Bush DAC90A output transformer.

[Retired]

Hi, I hope the following will save someone a couple of hours work. I recently stripped a Bush DAC90A output transformer for a friend and the winding details are thus: Primary 3,000 turns of 0.004" enameled copper wire measured over the enamel. 120T per layer. Secondary 80 turns of 0.019" enameled copper wire measured over the enamel. I'm just writing my notes up and thought this worth passing on. Col.

[ronbryan]

Thanks for posting the turns data, it will certainly help the re-winding process.

Is there insulation placed between every winding layer, or just between the primary and secondary of the transformer, and is it tape or Kraft paper?

Ron

[Retired]

Hi Ron,
Thanks for asking. Yes insulation between every layer and it was paper. I was lucky and found a replacement transformer to give to my friend so thankfully didn't have to wind this one. Given the number of primary turns and very small bobbin size I would have possibly only inserted insulation between every four or five layers because of the problem of running out of "window" space in the laminations and I would have fitted cheeks or flanges to the bobbin. I use the yellow tape as I've never been able to find a supplier for the paper and the tape can be a pain to use due to static? Col.

[MichaelR]

Hi Col,

Do you make your own "bobbins" ?

Mike

[Retired]

Hi Mike, Yes I make my own bobbins if the original is unusable. I have excellent workshop facilities and it's no problem for me to accurately measure the laminations then pop a bit of timber through my universal woodworker machining it to exact dimensions. I then wrap the "stick" with a layer of Sellotape. This in turn is then wrapped with insulation material and I've used common brown parcel paper and cereal boxes with success securing with craft PVA glue. The Sellotape allows a bit of clearance for mounting onto the laminations and prevents the new bobbin adhering to the stick. I ensure joints are positioned where they would be on the outside of the laminations also this applies to any tails and joints in insulating paper or tape. I need to think about the winding space available even at this early stage. The laminations have two "windows" and the size of these are fixed so anything that increases winding thickness needs to be positioned on the other two sides which are on the outside; I hope this makes sense. once the glue is dry then the new bobbin can be trimmed to exact length by once again measuring the laminations. One of my very first coils was winding a field winding for a Murphy A30C and these hold a tremendous amount of wire; I was pile winding adding insulation as I proceeded but very near completion the whole lot collapsed. I now add flanges or cheeks and during winding these are further supported with plywood flanges pre drilled or with slots for tails. Thin Paxolin is ideal for the cheeks and needs to be measured carefully marking out for the new bobbin (former). I cut the Paxolin using a powered Hegner fretsaw but this can be done with a standard fret or coping saw fitted with a fine blade. I drill four small holes in each cheek on the corners these allow insertion of the saw blade and give clearance for mounting on the bobbin. All sharp edges are eased either with a file or abrasive paper. The hole in each cheek is then filed to about 45 degrees on all four sides this increases the hole size on one side of each cheek only. both cheeks are then pushed into the center of the bobbin with the tapered holes facing outwards; next I add a small amount of epoxy around each end of the bobbin allowing it to extend inwards about 1/8"; now when the cheeks are pulled to their correct position the epoxy is scraped clean from the inner faces of the bobbin filling the taper and the excess is removed with a sharp knife as it starts to set. It's very important to get the cheeks perfectly aligned or they will wobble causing trouble during winding. By tapering the cheek holes I find it creates a stronger glue bond and using this method leaves the bobbin nice and clean ready for wire. one point I need to mention is that once the new bobbin has been glued and cut to size I apply a couple of coats of shellac before fitting the cheeks. The Paxolin cheeks need to be cut to the exact size where the laminations fit and I take a great deal of trouble over this; the other two sides on each cheek can be quite a bit deeper as the winding will be thicker here due to the tails and joints and it is very easy to mix up which way the bobbin fits the laminations because the hole is usually rectangular in shape. I've taken a few pictures hoping these will help; sorry about the quality. I modified my AVO winder by adding a threaded rod for bobbin mounting and this allows me to secure the supporting plywood flanges using the nut. All this sounds long winded but in fact is easy once the first bobbin has been made. Mains transformers used to scare me to death but I can now design and build my own from scratch but it takes a lot of time. A simple enough question Mike but I've expanded on the answer for other members wishing to have a go. Col.

[MichaelR]

Fascinating..... you are a true engineer Col. I will follow your instructions and try it.

Mike

[Retired]

Thanks Mike, As you live nearby you're welcome to pop in to visit me and I can show you my latest transformer; its the size of a radio set and I wound it to supply my 3 phase 415V into the garage. If you are interested I live in Fenay Bridge and please e-mail me. Col.

[davidw]

Originally Posted by Retired

my latest transformer; its the size of a radio set and I wound it to supply my 3 phase 415V into the garage.

Would that be the one in the latest "Bulletin"?

[Retired]

Hi David, Yes well spotted; Installing 3 phase 415V into my garage using my specially wound transformer was a very interesting project and saved me a great deal of money. For non BVWS members I obtained a huge scrap 3 phase transformer and re-wound it using household conduit wire 2.5mm for the primaries and 1.5mm for the secondaries I spent under £120 and by matching each machine to the transformer using capacitors can obtain full power; the transformer idles at 0.8A and is virtually silent in use. A friend in Wales informs me he was quoted by his local electricity supplier to supply 3 phase 415V into his two farmhouses at over £17,000. He has since gone the rotary converter route but now has a lot of trouble with noise as the converter kicks in and out during the night. If there is enough interest in this then I don't mind adding a new post with more detail but add that the method is copyright protected and anyone wishing to proceed would need to spend around £17 to purchase the 3 DVD's from the inventor; Douglas Arndt in America. The DVD's cover installing 3 phase at any voltage and at any power using three different methods; stator; rotary and transformer together with information on making a welder; the DVD's have a running time of 6 hours and are worth every penny. I've already helped a couple of guys out here in the UK with advice via e-mail after they bought the disics and have myself successfully tried both the stator and transformer methods.
Please sit back and enjoy the sample video's of Doug Arnd in action; I can highly recommend this guy to deal with.
http://unique3phase.com/
May I please add that I don't wind transformers for money; being fully retired it is just part of my hobby and I don't like to let members down when asked to do so. However with the onset of winter and the demand for Bush DAC90 output transformers I am considering having a go at producing a batch of coils as a project; if successful then I could possibly supply a coil unit on its own or for a little extra cost fitted. I stress this is yet just an idea as I don't have the correct wire so would have to source a supply. Coil winding is interesting but time consuming and it is the labour which costs. Any feedback on this would be appreciated. Col.

[kalee20]

Hi Col,

Thanks for write-up and description of methods used!

I've made bobbins from cereal packet cardboard, same as you. And I can't over-emphasize the need to make sure the bobbin flanges are well supported during winding, otherwise the pressure on the winding wire makes them bow outwards more and more as winding progresses. One bowed out so much I couldn't slide the laminations fully home; another just broke off the flange. I've not made that mistake since!

I use a rigid mandrel, usually made from wood cut to size, to support the rectangular centre hole. Quite often, I've found that the wound bobbin won't slide off, again with the pressure of wire, but when this happens I find that cycling the temperature between 0 and 100 deg C a few times eases things.

Keeping this on topic, the DAC90a transformer is generally regarded as having a fragile primary, but I'd not quite expected only 0.004" over enamel! That translates to about 44SWG, or 0.08mm, assuming Grade 2 thickness enamel. Not a lot to carry 27mA!

As the original had an interleaved primary, and modern enamels are more robust than 1950's materials, I would honestly suggest leaving out the interleaving and using the space saved to beef up the wire slightly. As long as the winding is even , so there's no chance of some of the final turns slipping down into contact with the initial turns (meaning the full primary AC voltage would appear with only two thicknesses of enamel as insulation), I reckon modern enamel would survive the audio voltage, and the heavier wire would give a more robust transformer. Good varnish impregnation, either vacuum or by brushing on a coat of varnish every few layers as it's wound, would also help.

[Boom]

Originally Posted by kalee20

As the original had an interleaved primary,

Can you clarify what this means? Thanks

[kalee20]

Interleaved winding means that turns ar wound, starting one side of the bobbin, turns lying side-by-side neatly, till the other side of the bobbin is reached. Then a layer of insulation is applied (often 1 turn of full-width Kraft paper plus a little overlap) and the next layer wound on top.

It makes for a neat winding, and has the advantage that the wire insulation has to withstand only a very small voltage, from one turn to the next. The rather higher voltage, from one layer to the next, is catered for by the interleaving insulation. Down side is a much bulkier winding.

[Boom]

Thanks for that information. This sounds like a fascinating part of the hobby.

Dave

[Retired]

Hi Kalee, That's a good explanation and I think we have both visited the same place; it's called despair. It's not amusing for a bobbin to burst when almost fully wound and as you rightly point out you only do it once.
I agree totally about the thickness of the wire and this is possibly why the primary is so fragile. Your point about using thicker wire is very valid and I would always use heavier wire where possible but care is needed with wire selection because too thick and the laminations won't fit. Leaving out the insulation between layers would possibly allow winding with 0.005" wire. The extra "thou" doesn't sound much but would make a heck of a difference. Obviously the insulation between primary and secondary would still be needed.
Yes Dave; it is fascinating and once you successfully wind your first transformer the process gets easier; I've made most of the mistakes by now and one mistake I made really stands out as it's a whopper and I'll own up to it here to save anyone else falling into the same black hole. I wound my first radio mains transformer after doing a lot of reading about turns/ratio and how these relate to core size of the laminations used. I made a perfect job of it and was ever so pleased thinking that wasn't so bad after all. Fitted to the chassis though it was only giving half the expected volts to the rectifier pins and it took ages for me to realize why. I didn't fully understand how the center tap worked and had wound for something like 300V as this was the rectifier volts when in fact I should have wound for 600V; the center tap then split this to 2 x 300V. I've only made this mistake once and I can hear you all laughing.
Once the basics are learnt and a bit of experience gained winding transformers becomes very interesting not only for radio work but for anything requiring voltage to be stepped up or down. Risk of electrocution is real though if working on mains voltage and can't be over stressed. Col.

[AlanBeckett]

Col,
If you're not bothered about (internal) authenticity then there are specialised very thin plastic tapes available for inter-layer insulation. However, getting your hands on reasonable quantities might be a bit difficult.
Alan

[Ed_Dinning]

Hi Gents,
I have re-wound several of these for various people, but started from a different point by looking at a modern substitute that would be visually similar.

The number 18 lamination by 14.8mm stack is within a few thou of the original lams and will fit into the original clamp. It can be bought with a single section moulded former.

I use UNISIL laminations as they will run at a higher flux density (reduced turns) and have a smoother curve going into saturation. I used 43 SWG double insulated wire for the primary and several layers of insulation (Type 56 tape) between pri and sec. This needs to be good as it is the main insulation barrier.
Sec is 0.45mm/26SWG.

I use 1 layer of type 56 tape as an air gap between the E & I lams to reduce saturation due to the anode current.

One of the other forum members was kind enough to test some of these units and found practically no noticable difference between my units and the originals in both frequency response and listening tests. They have also been tried on other similar sets with good results.
Winding on the moulded bobbin makes life much easier.

I'm presently working on a mains autotransformer for use on both the -90 and -90a. This will not give an isolated chassis set, but will allow it to run cooler, and can be used when the dropper is beyond repair. It is under evaluation by a forum member and I will report in due course.

Col, I hope I'm not teaching you to suck eggs, but was interested in your work with the transformers and various windings. PM me if you would like a rundown on methods of finding turns/volt empirically.

Ed

[Retired]

Hi, Now this is really getting interesting as I've been working on my own and struggling having to learn everything from scratch; I wish I had known about this forum and what a great bunch of guys (and gals) make up the membership. Alan thanks for the tip about tapes; I'd like to find a supplier of both tapes and the craft paper as the last time I bought ten rolls of the yellow inter layer tape in Huddersfield I got into a lot of bother; I innocently dropped into the company and asked the young lady in reception if I could possibly purchase some tape and she put a call out for a guy who greeted me with some hostility saying he had been in an important meeting and blamed me for interrupting it. I seldom swear and am always polite so didn't deserve this and having bought the tape have never returned so if anyone can point me to a supplier I would appreciate it.
Ed you certainly are not teaching me to suck eggs and any information is always warmly received by me because compared to the average member of this forum I really know zilch about electronics although I'm a quick learner and like to experiment. My background is in mechanical engineering and working life for me started as an apprentice mechanical engineer and I was working underground carrying engineers tools at the age of 15; (1963 before health and safety was invented) the first pit closed (Grange Ash) and then I was transferred to The Caphouse Colliery which is now the National Mining Museum. Many thanks Ed for adding this amount of detail and I'll be contacting you. Col.

[julie_m]

Originally Posted by Ed_Dinning

I'm presently working on a mains autotransformer for use on both the -90 and -90a. This will not give an isolated chassis set, but will allow it to run cooler, and can be used when the dropper is beyond repair. It is under evaluation by a forum member and I will report in due course.

That's an interesting idea.

Did anybody ever sell "economiser" transformers as retrofits for radios originally fitted with dropper resistors? Such a device ought to have paid for itself eventually, surely.

By the way, anyone thinking about making their own transformers could look in worse places than here: http://www.ludens.cl/Electron/Magnet.html No personal affiliation with the site, just found it explained everything a lot more clearly than anywhere else.

[Leon Crampin]

I would have thought that a transformer is only worth fitting in order to give mains isolation. What's the benefit of a mains autotransformer over a capacitive heater dropper?

If you pursue this project, it might be worth fitting a 5.5V 0.5A secondary so that 2 standard 6.5V 0.3A dial lamps (underrun) can be fitted.

When I restored a white DAC90A (cabinet prone to heat cracking above the dropper) I fitted a metal clad 900 Ohm resistor on the chassis in place of the major section of the dropper. This was a very effective means of removing the localised hot spot without changing the set electrically. The aluminium chassis is such a good heatsink that the overall temperature rise was small - change to set - 2 small holes.

Leon.

[kalee20]

Ed's right in that the output transformer primary-secondary in the DAC90a is part of the insulation system between the mains-related chassis and the hightly touchable metal loudspeaker mesh. So it makes sense to ensure good insulation here - sleeved leadout wires as they exit from the winding - 3 layers of primary-secondary insulation such as Scotch No 56 tape - and maybe a secondary winding which leaves a couple of millimetres margin from the bobbin cheeks. It won't comply with modern safety standards, but will at least be as good as the original.

For good measure, a primary-secondary insulation test of 2kV should be satisfied.