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LOPT/Flyback questions.

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Ash Small

Mon Sept 16 2013, 07:28PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I picked up 'job lot' of five assorted LOPTs/flybacks on Ebay over the summer, but I've only just got around to opening the box today. While the other two have cores at least as big as the biggest of the three pictured below, the three pictured below are interesting, as they seem to have the 'over-winding' on a leg of it's own, with just one other winding on that leg. The one with the smallest core (top one) is from a mid '70s Thorn 9000 series, I'm not sure about the others.

Does anyone know what the second winding on the leg with the HT 'over-winding' on it is for?.....Is it something to do with energy recovery from the deflection yokes, or something else?.....Could I use it as a primary, and remove the windings from the other leg?

I've not measured any resistances or anything yet. Hopefully someone who knows about these things can shed some light on the subject?

EDIT: I've searched for a shematic for a Thorn/Ferguson 9000/9600 series TV online, in order to try to find out myself, but so far I've not found anything.

Erlend^SE

Mon Sept 16 2013, 08:23PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

THat winding may be a heater winding.

Not too much use in heater without HV or HV without heater.

Probably around 6.3V RMS in the original system.

It could also be the winding connected to B+ and the horizontal output transistor (and the yoke via different components). Pulses of around 1 kV when giving out HV.

Maybe you can messure it? It can be something totally different.

Antonio

Mon Sept 16 2013, 08:46PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Apparently the large winding is the HV winding, and the other is the primary and probably other windings for measurement and generation of other voltages. It's not clear if the primary is in the same block of the secondary. You can use any as primary coil, but these transformers are designed for a quite high primary voltage (~100 V).

TwirlyWhirly555

Mon Sept 16 2013, 09:02PM

Registered Member #4104 Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122

I have the two bottom flybacks ,

The one with the Blue gel like stuff , I removed the primary completely and ran it with a ZVS driver with a 3+3 turn primary and on 30 volts supply , the amount of corona I got was crazy and the arc was over 7" long , The transformer did fail in the end though .

- The second flyback pictured running on 30V

Id take a wild guess and say the windings under the HV winding are either some sort of lower voltage wining or a form of feedback , with the primary kept on the other leg insulated as the primary would be run at a fairly high voltage .

The bottom one I have tried on a ZVS but it didn't seem to like running , But it did give some nice arcs . One of the windings on the opposite side of the HV winding is foil type wire between 2 sheets of plastic it fairly long aswell , not sure if its a screen or is a winding itself .

HDF49

Mon Sept 16 2013, 09:45PM

Registered Member #977 Joined: Thu Aug 30 2007, 06:57PM
Location: England
Posts: 74

I have the flyback on the bottom. On the left hand leg is a HV capacitor, the secondary is rectified and pretty bloody tough, I got some insane arccs with a halfbrudge and a zvs at 500 or so watts.

Proud Mary

Mon Sept 16 2013, 10:43PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The winding with the EHT O/P socket is in each case the EHT winding. It is not an 'over-winding' as it is not wound over anything - overwindings were chiefly a feature of valve era LOPTs, in which the other leg had to be used to provide the highly isolated filament supply for the EHT rectifier, a winding often consisting of one or two turns of EHT cable tacked in place with goop.

As these are not valve era LOPTs, there are no free floating filament windings for an EHT rectifier diode, so there is room enough to have the primary wound on one leg, and the secondary wound on the other.

One of the taps on the EHT winding will be the focus supply out.

The windings on the other leg will contain the primary, which may or may not be tapped, to provide autotransformer outputs, and quite possibly other isolated, possibly tapped, secondaries to feed a sub-circuit somewhere else in the telly. The ingenuity of LOPT designers is measured by the amount of work they can get the LOPT to do in providing one-off voltages for use elsewhere.

Ash Small

Tue Sept 17 2013, 10:21AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks for the replies, especially the info supplied by PM. That's a pretty impressive Jacob's ladder, Twirly.

HDF49 wrote ...

I have the flyback on the bottom. On the left hand leg is a HV capacitor, the secondary is rectified and pretty bloody tough, I got some insane arccs with a halfbrudge and a zvs at 500 or so watts.

Do you mean the one in the middle, HDF? Where you can clearly see the HV capacitor, or are you saying the bottom one also has a capacitor and is rectified?

TwirlyWhirly555 wrote ...

The bottom one I have tried on a ZVS but it didn't seem to like running , But it did give some nice arcs . One of the windings on the opposite side of the HV winding is foil type wire between 2 sheets of plastic it fairly long aswell , not sure if its a screen or is a winding itself .

Could it be a foil primary?

I'm guessing, by looking at it, that the top one (Thorn 9000) isn't rectified.

I'll probably remove the 'other' windings, and wind a primary of a few turns on the other leg myself, but I'm not planning on running these to destruction.

What's the easiest way to tell if a secondary is rectified? Presumably the voltage drop over any HV diode will render a DMM useless?

Antonio

Tue Sept 17 2013, 12:29PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

If there is no diode, you can measure the resistance of the winding, identical in both directions. If there is a diode string in series, you will measure open circuit, at least in one direction (probably both). To check if the diodes are good you need a power supply of several volts, a load resistor and a current meter. Connect all in series and see if you can measure some current with the winding in just one direction.

Proud Mary

Tue Sept 17 2013, 12:32PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Those LOPTs were designed to operate with external doublers/triplers. The diode-split LOPT was first used in a Bang & Olufson telly in 1977, and most manufacturers continued with the cheaper external multiplier system for some years after.

Ash Small

Tue Sept 17 2013, 12:59PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Proud Mary wrote ...

Those LOPTs were designed to operate with external doublers/triplers.

Thanks, that's what I thought. Any idea what the purpose of the HV capacitor is in the second photo in the OP?

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