Author Topic: Magnavox console / amp repair  (Read 3808 times)

PtrkLnk

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Magnavox console / amp repair
« on: July 17, 2016, 10:46:55 PM »
So recently my grandparents died and when going through their house there was an old Magnavox console from the early 60's that was going to get thrown away so I took it. It actually worked perfectly after I cleaned up the selectors on the top, but here the other day it stopped working and I am trying to figure out how to repair it. I have a decent amount of experience with electrical stuff however something this old that uses tubes is unfamiliar to me. I recorded this video to show what is going on with it since it's way easier then trying to explain it via text.

https://www.youtube.com/watch?v=3k2XbFPbi-g

Here are some pictures of the console and the sticker with the model number on it






Larry H

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Re: Magnavox console / amp repair
« Reply #1 on: July 17, 2016, 11:09:07 PM »
The tuner gives it away as a 1961 or 1962 model. 
--Larry

Consoleman

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Re: Magnavox console / amp repair
« Reply #2 on: July 18, 2016, 10:46:45 AM »
Don't power it on anymore until the electrolytic capacitors are replaced. There's a how-to guide in the Links section at the top of this site.If you put your location in your profile there may be someone nearby who can help and also test the tubes. Good luck!
Mark

AlexanderMartin

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Re: Magnavox console / amp repair
« Reply #3 on: July 18, 2016, 12:00:04 PM »
Sine the glow is caused by filaments, it seems the tube died OR the power transformer is s struggling.

Motorola Minion

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Re: Magnavox console / amp repair
« Reply #4 on: July 20, 2016, 03:36:39 PM »
Test your tubes asap and get us the black-stamped number from the amp chassis i.e. 93-01-00  the other stamped number is the date.

Work on the AMP chassis first - this is well worthy of your effort and we want to help. 
Tubes - Magical - Tubes

Dave

electra225

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Re: Magnavox console / amp repair
« Reply #5 on: July 20, 2016, 04:40:17 PM »
It is a 1962 "little" Imperial.  Probably has a version of the 9300 series amp chassis, with two horns and 15" woofers.  That is a dandy, one of the better models.  Tossing it would have been the crime of the century.  Good luck with it.
If it ain't broke, call me.  I can break it....

AlexanderMartin

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Re: Magnavox console / amp repair
« Reply #6 on: July 20, 2016, 06:56:11 PM »
I'll just do the obligatory beating of the dead horse and have that unit recapped and replace the dead tube, as all those tubes are series strung from the transformer filament winding and if the filament section was dead none would glow at all. Question is, what killed that tube's filament? Possible a coupling cap failed and fet the 100+ volts of DC into it?

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #7 on: July 23, 2016, 06:07:53 PM »
Well it has been a long week but I finally got a chance to take a look at the console. I took the amp out and took a picture of the back side which I have posted under this. I assume the two white cylinders are the capacitors that need to be replaced. There are those two small and one large disks which I believe are tantalum capacitors, do those need to be replaced as well? Also the number stamped on the amp is 6303-00 and next to that it has T-3 stamped on it.





PtrkLnk

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Re: Magnavox console / amp repair
« Reply #8 on: July 23, 2016, 06:14:38 PM »
Also as far as testing the tubes go, I don't personally have a way to test them. My location is Iowa City, Iowa, United States.

electra225

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Re: Magnavox console / amp repair
« Reply #9 on: July 23, 2016, 06:35:09 PM »
The tubes are not series strung from the transformer filament winding.  They are in parallel from the transformer filament winding.

If you do not have a tube tester, you can substitute a good tube to see if it helps anything.  You need to check all the 220K ohm grid resistors.  They tend to drift high, sometimes twice their value.  I like to just replace them and be done with it.  You need to check every resistor in the amp chassis, as they are not that great to begin with.  I like to replace resistors with 1% film resistors.  Check the cathode by-pass resistor and replace the cathode bypass capacitor with one about twice its rated value, something like a 47uf @ 50 volts.  You can restuff the can filter capacitor with Nichicon PZ series caps as they will all fit in the can.  I like to replace the cathode bypass cap and attach it to the chassis somewhere instead of leaving it in the can.  Of course, those old junky Good-All coupling caps need to go as well.  Those are good amplifiers, arguably the best model Magnavox ever built.  Replace any crossover caps to the horns you see.  That stereo will make you grin when you get it tuned up properly.
If it ain't broke, call me.  I can break it....

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #10 on: July 23, 2016, 06:52:18 PM »
I have to admit my knowledge of tube based electronics is not amazing, so I have a few questions. Which ones are the 200k ohm grid resistors? Also which ones are the cathode by-pass resistor and capacitor? Also which one is the can filter capacitor and which ones are the coupling capacitors?

Sorry for being so unaware of where all of these parts are located.

electra225

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Re: Magnavox console / amp repair
« Reply #11 on: July 23, 2016, 11:25:33 PM »
The first thing I would recommend you doing is to go to the top of this page, to "Downloads" and find the schematic for that amplifier.  It is a 9300 series, perhaps a 9302-00 or something similar.  Go to Magnavox then Amplifiers and find the schematic, download it and print it out.  The schematic drawing is the road map showing how the electricity flows thru the amplifier.  The coupling capacitors are the white capacitors between the various stages in the amplifier.  The can filter capacitor is the silver looking cylinder, on the top of the chassis, that houses the filter capacitors.  Probably there are four capacitors in that can.  Values like 40uf@350volts, and 40uf@350 volts and 30uf@350 volts and 25uf @ 50 volts, something like that.  You would replace these with individual capacitors.  You can either cut the can in half, remove the insides and replace that with individual capacitors or you can put a terminal strip under the chassis and mount individual capacitors to that and run the wires from the old cap to the terminal strip.  The 220K ohm grid resistors are coded red-red-yellow and go to the grids of the output tubes. 

Perhaps a better idea would be for you to tell in more detail what type of concern you have with the performance of your stereo.  I have serviced several Magnavox stereos and have yet to have one that did not work at all.  You need to be able to identify the parts before you can have any hope of repairing it successfully.  We will be happy to help anyway we can, but we can only go so far by electronic communication.  Good luck.
If it ain't broke, call me.  I can break it....

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #12 on: July 24, 2016, 12:07:55 AM »
Thanks. I will take a look at that schematic and see what I can figure out, but for the most part it makes sense now.

After I got the console it worked pretty well perfectly so I used it for a while, then one day I turned it on and it would just make a really loud buzz sound from the speakers no matter what volume I selected on the receiver or what input. I unplugged it and a week later when I went to take a look at it, I found that it made no sound at all except a crackling coming from the amp like you saw in the video I linked. Prior to me using it it was carried around a decent amount and was in the back of a truck for a several hour road trip so I wonder if that could have caused an issue with that tube?

electra225

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Re: Magnavox console / amp repair
« Reply #13 on: July 24, 2016, 07:16:05 AM »
I normally don't pay any attention to videos, but in this case I got it!  You seem to have a dead output tube.  And it apparently made noise and there may have been flashing in the tube.  And, even more seriously, there was possibly noise from the power transformer (?)  Now it does nothing? 

Based on that information, I would pull the rectifier ( the big tube that you noted was dusty) and see if the power transformer is still okay.  Do you have a Variac and or/dim bulb tester?  With the rectifier removed, you would slowly ramp up (increase) voltage and see if the dim bulb got brighter or if you noticed noise or smoke from the transformer.  If this test is negative, then I would check the voltages on the power transformer against those given in the service information.  If the transformer proves okay, then I would change every capacitor except ceramics in that amplifier.  Then I would test every resistor, including the 100 ohm power resistor and the cathode by-pass resistor.  My guess is that there is a short in the B+ line somewhere.  Since the rectifier in the amplifier also supplies power to the tuner and the multiplex adapter, you will have to go thru those as well before you can safely operate that instrument again.  Let's just hope you have not killed the power transformer running the stereo before it got checked out.  My earlier suggestions still hold true, I now have a better idea of what the issues are.  And, again, good luck.

The good news is that, based on the picture you posted, there is nothing evident on the transformer that looks serious.  Nothing has leaked out, nothing appears burnt.  And, my experience with Magnavox transformers is that, although they run pretty warm, mine have all proven robust.  You are gonna need to clear the transformer before you can do much else going forward.  More good news is that your 9300 series amplifier is the darling of the console rapers/guitar amp crowd, so you can find a replacement on ebay if you need one. 
If it ain't broke, call me.  I can break it....

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #14 on: July 24, 2016, 02:10:25 PM »
I am having a tough time tracking down a place selling those coupling capacitors. Am I correct that they are .047 uf 400 volt capacitors?


Also for the other capacitors in that can, how far off can the replacements be on the value and still be fine? For example, it calls for a 40 uf 450v capacitor, would a 47 uf 450v capacitor work or would that be too much?