70's electronic surgical cutter instrument repair

My system is a veterinary, she has a 70's electronic surgical cut, suitable for electrosurgery.

It's a LAEL with the front label electronic transitorized bistury STILUS.

 

She does not use this anymore, cause she had a new one. But last week she want to test it as a spare device. And she get a little shock.

The problem is that touching the ground (or the patient) you get a shock, and of course the patient to is been shocked.

I decide to open it and take a look at it.

First I notice the main transformer, so luckily the shock emitted by this device is not a power mains shock, but a secondary side one. Then I try investigate further.

I've notice a popped transistor. So I removed it.

I can not read the label on that transistor, cause it's popped and cause it seems marked twice or three times over. I just can read BC, and then something. But it does not help.

So, I measure voltage between the three join of the transistor. I suppose the opposite solder join are the emitter and collector, and the voltage between them it's 80V. So based on the device era a possible replacement is a BJT PNP one. I've a few 2N5401 here in my lab, they are PNP high-voltage transistor. Vceo it's -150V, that's enough for that. So cross finger and replace the original BJT with this one, placing emitter to Ground.

And it works!

I've tested from 0 to 10 gain, I can cut a piece of meat without problem, and without any shock.

Of course my sister will not use this device anymore, cause it's too dangerous to use. But at least it's a "on emergency" device to use.

Notes

• read risk disclaimer
• excuse my bad english