Replacing a transformer

[Schanz]

There's a related thread to this in the tech forum but I figured there are more radio guys here.
My type 80 rectifier will no longer light on my current project. I cannot detect any heater current (5v) at the socket. This transformer is not original to the radio and had leaked a bunch of wax prior to when I got it. Below is part of the diagram for the radio and a diagram for a new transformer on eBay. Would the new one be a suitable replacement? Current voltage now to the rectifier is 380.

[IMG][ /IMG]

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[sean]

That transformer would work. Your 6V filament tubes draw 2.2A and the 80 draws 2A from the 5V winding. Your plate current adds up to 63mA so this transformer is way capable of handling that.

Have you determined that the 5V winding on your transformer is no good?

[Schanz]

I've tested it several times and always get .001 to 0v on two legs. The other two legs read 378v. I haven't changed any of the transformer wiring and this radio was working nicely after I replaced all the caps. BTW, I do find 6.5v on the other winding.

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[bob91343]

Measure (with power and the 80 removed) the resistance of the 5V winding. It should be very low, less than 1 Ohm. If not, trace the connections and look for a broken or poorly soldered connection.

[Schanz]

wire to wire reads O.L. which I believe is infinite. The high side reads .390K. I checked at the solder joints as well. Wire doesn't feel broken. I could expose some bare wire near the transformer to make sure.

[Giulio Maiocco]

Hello,

Please bear in mind that old transformers usually did not use stranded wire as lead exit on their LV windings, but a lenght of tubing over the enamelled copper wire (at least, this is how old transformers in Italy were done) was the usual arrangement. What usually is the problem is the enamel is not scraped very well to take solder and to make a decent joint. Please, before condemmning the old PT, try to scrape the ends of the 80's filament winding to bare and shining copper, then retest.

I would be really surprised if this winding gave up the ghosts, as it employs few turns of a relatively heavy gauge magnet wire

Cheers

Giulio Maiocco

[Schanz]

I cut the wires loose, stripped back some insulation and retested ohms. Still nothing between the wires (0.L.). Measuring voltage to ground, I got 5V on one and 2.9 on the other.

[Giulio Maiocco]

Hello again,

Everything is possible, but what you have does not make sense to me. The magnet wire used to wind the 5V is thick, about a millimeter in diameter, so it's almost impossible to open it up. Is the wire exiting from the transformer a stranded wire? If so, there is a splice under the transformer's paper insulation that may be bad. If this is the scenario, cut back carefully the external layer of insulating paper on the power transformer and, after testing for continuity where the magnet wire joins the stranded wire, resolder the joints and check again.

The voltage test between the winding and ground does mean nothing, as this 5V winding must float with respect to ground to the full B+ voltage. So you should get 5V between the 2 wires composing the winding, voltages to ground are almost surely stray voltages.

Cheers

Giulio Maiocco

[maxhifi]

The scabby way to fix this would be to leave everything as-is, and solder a pair of 1N4007's and a dropping resistor in place of the 80. The silicon diodes won't need filamant voltage so the open winding becomes not needed. You can leave the 80 in place just for looks. You can figure out the resistor value by trial and error, start with say 200 ohms and work your way down.. a few wirewound 10W resistors would be a nice starting point. An 80 is more or less instant start anyway since it's directly heated. This is the $5 15 minute repair I'd go for, rather than all the nonsense involved in changing the whole power transformer.

[Schanz]

Thanks for your time Giulio. I'll try to be clear. I cut the wires where they were soldered at the socket. They are solid strand with a copper coating. Does it mean anything that a bunch of wax-like goo ran out of the transformer some time in the past?

Maxhifi, your solution is tempting. I may need some more details.

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[maxhifi]

http://archive.siliconchip.com.au/cm...1/article.html

scroll down to "rectifier replacements"

You will soon get the idea after reading.

[Schanz]

So to bypass the 80, I would need to know my target volts and amp draw, correct? Looks like 240 v at the other tubes, is that what I'm shooting for?

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[Giulio Maiocco]

You need at least 2 seriesed 1N4007 for each leg of the HT winding. Current handling of the 1N4007 is more than ample, but the problem is the reverse voltage you'll encounter in this application, so my suggestion of using 2 1N4007 in series.

Anyway, solid state diodes in vintage radios Might be ok as a test, but I would never ever consider this as a definitive solution, unless the rectifier is unobtanium, and even then I would think of an alternative tube to swap.

This is a very interesting fault with this 5V winding, too bad you are not in Italy, otherwise I would fix this for you in no time flat. The melted wax indicates the transformer ran warm in it's life, probably not helped by bad paper and 'lytic caps leaking like sieves!

Cheers

Giulio

[Schanz]

I appreciate everyone's help on this but I went ahead and ordered the transformer. Getting more $ in this radio than what is prudent but this was my first antique radio purchase and I really want it working. It's been sitting on the shelf far too long.

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[Schanz]

Installed last night. Works like a charm. May need to paint it black though.

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