Author Topic: Magnavox 64-01-00 hum balance replacement  (Read 392 times)

ALDesigns

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Magnavox 64-01-00 hum balance replacement
« on: October 05, 2018, 04:09:59 PM »
The hum balance pot has no effects if turned. It has a single wire attached and is soldered to the chassis, the service manual said a clarostat 39-200 would replace it. I bought one but it has 4 pins that look to be soldered to the chasis all pointing down. Does anyone have any advice on how to replace the original?
Thanks,
Aaron

electra225

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Re: Magnavox 64-01-00 hum balance replacement
« Reply #1 on: October 07, 2018, 09:12:04 AM »
The first fact I need to know is what problem are you working with?  I have never seen a bad hum control in my life.  They typically are found in the cathode circuit of audio tubes and control the bias.  You have not indicated what you have done to the chassis, i.e. replace caps, check resistors, etc.  If you have not done that, it would be good practice to do that before further troubleshooting.  My Imperial amp chassis had a burned cathode bias resistor and I have seen several other Magnavox amps with that condition.  What evidence do you have that the hum control is bad?
I don't need Google.  My wife says she knows everything.

ALDesigns

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Re: Magnavox 64-01-00 hum balance replacement
« Reply #2 on: October 07, 2018, 08:02:08 PM »
Thanks!
Here it goes.
Needed all the tubes so I got NOS matched pairs and quads. Replaced all wax caps with polypropylene caps of equal rating and higher voltage. Ecaps and cans have been replaced. Eveything was cleaned and solder joints were checked. The T2, bass output chnl 1, was sizzling and bubbling so I disconeccted it to see how the recap and new tubes would do. Everything sounded  great except minus the bass on chnl 1. Heavy hum even when the volume was down once it was nice and warm. Ordered a replacement transformer, NOS Merit A-2905 which I installed according to specs, blue and brown to plates and red to B+. The Merit however has a red blue and green, I connected green as if it were brown, unsure of colors since they have faded. Now I get great sound for about 2 minutes and the sound drops to a very low level and sounds tinny. I have rechecked all the tubes on my recapped mighty mite II, they all checked out. The unit has not had a chance to warm up enough with sound to tell about the hum balance since the new merit went in. I checked the sound dropping out on the phonograph, fm and am, and an auxillary in line from an ipod, same thing.
My tubes are all closely matched and before the new merit went in the hum balance dis nothing audible when adjusted.

electra225

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Re: Magnavox 64-01-00 hum balance replacement
« Reply #3 on: October 07, 2018, 10:47:01 PM »
What are the element voltages when the instrument is operating properly, then what are the same element voltages when it "sounds tinny."  We need voltages on the amplifier tubes at least, and probably on the rectifiers as well.  Sounds to me like you have a resistor or cap failing when it gets warm.  Have you replaced the cathode bias resistor and the cathode by pass capacitor specifically?  The bias resistor getting hot and going south could cause that problem.  You state the "T2" was "sizzling".  That is the bass channel audio output transformer.  A shorted or open cathode bias resistor or cap could kill the output transformer.  The reason I am harping on the bias resistor and bypass cap is that my Imperial had that very condition.  I was lucky and caught it before it fried the transformer.  I would strongly suggest you verify the cathode circuit being in good condition before you take out another transformer.  Don't check them.  REPLACE THEM!  IIRC, a 100 phm 5-watt resistor is the bias resistor and you can go up a tad with the bypass cap, to maybe 50uf at 50 volts or something close.  You did not mention replacing resistors.  The 220K ohm grid resistors in those amps are typically found drifted twice their rated value.  Check every resistor and replace any that are more than 10% off their rated value.  Replace with 1% or 5% resistors.  I replaced all the resistors in my Imperial amp.
I don't need Google.  My wife says she knows everything.

ALDesigns

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Re: Magnavox 64-01-00 hum balance replacement
« Reply #4 on: October 08, 2018, 05:18:54 AM »
Fantastic!
I will start looking into it in the next few days, weekends are usually when I can put hands on it. I will pull voltages as well, I have been taking notes and will relay any pertinent info.
Thank you very much!

electra225

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Re: Magnavox 64-01-00 hum balance replacement
« Reply #5 on: November 05, 2018, 03:14:56 PM »
Have you made any progress on this project?
I don't need Google.  My wife says she knows everything.

ALDesigns

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Re: Magnavox 64-01-00 hum balance replacement
« Reply #6 on: November 21, 2018, 07:51:18 PM »
I have not unfortunately.

Bill

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Re: Magnavox 64-01-00 hum balance replacement
« Reply #7 on: November 22, 2018, 06:57:39 AM »
Don't give up!

Bill