100v line transformer

[stuart_morgan_64]

What should I read on the primary, when I plug the transformer in the output transistors heat up quickly. Many thanks.

[jonnybear]

Would think it very unlikely to be a transformer fault, more likely to be a s/c transistor in the output stage.
John

[m0cemdave]

Originally Posted by stuart_morgan_64

What should I read on the primary.

A failure in a 100V line transformer is rare, but would be either an open circuit winding or a shorted turn.

The primary should read very low dc resistance. An inductance measurement would be needed to test for a short.

You could also test the transformer on its own with low voltage AC, say from a 6.3V heater supply, but be aware that it won't be very happy at 50Hz.

It's possible that something in the wiring is causing the amplifier to oscillate when the transformer is connected, so check for that too.

[Restoration73]

At switch on, after a second or so you should hear the protective relay click on. It
is there to protect the transformer in the event of transistor failure or a major transient.
If the relay does not click you can assume circuit failure.

[radiotechnician]

Without seeing more of the schematic, what you shouldn't see
is any DC on the transformer primary.

[Refugee]

I would disconnect all the output wiring from the transformer starting with that switch between the 70V and 100V taps and try it again with a speaker on the 8 ohm tap.

[stuart_morgan_64]

Ok thanks for the comments, it is reading about 8 ohms on the primary, the relay sometimes switches on, and sometimes doesn't, I cant see any oscillations on the output. I will try, and look at the suggestions above. I can post the full circuit, i will do that shortly.

[Herald1360]

8ohms resistive sounds rather inefficient for a 100V line transformer driven by a low impedance output transistor amp! Or is that the impedance seen when driving ac into the primary with the secondary correctly loaded?

DC wise, less than an ohm would be more like it.

[stuart_morgan_64]

8 Ohms is what i see with a DC resistance check, normal multimeter test.