Author Topic: Magnavox Stereo Theater 1MR418M restoration  (Read 61687 times)

ed from Baltimore

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #660 on: September 17, 2019, 09:32:33 PM »
    Oops, just saw your new post after I posted mine. Hopefully your horizontal hold control has a bad spot in it. Hope its something easily fixed.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #661 on: September 17, 2019, 09:55:33 PM »
I was in the process of connected the VCR when I started having an issue.  Connecting the VCR had nothing to do with it, because I didn't get it connected.  If I understand right, I shouldn't run the TV too long with a black screen.  I guess I'll check tubes and make sure I did not lose B+ for some reason.  I still have the same wattage draw as before, so I would think that I still had B+.  I'll test the horizontal oscillator tube, the damper tube, and the HO tube to see if they are still good.  I've never had sound. 

I was posting when you were posting.  Sorry.  Remember, Ed, this is the first TV I have ever messed with outside of dusting and changing tubes.  I am the rawest of amateurs.  I see the horizontal hold control (R3) as being a 50K ohm control that is in the grid circuit of the 6FQ7/6CG7 horizontal multi-vibrator tube, pin 7.  The brightness control, (R4) a 200K ohm pot, goes to pin 6 of the CRT.  I'll check those and the tubes I mentioned then go from there.  It acted like a dirty control to me, but what do I know. 
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ed from Baltimore

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #662 on: September 17, 2019, 10:15:56 PM »
        Good luck. I'm hoping the horizontal hold control just hit a dead spot. If the control is turned too far away from 15,750 Hz oscillator frequency, there won't be enough voltage to light the 1B3 HV rectifier filament as the HOT is kind of resonant tuned. Also, in the chassis 36 I've seen, the vertical oscillator runs on the horizontal Boost DC voltage, about 450 VDC  so if you have that boost voltage, you probably have sweep too. Your HV rectifier tube may have decided to die which would give a dark screen with only a slight lowering of wattage.  Just about the only way to measure it is to remove the tube and ohm out its filament. The filamenet glow is barely visible even in a dark room.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #663 on: September 17, 2019, 10:24:09 PM »
Is it normal for a horizontal hold control to "hit a dead spot" as a matter of course?  How long can I safely run the set powered up with a black screen?  I've read where that is not a good thing.

The schematic gives -15 volts on the grid of the horizontal multivibrator.  I'm guessing the horizontal hold control biases the tube and this makes the high voltage run?  Am I close?
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Bill

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #664 on: September 18, 2019, 06:03:29 AM »
Me, being the real dummy here, is it possible that the controls you were monkeying with still need more cleaning? 

Bill

ed from Baltimore

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #665 on: September 18, 2019, 08:02:35 AM »
          The schematic I'm looking at doesn't give a DC voltage on the 6CG7 grid pin 7 but it does give negative 30 volts DC on the grid of the horizontal output tube. I hope the chassis 36 is similar to yours.
          Well, the horizontal hold pot can get a dead spot on it if it is never adjusted in its years of operation, which is likely if the owners weren't knob twiddlers. The AFC dual diodes make the circuit like an FM tuner with AFC always on and only one radio station on the dial. The pot wiper eventually makes a bare spot on the one area the control is left sitting on. Just like a Magnavox console with spindly legs will eventually grind the carpet under the legs to nothing is not moved for 50 years. The rest of the pot gets 50 years of dust and oxidation on it without the self-cleaning action of occasional adjustment.
           Since the oscillator is free running with no TV station to synchronize to, only a little adjustment of the pot will send the frequency up and down over a super wide range. If the frequency is far enough away from 15,750 kHZ the various capacitors in parallel with the yoke coils and transformer windings will not be near resonance any more and there may not be enough 15 kHz AC to light the filament of the 1B3 HV rectifier tube and you will have low or no high voltage, and a dark screen even though your horizontal multi-vibrator is oscillating OK. A set with a non-lighting HV rectifier tube is not being hurt even though it has a black screen.
        Maybe check the DC voltage at the horizontal out tube grid, which the schematic I am looking at shows negative 30 VDC. If you have that negative 30 volts on the grid you should be able to leave the seet powered up while troubleshooting even with a dark screen. There is a rare failure of the HV rectifier tube where the filament sags and rests against the plate, which then puts the high AC high voltage across the filter capacitor composed of the aquadag coating on the inside and outside glass of the back of he CRT. But that failure would cause your watt meter to read higher than otherwise because that capacitor is a heavy load on the transformer.
          The horizontal control indirectly generates the HV by adjusting the RC time constant in the multivibrator to put its free running frequency close to 15,750 Hz, and then the AFC diodes and circuit lock it into the exact TV station frequency. If you have your negative 30 VDC on the output tube's grid you might vary the pot slowly back and forth and see if the raster comes back but it has to be close to the right frequency for a few seconds to give the 1B3 filament time to light up. But if you don't have that negative 30 VDC on that grid don't leave the set on for too long. Even with the large resistor in the screen grid circuit limiting the screen voltage and current, the tube is still derawing a heavier than normal DC current without that negative 30 VDC on the control grid.
            The bias on the output tube grid results from the grid-cathode acting like an AM detector diode like in an AM radio. It's presence means that a large AC voltage is capacitor coupled to the grid and that would only be there if the horizontal multi-vibrator is functioning, at least at the right voltage but not necessarily the right frequency.  To get the HV on the CRT you need the right voltage and close to the right frequency.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #666 on: September 18, 2019, 08:39:16 AM »
The picture tube in this TV has a wafer socket on it.  The only key there is on it is a dimple in the glass.  I dreamed last night that I got the plug one pin off on the CRT.  Would that be possible in real life, and what would the effect of that be? 

I'll check the horizontal hold.  Would that control be open if it had a bad spot in it, or only open in the bad spot, yet good elsewise? 

The schematic I have shows -15 on pin 7 of the 6CG7.  The HV rectifier is a 1K3, and it was new when I worked on the chassis, but not since the failure.  The C36 chassis are very similar, the main difference being the audio circuits and the LV power supply.  You have to watch for the notation "some Theater models".  I own a "Some Theater" model.  This thing also does not have audio.  I read somewhere that if you have no picture and no sound, a certain tube might be the cause.  6GC5, maybe? 

First thing I'm going to do is to make sure the CRT is lighting.  Then I'll test tubes.  Then I'll check the controls. Then I'll punt.  Thanks, Ed.  I'm going to need a hand with this one.   ;) :)
I don't need Google.  My wife says she knows everything.

19and41

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #667 on: September 18, 2019, 08:53:29 AM »
Those adjustment controls have rested for a long time at a specific setting, with tar, nicotine and other pollutants covering the rest of the controls surfaces.  If unsure if there is a bad spot, you could disconnect and do a resistance and continuity check and clean of them.
"Any sufficiently advanced technology is indistinguishable from magic."
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electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #668 on: September 18, 2019, 09:37:10 AM »
That's next.  I'll clean and test all four of the "operator available" controls and see what that does.  I have seen volume controls completely kill a radio, so I assume the controls on this TV could do the same?  I understand that if the horizontal oscillator is running too far off frequency, it will kill the HV rectifier.  The horizontal hold seems to be dead over the full range of the control. 
I don't need Google.  My wife says she knows everything.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #669 on: September 18, 2019, 10:51:26 AM »
I see there is a horizontal frequency control in the cathode circuit of the 6CG7.  There is a procedure listed in the Magnavox version of the C36 chassis schematic for adjusting this control.  It says to turn the horizontal hold control to mid-way, then adjust the horizontal frequency control.  Could it be possible that after I changed components in the horizontal circuit, caps, tubes, etc., that the horizontal is running so far off-frequency that it has killed the HV rectifier?  Before the screen went black, the horizontal hold control was not having much effect.  Now, I need to figure out a way to troubleshoot this.  How do I determine which control is the cutprit?  That horizontal frequency control could also be dirty or have tinwhiskers.  Horizontal width is a coil, L22, not a pot?  There is a width control on the chassis.  Where am I overlooking it on the schematic?....... :-[
I don't need Google.  My wife says she knows everything.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #670 on: September 19, 2019, 12:48:43 PM »
I have the raster back!!   :) :) :) :) :) :) :) :) :) :) :) :) :) :) :) :) :)

I have no idea what I did to fix it.  First thing I'm gonna say is that I got two pieces of REALLY BAD advice on another forum.  Those were not to clean the controls, lest you wreck them and to remove all the unused pins from the new damper tube socket.  This morning, I washed the wadding out of the controls with brake cleaner.  Then I gave them a good dousing with WD-40.  The resistance of the contrast, brightness, horiz. hold and vert. hold controls were all over the place.  I really believe the brightness control was the one that killed the picture tube.  It sent the tube to full darkness.  It showed shorted to ground when I first checked it.  It is 200K ohm when it is right.  The damper tube was REALLY loose in its socket.  I tightened the pins best I could.  If I can't find the pins I removed from this socket (I think I saved them), then I'll get another socket and leave the unused pins in this time.  The damper tube lays on its side, not a very good idea to begin with.  The extra pins help stabilize the tube.  I still don't have any audio and the horizontal is unstable.  I understand why the TV guys call these things "Maggotbox".  This is not a very good TV.  I don't believe I'll ever get this thing where it will be dependable for frequent use.  If I can get it good enough for OCCASIONAL use, that's all I can hope for.
I don't need Google.  My wife says she knows everything.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #671 on: September 19, 2019, 03:03:46 PM »
My next foray is into the video amplifier stage.  This is where audio and video are separated.  I can see in the picture tube a "scratching" or disturbance when I change TV channels, but I can't hear that.  I may have an issue in the tuner, but I should be able to hear that noise between channels.  I am almost done troubleshooting until I get a test socket....... :-[
I don't need Google.  My wife says she knows everything.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #672 on: September 19, 2019, 04:35:53 PM »
I have absolutely NO audio going from the TV chassis to the tuner.  I can't even get noise when I wiggle tubes.  I'm about 90% convinced I need to troubleshoot the tuner next.  I have no picture and no audio.  Connecting the VCR does nothing.  I'm getting retrace lines, and I can't tell if the horizontal is finally stable or not.  I have an extra 20 watts draw on the line now.  The picture tube is brighter and the brightness control actually works.  I can't really tell anything about the contrast control working at this point.  I'm really glad to see the picture tube lighting.......... ;) :) :)
I don't need Google.  My wife says she knows everything.

Harbourmaster

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #673 on: September 19, 2019, 05:27:58 PM »
One small victory at a time...   :)
-- Aloha, Ken

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ed from Baltimore

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #674 on: September 19, 2019, 07:20:48 PM »
           That is great news, Greg. It's a pleasure to try and help you get this massive ST going because you simply never give up !!.   I will not even try and comment on the kind of advice you have been getting from those "other" forums. Stick with us ! 
           If that damper tube socket is good and clean, and it must be if you took it to pieces enough to get the unused pins out, I wouldn't replace it unless you can find a socket made for another damper tube somewhere. The damper tube pins have the highest voltage spikes and DC voltages anywhere in the set except maybe the HV rectifier. You may even see that it's a different color material than the other tubes sockets in that set. Its filament current is probably highest also.
        Don't forget to replace that super high value resistor from +DC to the AGC line. Now  I see it is 9.1 or 9.6 Meg Ohm in the Magnavox 36 chassis I'm looking at. If that resistor is open (an old carbon resistor that high in value will open quickly if it has DC voltage on it) it will shut off the tuner and video IF tubes. You will get a blank raster and no sound at all.
      The tube that is the audio output tube in the chassis is still used even in the ST but not for audio circuits. It is used simply as a voltage dropping resistor to lower the +DC to about half its value--from 280 VDC to 150 VDC. If it goes bad somewhere, the +150 VDC goes away, and your tuner tubes, video IF tubes and sound IF tubes lose their DC plate voltages. So check the cathode of the audio output tube (it may be labelled voltage divide tube on your chassis) for +150 VDC.
         I hope the schematic I'm looking at is similar to your set. Is your schematic somewhere in the downloads section?
              Good luck !!