Author Topic: Noise in output on Magnavox Symphony  (Read 2231 times)

electra225

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Noise in output on Magnavox Symphony
« on: November 14, 2014, 12:01:13 PM »
My beloved Magnavox Symphony stereo with the 9302 amp chassis has developed a bug.  A steady "plop, plop" sound, at about one second intervals, from Channel 2 only.  (The RH channel, to you non-Magnavox guys).  It only occurs after the instrument is screeching hot, after running for several hours.  The amp chassis is positioned directly under the tuner, so the tuner serves as a giant heat sink for the amp.  I took the amp chassis out of the cabinet and went back over all my previous work, with no change.  I switched and swapped tubes, with no change.  I replaced the rectifier with another, no change.  I wiggled tubes and cleaned the sockets, no change.  I subbed all three 6EU7's, always suspect in situations like this, no change.  I cleaned all the controls with De-Oxit and found the problem to be with the function switch.  Apparently, there was just enough of a dirty contact, that, once fully warm, it would cause the amp to act wonky.  The noise occurred in any position of the function switch.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

Pat L

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Re: Noise in output on Magnavox Symphony
« Reply #1 on: November 14, 2014, 01:24:37 PM »
Very good find, thanks for sharing the solution. I'm going to put this into my bag of tricks.

Harbourmaster

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Re: Noise in output on Magnavox Symphony
« Reply #2 on: November 14, 2014, 01:31:11 PM »
That reminds me... I need to order some FaderLube from Caig to see if I can get that damn Fisher console to play in stereo without having to go through the hassle of unsoldering the Stereo/Mono switch and tearing it apart to try and make it work.
-- Aloha, Ken

No Console Left Behind!

Larry H

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Re: Noise in output on Magnavox Symphony
« Reply #3 on: November 14, 2014, 10:12:18 PM »
Anytime there is a bandswitch, inspect it closely to see if any of the rotating plates on the switch are black.  If you find any like that, erase all the black with a pencil eraser you can get at Hobby Lobby.  Then, dip a soft brush in alcohol and clean the bandswitch with that.  Makes everything work lots better.

I have a customer's 1959 stereo console here right now with a dirty bandswitch.  Time for another erasing job. :)
--Larry

electra225

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Re: Noise in output on Magnavox Symphony
« Reply #4 on: November 14, 2014, 10:23:15 PM »
The function switch in all the Magnavox instruments I have worked on, appear to be well made and seldom cause trouble.  When I redid this instrument, I cleaned all the controls thoroughly.  I had heard about your trick, Larry, and actually have done that with excellent results on all my Magnavox instruments, including this one.  I think the problem with this one is that it gets so blamed hot.  Cabinet ventilation is the Achilles' heel for at least all my Magnavoxes.  ANY small amount of dust or gunge of any kind, has the potential, combined with heat, to cause a problem.  This is a first for me.  I have never had an issue with a control on a Magnavox.  The worst part about this job was getting the tuner out.  Vertically mounted Magnavox tuners can be a pain to service.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

electra225

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Re: Noise in output on Magnavox Symphony
« Reply #5 on: November 14, 2014, 10:27:40 PM »
Fill us in on your Fisher, Ken.  What kind of issue are you having, or have I missed the discussion?
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

Harbourmaster

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Re: Noise in output on Magnavox Symphony
« Reply #6 on: November 15, 2014, 01:33:53 AM »
66 Fisher Futura (all SS unfortunately)

So I have replaced a couple of obviously bad (leaking) caps. The issue is that I do not get any sound out of the left channel when the unit is switched into "Stereo" Mode.  When switched to Mono I get sound out of both sets of speakers. I was told by the knowledgable folks over on the Fisher Forum at AK that this would indicate that the switch is the culprit and not bad transistors. 

I have so far attempted to clean the switches using Deoxit but without any improvement.  The switch in question is a pushbutton type rather than a normal pot and I am led to believe that they are prone to this sort of problem and that "Faderlube" from Caig is specifically designed for pushbutton and slider type controls and may solve my problem without disassembly.

Here are some pictures to give you an idea of what I'm working with.

 
-- Aloha, Ken

No Console Left Behind!

electra225

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Re: Noise in output on Magnavox Symphony
« Reply #7 on: November 15, 2014, 03:01:02 PM »
I might just add that you might not be out of the woods as far as your dead channel is concerned.  I have seen solid state equipment where the two channels are simply hooked together in mono, so the speakers on the dead channel are powered by the good one.  Have you absolutely verified that the "dead" channel is really dead? 
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

Harbourmaster

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Re: Noise in output on Magnavox Symphony
« Reply #8 on: November 17, 2014, 08:10:55 PM »
I had thought the same thing, which is why I asked some folks that "should be" more knowledgeable about the equipment than I am.

To answer your question directly No I have not yet been able to determine if in fact one channel is dead or not.  I'm not really that savvy when it comes to solid state stuff. I did purchase a B&K Model 520 Transistor tester last week that should help me determine if in fact any of the transistors in the audio section are bad, without having to remove them from the chassis for testing.

-- Aloha, Ken

No Console Left Behind!

electra225

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Re: Noise in output on Magnavox Symphony
« Reply #9 on: November 17, 2014, 11:55:28 PM »
And, Ken, I am not much on SS myself.  Seems to me that you could determine if the channel was dead and not involve transistors.  Looks like you should be able to find the signal on the volume control and tell if a channel was dead or not.  This is just a guess on my part, but there should be a simple way somehow.  Good luck.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.