Rewinding a 100va transformer

[dr pepper]

My old 'scope blew up, smoke and flames.
Not wanting to connect my nicer 'scope to smps's running of the mains I've decided to fix the old dartronic, the transformer is toast, so I plan to rewind it.

Not a job for someone impatient, I've used a long knife blade and a teeny hammer to peel off all the E I laminations, took near 2 hours.

At first I thought the primary which I think is the one thats failed is on the very outer edge, but on closer inspection now the cores removed, it looks like 1/2 of it is on the outside and the other 1/2 is next to the core, and it looks also toasted, bummer, so I'll have to rewind the 'ole lot.
To make things worse I dont know the original o/p voltages, I'll have to go by turns.

 

[KeepItSimpleStupid]

Maybe you should invest in an isolation amplifer. I picked one of these (phillips PM8940) **broken link removed** off the bay.

I actually haven't tested it yet, but it was a good deal. It needed a relatively unusual screw and it does require some relabeling.

 

[rumpfy]

I'm with you doc; you're on the right tram
just barge ahead; do the homework, count the turns or weigh the copper and calculate; dont despair over the rewind; the old insulation was probably dodgy anyway. The split wind isn't that unusual. A 230/115 arrangement? The biggest problem will be (probably) the old wire was to SWG (or whatever) and the new wire will be metric. The problems we get to solve!!!
When the job is done, as it WIIL BE, you will have earned heaps of bragging rights.

 

[Colin]

Firstly count the turns then work out the output voltages.
Then look on the web for a transformer. Don't bother winding it yourself as it will hummmmm.

 

[tcmtech]

Any idea on the output voltages?

Good chance the old transformer/power supply can be replaced with newer stuff rather than trying to do a rewind of the old transformer.

 

[dr pepper]

Not eaxctly no, and I dont have data, but the tube heater will need ac, I tried to run a crt tube from hf ac off a switcher and it wouldnt focus, dc isnt great for them either, so I will need a ac source, ie transformer.

I have the core out so I'm going to pull it apart, I have a mic to measure the wires, I also have some spray dielectric varnish, I'll clean up the lams and spray them before reassembly, as for the hum this trans has only 2 of 6 possible bolts holding it together so I'll put all 6 in, I can stand a little buzz.

Winding primaries next to the core and on the outside is common practice with switchers as it reduces leakage, didnt expect to see it on a 50hz iron lump.

I lost one iron E and i section tearing it down, I dont think that's make much diffo, but once I have the turns counted I'll run it through the transformer equation, if its close to saturation I'll put a few extra turns on everything.

 

[tcmtech]

Okay that basically didn't answer anything.

Tube heaters are typically 6.3 volts and not all that fussy about a +- a few 10th's variances.

The rest of the unit more than likely rectifies the output from the transformer to create one or more DC voltages some place being I have serious doubts that the whole unit ran off of an assortment of straight AC voltages at line frequencies.

 

[dr pepper]

Course not, I was talking about the trans itself.
I guess theres a supply for vertical amplifiers, a supply for horizontal deflection and maybe a couple of ht's for the anode & focus grid etc.
I have no idea of the voltages.
I spose you could if you wanted to spend large amounts of time build a switcher to supply the dc supplies and have a wound trans for the heater, that however would probably take a whole lot longer.

 

[tcmtech]

What I am thinking was to just repurpose a tube amp or old TV power transformer to replace your burned up one. Odds are if the upper level voltages and basic number outputs were known finding a replacement would not be all that difficult.

Odds are a number of the members here would likely have a few good used ones someone would part with for next to nothing if they knew what you were working with.

I for one have a pile of old tube amp and TV power transformers laying out in my shop that I have collected over the years.

 

[dr pepper]

Yes probably not a bad idea, this tranny does have 6 o/p's though.
I might try stuffing 50hz at 6.3v up the heater winding and see if I get any other o/p's, if the duff windings are not shorted that might work, that said the 'scope did blow a fuse, I'll be able to disconnect the outer winding now the bobbin is out, but if the inner coil is duff too I'm not gonna be able to find out that way.

 

[tcmtech]

Is that 6 different secondary sets or 6 leads?

 

[dr pepper]

6 secondarys.

 

[gary350]

If your talking about a power transformer with 6 windings then you can determine voltage and current of the secondary windings by the tube numbers. One winding is the primary so the other 5 windings will be the secondaries. Often secondary windings are 5 volts for the rectifier tube, 6 and 12 volts for the other tubes and high voltage 250 to 600 for B+ voltage. Your transformer might have two 6 volt windings if there are no tubes with a voltage higher than 12 volts or nothing lower than 5 volts. What is the voltage of the scope cathode ray tube.

Look at the tubes, use your tube manual to determine the current of the tubes. You should be able to trace the wires to each tube to determine how many tubes are on each filament circuit, then add up the current for those tubes to determine amp rating for that circuit. The rectifier tube will have its own circuit, all the 7 pin tubes have their own circuit, all the 9 pin have their own circuit and all the 8 pin have their own circuit, the CRT will have its own circuit for filament too.

Make a circuit drawing starting at the power supply tube this will tell you a lot about what you have.

There are lots of people selling power transformers on ebay but very few have a clue what they are selling. Often all the buyer knows is the transformer has a certain number of wires and it measures 4" square and weights 6 lbs. About 2 years ago I needed a power transformer and ended up buying several unknown transformers on ebay to try and find what I needed. I had to test them all and mark the windings with correct voltage to resell them at a higher price to get my money back. I never did find what I needed. I resold all the transformers for 2 times what I paid because I knew the voltages, it was no longer an unknown transformer.

 

[chemelec]

Why don't you try to find a Schematic and or Manual for that Scope.

Than it will give you a Much Better understanding of the Power Transformer Requirements.
Possibly even the Actual Voltages from the Transformer.

 

[dr pepper]

I have yet to find any technical data for dartronic, I htink they were made in small qty's for the uk market.
Sorry I shoudl have said sooner, this 'scope has a tube for the display, but the rest of the 'scope is transistorized.

I'm pulling aprt the bobbin now, to be precise there are:

2 centre tapped secondaries for + and - rails.

3 single (pair of leads) ac o/p's, one of which is dedicated to the crt heater.

As far as I can tell vert and horizontal amplifiers/oscillators use the 2 +/- rails and the ac's feed the tube, so theres probably heater, anode voltage and focus voltage.

The last couple of times I used the 'scope when it still worked, the focus control had hardly any effect, so if the primary is not the issue then its looking like one of the ht's, makes sense, the highest voltage windings are going to fail first.

Not unexpectedly the bobbin is a right mess, a congealed matted mess, I think I'll cut a good section of the wires out and count the turns end on, looks like a couple of thousand on the ht's, if I get really stuck I'll look at the smoothing cap voltage ratings.

 

[itsthatidiotagain]

I have yet to find any technical data for dartronic, I htink they were made in small qty's for the uk market.

Don't suppose you have any pictures of the scope? It may be a re-badged something-else-oscope.

 

[chemelec]

There is a "Dartronic" Company in the USA.
1-800-298-8936
But no idea if it could be the same company as your scope.
And that 1-800 number is Not Accessable from Canada, so I can't call them.

 

[alec_t]

I have no idea of the voltages.

You may be able to get somewhere in the ball-park by checking the working voltage ratings of any caps (especially electros) in identifiable circuit areas associated with the power rails.

 

[chemelec]

Also, The Scopes Cathode Ray Tube should have a Part Number on it.
And if you look up its Spec's, it should tell you the Typical DC Supply Voltages, as well as Filament voltage and Current.

 

[dr pepper]

Thems usefull pieces of advise thems is.
I'll do that anyway, if everything ties up I have a good idea its right.

I can take a photo when I get home, yes it could be a re-badge.