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Repairing an old tympanometer (medical device to test ear drum function)

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I am new to the forum and it's been a couple of years that I am learning about electronics.
I have a >20 years old tympanometer which is a medical device for testing ear drum function. It has basically a probe that the doctor puts in the patient's ear and there is a pump that measure the flexibility of the ear drum and produces a chart similar to a normal distribution chart from statistics.
It's been a while now that the behavior is erratic. I turn it on and it works fine. The next day I turn it on and although it seems to work fine, when I try to work with it, it is idle. It does not detect that the probe is in the ear and does not start pumping.
If I fiddle with the probe's socket it seems to work for a while but it is a matter of time to start the problems again.
It has been sent for repair a couple of times but I honestly believe that the repair-man did not try too much since it is the same man who will sell me a new one (about 2-4000 euros).
Anyway I bypassed the socket and soldered the connections in a last effort to check if the loose connection is the problem. Same behaviour. One day it works the next it doesn't.
I inspected the pcb in order to chech for faulty capacitors but they all look fine, no leaks or bumps.
The only component that looks suspicious is a transistor which has a black stain around the area of the heat sink that it is attached to.
I am attaching a link with photos.
Does this transistor look faulty to you?
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