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 Post subject: A very Unusual Problem--Tube Type Console
PostPosted: Aug Thu 12, 2021 10:36 pm 
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Joined: Mar Sun 29, 2009 3:34 am
Posts: 1017
Location: chancellor al.
At risk of repeating myself (I think I posted this with A, Radio Classified about 15 yrs ag0).
I was repairing a very popular well made (Magnavox?) that had a strange audio problem-
after I had replaced defective parts. This was a rare circuit-both spkr voice coil leads were above ground. I decided to test output transformer and found leak. between plate/B+ and voice coil-about 200VDC. What are the odds?

Harold


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 Post subject: Re: A very Unusual Problem--Tube Type Console
PostPosted: Mar Sat 26, 2022 5:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11583
Location: Beaver Falls, PA. USA
I saw that several years ago, in an old Allen organ amplifier; the output transformer shorted primary to secondary, which put a dead short on the B+. Fortunately, I had a spare transformer. Another amplifier, out of a Seeburg jukebox, had an output transformer short from the primary to the core.

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 Post subject: Re: A very Unusual Problem--Tube Type Console
PostPosted: Mar Sat 26, 2022 8:32 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 9042
Location: Near Brandon, Iowa
What model Magnavox are you specifically referring to? Some degenerative feedback schemes involved the output transformer secondary winding.


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 Post subject: Re: A very Unusual Problem--Tube Type Console
PostPosted: Mar Sun 27, 2022 12:58 am 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 11767
Location: Long Island NY
Transformers can and do fail for a variety of reasons but one of the most common ways output transformers get wiped out in tube equipment, is when somebody leaves the speaker voice coil disconnected. Recall that the load on the secondary of a transformer is reflected back to the primary and reduces its inductance by demagnetizing it. If the load on the secondary goes to zero because somebody unplugs or disconnects the speaker, the primary inductance becomes much higher. The plate current swings from the output tubes then produce huge voltage spikes that kill the insulation between primary and secondary or primary to the core.

It's always safer to short out the speaker terminals of a tube amplifer than to leave them open. Solid state amplifiers are the exact opposite, you never want to short their speaker terminals out but there's usually little risk in leaving them open.

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