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Old 21st Jan 2018, 5:18 pm   #1
Keith
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Default Bush TV22 LOPT problem

Well I finally took the plunge and acquired a Bush TV22. This one is the version with EF80s in the RF section and also has been fitted with a 2 channel tuner, which I understand was quite a common modification. I have partially dismantled the set and thought the LOPT looked a bit rough so did a few resistance measurements. All seemed OK until I got to the overwind which, sadly, appears to be o/c. The actual winding, where visible through cracks in the pitch, doesn't look cooked so I wonder where these usually fail. I'm guessing it's most likely next to the former, in which case, what are my chances of removing the whole winding and finding the break? Any advice would be appreciated.
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Old 21st Jan 2018, 5:57 pm   #2
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Default Re: Bush TV22 LOPT problem

That looks to have been quite hot at some point to shed it wax that much. Don't condemn the overwind just yet, It could be just an end to find and re attach though so look hard through a loupe or good magnifier. I use a pin to help flick off small bits of pitch.
Don't take it as gospel but I think the inner start of winding is here in picture, its normally under some tape for first 3/4 inch away from winding. other end is to the tag the rectifier is monted on.
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Old 21st Jan 2018, 6:36 pm   #3
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Default Re: Bush TV22 LOPT problem

As Freya has said, particularly check the presence of the inner winding connection. Disturbance to the pitch covering in the past could have broken it. The wire is very fine, hard to spot, and indicated by the left pointer in the illustration attached. More HERE .


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Old 21st Jan 2018, 7:53 pm   #4
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Default Re: Bush TV22 LOPT problem

You may notice the difference in lamination shape of Panrock`s LOPT compared to yours.
The type you have is the "I" pattern which is slightly longer and the later type, whereas Panrock`s is the early "T" shape which was considered wasteful.
There is another version which has a ceramic former which has the heater winding on, I believe these are the best type.
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Old 21st Jan 2018, 10:20 pm   #5
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Default Re: Bush TV22 LOPT problem

I think as the pitch hardens cracks & shrinks it is responsible for breaking the wire. I would put the transformer in mineral turps bath (or equivalent product UK) for a few days, dissolve off all the pitch, then the view of the winding will be good enough to see where the wire broke and possibly repair it. This one though does look like it get very hot at some point. The assembly can be then be dipped in marine spar varnish after that and it will have a nice clean new look to it too. As the pitch is dissolving away a very gentle brush with a soft artists brush can speed that up, but nothing more than that. Never pick at the hard pitch, or that can do more damage to the overwind. I would also avoid heating it in attempts to remove the pitch.
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Old 21st Jan 2018, 11:15 pm   #6
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Default Re: Bush TV22 LOPT problem

Hello.
We used many pitch covered transformers in industry, often they failed due to overloads and the damp conditions they were subjected too. Back in the workshop they were heated up to remove the pitch (bitumen) coating. Chemicals were not allowed in our industry as any flash off was considered a fire risk. Put your transformer in the oven with a drip tray underneath, this is the best way and it will also drive out any retained moisture. Aim for 120-150c.
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Old 22nd Jan 2018, 12:16 am   #7
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Default Re: Bush TV22 LOPT problem

Thank you all for your replies - most encouraging. I shall attempt to find the break in the winding. Hopefully it will be at one of the leadouts. Removing the pitch seems to be the first priority although there seem to be different views of the best method. I shall try localised application of white spirit initially assuming this to be less risky than heat.
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Old 22nd Jan 2018, 1:21 am   #8
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Default Re: Bush TV22 LOPT problem

Old hardened pitch needs to be raised to a very high temperature to get it to melt and sometimes, because its own natural solvents are grossly depleted, it wont liquify and it will turn into a very hard mass and you'll end up baking it on...in short don't do it, it is really bad advice to remove old pitch with heat, though some people have got away with this method in some cases doesn't make it the better method. Your transformer has already suffered some heating effects, do you really want to make that worse.

With the white spirits (turps), you will find that if you apply some to the surface as a test, little will appear to happen initially as the hardened pitch resists the solvent, initially. You have to submerge the transformer in a bath of it and wait very patiently. After 1 day you will start to see the effects as the pitch enters the solvent to colour it brown, as each day goes by the process accelerates. You cannot judge the benefit of the turps with a quick test. It may even take a week, but you will remove the pitch with no damage to the transformer's windings.
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Old 22nd Jan 2018, 6:17 am   #9
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Default Re: Bush TV22 LOPT problem

Hi Gents, sorry to add damper to the thread, but I suspect that the primary windings may well also be cooked (but much easier to rewind).
Try a resistance test between windings to look for leakage.
The paper insulation often carbonises and leaks with the higher voltages seen in operation.

Ed
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Old 22nd Jan 2018, 6:54 am   #10
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Default Re: Bush TV22 LOPT problem

Turpentine is a natural bio product and expensive.
Turps substitute is usually white spirit, a distillate of oil and cheaper.
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Old 22nd Jan 2018, 7:08 am   #11
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Default Re: Bush TV22 LOPT problem

We have two sorts of "turps" available, either mineral turpentine or vegetable turpentine. The latter smells a lot like pine needles.

The one I know for sure that works to dissolve the pitch is Mineral Turpentine.

I'm not sure of the equivalent product in the UK. Normally I buy the bottles made by BP, here is their data sheet:

https://www.ils.co.nz/ils/assets/dow...TDS/684218.pdf

as noted on the sheet it is referred to as "Petropine" or "Turps".

So its just a matter of finding the equivalent UK product, or close, now you have the data sheet.

Ed may be right about the windings, you will only know when the pitch has been dissolved away and you can get a better look at the windings.
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Old 22nd Jan 2018, 10:20 am   #12
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Default Re: Bush TV22 LOPT problem

Lead out wires are very often where the break is. I had a transformer with a multiple sectioned overwind [VIDOR 369A] The leads passed through a nasty sticky sleeving and every one was rotted through. Some careful work with some very fine abrasive paper and short lengths of 5amp fuse wire resulted in a very satisfactory repair.

The adhesive on that old style retaining tape can also rot the fine wire over decades of damp storage.

Please don't pull the chassis to bits! Get a glimmer of hope on the screen and work forward from that.

Cleaning is a KILLER and will result in fine wires being dislodged and multiple faults produced. Dust and dirt won't stop it working. Good luck with it. John.
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Old 22nd Jan 2018, 10:47 am   #13
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Default Re: Bush TV22 LOPT problem

Lots of useful advice there so many thanks again. Sounds like soaking in white spirit (which seems to be turpentine substitute) is the first requirement. I did try approaching a small outcrop of the pitch with a soldering iron and can confirm Argus25's findings.

I certainly don't intend to pull the chassis to bits at this stage. I need to get some indication that the CRT is usable before I proceed to major restoration and so I need to get the line output stage/EHT functioning first.

Thanks for your suggestion, Ed. I'll get the Megger out.
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Old 22nd Jan 2018, 10:24 pm   #14
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Default Re: Bush TV22 LOPT problem

Hi,
I have now successfully removed the coating from two LOPT's using Argus's method of soaking in white spirit. According to the internet, White spirit seems to be the UK equivalent of mineral turps.
The first transformer was heavily wax coated. I removed the bulk of this by placing in a small oven at 100 degrees or so to soften the wax which I then removed with kitchen roll. It was then submerged in white spirit but still took several days for all the wax to come off.
Really not liking the idea of heat removal, the second transformer, which I am still working on, I just used the white spirit. The EHT winding had a very thick coating of either pitch or black wax, I'm not quite sure. This has taken about three days and a change of white spirit midway- it seemed to loose it's effectiveness. The winding is now clean and ready for me to try and repair.
Hope that helps,
All the best
Nick
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Old 23rd Jan 2018, 5:27 am   #15
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Default Re: Bush TV22 LOPT problem

Quote:
I have now successfully removed the coating from two LOPT's using Argus's method of soaking in white spirit. According to the internet, White spirit seems to be the UK equivalent of mineral turps.
Well done. Of course before you power the transformer up in circuit to generate EHT it must be re-coated. If it is powered in a set with no coating at all, corona discharge is a big risk.

Buy a tin of high quality marine grade spar varnish (or standard transformer varnish if you can get it) Drop the transformer into it and leave overnight, then hang it up to dry. Once you figure out how long the particular varnish takes to go tacky and not run, you can repeat dip it 10 times or more until you have built up a thick coat at least 0.5mm or more thick over the transformer & overwind surfaces. When you have built a a good thickness leave it for a week to dry. Then install in the set.

I use this product (but I don't know the equivalent in the UK), excuse the ebay reference:

https://www.ebay.com.au/itm/Feast-Wa...AAAOSwt5hYZMBs
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Old 23rd Jan 2018, 7:30 am   #16
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Default Re: Bush TV22 LOPT problem

I have not used "spar" varnish but Polyurethane clear varnish does what it says on the tin (but not as an insulator).
Not tried on an overwind though. I get the windings warm before the first dip, as they cool the varnish is drawn in to some extent. Done lots of transformers, armatures, stators and chokes.
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Old 23rd Jan 2018, 8:42 am   #17
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Default Re: Bush TV22 LOPT problem

Spar varnish is really just thicker with the ability to resist knocks to some degree, it does however crack eventually with heat. Also remember that if the transformer doesn't work or fails after this treatment, it would be a real mess to rewind, I would love to see Mike B`s face.
Why not but a tin of anti corona spray from RS, two or three coats of this would be far more reliable ?
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Old 23rd Jan 2018, 11:19 am   #18
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Default Re: Bush TV22 LOPT problem

I have restored a number of pitch coated line output transformers with the described method and have not had a single failure, or seen any cracking in the varnish over 20 years. Cracking, is primarily due to UV exposure,not heat, which the varnish does not get in this application.

Also, I could add, this is not my idea. I got it after studying the H output transformers used in professional Marconi television video monitors.

You could always go back to square 1 and re-coat it with pitch or wax if you wanted an inferior result and for history to repeat itself.

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Old 23rd Jan 2018, 3:05 pm   #19
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Default Re: Bush TV22 LOPT problem

My LOPT is now sitting in a jar of white spirit which does seem to be slowly softening the pitch/wax. I give it a bit of agitation every now and again and very light brushing with a paint brush. Before I dunked it I tried the Megger (only 250V mind) between the primary windings which indicated no leakage so fingers crossed.

In the meantime, I've been re-forming the electolytics (OK so far) and examining some of the very dodgy repair work done over the years. Can anyone tell me the correct value of C24 on the line scan coil? My Molloy and Poole shows a 2uF electrolytic but someone has fitted a 0.25uF 350V component (which is now leaky).
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Old 23rd Jan 2018, 10:40 pm   #20
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Default Re: Bush TV22 LOPT problem

Quote:
Originally Posted by Keith View Post
Can anyone tell me the correct value of C24 on the line scan coil? My Molloy and Poole shows a 2uF electrolytic but someone has fitted a 0.25uF 350V component (which is now leaky).
If it is the line yoke coupling capacitor (2uF electrolytic) you are referring to mounted on the yoke, the smart move is to replace it with a 2.2uF 400V Solen Fast brand MKP cap. These are black with white writing.

It was a design blunder (in my view) to use an electrolytic as a yoke coupling capacitor (others may argue) but technical issues of ESR aside, the electros never last. You will often see these leaking with corrosion around the end with the seal, but if you go to an MKP, you will never have any trouble with it in the future and the circuit function will remain normal over time. Photo of one sitting on the yoke attached.
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