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JABCO : Line Output Transformer Tester

 
crustytv
(@crustytv)
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My method of testing Line Output Transformers has always been to use an oscilloscope to ring the LOPT. This  has proved more than adequate over the years I've been active and has proved one or two faulty ones, including a Thorn EHT "jelly-pot". Therefore you might ask why bother with this JABCO tester.

Well two reasons, first I'm quite taken with JABCO products especially the Thorn 3000/3500 test rig which is used constantly and has proved to be invaluable. Second reason JABCO were a local company based in Shoreham Road Brighton, the town I was born in and spent 44 years there before relocating northwards. Therefore when one of these LOPT testers came up for grabs, I couldn't let it go.

The LOPT tester sitting with the 3000/3500 test rig

jab-1.jpgjab-2.jpg

I guess the first thing to do is see if it still works and can be trusted. For this test I will use a known good PYE 691 LOPT, demonstrate it is actually OK by doing a ring test on the scope, then simulate a shorted turn by wrapping a wire around the overwind. Repeat all this with the JABCO tester.

Hooked up to the scope, ring trace confirms a good LOPT

jab-3.jpg

Now connect the JABCO, it also confirms LOPT good ( green light)

jab-4.jpg

Simulate a shorted turn by wrapping a wire around the overwind

As can be seen the scopes ring trace is totally dampened, confirming shorted turn

jab-5.jpg

Connect the JABCO, it too agrees a shorted turn has been located (red light)

jab-6.jpg

Lets have a look inside.

Not a lot to it, a TX, bridge rectifier, two i.c's a couple of presets, some caps, diodes and resistors. I thought it might be loosely based on a device that appeared in "Television". The Television circuit used one i.c and a transistor whereas this uses two i.c's and is mains powered, there is also a label stating its their own patented design.

jab-7.jpg

Its got me wondering what else JABCO made for the trade, wish I could find out more about them. I wonder if they had a product catalogue. This is likely get used a fair bit especially when I can't be bothered to set up the scope. Could also be handy for trips to buy TV's, simple to stick in your coat pocket.

Hope the thread was of some interest.

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Topic starter Posted : 07/06/2017 1:11 pm
Marc
 Marc
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Hi Chris,

It's funny you should come up with the JABCO tester as I've been looking to build the LOPT tester from 'Television' . My Tek 2230 doesn't have the usual 2v or 5v hook on calibration point instead there is a tiny probe socket which you can't really connect anything else to hence the thought of the 'Television' project.

I know the project uses a 4001 chip, what chip is used in the JABCO ?

Marc. 

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Posted : 07/06/2017 1:34 pm
Nuvistor
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I have heard about LOPTX testers but never used one, always used the scope like yourself. I have not heard of the JABCO one though.

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Posted : 07/06/2017 1:35 pm
crustytv
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Marc Said

My Tek 2230 doesn't have the usual 2v or 5v hook on calibration point instead there is a tiny probe socket which you can't really connect anything else to

Marc.   

Why let that stop you?

Of course if you want to purchase all the parts required and build a LOPT tester then it would be interesting and educational. However why not just use a function generator, if you don't have one you can pick cheap Chinese ones up for under a fiver or the one I have here for just under £50

Here I am doing a ring test and not using my scopes internal cal point but my function generator to feed in a 1kHz 5V square wave to the LOPT

extsig.jpg

Or far easier than that and for just a few pence, bread board a 1kHz square wave generator (see below) using a 555 timer i.c. You could have this up and running in two mins.

Image result for 1khz square wave using 555 timer

knockup.jpg

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Topic starter Posted : 07/06/2017 2:35 pm
Anonymous
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Chris said
Therefore you might ask why bother with this JABCO tester.

Tools that work are always welcome on my bench.

Lets have a look inside.

Not a lot to it, a TX, bridge rectifier, two i.c's a couple of presets, some caps, diodes and resistors. 

The key here is that the tool has 'Go' and 'No-Go' indicators. Which begs the question "How does that work?"

We need a pulse to drive the LOPT under test, and we need to understand the result. A good coil (LOPT in this case) will ring, and the voltage peaks will be much greater than the driving signal, which doesn't have to be very big, probably will work with only one volt p-p driving it. A shorted turn on the LOPT will dampen the ringing, as shown by your 'scope.

Now if we said that a multiplication of, say, ten or more was good, a detector and comparator would suffice. The output feeding the green LED, and turning off the red LED. The ratio of ring voltage to driving voltage is called the Q-Factor.

Mains power is not needed; a battery would work, and make this pocket sized for field checking where's there no mains available.

As a further refinement, could a single turn of wire on the LOPT work? Then no electrical connections to the LOPT under test would be needed; just thread an insulated wire through the LOPT core's window.

Hope the thread was of some interest.  

Indeed!

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Posted : 07/06/2017 7:12 pm
Marc
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Hi Chris,

Thanks for your thoughts, you've raised several good points there. I suppose I was leaning to the 'Television' project as I think I may have most of the parts to hand but as you point out function generators are so cheap now. I shall mull over the options. hmm_gif

Cheers, Marc.

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Posted : 07/06/2017 7:32 pm
Cathovisor
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I built a Television design from the early/mid-80s which used an op-amp (a 301 IIRC) which worked on the principle that any coil that had shorted turns would damp an oscillator and stop it, thus extinguishing a light.

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Posted : 07/06/2017 10:46 pm
Anonymous
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Cathovisor said
I built a Television design from the early/mid-80s which used an op-amp (a 301 IIRC)

Do you have the circuit diagram?

which worked on the principle that any coil that had shorted turns would damp an oscillator and stop it, thus extinguishing a light.  

I thought about the LOPT "ringing voltage" lighting a small neon bulb. Only good coils give a light.

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Posted : 07/06/2017 10:53 pm
Cathovisor
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marc said
Hi Chris,

Thanks for your thoughts, you've raised several good points there. I suppose I was leaning to the 'Television' project as I think I may have most of the parts to hand but as you point out function generators are so cheap now. I shall mull over the options. hmm_gif

Cheers, Marc.  

There is also this rather nice device:

https://www.flippers.com/blue-fbt.html

ISTR I bought the bits for one. Where I've put them is another matter.

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Posted : 08/06/2017 6:01 pm