Motorola Minion

  • Hero Member
  • *****
  • Posts: 841
  • Southern Pennsylvania
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #300 on: September 18, 2018, 02:17:28 PM »
Probably OK, and you don't want to try finding the exact inductance value online. Just be careful cleaning around any coils. Those are not easy to repair, so use a long-bristle 1" wide paint brush to get all the dust off.

I remember opening old TVs to see dust much worse than that, mostly from houses too near a road, farm field, or with the heat register near the TV on a forced air system. There were literally several-inch long icicles of dust hanging off the CRT leads in one set.
Tubes - Magical - Tubes

Dave

electra225

  • Hero Member
  • *****
  • Posts: 2486
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #301 on: September 18, 2018, 03:23:05 PM »
That coil is called an RF coil on the schematic.  What purpose does it serve?

How would I test it to make sure it is good?  There are no specifications given for it on the parts list.  Just check for continuity?
I don't need Google.  My wife says she knows everything.

electra225

  • Hero Member
  • *****
  • Posts: 2486
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #302 on: October 24, 2018, 07:27:35 PM »
As i mentioned previously, my intention is to do a component level restoration on my TV chassis.  This means to replace all the resistors, all the capacitors except ceramics and replace all the tubes with NOS RCA tubes.  The jury is still out on the CRT.  I plan to clean the tuner when I get the TV chassis done.  Really the only channel that needs to work dependably is Channel 3, so I'm not going very deep into the tuner. 


The guys who work on TV's regularly advocate the replacement of one or two components, then powering up to see if a mistake has been made.  This seems like a lot of extra work and even a good way for ol' fumble fingers me to mess something up, connecting and disconnecting things several times.  This is a printed circuit chassis.  I don't see how I can make a wiring error, replacing one component at a time on a PC chassis.  So my plan is to divide the chassis up into five parts, LV power supply, HV power supply, sweep, IF, and audio.  I will get the parts for a section at a time so as not to have a pile of parts and increase chances for confusion.  I will start a chart as I work.  I want to take resistance measurements and note them on the chart.  Then remove each component, measure it, note its position in the chassis, then do the same for the replacement component.  Then, when I get done and I power up, if I have an issue, I'll at least know what I had when I started, what I did, and won't leave anything to memory.  This chassis should either work perfectly on power up, or it will never work properly at all.  It will essentially be a brand new 1962 Magnavox TV chassis. 


I'm reseaching the RF chokes in the HV power supply.  Makes no sense at this point.


You may remember that, when I was researching codes, we found the MPX adapter with a "253" date code.  There are not 53 weeks in a year, so the code made no sense.  I have since discovered that some date codes were changed to reflect the month they were built, not just by the week they were built.  So, apparently, if one encounters a date code with "too many weeks" this method of dating by month should be considered.  Basically, the date code on the MPX is December ("53) of 1962 ("2").  This lends credibility to my theory that my Stereo Theater started its life as a model 417.  The basic difference between the 417 and the 418 is remote control.  The amp and speakers are dated July 1962, while the tuner, MPX, remote chassis and CRT are dated December 1962.  The 417 code is printed under the amp chassis.  Five knob tuners are used in the 417, while a six-knob tuner is used in the 418 in 1962.  Strange and confusing.  Typical late tube Magnavox practice.
I don't need Google.  My wife says she knows everything.

TC Chris

  • Hero Member
  • *****
  • Posts: 1465
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #303 on: October 24, 2018, 07:57:59 PM »
OK, I'm buffaloed.  How does "53" relate to December? 

Chris Campbell

electra225

  • Hero Member
  • *****
  • Posts: 2486
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #304 on: October 24, 2018, 10:50:27 PM »
I'm not sure I understand all I know about it, but there are codes that indicate the month an item was built, versus the weekly codes more commonly used.  I'm further not certain that each manufacturer adhered to a common code.  They may have modified them for their own personal use.  My understanding is that when you get a date code that is greater than the number of weeks in a year, the monthly codes may be considered.  I asked someone who is more knowledgeable about date codes than I am about the MPX in my ST having a "53rd week" date code.  I was informed about the monthly codes and was informed that "253" would have been December, 1962.  Again, research on this continues.  I want to find the codes for the 12 months so this makes more sense.  This is one of those subjects that presents itself as a part of restoration.
I don't need Google.  My wife says she knows everything.

electra225

  • Hero Member
  • *****
  • Posts: 2486
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #305 on: November 05, 2018, 06:02:23 PM »
I have the TV chassis set up on a card table in my den.  I have my VTVM and a little cheapo VOM at the ready for resistance measurements.  I have printed the resistance chart given in the repair documents for reference.  I can make notes and whatever else I need to for documentation without defacing the original documents.  I have started measuring resistances in the LV power supply.  I have a chart that I will note resistance readings on to be compared with what they should be.  I will highlight any major discrepencies from spec with my yellow highlight marker.  I will make a chart for each section of the chassis: LV, sound, IF, sweep, and HV.  I will replace components one section at a time.  So far, I have found bad socket connections on the PCB on the 6GC5, a bad damper socket.  I'm not done with the LV power supply section yet, although I haven't found anything that stands out as being amiss there at this point.
I don't need Google.  My wife says she knows everything.

Bill

  • Hero Member
  • *****
  • Posts: 841
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #306 on: November 06, 2018, 05:59:30 PM »
Sounds like you have a good start Greg, and are doing everything you can to make this a working TV.  :)  I know I will be interested with your progress, and I'm sure there are others on the forum. 

Bill


TC Chris

  • Hero Member
  • *****
  • Posts: 1465
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #307 on: November 06, 2018, 06:29:04 PM »
I enjoy reading these updates.  They give a bit of vicarious fun to those of us who don;t have time (or knowledge, for that matter) for a such a big undertaking.  I've got a 1951 Crosley that will be a someday project.

Let's see if the image attaches.

Chris Campbell

TC Chris

  • Hero Member
  • *****
  • Posts: 1465
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #308 on: November 06, 2018, 06:33:53 PM »
Trying again on the Crosley image.  Can't make it work.  Here's a link:

http://www.tvhistory.tv/1951-Crosley-S11459MU.JPG

Chris

electra225

  • Hero Member
  • *****
  • Posts: 2486
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #309 on: November 06, 2018, 09:48:41 PM »
I'm still trying to figure out what the RF chokes in the HV power supply are supposed to do, how to troubleshoot them, how to check them, etc.  I need to know more about them than I do now so I'll know how to handle them.
I don't need Google.  My wife says she knows everything.

electra225

  • Hero Member
  • *****
  • Posts: 2486
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #310 on: November 11, 2018, 03:57:27 PM »
I also need to Google "quadrature coil".  Sounds mysterious.  This particular series of chassis, the 36 series, is said to have a bug in the audio, an intermittent bug no less, caused by and connected to said quadrature coil.  I need to find it, then figure out how to test it.   :-[


I have decided, out of respect for the quadrature coil and a couple other components I have no idea about, I'm going to start replacing parts in the HV section.  I recognize more parts there.  Common old caps and resistors.  And I need to replace the damper socket, so this decision was logical.  I sure hope I'm not doing more harm than good.  Everybody reading this knows more about this TV chassis than I do.  I've been doing LOTS of reading. 
I don't need Google.  My wife says she knows everything.

TC Chris

  • Hero Member
  • *****
  • Posts: 1465
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #311 on: November 11, 2018, 04:04:06 PM »
It's the coil used in the detector, isn't it?  It's a method of FM demod.

Chris Campbell

ed from Baltimore

  • Full Member
  • ***
  • Posts: 224
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #312 on: November 12, 2018, 05:36:28 AM »
I think the RF choke going to the damper tube socket is there to "clean up" the heavy voltage and current spikes that are generated by the horizontal output tube which is basically a class C power amp circuit like in an AM transmitter but without a tuned circuit in the plate circuit. I don't know if it increases the efficiency of the circuit, if it keeps interference out of the power supply, or the tuner or a combination of the above. The same kind of choke is used to isolate the tuned circuit RF currents from getting back into the power supply in AM transmitters. The "scramble" coil winding and the multiple stacks gives it a high RF impedance over a wide frequency range to eliminate spurious "suck out" resonances. If it looks overheated, a circuit problem is causing it to absorb an oscillation that it was supposed to prevent from occurring. It doesn't look overheated in the picture.
    Not sure how to measure it except for DC resistance. Its one of the "do not measure"

ed from Baltimore

  • Full Member
  • ***
  • Posts: 224
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #313 on: November 12, 2018, 06:16:17 AM »
     the "do not measure" warnings on a TV schematic include the horizontal output tubes plate cap, the damper tube cathode and some of the horizontal deflection coil wires. Just touching voltmeter leads or a scope probe to them affects the circuit operation. The vertical output tube plate circuit is also a "do not measure".
      The quadrature coil is in a tuned circuit that goes to a special grid in a quadrature detector circuit. Sometimes its called a "locked oscillator" detector. The tube is its own limiter stage and also the FM detector. Its supposed to be so sensitive that it gets noise free audio when the signal is too weak to look like anything but snow on the CRT.
The special grid in the tube gets a 4.5 MHz sine wave voltage on it without being connected to anything other than the tuned circuit "quadrature" coil and a capacitor in parallel. I think just the interelectrode capacitance of the tube electrodes supplies enough coupling to generate the RF voltage on the coil. the voltage is 90 degrees out of phase with the FM signal applied to the normal control grid because the current through the interelectrode capacitance is 90 degrees apart from the voltage. It takes the place of the coupling capacitor from primary to secondary windings in an FM discriminator or the third coupling coil in a ratio detector. The secondary tuned circuit goes above and below 90 degree phase difference with the primary and the diode AM detectors convert the amplitude differences into rectified audio.
       I think

ed from Baltimore

  • Full Member
  • ***
  • Posts: 224
    • View Profile
Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #314 on: November 12, 2018, 06:47:07 AM »
   I don't think I've ever seen an alignment procedure using an FM generator and scope for this circuit. The tuning is thrown off even with short low C leads. They usually have you insert an RF attenuator between the tv signal antenna and the tuner input, and weaken the signal until the sound is noisy or distorted then adjust the quadrature coil for least noise or distortion. weaken the signal more,  then readjust the coil tuning and keep repeating until no improvement is possible. Then restore a strong signal with lots of detail, like small lettering on the screen and readjust slightly to null out any buzz noise. That's what the photofacts usually say.