strange transformer failure mode

[capslock]

Just ebayed a defective JVC tuner from the mid 80s. It had a small EI dual chamber transformer which had no filter, fuse, switch whatever on the primary side. There was one secondary, with the rectifier and lytic also always powered. Turned out the primary had gone open circuit, not only at the pins but also at the wires coming out of the primary coil. Also, there were no signs of burns.

The label on the back side of the case read 220 V / 11 W. Wonder if a surge or just permanent exposure to 235 V killed the thing. Also, 11 W seems a bit much for such a small transformer.

Oh, the voltage reg is a speciality, too. AN7812-whatever, has a TO-220 case with four (!) pins.

[peranders]

Originally posted by capslock
Turned out the primary had gone open circuit, not only at the pins but also at the wires coming out of the primary coil. Also, there were no signs of burns.

I have seen (and replaced) many door bell transformers (located out int the country side) with lightning damages. One typical damage was blown away primary wires!

If you are lucky the tuner is alive and you only have to fix some power.

[joensd]

A lot of transformers in consumer electronics have temperature fuses in series with the primary. Sometimes they are visible and you can even get them out and exchange. (just look carefully around the tranny)
But sometimes you can´t...

Regards
Jens

[capslock]

Well, I have unsoldered the tranny and I could almost look at the primary winding except that it is covered by yellow tape. Would the fuse hide underneath that??

[janneman]

Originally posted by capslock
[snip]Oh, the voltage reg is a speciality, too. AN7812-whatever, has a TO-220 case with four (!) pins.

I know those, have a few myself from old times. The 4th pin (the others are in, out, gnd) is the input to the error amp. That way, you can set the output voltage with a resistive divider between output and gnd with the tap going to this pin.

Jan Didden

[TheFettler]

Would the fuse hide underneath that??

sometimes, yes.

I have seen thermal fuses 'embedded' in the primary winding to maintain tight thermal contact with the primary.

Thats not to say that there *is* one there, but there may be.

ray

[TheFettler]

... another thought - look closely at the primary wiring where it connects to the tranny pins. If one wire looks thicker than the other(s) , it may indicate the presence of a thermal fuse under the tape, since the 'odd' wire probably won't be copper coloured ( it'll be tinned ) , or enamelled, as it's the direct wire from the thermal fuse.

The thermal fuses sometimes are quite small, not much larger than a grain of rice , or sometimes they look like rectangular caps.

ray

[capslock]

Yes, there was a tinned wire. I will check tonight.

[capslock]

Yupp, that must have been the fused. It seems to sit right on the core, underneath the primary winding, so there is no chance of replacing it. I soldered in a 50 mA slow blow fuse, wonder if that is sufficient...

The pin assignment was pretty unusual, but it turned out there is a connector that gives me another 10% extra turns, so that must be the 240 V setting -> Tuner works fine!!!

[TheFettler]

good.

Now you know that the thing works it would be a good safety point to replace the transformer, because that one has exceeded its safe thermal operating area ( hence the thermal fuse failure ).
There's always a danger that the primary could have been damaged, and running without thermal protection could risk a fire..
Thermal fuses in transformers are usually rated at 130 degrees C , at least the ones I've seen have been.

On the other hand it could work fine for years , but I thought I should point out the risks.

it most probably failed due to the overvoltage and continous power supply connection.

ray