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 Post subject: How hot should a flyback get?
PostPosted: Mar Sat 03, 2018 12:40 pm 
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Joined: May Sun 07, 2017 11:35 am
Posts: 384
While working on intermittent sync fault on a mid 50's HMV chassis (turned out to be a leaky 220pf MICA cap), with just over an hour's running the set started losing width. All the usual suspects already replaced, hor OP tube current up to 190mA when it had been running 140mA. To my horror, I saw a wisp of smoke coming off the flyback primary winding. So after killing the power I thought, "OK, it's kill or cure, I'll douse it with CRC". After I gave the winding a really good soak, to make absolutely certain and while it was cooling down I replaced some 30% high resistors around the hor driver (RCA Synchrolock circuit). I fired it up and checked the drive waveform. Hor OP now running under 140mA as before.

It's now been running perfectly for about 5 hours now, nice bright pic with no rise in the OP tube current and no loss of width or HV.

But the flyback primary winding and particularly its ferrite core are running very hot, I guess about 70 degrees C at a room ambient of 23 degrees C. Too hot to touch for more than a few seconds.

I've never paid any attention to flyback running temps before. Anyone know what to expect?

It's a standard 50s- vintage transformer, 90 degree deflection, honeycomb wound DCC insulated litz wire, wax sealed, primary under the EHT overwind with a corrugated spacer in between. This one, 60+ years old, was made by Rola in Australia but you've seen the type.

Did I actually save the flyback with the CRC? Should it run that hot?


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 Post subject: Re: How hot should a flyback get?
PostPosted: Mar Sat 03, 2018 8:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1720
Location: Lafayette, CO
Mid-50's.....by then manufacturers really had the physics down on flybacks. That wisp of smoke was a hot spot apparently filled by whatever oil you used. If it works, great! Zenith had a lot of failures with their early ones and the traditional FLY1 RCA used in their 630-chassis were rock solid but needed a doubler when they went to larger screens. If l were you l would look for another fly just in case....Craig


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 Post subject: Re: How hot should a flyback get?
PostPosted: Mar Sat 03, 2018 10:06 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 671
Location: Pasadena CA USA
What is CRC?


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 Post subject: Re: How hot should a flyback get?
PostPosted: Mar Sun 04, 2018 1:04 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 384
CRC 2.26 water displacement liquid.

On the can it says " Makes things work again"! I have brought cellphones dropped into swimming pools back to life with this stuff...

Re the flyback itself, this brand was used by a number of TV manufacturers in the 50s and 60s. Pretty reliable but they were known to catch fire.

If there was still a shorted turn I'd expect a loss of width and HV but this thing is fully scanning my 110 degree bench test CRT.


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 Post subject: Re: How hot should a flyback get?
PostPosted: Mar Sun 04, 2018 6:06 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 384
Hmmm! I think I found a reason for the extra heat...

After replacing two can electros that had started to leak electrolyte, I checked the B+ at the rectifier cathodes and got 265 volts.
That sounds OK but the circuit says 245 volts.
Checked the transformer tap - it was set to 220v! (Mains power in Oz is 240 volts).
Fixing that reduced the width to a realistic level on my 110 degree test tube. Hor OP current is now 130mA and after a hours running you can hold a finger on the flyback core for more than 10 seconds. It's hot, but not as hot as it was.

I didn't notice this earlier because the old rectifiers (a pair of 6N3s) were well down, B+ running at 180v, while I was waiting for some NOS 6N3s to arrive.


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 Post subject: Re: How hot should a flyback get?
PostPosted: Mar Fri 09, 2018 10:49 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 384
I measured it with a laser thermometer. Results:

Run Time (mins)......Temp deg C........Temp deg F
............................. Core....Winding....Core.....Winding
=================================================
20............................70........62..........158.......143
40............................76........68..........169.......156
60............................77........71..........171.......159

Ambient temp 23 deg C 73 deg F

It seems to stabilise at 60 mins runtime.

Apparently the type of ferrite used in these things has a max working temp of 175 deg C and a curie point of 210 deg C
so I think we are well inside safe limits.

I'm surprised that in 50 years I have never noticed how hot they run.....

BTW, is there a better way of formatting a table with this editor?


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