Author Topic: Magnavox Stereo Theater 1MR418M restoration  (Read 6472 times)

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #285 on: August 23, 2018, 12:13:17 PM »
While waiting on more detailed service information to arrive, I have been looking at the TV chassis.  I have identified at least one tube socket on the 6CG5 audio tube that has bad soldering.  I have read a post on another forum where a member has replaced every resistor and every capacitor that was not ceramic on a Philco Predicta chassis.  I initially believed this to be radical overkill, but, on looking more closely, I see the wisdom in doing that.  I'm considering doing that on this Magnavox chassis.  The Predicta restorer has found over 50% of the resistors in that chassis drifted out of tolerance.  The Magnavox chassis consists of four main PCB's.  Audio, IF and video, vertical, and horizontal and high voltage.  The chassis unplugs from other components and is probably one of the easiest to access and work with you'll find.  Since I am learning about TV repair, going thru this chassis component by component would be a good way to learn.
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Motorola Minion

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #286 on: August 23, 2018, 02:56:40 PM »
Greg,  Some of the VK artisans do not want anything left to chance, especially on rare models and those that can bring real money. I have been on there for 7 years and prefer to assist the timid and learn from the bold. For those of us with limited bench time and more pedestrian TV models, I do full-recapping with a few exceptions like leaving ceramic bypass caps alone, etc. TVs have so many resistors, its often easier to recap then power up and measure DC voltages on the tubes to sniff out any bad resistors.

My concern always begins with the AC plug, so I install safety caps from both sides of AC line to chassis, add a fuse (1.5 to 2 times steady amp draw) and surge limiting thermistor to reduce tube filament inrush, finishing with a new grounded AC cord. The way some of those tiny AC plugs look with the green ooze around the prongs and cord, I cannot take chances. If I see it, I have to deal with it.

A TV this age needs every electrolytic capacitor replaced, though some VK'ers have had success "reforming" them with gradual power-up and prolonged burn-in. Electro. is all caps 1 uf and higher. Anything from .001 uf up to .47 uf are probably wax-paper and must also be replaced. I do not stuff cans, just disconnect and leave them be. The replacements are much smaller and will be out of sight under (or behind) that chassis anyway. The Magnavox PC boards are pretty rugged but use a 40 watt or smaller soldering pencil and solder wick if you do not have a solder-sucker of some kind.

Based on what I read HERE, I was picky restoring the 93 series amp and 57 preamp-tuner on my '62 Normandy Symphony. I just have not put it together though its already in my attic-lounge, waiting for the room to be done. Resistors drifted beyond 10% were replaced, all the paper and electro caps and I decided a few ceramic disc coupling caps would be better off as mylar. Ceramics being temperature and vibration sensitive, have no place where frequency response could be affected.
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Dave

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #287 on: August 23, 2018, 03:26:37 PM »
I'm trying to avoid the "recap then power up and see what happens" scenario.  I only want to fool with high voltage a minimum of times.  I mentioned the restoration done on the Predicta not as a criticism, but as an example of what my thinking was on my Magnavox.  I do not want to charge and discharge a CRT.  Not even once, if I can avoid it.  I want to replace caps and resistors and whatever else may be needed so I can power up once and maybe make minor adjustments, like I do on a radio.  The thought of messing with discharging that CRT keeps me up at night. 

I appreciate your help with this and for being patient enough to coach me thru the process.  I can see where I might get in to TV repair, once I get my head around discharging the CRT.  I have watched YouTube videos and it looks so easy.  I did not like getting zapped by automobile ignition and I learned what to stay away from.  It is difficult to repair something if I'm afraid of it.  I am petrified of high voltage.   :-[

I routinely replace all the components you mention on your Symphony project.  The 220K ohm grid resistors are another component notorious for drifting high.  I replace the cathode bypass capacitor and the cathode bias resistor like I would any other cap in that amp.  There is a double 20uf@25 volt cathode bypass cap on the 6EU7 in the tuner and the double 30uf@350 volt plate bypass cap that need replaced in the tuner.  There are a couple paper caps hidden under the shield on the FM tuner that I replace.  If those tuners do not have AFC, I like to replace the 6EA8 in the tuner with a 6GH8A.  That helps with FM drift.  I check every resistor.  Magnavox did not use the best resistors I have encountered.  They are prone to drift high.  My opinion is you did not get carried away.  I have been accused of "overthinking" this project, but I want to do it right.  The first and only time.   ;)
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #288 on: August 29, 2018, 11:31:58 AM »
Dave was good enough to send me a Sams file for the TV chassis in my Stereo Theater.  Sams #591, folder 2.  I believe it to be a dead wringer for the chassis in my ST, minus the features that are exclusive to a Stereo Theater.  Case in point is the audio output transformer in the upper left part of the chassis shown in the print.  My chassis does not have that transformer, due to the fact that the stereo provides the audio for my TV.  The folder shows an alternative audio board, which is like the one I have.  The 6GC5 audio output tube becomes a cathode follower in my chassis.  The extra 6EU7 in the radio tuner does audio duty.  Where the output transformer is located in the print is where the amp plugs in and where the audio output cable for the TV chassis is located.  The two setups are really not that much different.  I believe this will work for any Stereo Theater with a 36 series TV chassis, regardless of tuner or remote control setup.  Next, we need a print for the 77 series tuner.  That will answer lots of questions about the remote.

I appreciate Dave taking the time to copy the print and send it to me.   :)

If anyone else has anything on this Stereo Theater, I would appreciate knowing about it.  It does not matter if it seems redundant.  Sometimes there are revisions or additions from one print to the other than can help someone, if not me in this project.  We need to accumulate all we can find on not only my Stereo Theater, but for any Stereo Theater.  We can compile the information and it will help someone down the line when they work on one.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

Motorola Minion

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #289 on: August 29, 2018, 03:00:45 PM »
Greg,  I'm pretty sure I can find the 77 series tuner schematic to downloads. My 1962 1ST659 has a 79 tuner which sure looks different.

I was not about to upload a TV schematic to the site, though the Early Television Foundation Museum is happy to accept anything I scan, as long as its a TV. ::)
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Dave

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #290 on: August 29, 2018, 10:39:02 PM »
Is the 79 series tuner a five knobber?

Having a drawing for the 77 series tuner and knowing its relationship to the remote control would be a god-send at this point, Dave.  Thank you for your help with all this.  I have the tuner sorted, but the remote control is a mystery to me at this point.

I agree that downloading a TV schematic is not something I'd want to do.  My suggestion would be to send whomever needs one like you did to me.  I believe the Sams 591 will work for almost any black and white TV found in a Stereo Theater.  I won't swear to the very earliest 1959 models being the same.  The ones I know of on this forum would likely be served by Sams 591.
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electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #291 on: August 29, 2018, 11:18:07 PM »
I received several downloads Paul sent me on how the 36 series TV chassis is supposed to work and how to diagnose problems.  Also detailed information on how the remote control system works and how to diagnose problems with it.  The files are pretty good sized, and contain the Mother Lode of information on these systems. 

Thank you, Paul for your kind consideration in helping us figure these things out.  I appreciate your taking the time to send me this documentation.
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TC Chris

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #292 on: August 30, 2018, 05:00:37 PM »
The farther we get from the time when these things were new, the more important it is that service and design info is preserved and distributed so it's saved in multiple places.  It's a great service to the future when people make the information available.

Chris Campbell

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #293 on: September 02, 2018, 11:04:40 AM »
When a person gets heavily involved with anything Magnavox, accumulating information is part of the challenge. 

I have also recently learned another tidbit that might be well to share.  A local person brought me his "tube stereo" (Magnavox Astro-Sonic) for me to "work on and get it running."  I don't do that for other people and told him so.  During our visit, I noticed that the knob on one of the sliding lids had broken out of the lid.  Apparently the lid slid open sometime during transport hard enough to break the knob out of the tempered Masonite lid.  Jamming the doors shut with a hunk of Styrofoam then wrapping the cabinet in a blanket and shrink wrapping the whole thing would prevent such a tragedy.  The plastic clips on the front sliding doors on Magnavox instruments is made of unobtanium.  I had transported several Magnavox instruments and never considered damage from the doors moving during transport.
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electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #294 on: September 15, 2018, 12:21:25 PM »
I have been working in the garage, cleaning up a bit (okay, a LOT!) before winter.  Fall cleaning, if you will.   ;)

I have a stack of yard sale records that I play both to test changers and to play on instruments with styli of unknown condition.  I don't care if they get ruined.  I have album "Aloha Moods" by the Longines Symphonette Society, on RCA Special Products label.  Although no artist if given for the music on this record "artists of known reputation" can be easily identified on this album as being Lawrence Welk and his orchestra.  Clearly this is the case.  No other orchestra played Hawaiian music with a harpsichord and an accordion.  The background vocals are the Glee Club.  Norma Zimmer's soprano and Jim Robert's baritone are unmistakable.  This record gets processed to digital before it gets ruined.  The fidelity and stereo separation are remarkable.  But I digress.....

I learned one other notable fact this morning.  The boys and girls at Magnavox may have known what they were doing using 12" speakers in this Stereo Theater.  I have threatened, from the beginning of this project, to replace the 12's with a set of 15's I have.  Now I'm not so sure about the soundness of that choice.  This thing still needs the crossover networks rebuilt, but it sounds pretty good now.  The poor old changer grinds and rumbles from years of neglect, but it works absolutely perfectly.  It changes records, drops the tone arm in the correct spot every time and shuts off when the last record has played.  The drive tire has to be rock hard.  I'll go through it like I do the others, but one hates to fool with something that works so well.  Even in less than ideal condition, this old ST sounds ALMOST as good as my Symphony.  That is a high bar in my book.   :)

I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #295 on: September 17, 2018, 02:23:25 PM »
I pulled the TV chassis this morning.  Really not all that bad as far as dirt goes.  Lots of nicotine on the picture tube.  I have identified at least two trouble spots on the chassis.  There are several broken tube pin connections to the PCB around the 6GC5 cathode follower tube.  This tube powers both the audio and video.  And the burned tube socket on the damper tube.  Methinks this TV was on the blink when the stereo was mothballed.

I see Magnavox continues their "modular" concepts on the way the picture tube is mounted into the cabinet.  One can either remove the brackets from the CRT, you can remove the brackets and CRT from off the wooden crossmember, or you can remove the crossmember with the brackets and the CRT all in one chunk.  Removing the chassis requires disconnecting wiring and removing five bolts.  Piece of cake.  Now if I can just get it repaired properly so it stands a chance of actually working....... :-[
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Bill

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #296 on: September 17, 2018, 08:36:17 PM »
So that's what it looks like out of the cabinet.  :)  A little scrub a dub, dub, and I will look like new. 

Bill


electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #297 on: September 17, 2018, 10:54:28 PM »
I'm planning on using 91% rubbing alcohol on it to clean.  I'm not brave enough yet to put it in the car wash like the TV experts do.  Alcohol allegedly cleans well and will not remove markings on the chassis.  The plan is to print a copy of the resistance chart and run the resistance on the chassis.  Then I can print another copy and make measurements as I replace the old components.  This will let me know early should a mistake occur.  I can fix it before I get too far along.  I have some questions on the best caps to use in the high voltage.  I plan on using orange drop caps to replace those white Goodall caps on the high voltage board.  I want to replace 1/2 watt resistors with one watt, one watt resistors with two watt.  Replace up to 600 voltcaps with 1000 volt and 1000 and 1400 volt with 1600.  Hey, if this plan works on a Predicta, it should work on a Magnavox.   :)

Bill, you can probably get in on some of this in the winter time in AZ if you'd like.   ;)
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Bill

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #298 on: September 18, 2018, 07:08:33 AM »
So you did decide to take the ST, TV to AZ.  :)  If I come over I can watch and learn.  I need the learning part.

Bill


electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #299 on: September 18, 2018, 08:54:11 AM »
In the fourth picture I posted above, there is an RF choke that comes off the damper socket.  The choke looks "breakdownable" to me.  Wonder if it is still okay?
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.