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Old 9th Apr 2019, 2:51 pm   #1
greenstar
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Default Transformer - is it the end?

HI folks, appreciate your help again.
I have a substantial transformer I would like to use for a power supply for use with military sets (in this case an R1155, but potentially others). I picked it out from my accumulation because it supplies more than the standard 5 valve set psu transformer - 200mA HT and 5A at 6.3v. Windings show continuity as expected and output measures about right. But the thing shows signs of distress - a soft hum with mains applied, a little crackling, a whiff of smoke, and heating up, warm to the touch. I only had mains on it for a few seconds. It doesn't seem to show signs of past overheating. It's been stored a few months after arriving from a Harpenden lot, in a shed which is reasonably dry.

Is it beyond hope? Is it likely just damp? Stick it in the oven?
Thanks,
Tony
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Old 9th Apr 2019, 3:16 pm   #2
Boater Sam
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Default Re: Transformer - is it the end?

Check for tracking between the tags, or 2 down to the core, nothing dropped in to short? Otherwise it sounds like a shorted turn.
Where did it smoke from?
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Old 9th Apr 2019, 3:57 pm   #3
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Default Re: Transformer - is it the end?

Are you sure that you have it strapped for 240V input rather than 115V? I can't read the printing on the insulating tape in your photo.

Ron
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Old 9th Apr 2019, 4:43 pm   #4
greenstar
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Default Re: Transformer - is it the end?

I did get it wrong initially, but so far as I can see it's correct. When I got it, tabs 4 and 5 were joined which I removed as I could not see why.
On the primary side: 1/5 2/6 3/7 4/8 101-110-119
also: 1-2/5-6 202V, 1-2/5-7 211V, 1-3/5-7 220V, 1-3/5-8 228V, 1-4/5-8 238V.
Tell me if I've done something idiotic!

Sam, There's nothing obvious shorting the tags and no sign of tracking.
Appears an infinite resistance from 2 to the laminations.
Measuring from secondary to laminations, the meter just twitches on the highest range,

Last edited by greenstar; 9th Apr 2019 at 4:58 pm.
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Old 9th Apr 2019, 5:09 pm   #5
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Default Re: Transformer - is it the end?

It looks to me as if the transformer needs two separate 120V primary windings joined in series for 238V. Remove all existing primary links. Just link tags 4 and 5 and apply 240V mains between 1 and 8. If you have a variac apply a low voltage to start with and increase it slowly to 240V. If not, make sure you have a 1A or 2A fuse in the plug top.

Ron
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Old 9th Apr 2019, 6:01 pm   #6
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Default Re: Transformer - is it the end?

Thanks Ron, that explains why they were joined. I will try this in a couple of days and report back - I do have a variac.
Tony
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Old 9th Apr 2019, 6:15 pm   #7
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Default Re: Transformer - is it the end?

That is normal for a twin primary transformer. Usually both windings are 0-120 and 0-120. For 120 volts the winding are connected in phase in parallel so 0-0 and 120-120. For 240V operation the windings are connected in phase in series so 120 connected to 0 (inner tags) and 0-120 (outer tags) to the mains.

Connect it as Ron says and hopefully it will be OK.
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Old 9th Apr 2019, 9:19 pm   #8
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Default Re: Transformer - is it the end?

You say it has a 6.3 secondary. Feed in 6.3 from a suitable source, and measure everything from there. That way, no risk of wrong connections or voltages whilst you check.
Once you are sure, connect to mains with no secondaries and it should not take significant current, hum or overheat. If it does any/all of those, you have shorted turns.
Les.
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Old 10th Apr 2019, 3:10 am   #9
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Default Re: Transformer - is it the end?

Hello,
What I think you have on the primary side is two identical 119 volt windings.
One winding goes between taps 1 an 4 with intermediate tappings at 2 and 3 and the second winding is between 5 and 8 with with intermediate tappings at 6 and 7.
The voltage between taps 1 and 2 and between 5 and 6 is 101 volts, between 2 and 3 and 6 and 7 is 9 volts and between 3 and 4 and 7 and 8 is also 9 volts.

(1 -101V- 2 -9V- 3 -9V- 4) (5 -101V- 6 -9V- 7 -9V- 8)

For low voltage use connect the two windings in parallel : 1 to 5 and 2 to 6 for 101V , 1 to 5 and 3 to 7 for 110V and 1 to 5 and 4 to 8 for 119V
For high voltage use series connection; connect 4 to 5 and connect supply to 1 and 8 for 238V, connect 2 to 5 and supply to 1 and 6 for 202V etc as indicated on the transformer.
Yours, Richard
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Old 10th Apr 2019, 9:48 am   #10
Maarten
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Default Re: Transformer - is it the end?

If 4/5 were joined, and you disconnected them before applying voltage to the transformer, nothing should have happened. To which terminals did you connect mains voltage?
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Old 11th Apr 2019, 10:01 am   #11
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Default Re: Transformer - is it the end?

This is what I think it is like (As previously posted by everyone else).
It might help you visualise the whole thing.

If the picture you posted with the two brown wires (tags 5 and 7) were your mains connections, then you have put 240 volts across a 110 volt 'section'.
It might be ok if it was brief...

Again as said before, link 4 and 5 together and put 240 volts on tags 1 and 8.

Alan
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Old 11th Apr 2019, 5:13 pm   #12
greenstar
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Default Re: Transformer - is it the end?

I have simply connected as advised by Ron and Alan - join 4 and 5, mains to 1 and 8. Thanks all for the explanations. Alan's sketch helps immensely. It's a challenge, sussing old transformers, but at least this one had markings. Starting with a low ac voltage applied to the secondaries makes sense.
But it has just operated as marked, without any sounds or smells of distress. Hopefully it will live a long while yet! Is it worth testing with a load?
Tony
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Old 11th Apr 2019, 8:35 pm   #13
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Default Re: Transformer - is it the end?

Hi Tony, if an hours test with no load shows only slight warming then it is worth trying with a load.

Ed
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Old 12th Apr 2019, 6:34 pm   #14
greenstar
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Default Re: Transformer - is it the end?

Thanks Ed, I ran it today as you said for an hour, no load, and no discernible warming. I then connected a 25W filament bulb across the output, and ran again for 90 minutes, after which it was slightly warm. Looks like it will be ok.
Tony
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Old 12th Apr 2019, 6:39 pm   #15
Boater Sam
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Default Re: Transformer - is it the end?

At least it will be dried out now!
Sorry to panic you earlier, its the mention of smoke that got me going, I gave up 6 years ago.
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