Hello,
Have you looked for a fix for the dead CISCO 7940 or 7960 IP phone?
This is for the fixing of the more rare "dead, no power" symptom on these models. Not to be mistaken with the boot loops or other firmware flash issues.
These models get in recycle quite often in big organization, it seems the POE hardware glitches and kills them. Used to be expensive items so it's definitely worth fixing them. This is not about the hook contact or broken back stand issues. Those are so easy to fix.
This is about the completely dead symptom, with no LED blinking after plug into power.
There is no schematic available but the OnSemi app note for the main CS5124 PWM controller gives a hint on how Cisco was using them in their design.
The design has a flaw, the POE power goes thru that little SMD Mosfet MD transistor to the CS5124. If that transistor fails ( short) it will put the whole 48V on this chip. The max voltage this chip works is around 20V
This is why sometimes the failure of the MF (the SMD code on it) transistor will kill the PWM controller also. Most of the times no, but there is a chance.
Replacement for the MF transistor ( this is not the bipolar 2SA1235, that you can find googling for that MD smd code) is most plausible the 2N7002 N channel Mosfet .
This can be harvested from numerous PC mother boards, Apple motherboards also. All of them have some similar version, marking code 702 or K70 or K72 - an equivalent. You just need to search.
Also you can buy them, at around 0.5 $ each.
The PWM controller is a bit more expensive and not much available. The 7910 models are a good donor board for them, in case you have access to them for parts.
That's it, let's save the planet one item at a time.

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