Author Topic: Magnavox Stereo Theater 1MR418M restoration  (Read 31815 times)

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #630 on: September 11, 2019, 08:01:34 PM »
I remember reading somewhere that the circuit breaker in TV sets only protected the high voltage section of the TV.  There is a fuse usually found in the low voltage section of the TV for the filaments and that.  I don't see a fuse in this one.  There was no fuse in the amp.  I believe I may have a plan, borne out of desperation.  I'm going to assume the PT for the amp does not power the TV.  The PT on the TV chassis is twice the size of the one in the amp.  The TV chassis has its own solid state rectifiers and filter components.  My plan is to restore the amp chassis like it was before I put the fuse in.  Take the fuse completely out of the line.  The line cord goes to a terminal strip where the wires for the PT are connected.  Then I'll break a connection between that terminal strip and the primary wires and insert the fuse there.  I'll still be protecting the PT with the fuse, but I'll also be able to power the TV chassis, being that it is now around or ahead of the fuse.  I need a complete schematic for the Stereo Theater in the worst way, but I'm told such an animal does not exist. 

Another point to ponder.  The TV chassis plugs into the tuner chassis with a regular line plug.  I figured one side of that line cord was hot, the function switch in the tuner completed the circuit.  Apparently that is not the case.  This is what threw me off, guessing how Magnavox wired this thing.  The amp powers the tuner, but there has to be an extra wire from the power cord on the amp that powers up the low voltage in the TV.  Strange......

How far off am I?
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electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #631 on: September 11, 2019, 08:48:36 PM »
The copy of the schematic for the C36-20 chassis I have is smudged.  That said, I don't see any relationship between the power in the TV chassis to the amp chassis.  This Stereo Theater has a replacement power transformer in there now.  The TV has not worked in who-knows-how-many years.  I have to wonder if the problem I'm having has been there for eons.  Whatever burned up the original transformer may still be there, and would have taken this one out had it not been for the fuse I put in.  The amp does power the tuner chassis.  The TV chassis plugs into the tuner chassis.  What does the fuse in the amp chassis power cord have to do with the TV chassis?   ::) :-[ :-\ :o

I'm thinking out loud here, so please bear with me.  Could this be an issue with the remote control as well?  I believe my first step is to disconnect the TV completely from the tuner, then power up the TV separately using the wattmeter and an extension cord.  This will give me an indication if the TV has a problem.  Then, with the TV completely disconnected from the tuner (power cord and audio cable) see if the fuse blows in the amp when I turn the function switch to "TV".  (I still have not replaced the three dial light bulbs in the tuner, so they are not causing a problem).  The stereo works just fine until I turn the function switch to "TV".  There is a switch to turn the remote control amp off and on.  It has never been "on" since I have been working with this thing.  Everything in this Stereo Theater is powered by the amp chassis except the TV chassis, if I'm reading right.  I need to order a truck load of 2-amp fuses and I need to see if I can obtain some more service literature on this Stereo Theater. 
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SeniorSteve

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #632 on: September 12, 2019, 08:35:54 AM »
If the audio portion of the stereo theater works correctly and the fuse blows only after you turn on the tv, it means that the fuse is trying to "protect" both the audio amp and tv.  Check your wiring to make sure the fuse is wired on the primary of the transformer on the audio amp only.  The best way to do this is to disconnect one lead of the primary of the transformer and connect it to one side of the fuse, then the other side of the fuse would go to the point where you disconnected the transformer lead.  My gut feeling is the added fuse is not protecting just the power transformer on the amplifier, the TV current is going through it as well.  With all of the work you have put into this, I hate to have you get discouraged.

Steve

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #633 on: September 12, 2019, 10:17:58 AM »
I'm not discouraged, I'm just STUCK.  I can't go much further without more information on this thing.  This is part of restoration.  This is like getting a '56 Packard into the shop with the torsion bars all the way down.  Getting it UP so I could fix it was the bigger task.....

I don't have any information at all on the radio tuner.  I have factory documentation for the C36 series TV chassis, and it has served me well up to this point.  Now I need specific documentation on the C36-20 chassis.  And it would be helpful to have some type of documentation on this Stereo Theater in its entirety, something I have not yet found. 

Steve, like you, I wondered if there was some power supply for the TV chassis that ran thru the amp chassis.  At this point, subject to change, I don't think that is the case.  The TV chassis appears to be powered thru the circuit breaker on the interlock board.  The "power cord" that goes to the tuner from the TV chassis appears, again subject to change, to be nothing more than a convenient way to provide a switch for power to the TV chassis.  Then there is the single wire with an RCA jack that carries audio from the TV chassis to the radio tuner.  The notes I made on the tuner do not indicate that I noticed anything amiss.  I remember seeing a switch ganged to the function switch in something I've worked on, perhaps this chassis.  And that would make sense.  My theory at this point is that something is shorting or backfeeding the 6-volt winding in the power transformer in the amp chassis, causing the fuse to blow.  Since there was no fuse in this amp when I got it, I'm almost certain that this exact issue I am chasing is what toasted the original power transformer.  I'll bet a TV repair shop in Detroit found the transformer bad, found the short and either wanted too much to repair it, or did not figure something "Old" was worth putting the money into and advised their customer likewise.  The power transformer was replaced so the stereo would work, and the TV was abandoned.  There were a myriad of issues with the TV chassis, so I wonder if the issues with the TV chassis caused this issue or if the two are just coincidence.  The issue I had with dial light bulbs should have given me a clue.  I have since tried these three bulbs in another device and have found they are good.  The wiring for the bulbs is what I suspect of somehow being the problem.  When the function switch is turned to "TV" something backfeeds or shorts the wiring to the dial lights and pops the fuse in the amp chassis?  How much weirder of a problem could one imagine?

I believe I will change the orientation of the fuse.  Do like Steve suggested.  That will do the same job of protecting the transformer and should eliminate any possibility that I'm wrong about how power goes to the TV.
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electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #634 on: September 12, 2019, 02:25:45 PM »
I'll re-orient the fuse, but I'm not sure it's the problem.  The extra wire is for the record player motor.  It goes to pin 5 of the Molex connector.  The switch I remembered that was ganged to the function switch is also for the record player motor.  I'm back to chasing a legitimate short.  I'll see if I can get some information on the 77-series tuner and go from there.
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SeniorSteve

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #635 on: September 12, 2019, 03:43:24 PM »
If the TV is powered by the plug on the tuner, pull that plug from the TV to the tuner and see if the fuse blows when you select the TV position, it shouldn't.  This will tell you if the TV power is being run through the fuse.  If indeed you have a short, the fuse will blow anyway.  I'm trying to figure out logically how to determine if the TV power is being run through the fuse.  Keep reporting and I'll think of another way to test.

Steve

TC Chris

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #636 on: September 12, 2019, 07:09:31 PM »
Here's another suggestion.  Doesn't all the power to the TV chassis have to go through its own power transformer?  Why not just disconnect the TV power transformer primary and wire a cord to it, with maybe a switch and fuse, and power it up independently?  That would be a way of determining if the video circuits were doing anything, or perhaps if there were some internal TV-only short.  It's possible that something could have got wired just exactly wrong. Not that I've ever done anything dumb like that, of course....

Chris Campbell

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #637 on: September 12, 2019, 10:34:46 PM »
The plug from the TV to the tuner has got to be to the switch.  The primary of the PT for the TV chassis gets its power thru the circuit breaker at the interlock.  I need to get more 2-amp fuses before I can test much else.  I don't want to use a larger fuse.  Then I'll unplug the wire from the TV to tuner and see if the fuse holds.  If it does, I'll short-check the TV chassis.  When I went thru it last winter, all the resistances checked per the schematic.  If the fuse blows with the TV disconnected from the tuner, something is going on in the tuner, it would seem to me.  How the amp chassis has anything to do with the power supply of the TV chassis is still a mystery.
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electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #638 on: September 13, 2019, 11:06:49 AM »
I have been tracing wires this morning until my eyes are gonna bleed.   ::)

I MAY have found a possible connection between the TV power and the amp chassis.  THE REMOTE CONTROL!  The amp chassis powers the remote control.  And since the remote control chassis is solid state, and by trying to follow the wires best I can, I believe the remote control is powered by a SECOND six volt tap on the power transformer in the amp chassis.  The amp chassis schematic shows a second six volt tap that powers the 6EU7 filament separately from the rest of the filaments in the tuner.  I suspect one of the  six volt taps is powering the remote control chassis.  Being that it is solid state, we wouldn't need high voltage in the remote.  (I am searching for a schematic that may not exist for this thing!) Something as simple as a set of stuck points in the remote control may be causing a short in TV power.  I need to divorce the TV chassis completely from the stereo in order to test it.  Then I need to try to separate the remote control from the stereo, then test the stereo and the TV together, if the TV works okay separately.  There is an on-off switch for the remote control on the rear of the cabinet, next to the interlock board.  The remote has been turned off, and still is.  The audio goes thru the remote control chassis and thru this switch.  The remote only controls TV functions, on-off, volume in three steps, and channel selector.  While it does not change stations in the tuner, nor does it work with the record changer, it appears to me that the remote will turn off power and change volume in the tuner, since the remote works with the tuner, not the TV.  So, if there was a short somewhere in the remote chassis, that would be in series with the tuner and TV.  Wow!

For all you out there in the cloud who are following this Stereo Theater restoration, and to those on this forum who are following it and have given so generously of their time and talent, I thank you for your interest and for your patience.  This is new territory for me.  And, at least for this part, I'm flying blind without the proper documentation.  Anyone who has information on this instrument and would be kind enough to make that information available, it would be greatly appreciated.  I'll get this thing going, eventually.  I may have to run every single wire in it to figure out how it works, but I'll get it.   
I don't need Google.  My wife says she knows everything.

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #639 on: September 13, 2019, 11:43:20 AM »
I see one version of the remote control chassis schematic that has 110 volts to it and shows a power transformer on the remote chassis.  That would shoot my theory of the amp chassis powering the remote chassis via the amp power transformer, but how do they get the 110 volts to the remote chassis? 
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ed from Baltimore

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #640 on: September 14, 2019, 02:56:37 PM »
           Just out of curiosity I looked at the primary side switching of some Magnavox chassis of the late 50s and early 60s under the downloads section on this forum. I was checking for fuses, circuit breakers, switches, remote control relays etc. A TV chassis 36 I think, had a circuit breaker in series with the transformer primary, along with the on-off switch, a length of # 24 wire in series with the filament winding as a filament circuit fuse, and a 5 ohm 10 watt resistor in series with the secondary winding to the silicon diodes and filter capacitors of the 280 VDC.  There were no other fuses or limiting resistors in series with the damper tube or horizontal output tube or the "Boost" voltage. The circuit breaker looked like it was next to the power cord interlock and would be resettable without taking the back off the set.
               What I was wondering, Greg is does it look like Magnavox took this chassis, changed only the location of the tuner and controls, added the motors for remote channel selection and led the audio output of the FM detector off the chassis to remote volume motor circuits elsewhere and changed almost nothing else, including fusing and circuit breaker ?  This particular schematic shows only 19 inch and 23 inch CRTs on it, but if the 27 inch CRT has the same deflection angle, they could use the same yoke and deflection circuits, maybe a higher CRT anode high voltage and current. I'm assuming you could get the ST with different size picture tubes and also that remote control wasn't standard in all STs.
            Well then I looked at the schematic for the 1958 or so Concert Grand, with remote control and separate amp and tuner and remote chassis. I see that nothing in this CG looks anything like your ST but I figured hat the schematics for the remote control would at least indicate how Magnavox grafted in the  Remote chassis and maybe they adapted the old circuits as best they could for the ST.  It was the most confusing thing I've ever seen. I never was able to figure out what controlled what in the 117 volt primary circuits to the radio chassis and remote chassis and the paths for the audio signal is just as confusing. I truly hope you can find the exact wiring schematic for your particular ST but I bet they were changing things on the fly to correct problems or complaints. 
          It looked like the remote chassis, in this CG model, at least, had its own power transformer for DC plate voltages and filaments to stay active even when everything else was shut off, but with no fusing at all, which is scary because this remote chassis may stay powered up 24 hours a day if the owner didn't bother using a master off switch, wherever that was. and however it might be labelled. There was a 24 VAC winding that looks like it powered some relays and some selector motors for remote control rotation of pots, and there looked like other relays that wouldn't be in your ST that were powered by DC plate voltage in the FM-AM chassis for radio station searching, and another 117 volt relay in the record changer for record reject.
          I don't think any fuse, even the slow-blow ones, can handle the kind of turn-on surge currents that the cold tube filaments, and the super low resistance of power silicon rectifier diodes charging large electrolytic filter caps. would draw. That is probably why they went to the thermal circuit breakers with manual reset. Those circuit breakers were their own surge limiters and can keep from tripping  for many seconds if they make the thermal element large enough, it just has to trip faster than the transformer would overheat.
             If your TV horizontal output stage is wired like the one in the download, there is a large resistor in series with the screen grid of the horizontal output tube going to 280 VDC and it looks like it lowers the screen grid voltage to 140 volts or so. That should protect the tube and HO transformer if the oscillator stopped working or if you were bringing the voltage up slow on a variac. The older TV sets that connected  the screen grids directly to DC voltage without the voltage dropping resistor are the ones that will melt down if the horizontal oscillator tube decides to burn out.  Those were the ones that usually had separate fuses for the horizontal circuit--they had to.
               
                 

electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #641 on: September 14, 2019, 03:39:40 PM »
As far as I have been able to determine, the generic C36 series TV chassis schematic will work for 99% of this TV chassis.  The change is in the power supply.  And most of that difference is ahead of the circuit breaker to the TV chassis.  I have a "close" schematic for the TV chassis, but not an "exact."  I also have a "close" schematic for the remote control.  The circuit breaker at first appeared as if it received power at the interlock.  I have found that to not be the case.  At this point, it appears the power cord coming out of the tuner may actually provide line voltage to the power transformer on the TV chassis.  How line voltage gets to the tuner is the next question.  The logical assumption would be that the tuner gets line voltage via the amp chassis.  I have not found any extra wires in the amp chassis to confirm this.  Only one for the phono motor.  There appears to be two ganged switches ganged to the function switch in the tuner.  Is it possible that the wire that supplies line voltage to the phono motor also provides line voltage to the TV chassis?
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electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #642 on: September 14, 2019, 05:57:16 PM »
I'm back to having the stereo playing again.  I'm not sure I know anymore than I did.  I made some notes on schematics about what I found and about the fuse in the amp transformer primary.  There are several switches and micro switches in this thing.  I count four switches to control the TV alone.  Five if you count the function switch, which controls audio.  The wiring appears to be in beautiful condition.  I'm grateful for Magnavox's brightly colored plastic wiring without stripes or dots.  Next is to verify I have eliminated the issue with dial lamps, then see if the TV chassis will work.  I've about had all the fun I can stand for one day.  Well, maybe a couple more little things...... ::) ;) :)
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19and41

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #643 on: September 14, 2019, 09:45:43 PM »
I read back through the saga to today.  Sorry about the trouble you're having.  I wish there were more dab hands with CRT televisions as there once were, to seek advice from.  Greg, you've done valiant work in getting the set to this point.  I have some long life #47 's available should you need them.
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electra225

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Re: Magnavox Stereo Theater 1MR418M restoration
« Reply #644 on: September 14, 2019, 10:44:18 PM »
It's good to hear from you and thank you for the kind words, Rex.  I may finally have this down to just a couple areas of concern.  Seeing the CRT light with a raster and some noise in the audio would be "magnificent" about now.
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