The main purpose of this transistor is a classic linear voltage regulator for a PSU fan depending on thermal sensor. Thus it dissipates around 0,9 Watts on average fan speed, which is too much for TO-92 package. It is better to replace it with something more powerful like BD139 in TO-126 case.
I had the same problem, so i drilled a hole in the secondary circuit heatsink and installed BD139-10 here. It has a plastic case, so isolating is not needed.

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The appreciation what you get is a deserving return or rather 'fringe benefits' in the profession.

The transformer rewinding depends on whether it is Ferrite Core or Iron Core. Iron core can be dismantled easily, but not Ferrite. It might break, as it is fixed with glue. We may have to go for a similar former. Coil can be wound by hand. But you should take length and gauge of wire removed for exact replacement. (Not number of windings)

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Thank you dear colleague for your too kind comment! In the past sometimes Jestine's newsletters turned up in my 'unwanted mail/spam box'for some reason. But after I placed them back in the 'mail inbox' they again always are received without any problem. The transformer coil repair tips you gave in the comments after your recent article sure will come in handy in case it is the transformer that is defect. But I'm glad it was just a gone bad transistor. I wouldn't know how to easily rewind a transformer coil anyway, and dismantling any transformer core isn't easy I guess?

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Addendum: I repeated above breakdown voltage tests on my new S8050 Bipolar NPN transistor with both my perfect working DY294 Testers to make sure they both were working fine and are giving the same results which they did!
So it must have been a bad transistor pincontact that previously lead to not reaching the correct breakdown voltage on the 200V and the 1000 (over 1600V!) testscales.
Now I got even better results than those given in the datasheets (depending on the manufacturer the voltages are often not the same) and I got normal acceptable values!.

So as in the manual is written just use only the E and C connections on your DY294 Meter/Tester to test the BREAKDOWN voltage of any Diode/Triode, and connect in case of a NPN triode like my 8050:
1. For the Vcbo (collector-base) breakdown voltage test, simply connect the Collector of the transistor to the C onnection on the DY294, and the Base of the transistor to the E connection of the DY294.
2. For the Vebo (emitter-base) breakdown voltage test, connect the Base to the E connection of the DY294, and the Emitter to the C connection of the DY294.
3. For the Vceo (emitter-collector) breakdown voltage test, put the Emitter on the E connection and the Collector to the C connection of your DY294.

And press the test button on the 200V or 1000V scale selections to read the breakdown voltage on the display. The breakdown voltage normally kills any semiconductor but in our DY294 its is a protected voltage test which doesn't destroy your transistor/diode (or capacitor)! But never touch any wires of contacts while pressing the test button!
In case of a PNP transistor you need to reverse the Base and Emittor transistor connections because the current (the arrow in the emitter symbol) flows in the other direction.

There is also a 4th Breakdown Vces test but that is only a Vceo test with the base connected the the emitter.

My DY294 testers gave a Vcbo of 81V (30,40 or 45V depending on what manufacturer), a Vebo of 11V (5, or 6V depending on the manufacturer), and a Veco voltage of 26V (25V in most datasheets).
So everything works as is to be expected! Problem solved.

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This repair indeed took some valuable time to solve the nasty Fan problem Christopher. Luckily it didn't make me sleep any worse. Now that it is finally fixed does help of course! Our gut feeling can help many times, but in this case I was glad that I still kept that months before taken old snapshot with the measured Hfe 18 on it which was the key in solving the problem. Cheers!

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Indeed, but why would you look for an equivalent transistor if you have the exact type in store with the same pin connections. 100 new S8050's only cost about 1 Euro/dollar free shipping!

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I'm afraid that it was not as easy as you think it was Robert. Because the transistor worked at first as a transistor, but just badly with a too poor Hfe, and it probably only died months later when I heated the board to check if it would start the Fan. (in my search for the temperature sensor that wasn't there).

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That is easy to explain but only if you know how a transistor works Imoudu.
It is the multiplication factor of the current through the collector compared to the much lower current that goes through the base (Ib) of your transistor to make the higher Ic (collector current) possible. And the emitter current is the [ Ib times the Hfe factor plus 1 x Ib ] current. So a Ie current of Ib x (Hfe+1). Your shorted transistor doesn't have a Hfe or neither a good Ib, Ic or Ib current anymore because it only conducts a shorting current. You have to replace the internal shorting 8050 by a good intact 8050 that still works by giving a decent Ic and Ie current after it receives a small Ib current on its base. Transistors normally are current amplifiers. Mosfets are controlled by changing voltages on their gates (base of a Mosfet) and so are voltage switched amplifiers. Processors contain millions of tiny mosfets and the faster they all change from on state to going off the hotter your processor gets! And they only consume energy when their situation of being an open connection or a conducting mosfet changes.

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Albert, test that transistor that your other meters say is not damaged by your DY294 with a VOM(with X10K), and see if it passes.
If a transistor, FET, diode, etc.. passes with my other meters but fails with my VOM, I will believe my VOM every time!

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