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Crude LOPTX testing.

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Crude LOPTX testing.

Post by Rebel Rafter » Mon Nov 16, 2015 10:38 pm

Hi, folks, RR here. I have a non working Telequipment D32 scope for repair which a friend of mine got from ebay. For those not familiar it's a portable scope with ni-cad batteries, with a push-pull inverter using a pot core transformer to provide all the different voltage rails, including the EHT. I've removed the batteries as they're all original from 1972 and therefore well knackered, and I've modified the power input arrangement so the scope can run from the mains without overvoltage in the absence of the batteries. But there is no output at all from the transformer so I suspect it of having shorted turns, most likely in the EHT winding, as I can't find any shorts on the outputs or any shorted rectifiers or capacitors, etc. So I took the transformer out and thought about how I could test it without my ringing device which has gone missing. So I thought what about the "rush" from a blank FM frequency on an old ghetto blaster I've got, so I connected one of the low voltage windings of the pot core to a speaker output without the speaker connected and I connected a mains neon to the EHT output and turned up the volume and the treble and guess what, the neon lit up all nice and bright! and I got several volts from the other windings too. The audio output is about 7 watts. So does this mean the transformer is ok, or is it just giving me false hopes? I thought I'd mention this as I thought this might be a possible means of testing old TV line output transformers, though I think you'll need a few more megohms in series with your neon for testing TV LOPT's, especially colour ones. RR.

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Re: Crude LOPTX testing.

Post by line sync » Mon Nov 16, 2015 11:22 pm

That's a very interesting way of testing a lopt.
Sorry I cannot offer any help with your scope.


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Re: Crude LOPTX testing.

Post by Cathovisor » Mon Nov 16, 2015 11:49 pm

The problem is that line transformers are operated under different conditions to other transformers: they are pulsed by switching the supply to the primary and usually form part of a resonant system, which is why they are usually "rung" to test them - any shorted turns will damp their inherent tendency to ring.

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Re: Crude LOPTX testing.

Post by crustytv » Tue Nov 17, 2015 10:56 am

Rebel Rafter wrote: I took the transformer out and thought about how I could test it without my ringing device which has gone missing.

So why not use a scope assuming you have one to hand. It seems all a bit convoluted when a simple ring via a scope would suffice.

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Re: Crude LOPTX testing.

Post by Rebel Rafter » Tue Nov 17, 2015 12:42 pm

Hi, folks, RR here. Thanks for that article, I've printed it out, I'll have a read later. I've got a scope here, a nice big Roband one. I didn't know you can ring a transformer using the calibrator, my scope has one built in. I normally use a device I built from an article in "Television" magazine, but I can't remember which issue it was in. It's basically an oscillator driving a high voltage power transistor with capacitors which can be switched in or out across the coil under test. It works with all sorts of stuff, not just line output transformers. The coil being tested forms the collector load for the transistor. RR.

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Re: Crude LOPTX testing.

Post by Wolfie » Tue Nov 17, 2015 2:03 pm

That was amazing just how a single wire loosely wrapped around could dampen the ringing so effectively.

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