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 Post subject: Hot flyback core on my Magnavox CTC12 clone color tv
PostPosted: Feb Sat 08, 2020 2:20 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 527
Location: Waynesboro and Richmond, VA , USA
The flyback in my Magnavox CTC12 clone "roundie" color tv gets hot but mainly at the core which gets really hot to the touch after a couple of hours of playtime. Is this normal? The "doughnut" itself only gets slightly warm and not soft but there has been some minor drippage that seems to have come from the center.

The flyback is a NOS replacement with less than 20 hours on it. The efficiency coil has been properly adjusted, the sweep and rectifier tubes replaced, and the hv has been adjusted but can only get 21kv out of it. There is no blooming, the picture is nice and sharp and it plays perfectly. The only thing besides the heat problem and the slightly low hv, is the filament of the hv rect tube seems a little bright to me. It was always my thought that the filament should only be seen in a dark room.

Any feedback you all could give me on these questions will surely be appreciated as i don't want to ruin the new flyback.

Thanks!


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 Post subject: Re: Hot flyback core on my Magnavox CTC12 clone color tv
PostPosted: Feb Sat 08, 2020 5:16 am 
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Joined: Dec Mon 23, 2019 5:52 pm
Posts: 267
When I have run old flybacks whether NOS or original that haven't been operated in a long time they often run very hot and drip wax for awhile. Sometimes it takes many cycles of use, then cool, until they "dry out" and then behave. Someone posted once about baking the moisture out of them in an oven, but I've never tried that.


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 Post subject: Re: Hot flyback core on my Magnavox CTC12 clone color tv
PostPosted: Feb Sat 08, 2020 11:56 pm 
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Joined: May Sun 07, 2017 11:35 am
Posts: 1043
Location: Belrose, NSW, Australia
Yes rcadanny I think you might be right.

I had a particularly hot flyback core on an HMV. For some reason I hit it with CRC spray while still hot, the CRC smoked!

But when I tried it again the next day all was well. Maybe moisture entering the porous core?

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Wax, paper, bitumen, cotton, high voltages - what could possibly go wrong?


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 Post subject: Re: Hot flyback core on my Magnavox CTC12 clone color tv
PostPosted: Feb Sun 09, 2020 10:37 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 777
Generally most Line output transformers have ferrite cores (there are some exceptions in older sets).

Core losses, which generate heat in the core, are Hysteresis, Magnetic losses and Eddy current losses. But in ferrite cores the eddy current losses are negligible because the magnetic elements are well insulated from each other, so that just leaves the first two losses, which have frequency dependent and frequency independent properties. Hysteresis loss is frequency independent, while the magnetic losses are frequency dependent as I recall.

The magnetic losses are generally measured in Joules per cubic meter and represent the area inside the B-H loop for the core. The higher the magnetizing forces H (from the currents in the coils around the core), the wider the B-H loop and the greater the losses, especially if the core is pushed harder.

But when a core heats up of course, you need to distinguish between heat generated in the core itself, due to those losses, and heat generated in the windings (I^2.R losses) which will heat the core by conduction of heat, from the windings to the core.

So really it is more important to know the winding temperature than the core temperature. A transformer will not fail from a hot core, they fail from a hot winding and insulation failure of the enamel copper wire of those windings.

If your set is working ok, most likely the transformer will be ok, unless say something is overloading one of its outputs and overheating a winding. But that would be unlikely, if everything appears to be normal otherwise.


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 Post subject: Re: Hot flyback core on my Magnavox CTC12 clone color tv
PostPosted: Feb Sun 09, 2020 11:06 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 1043
Location: Belrose, NSW, Australia
All this is true, which is why I found the core heating (and corresponding width reduction) strange at the time, and the "fix" even more inexplicable. Ferrites vary widely, maybe the type used here was only marginally suitable. At the time I wondered if the ferrite's characteristics were being degraded by heat. A kind of thermal runaway.

On reflection, it's more probable that losses in the fairly hygroscopic (cotton insulated) DCC-honeycomb-wound EHT overwind due to moisture ingress (which I seemed to have addressed with the soaking of CRC) was the more likely explanation.

Back in the day, EHT cage fires occasionally happened in HMV F-series chassis that mostly used Rola flybacks.

But the owner of the subject TV has been using it regularly for a couple of years now and has had no trouble. On the spray can it says "makes things work again"! In this case I can vouch for that!

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Wax, paper, bitumen, cotton, high voltages - what could possibly go wrong?


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 Post subject: Re: Hot flyback core on my Magnavox CTC12 clone color tv
PostPosted: Feb Tue 11, 2020 10:56 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 9459
Location: Beaver Falls, PA. USA
What does the cathode current of the horizontal output tube measure?

If you can only get 21kv out of it, the first thing to check is the 6BK4 and its circuit; a problem there could be loading down the HV. Also, try another 3A3.

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