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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 24th Aug 2018, 2:57 pm   #1
Nanozeugma
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Default Transformers, Mains and Output.

Hi.
Can anyone give an answer as to why certain manufacturers of valve radios decided it was a good idea to coat or dip the transformers in pitch - and most didn't bother (certain PYE and MURPHY sets spring to mind).

I wondered if it was in an attempt to discourage moisture ingress - and the dreaded green spot disease? It mut be said that the examples I have seen were intended for the domestic market and not "tropicalised".

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Old 24th Aug 2018, 3:26 pm   #2
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Arrow Re: Transformers, Mains and Output.

My understanding. With pitch, moisture kept out, but adding pitch incurs a cost. Without pitch, moisture can get in, but above cost then not incurred. Also of consideration is that if the entire transformer is encapsulated in pitch, for a given power throughput, the temperature of the core - and thus the windings - will be increased. A sensible design should take that into account.
As is the case with any engineering project, a trade-off amongst cost, reliability / maintenance and quality, provided the essential requirements of form, fit and function are met.

Al.
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Old 24th Aug 2018, 4:11 pm   #3
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Default Re: Transformers, Mains and Output.

In addition to what has already be stated, and if my memory is correct it was to reduce vibration caused by the effect of magnétostriction.
Cheers John
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Old 24th Aug 2018, 6:30 pm   #4
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Default Re: Transformers, Mains and Output.

Pitch is an amazing high molecular weight hydrocarbon. It's what you get when all the oil and petrol has been taken away (generally it's a petroleum product but it can be made from plants). It is an excellent insulator, has a high dielectric strength, it totally water repellant (which is why it was used to seal roads and the hulls on boats) and makes a great coating for transformers.....but it has a limited life span for this application, just as it has for tar sealed roads and folks in that industry will tell you about the life cycle of a Tarmac road.

The problem happens as it ages. It contains a range of higher and lower molecular weight hydrocarbons. The lower mw ones slowly leave and as a result the pitch hardens, shrinks and cracks.

If it is used to coat iron core transformers, it separates from the iron lamination surfaces, lets air and moisture in so rust occurs.

In transformers with fine link wires, they can get stressed and break. With TV line output transformers, corona discharges can occur as the over winds get exposed to atmospheric moisture.

Pitch coatings, depending on temperature are tacky and stick to airborne dust.

In reality it's a cheap, ugly and medium term solution to protecting items like transformers. I give it about a 15 to 25 year life span, after that it's downhill faster than a ball bearing on a polished 45 degree slope. You are much better off with an oil filled transformer than a pitch filled one, but the latter does not require a seal, so it's cheaper to make.

Hot pitch can melt too, but ironically if it is very old pitch, if you heat it , it hardens and bakes on, and will not flow. Many a pitch coated transformer has been damaged trying to melt off or crack off old pitch.

It is best in vintage radio work, if you see pitch coated objects, to dissolve it off in a tank of mineral turpentine over a few days (white spirits in UK) then replace it with transformer grade or marine spar varnish and if you can organise at least a 500mmHg vacuum, all the better to help the replacement varnish impregnate.

Last edited by Argus25; 24th Aug 2018 at 6:38 pm. Reason: Typo
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Old 24th Aug 2018, 8:17 pm   #5
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Default Re: Transformers, Mains and Output.

Vacuum impregnation with varnish is definitely the way to go for high quality transformers.

There are still a lot of transformers ion the surplus market with oil-filled cans and ceramig/glass feedthroughs. The snag is that in some of them the oil is pased on PCBs (poly-chlorinated bi-phenyls) which are bad enough as they are, but turn to dioxins if they burn. If you have any oil filled transformers, just don't risk breaching their sealing.

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Old 25th Aug 2018, 3:24 am   #6
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Default Re: Transformers, Mains and Output.

One quick way to get your hands on excellent transformer oil if you need it to repair oil filled transformers or capacitors is to buy auto ignition coils and just sacrifice the coil. I have used this as replacement oil for a number of high voltage projects such as probes and a lopty repair. It's likely that any coils made over the last 20 or 30 years use PCB free oil. But all chemicals should be handled carefully to avoid skin contact and used in a ventilated area to avoid inhaling fumes anyway.
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Old 25th Aug 2018, 8:33 am   #7
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Default Re: Transformers, Mains and Output.

Quote:
Originally Posted by Argus25 View Post

The problem happens as it ages. It contains a range of higher and lower molecular weight hydrocarbons. The lower mw ones slowly leave and as a result the pitch hardens, shrinks and cracks.

If it is used to coat iron core transformers, it separates from the iron lamination surfaces, lets air and moisture in so rust occurs.

In transformers with fine link wires, they can get stressed and break. With TV line output transformers, corona discharges can occur as the over winds get exposed to atmospheric moisture.

In reality it's a cheap, ugly and medium term solution to protecting items like transformers. I give it about a 15 to 25 year life span, after that it's downhill faster than a ball bearing on a polished 45 degree slope.
....which is exactly what I have noticed in my Pye TV. The LOPT is heavily coated in pitch and bits are now breaking off. If your 25 year max lifespan is correct, this one has exceeded it by another 25 years....!

Quote:
Originally Posted by Argus25 View Post
Many a pitch coated transformer has been damaged trying to melt off or crack off old pitch.

It is best in vintage radio work, if you see pitch coated objects, to dissolve it off in a tank of mineral turpentine over a few days (white spirits in UK) then replace it with transformer grade or marine spar varnish and if you can organise at least a 500mmHg vacuum, all the better to help the replacement varnish impregnate.
The transformer isn't arcing or doing anything untoward at the moment so I've left well alone but I was wondering about the possibility of dissolving the pitch and dipping in varnish....maybe the white spirit method is the way to go.
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Old 25th Aug 2018, 11:41 am   #8
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Default Re: Transformers, Mains and Output.

I am always amused to see the pitch puddles and runs around the base of an old wooden telephone pole. Not sure how long they take to form.
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Old 25th Aug 2018, 3:26 pm   #9
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Default Re: Transformers, Mains and Output.

Quote:
Originally Posted by Sideband View Post
... I was wondering about the possibility of dissolving the pitch and dipping in varnish ... maybe the white spirit method is the way to go.
I'd be a bit nervous about the white spirit affecting other elements of the transformer, but it will depend entirely on its construction details. An old trick which I've used in the past is to seal the transformer into a polythene bag and leave it in the freezer over night. The following morning the pitch will be brittle enough to chip off pretty quickly.

The downside is that the transformer will be very cold so it's best if you can do the job in the driest air possible (outdoors in clear weather can be quite dry first thing) rather than in, say, a steamy kitchen. This will minimise the amount of water that ends up condensing into the transformer. Putting it back into a sealed poly bag while it warms back to room temperature can help too.

Cheers,

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Old 26th Aug 2018, 1:40 am   #10
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Default Re: Transformers, Mains and Output.

I have dissolved the pitch off many transformers using Australian made mineral turpentine, it's a BP product sold at hardware and petrol stations. I have never seen any damage yet to the windings or insulating materials that compose any lopty or iron cored transformer. Also mineral turpentine is also the thinner for the varnish applied and if there is residue of the turps deep in the wave wound over wind it actually helps the varnish impregnate. (As for the UK's White spirits I'm not as certain)

I would caution at any attempt to mechanically pick of the pitch, regardless of its temperature. It wraps around the fine overwind wires and it will break the fine wire there . There is no need for heating or mechanical forces to solve this pitch lopty issue. It's a molecular problem.

I would have published this solution years ago, but until recently I had no idea about the pitch problems people were having, and because I had solved this issue I thought everyone else would have too. I was taught when I was about 14 years old that the solvent for pitch was mineral turpentine ( it has been used across a century to clean road tar off motor vehicles) and as a teenager had prepared different dilutions of pitch for various projects with a range of viscosities.

All you need to do is drop the transformer in a bath of it and be patient because hardened pitch, initially, is slow to start to dissolve, but the process speeds up over days. A very gentle stroke with a very soft artist's brush is ok if compelled to use force, and renewing the turps in the bath helps to view the progress.

Of course if a transformer is working fine you may want to leave it alone anyway.
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Old 26th Aug 2018, 9:22 pm   #11
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Default Re: Transformers, Mains and Output.

Hi Gents, in the UK it may be worth checking out "tar spot remover" as sold in motoring shops. That could well be mineral turpentine.

Ed
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Old 26th Aug 2018, 9:40 pm   #12
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Default Re: Transformers, Mains and Output.

Or "turpentine substitute" / "white spirit".
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Old 27th Aug 2018, 8:42 am   #13
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Default Re: Transformers, Mains and Output.

It's as well to be vigilant when buying.
'Turps subs' (white spirit) is gradually being replaced by 'white spirit substitute', which may or may not dissolve pitch - I haven't tried. It's certainly not as good as the real thing for paint removal.
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Old 27th Aug 2018, 8:57 am   #14
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Default Re: Transformers, Mains and Output.

I have used paraffin to dissolve pitch. It works quickly and brushing a bobbin with it enables slow unwinding.

I would think that any petroleum product would do.
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Old 27th Aug 2018, 9:26 am   #15
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Default Re: Transformers, Mains and Output.

Quote:
Originally Posted by Argus25 View Post

Of course if a transformer is working fine you may want to leave it alone anyway.
....and that is the problem!! If a new transformer was still available, I'd take the gamble to remove the pitch from the old one and re-varnish. If the transformer was then no good I'd just have to replace it. Now of course I don't have that option so I'll leave it alone for now.....

It would appear that White Spirit sold over here is 'Mineral Turpentine' as sold in US/Can/Aus according to Wikipedia.

https://en.wikipedia.org/wiki/White_spirit
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Last edited by Sideband; 27th Aug 2018 at 9:36 am.
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