Hello, I have the above radio which is working well, all caps replaced and resistors checked and replaced as necessary and is drawing slightly lower current than advertised.

The problem is I found some wax had melted from the transformer which I cleaned off and after running for a hour or so the transformer gets very hot and starts melting the wax again. About 63C with an ambient temperature of 24C.

The transformer has a single primary and a single winding secondary and the filament winding, no taps or centre taps.

The HT after the 6V4 is 252 VDC and should be 230, the filament voltage is 5.7 and should be 6.3 VAC, this is with a supply voltage of 230 VAC as per the radios rating.

With the valves and pilot lamp removed the transformer pulls 153 mA. The HT from the secondary is 242 VAC and the filament is only 6.3 VAC. I would think the filament unloaded would be closer to 7V

The bottom line is the secondary output is too high and the filament is too low plus the transformer gets too hot. Any thoughts as to what is happening in the transformer?

Thanks

Seems a bit like the primary winding has a few adjacent winding shorted turns.
This would explain the excessive virtual idle current.
And higher secondary voltage as the ratio between number of primary turns to secondary turns, has now increased.
But then again, NZ transformer, maybe designed more towards the lower part of 230V.
JJ

Johhny is probably right.

However to be sure, DISCONNECT all the secondary leads so they are not connected to anything and then see if the idle current and temp is excessive.
I have had things like valve sockets or wiring short and put heavy load on the secondaries.

If its still crook then I unbolt the tranny and hook it up free of the radio on the bench and see what sort of current it still pulls.
Lat possible thing to look for is any shorts between leads coming out of windings.

Good luck.
Fred.

Thanks Johnny and Fred, I also wondered if the primary may have shorted turns but can't understand how that would lead to a low Filament voltage.

I will lift the wires from the transformer and check the current draw, thanks Fred.

One of the things I do if the secondaries are isolated, is to earth leakage test all of the windings. Normally its only the primary to be tested.

I also take the opportunity to test between windings to ensure that there is no internal failure causing them to short together.

Transformers have a tendency to create hum, or literally "growl" if shorted.

1937 spec. quotes 55 degrees as maximum allowable for a mains transformer.

Marc

Hi Tippy, funny things transformers!

It is possible to get an interlayer short that effectively removes primary turns.
That can alter the turns ratio and so the nominal voltage off each secondary can be greater than what the original was.
Trouble is the shorted turns sit there gently frying from the "short circuit" current.
That level of current may not give smoke straight away, just warming heat.
If you leave it, the tranny soldiers on and sooner or later gets more shorts and more heating and then the MAGIC SMOKE finally comes out.

Getting the tranny by itself on the bench so everything external and into the windings can be excluded from fault is the way to go.
As Marcc says you use all the tools at your disposal, measure continuities, leakages, resistances, currents voltages looking for something non logic, before condenming.

I have had a "bad" tranny that had an insulation break loading up a heater secondary, that made the tranny hot and had low heater volts.

I has a tranny where an unused primary tap shorted in the sleeving to the HT centre tap.
Symptom was heating and silly volts, fix the sleeving on the leads (all twisted together and jammed through a chassis hole) and tranny was back to spec.

Fred.

After decades of fixing one does come up with many ways of making life easier. However, nothing is achieved without methodology and learning. Shotgun approach never really works.

Initially powering is a recipe for disaster. As a fixer I am expected to perform miracles, after someone has done that. You must learn to assess it understand the risks and deal with them before powering. There are two radios and a Philips 3003 PSU None of these were powered first. The 3003 needs work it has been tested, not powered. Exuberance can do a lot of damage.

One can acquire equipment, but unless you know how to apply it and understand what is is showing you, the value of it is lost. Learning how the thing works, invariably gives an insight as to why it wont.

One of the shortcuts precipitated by capacitors & other weird happenings, relevant here, was the construction of a reformer with stepped voltages, as part of the bench HV PSU. This not only to check & refresh electrolytic caps before use, but to catch out those that fail in service, but power in steps a "B" rail that is suspected of carrying a short (set source power disconnected). PSU rails are fused or have circuit breakers; However the reformers solid state Chip will lock up on overload.

Many ways to skin a cat.

Thanks everyone, I have the transformer disconnected and it checks out OK with the limited information and testing I can do. I am confident that it has a short somewhere.

I'm going in...

With everything disconnected except for the primary, then after being on for some hours it should only get barely warm.
Otherwise it's definitely faulty. JJ

Thanks to everyone for your input.

It turned out the voltages were most likely correct, there was no less than 4 versions of the same chassis with different voltages, the changes to the circuit were minimal but the voltages varied by at least 30 volts.

I rewound the transformer, the old wire was overheated and smelled burnt, the enamel coating wiped off with my fingers. After rewinding the voltages are correct for my chassis and the transformer runs cool as a cucumber.

Thanks again.

Hi Tippy, well done, the tranny just had shorting turns as most suspected.
That answers the question and rounds the repair up nicely.
Did you have any particular problems with the re-wind?

Fred.

Hi Fred,

The rewind was straight forward with no complications, this was my second transformer rewind and I'm getting more comfortable doing it. I downloaded an Excel spreadsheet to help with the calculations which helped.

Thanks Fred