Author Topic: Magnavox Stereo Theater 1MR418M restoration  (Read 31811 times)

ed from Baltimore

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #615 on: September 09, 2019, 08:13:52 PM »
               That sure does sound high. I think the RCA color TV chassis we had in 1960 wattage was in the 500s, and you could feel how warm the metal cabinet top got after a few hours of operation. It did not have the UHF or remote options.
           Maybe they included every option, like MPX, remote, and UHF ? That would get it a little closer. Was their a tape recorder option for this model ?

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #616 on: September 09, 2019, 11:23:35 PM »
No tape recorder option.  The remote control is not figured in my calculations, but it is solid state.  How can it draw the nearly 300 watts difference between my measurements and what the model number sticker says?  The 165 watts for the TV is what Sams gives in its drawing.  Sams has been known to make mistakes.  That spec is also for a 24" picture tube.  So, let's give it the benefit of the doubt and say 200 watts for the TV chassis before I pull the plug.  That makes a total of 485 watts to the end of the world for the Stereo Theater.  From what I have been able to learn, most all-tube color TV's of the middle 1960's range (roundies) run like 500 watts fully warm.  The wattage on the Stereo Theater slowly climbs as the components start getting warm.  155 watts was as high as it got, and that after running about 30 minutes. 
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electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #617 on: September 10, 2019, 10:34:52 AM »
The test drive of the TV chassis did not go well....... :-[

I turned on the stereo and it ran for maybe ten minutes.  I looked everything over again for the umpteenth time and decided to turn the TV on and see what happens.  What happened was the fuse in the amp blew.  I took the amp out and put a 3 amp fuse in it, which was all I had.  The lights came on, then that fuse blew.  I have an issue somewhere.  I'm not sure it has anything to do with the TV yet.  I'll keep you posted.
I don't need Google.  My wife says she knows everything.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #618 on: September 10, 2019, 02:36:54 PM »
This is not looking very good.  I pulled the rectifier.  I put another 3 amp fuse in.  The dial lights came on, then the fuse went again.  Resistance testing reveals no shorts in B+.  I'm beginning to wonder if I have lost the power transformer on the amplifier chassis.  You may recall the one there now is a replacement.  It is not as physically large as the original.  The thing ran hot enough to fry eggs on after running about three hours.  With the rectifier pulled, we'll have to look in the 6 volt circuit, since that is the only one with a load with the rectifier out.  That included all the tube filaments and the dial lights.  Whatever happened was a catastrophic failure.  I have put maybe 100 hours on this instrument since I went thru it.  I don't remember having a power transformer go out in use before.  At least I know the fuse does what it is supposed to do, and it may have saved other problems, even if the PT is toast.  I have not been a fan of this PT, but figured if it worked, it would be good enough.  I don't smell anything nor do I see anything that has been leaking, nor anything that looks like it has been hot.  Better this happened at this stage then after it was done.  I'll check the schematic and see what the ratings are on the PT.  This isn't the end of the world, just one of those things that makes this hobby exciting. Now I'm beginning to wonder what happened to the original PT.
I don't need Google.  My wife says she knows everything.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #619 on: September 10, 2019, 05:33:41 PM »
Let's remember the theme for this restoration: "Gloriously Complicated".   :) :)

Let's remember another famous saying: "Even a blind squirrel finds an acorn occasionally".   ;) ;) :)

I expected this stereo to have a bad power transformer.  It was dead as a hammer.  It went out suddenly when it was in operation.  However, there was no overheated transformer, there was no noise, there was no magic smoke released, no tar running out, no burned wires.  It would pop fuses as fast as you could put them in.  I had to know for sure.

I ran resistance measurements on the PT and found noting amiss.  Since removing the rectifier did not clear the short, I decided to see what I could find in the low voltage taps on the power transformer.  The 5 volt tap was easy.  That was the rectifier.  The next one, the 6-volt tap, was not so simple.  I decided to pull all 19 tubes in the amp, MPX, and tuner, along with all the dial and pilot light bulbs.  This done, I put in a 2-amp fuse and applied power.  The PT with no load, by the way, pulls a little over 9 watts.  The fuse held.  No noise from the PT, no smoke, nothing amiss.  Then I started putting the tubes back in, one at a time, with power applied to the chassis, but no rectifier.  Tubes in the amp, MPX and tuner draw 65 watts.  With all the tubes back in, the fuse still held.  I put the rectifier back in and had instant music.  Like solid state.  What was the problem?  One of the three bulbs in the tuner has a dead short.  My guess is it's the one in the tuning meter, but that is just a guess.  I put all new bulbs in when I had the tuner out.  Thank God I had a fuse in the primary of the PT, or I could have taken out the PT for certain over a shorted 20 cent bulb.   ::) :-[ :-\ >:( ;) :)

I still don't know if the TV works.  After all this, I may wait for another day.   ::) ;) :)
I don't need Google.  My wife says she knows everything.

TC Chris

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #620 on: September 10, 2019, 06:20:05 PM »
That's a valuable lesson for all of us, and thank you for sharing it.  I have not been adding fuses in the primary in my restorations, mostly because it requires mental effort and I can be lazy sometimes after work.

Maybe somebody can give us a short tutorial  on sizing fuses for the power transformer? 

There's the whole issue of inrush current as filaments are cold and the filter caps charge up, and maybe a new issue of higher current draw at higher modern line voltages.  Is it enough to just take wattage from the label or schematic and add 50%?

One last point--most light bulbs fail either when brand new or when very old.  It's a bell-shaped curve with few failures once you get past "youth."  Maybe it's otherwise in those AA5 sets where the dial light looks like it's going to self-destruct at turn-on.

Chris Campbell

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #621 on: September 10, 2019, 06:41:50 PM »
The way I determine how big a fuse to use is not very scientific.  I find out how many watts it draws, then divide watts by input voltage to get amperage.  So, if I have 120 volts line voltage and I have 180 watts draw on the device, I figure a 2-amp slo-blow fuse would be about right.  The Stereo Theater runs 155 watts and line voltage today was 121.  2 amps draw at that line voltage would be 242 watts.  The two-amp fuse I had in the Stereo Theater amp did its job.  This value gives a little headroom for warming up cold tubes.  I used a 1-amp fuse in the smaller stereos.  I have a two-amp fuse in each amp on the Concert Grand.  I put a one-amp fuse in the tuner power supply on the CG.  The Imperial has a two amp fuse.  The Symphony has a two-amp fuse.  I put a 1-amp in the little GE radio I redid for a neighbor.  I am putting one-amp fuses in the little GE stereos I have.  Anybody who rebuilds a device with a power transformer without putting a fuse in the primary of the PT is on a fool's errand as far as I'm concerned.  This is the second time in recent history I know for sure of a fuse saving a power transformer. 

Since I received my watt meter for my birthday, I don't know what it cost.  The ones I priced were in the $20-$30 range.  They are worth their weight in gold.  It is easy to tell how much power something uses to figure how big a fuse to use.  I would highly recommend such a device if one is working with devices that  use more than about 75 watts of power.  Under that, a dim bulb would be just fine.  A watt meter will not limit current like a dim bulb, but it does not need to.  You can pull the plug when you see wattage becoming excessive. 

I'm not going to pat myself on the back precipitously.  I want to run thru a couple warming and cooling off cycles to be sure a shorted dial bulb is the only issue.  I'm not out of the woods yet.

As far as inrush current is concerned, it's interesting to watch the rectifier starting to conduct.  The Stereo Theater will peak at about 176 watts while the rectifier is starting to conduct and the filter caps are charging.  Then the wattage falls a bit, rises a bit, then settles down to around 155 watts, sometimes a watt or two more.   

The new bulbs I put in the tuner were from Radio Daze.  I bought a couple boxes of #47 bulbs specifically for this project.  I'm a bit hesitant using more of them.  I may go to NAPA and pay the big money ($3) for ten Sylvania bulbs.  What say you all about this?  Anybody else had experience with bad bulbs lately?
I don't need Google.  My wife says she knows everything.

Bill

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #622 on: September 10, 2019, 08:08:16 PM »
First congrats on the simple fix!  :)  Good thoughts that does the trick.  ;D  Second, your formula is what I use to figure the size of a fuse.  As I have said many times before on the forum installing a fuse saved my butt, and my PT.  :) :)  I install a fuse in just about everything I rebuild.

There are good, better, and best when it comes to bulbs.  Most today come from China, but even China has ultra cheap, and better quality.  I have found, and this comes from experience in the auto industry, that brand names like Sylvania, and others, are usually better than say a store brand, or no name brand.  I can remember when GM started getting their replacement bulbs from China, they were the cheap ones because the bean counters were trying to save money.  :o  It backfired....Replacement parts, if installed by a dealer have a warranty including labor.  We were replacing those brand new bulbs left and right within a month of installation, and GM was paying.  All car companies hate paying warranty claims, so needless to say they changed  suppliers very quickly.  ::) ;) :)

Good luck, and I hope you can move on to the TV.  ;) ;) ;) :)

Bill


 

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #623 on: September 10, 2019, 08:27:22 PM »
I appreciate the input about bulbs.  This may be one time that being cheap cost me in the long run.   ::)

I believe you understand how it feels to work on something, get it running and actually use it for a reasonable amount of time, then have it shoot craps without warning or without apparent reason.  That is not a good feeling.  I thought it was dead for good and that I had killed it.  My method of troubleshooting was not very scientific.   ;)

I'll make sure the ST still wants to work then I'll move on to the TV. 
I don't need Google.  My wife says she knows everything.

Bill

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #624 on: September 10, 2019, 08:40:25 PM »
I thought your method of trouble shooting was very much in a logical order.  End result, you found the problem, and it was not your fault.  Well maybe, you bought cheap bulbs.  ::) ::) ::) ;) ;) :) ;D ;D  Sorry, I couldn't resist.  ;) :)  Remember I'm cheap too.  :o ::)

Bill


electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #625 on: September 10, 2019, 08:46:34 PM »
A simple fix.  Perhaps TOO simple.  It took all day, since I neglected to post how much I had to take apart to get the 6CY5 tube back in the tuner.  It came out pretty easy.  The TV should not have anything to do, power wise, with the tuner nor with the amp. 
I don't need Google.  My wife says she knows everything.

ed from Baltimore

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #626 on: September 11, 2019, 01:07:10 AM »
             Good for you Greg. You found that bad pilot light a lot, lot faster than I would have, that's for sure. I'm the guy that took a cylinder head off of a Triumph TR-4 engine because cylinders 3 and 4 weren't firing and I was convinced it had a blown head gasket. If I had only removed the air cleaners to the twin SU carburetors first, I would have seen that an aftermarket foam rubber air cleaner element had gotten sucked into a carburetor throat and was flooding cylinders  3 and 4.  I was "trying to save time" (oh, the irony !!)  by leaving the manifold and carbs and air cleaners  attached to each other.
            Just last week, suddenly called back to work,  I had a General Radio VTVM with 2 fuses, one in each AC line wire but not blown,  not even lighting its pilot lamp or the 6 volt tube filaments, and yet it had + and - 250 VDC and the voltage regulator glow tubes were lit.  It smelled a little like heated metal when it was on for a while, but the panel meter refused to move with zero adjustment. But the power transformer must've been good, at least it had DC power supplies up. I never did find out why the pilot lamp and 6 volt tube filaments wouldn't light as we had no time to fix it. We just used a DVM instead.
              It took me forever to realize it wasn't the line cord, fuse, on-off switch, etc. Maybe it had an open filament winding ?

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #627 on: September 11, 2019, 08:55:18 AM »
While I certainly appreciate the kind words, let's reserve our congratulations and back patting until we see that TV screen light.  I'm not 100% convinced we have heard the rest of the story on this thing yet.  I was putzing with the TV controls yesterday when all hell broke loose.  I remember I turned the function switch to "TV", maybe heard a hum (?), then everything went silent.  A pilot light bulb could have caused the hum, since it is in the filament circuit.  I have never had a pilot light bulb fail like that.  Usually they just don't work when you turn them on, or they pop when you turn them on.  I have checked those bulbs with an ohmmeter and they check like normal, good bulbs.  I know, though, that the fuse in the amp will do its job.  There is no power shared between the amp and the TV chassis.  The function switch serves as the power switch for the TV.

If I could figure out the specs on the power transformer in the amp, I'd entertain the notion of replacing it anyway.  It is too small for the opening in the chassis and looks odd.  And a transformer designed to run on 125 volts with high temperature insulation would be a benefit.  Surely, I could find a good, heavy power transformer for under $100.  All that is given on the PT in Sams and in the Magnavox literature I have is a part number.  Lot of good that does 60 years after it has become obsolete.
I don't need Google.  My wife says she knows everything.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #628 on: September 11, 2019, 07:37:48 PM »
Now I'm stuck...... ::) :-[ :-\ ;)

I tried the TV again.  And had a giant fail again.  I believe this problem is my doing.

This is a long story, and I apologize if this is long winded.  There is an interlock block where the power comes into the instrument.  The power cord has an interlock on it, typical of TV power cords.  There is a circuit breaker that goes to the TV chassis and a double black wire that goes to the plug for the amp located on the TV chassis.  The wire for the amp goes around the circuit breaker.  I was under the impression that the TV was powered thru the circuit breaker and the amp was powered around the circuit breaker.  I do not have a diagram or literature on this portion of the Stereo Theater.  Now I'm beginning to believe that the circuit breaker only protects the high voltage section of the TV chassis, while the line voltage, or low voltage for the TV chassis runs thru the amp chassis.  The stereo will play just fine until I turn on the TV.  When the wattage gets over 200, it pops the 2-amp fuse in the amp chassis.  I need to find the wire that powers the TV chassis and route it in front of the fuse.  I put the fuse in one side of the line, and it's not enough to protect the amp, yet power up the TV chassis.  I don't want to have to pull the radio tuner to chase those wires.  I'm open to input, here, guys.  I believe, for the purposes of testing, I'll replace the fuse in the amp, make sure the stereo still works, then power the TV chassis up separately.  I can unplug the TV chassis from the radio tuner and power it with an extension cord. 
I don't need Google.  My wife says she knows everything.

TC Chris

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #629 on: September 11, 2019, 07:49:00 PM »
I can't recall seeing a circuit breaker on consumer goods in anything but  a primary circuit.  I doubt it's in a HV circuit.  In places like that they would likely have a non-user-serviceable device, one requiring the service guy to come out and fix the underlying problem.  But those are just semi-ignorant guesses. 

Chris Campbell