Transformer Rewinding Service

I have been leaning into the rebuild direction, so a few questions to Ian, Bryan and the rest of the Forum members here. I doubt I could actually obtain the build sheet for this transformer, so hand unwinding the unit and counting the turns would have to be the way to go on this rebuild project. Opinions. Of course I would take some DC resistance readings before unwinding the HV filter choke, but no inductance meter here at the QTH. I am guessing with the same number of winding when using the same sized wire as the original unit, plus having the same DC measurement when finished, that should be in the ball park inductance wise. Is Nomex paper and their laminates the paper to be used here, or way to overkill? The Nomex laminate might be used anyway in some heavy coats to make sure the HV arcing problem of the Collins original unit never happens after the rebuild. TNX for all of the great info and the money saved can be used on powder coating the cabinet pieces. I am not worried about pinching pennies as this project isn't going to be a money maker to be sure.

 

Mercury’s “bread and butter” has been the Audiophile and Music Industry.
They frequently write articles for the market’s industry trades.
https://www.mercurymagnetics.com/

While they have performed commercial/industrial/government work and have ventured into automotive/marine market — they have skipped the commercial/amateur radio niche that Peter Dahl use to occupy. Since Hammond bought the P. Dahl database, they can fulfill the commercial/government/military needs ... and radio amateurs with financial ability.

 

If you're just looking to repair and/or replace just the one component then rewinding it yourself is probably the way to go. That way it remains as original as possible.
If there's a possibility that other wound components need attention and originality is not so much of a priority, then a complete re-design using modern techniques (and components) might be an option. That was the option I selected with the Henry 2k I just rebuilt as there were too many unknowns in the rest of the amplifier as it had been messed about with and unused for many years.

If you decide to rewind yourself, take lots of pictures as you dismantle it. When you are actually unwinding rig something up to count each layer. I was amazed that both transformers I've pulled apart the turns per layer counts were not the same, sometimes as much as +/- 2 turns but the overall count per winding half is always correct. I used a $10 electronic counter from china to do the counting. Make lots of notes of dimensions as you go and when rebuilding put it back together as close as possible to the original as you can.

One of the issues when rebuilding is the thickness of the insulation used then and now. The Henry plate transformer used what looks like thin brown paper between each layer. It measured (after being squashed for nearly 60 years) 0.002 inches thick and the secondaries were really tightly wound. The minimum thickness available of Nomex 410 is 0.002 inches thick and is not readily available. The people I used: www.baewire.com/category-s/135.htm only stock it down to 0.005 inches. Space becomes a problem especially on a first attempt!
I used Nomex-Mylar-Nomex solely for the new-build and self-adhesive Mylar tape for anchoring the windings. The thinnest available is 0.007 inch so it takes up much more space but has a dielectric strength of 11kv
Another thing to watch out for is that modern copper wire comes in more than one flavour. I use Temco heavy build 200 deg C enameled wire. The insulation is slightly thicker than wire used in antique transformers/chokes. It's not a lot but needs to be taken into account when deciding if the 40 layers of wire and 40 layers of insulation will fit in the lamination window you are working with.

Can you see where it is arcing? Is it arcing to the frame or inter winding?Maybe you can add some extra insulation and maybe some heavy motor varnish to it if you can remove the carbon track the arcing has left.

Ian

 

With my luck (skill) I'd run out of room at layer 38...

 

TNX for the reply Mr. Ian. In my case, I won't need to worry about the size of the rewound unit, as there is plenty of room in the PS box. The original Collins HV choke had problems with the HV on the outer edges of the Xformer arcing over to the mounting bolts to the Xformer. I will make sure the choke is laminated. As I sure don't want the original known problem with the HV choke to rear it's ugly head, that destroys the PA plate Xformer also, if the HV choke goes South for some reason.

Mr. Ian, in your project you used a China counter in your re-winding effort. Was that counter a linear ft. counter? As you know, when unwinding the Xformer the amount of linear ft. wire needed to make 1 complete revolution of the core, will become less as you unwind it. So if you go by the actual linear foot measurement/size of the original X former winding that would provide an additional benchmark, along with actual winding count. opinions. TNX to the Forum for all of the help.

 

In my most recent rewind, I used NMN333 "paper" with heavy polyimide (aka Kapton™) coated wire. The transformer came from a scrapped medical equipment PS, and uses a split bobbin (very low pri-sec leakage). You want good insulation/leakage in a HV xfmr.

Manfred XQ6FOD has info about magnetics here (link) and practical winding info here (link). The latter has a nifty Excel spreadsheet calculator that can help figure out if it'll work BEFORE attempting a re/wind.

When I was done, I had some magnetostrictive noise. A local transformer shop vacuum-pressure impregnated my transformer w/ a polyester resin. That greatly reduced the noise.

 

No it doesn't count feet. only turns. You can use Manfred's spreadsheet to work out the length of wire needed or just estimate based on the length of one complete turn at the outside and one complete turn at the center averaged and multiplied by the number of turns. It's close enough. You will almost certainly end up buying more than you need due to the way it's sold.

This is another reason to go with C cores and starting from scratch. Once they're clamped properly they are virtually silent. Mind you they make an amazing noise if you just clamp them together using woodworking clamps for testing!

 

Mine was a REWIND. and runs quiet.

 

Exactly the point I was making with starting from scratch. No soaking or no baking, you simply bolt the cores together. The only varnish requirement is at the ends of the windings to keep dirt, moisture etc. out

 

I kinda wondered why I haven't heard more about them in other applications. They do clearly mention the fact that they are fully capable and do serve many other industries. But as you say their main bread and butter is indeed premium audio output transformers.
I only mention them as an alternative to the other popular "go-to" rewind service, because I noticed Mercury Magnetics has offered on the bottom of their products menu, transformer restoration and rewind services as well as custom wound to your specifications services. They have advertised "one off" or will do custom OEM production runs.
Of course the literature always says please call for details....
Call for details usually says to me that it is going to be so expensive that they won't publish it...
At least this was the thought I did have considering that Mercury has gone to considerable effort in marketing their name and creating the name association with a premium high end product. However, speaking casually with a rep once after placing several modest orders, I informally inquired about what was involved to rewind a very desirable and highly sought after transformer from a rare expensive vintage amplifier. While they do have their own production replacement the cost to rewind the stock original and keep the amp 100% vintage period correct was not too far out of the park to be unreasonable. Not too far off from their own production unit.
While they cater to the niche boutique ($$$) guitar amplification crowd, they may be another alternative to the shrinking far and few others that will provide such services. How competitively? I have not the experience to know or say... But it didn't seem so far out as I may have thought it might be... Fortunately I ended up not having to further follow up or go down that road.... YMMV