Voltcraft MF-90 - material moisture meter showing 0%

[shianne]

Hello,

colleagues have brought over a material moisture meter type Voltcraft MF-90. Error description: It is always displayed 0%.

The oscilloscope indicates that there is no signal at the sensor tip. After opening, this can also be confirmed at transistor Q5. Q5 seems to belong to the oscillator.
Specific tapping with the wet fingertip showed that the main IC seems to measure in principle.

My first guess was a defective Q5 (maybe ESD). It says BF199 on it, so I assume it is a BF199 (Q5-BF199.jpg). Measurements with the diode tester are confusing - there must be something defective.
Still, it looks more like PNP than NPN... (Voltcraft-MF90-BF199-diodetest.jpg)

Does anyone have an explanation?


A test with a SF245 (GDR replacement type ) showed no oscillations too. So I draw out the circuit diagram after all. (Voltcraft-MF90-Osc3.jpg)
Edit: Pinout of transformer T1 (T1-Pinout.jpg).

Hm, that looks like a Hartley oscillator. And NPN type for Q5 is OK.
Is the transformer defective?

Does anyone have any other approaches or hints before I desolder the transformer and disassemble it if necessary?

For the sake of completeness  two images from the PCB (PCB-LS.jpg and PCB-BS.jpg).


Best wishes,

shianne

(Yes, I can buy such a meter new, but anyone can do that  )

[shianne]

And here is the follow-up:

While measuring the transformer I could not find something unnormal.

So I tried another SF245 - it does not work.
Tried a SS9014 - voila, oscillations.

The measured values seem to be plausible, but I need to check them against another meter. Next year ;-)

I am still not sure about the reason for the failure