Main transformer sudden death forensic, looking for cause.

[anotherlin]

Hi everyone,

I just have busted main transformer :

https://www.eevblog.com/forum/product-reviews-photos-and-discussion/hameg-hm8130-function-generator-teardown-and-repair/

I just unsoldered today and double checked it.
It has two primary wirings for 110v (parallel them) and 220v (put them in series). One primary has 43ohms and the other one 10kOhm, this one is broken. If plugged, the main fuse keeps blowing.
The secondaries seems to have "normal readings" of a few ohms.

Hopefully, Hameg stocks the spare, so I can replace it.

Yet, I'd like to find out what caused it to break. So as not to buy a new transformer and break it to.

I was basically using the generator and it just shuts down (fuse blown), some sort of sudden death.
There is a line filter before the transformer, I've checked it, it seems ok.
All secondaries are connected to the usual rectifier and filter caps. They seem ok too.
Then they all go to voltage regulators, standard 7805, 7815, 7915, and L200.

I'm a little bit puzzled as the 78xx series are supposed to have current limiting. So even in case of a short on the main board, the transformer should be protected.

Any idea ?

Best regards,

[SeanB]

Shorted diodes, shorted capacitors ( including the ones across the diodes) or open diodes can all either cause the transformer to be overloaded or to have a DC current flow through it ( causes overheating as the core material saturates) and eventually it is hot enough that the varnish breaks down and then crisp time. Could also be a faulty unit that had a shorted turn somewhere that took a few years to break down and become a problem.

If you are up to it RS probably still has a DIY transformer kit. You get the core with a prewound and tested primary, and you buy the required wire and wind the secondaries you need, then assemble the core and bolt it up and use it.

[ciccio]

Is the transformer used at 230 V?
If so, the measured resistance values are obviously indicating one (or both) faulty primary, but are not directly leading to a blown fuse.  In fact, one open primary winding will be in series with the other, resulting in non current drawn.
Did you check if there is some form of continuity between primary windings?
Regards