Help identify part

[naujoks]

I have a Panasonic KX-T3910 telephone from around 1988 that has a blown transistor.
Unfortunately the part description was partially destroyed by burning out. It starts with D199.
The schematics I found are quite blurry and also don't list the name. The transistor sits in the power regulation section. It's Q8 in the schematic
The phone gets 12V at DC in.
Can anyone tell me with this information, which kind of replacement I could get for this part?

[wraper]

Should be 2SD199x, like 2SD1994, 2SD1993...

[naujoks]

Is there a modern equivalent for it?

[wraper]

Something like BC547 should work, although it has different pin order like the vast majority of BJT these days. Also I don't think it's Q8 in schematic which is PNP. 2SD... (which it certainly is) are NPN.

[naujoks]

Yes, I think that might be right. The phone I have is actually KX t3910h.
If I try out a BC547 but put it in in the wrong orientation, will it blow up the phone or the transistor?

[wraper]

Depends on how it's used. Original should have ECB pin order. Also the initial issue might be somewhere else, it might have emitted magic smoke due to overload, not by itself.

[wraper]

I've looked into some datasheets and it might be more powerful than BC547 depending on the last part number digit.  You could try cleaning it with alcohol to see if there is something left of top part of the last digit.

[naujoks]

Unfortunately the last two digits were where the blow out occured.
I jumped in and soldered in a BC547, with reversed BCE, as suggested. And it's charging the handset now. I'll try out the rest once it's been charging for a bit.
I powered it with a bench power supply and it's drawing 0.231A. That should be within specs for this transistor?

[wraper]

Unfortunately the last two digits were where the blow out occured.

IMHO looks like top part of the last digit might be visible if magic smoke is cleaned off.

[wraper]

I jumped in and soldered in a BC547, with reversed BCE, as suggested.

I did not suggest to reverse it as it has CBE pinout as pin order is different, not simply reversed. With such substitutes I put it with 90^o angle and bend base terminal backward, emitter forward, collector to the center, so you get ECB in a straight line.

[naujoks]

Oh dear. How come it's working anyway?
I tried the phone without using the handset, just using the internal number pad, speaker and microphone, and that's all working.
The charging light for the handset is lit, but the handset doesn't work.
I examined the battery, knowing that the owner had changed it not too long ago.
It's an aftermarket battery, the original is supposed to be a KX-A36A. I measured the voltage on it, and had reversed polarity!
Turns out that the connector has two separate plugs and it's possible to plug in the battery with reversed polarity. There's no indication on the headset where +/- is supposed to go.
So the question is, was the battery wrongly plugged in or is the wrongly put in transistor causing the reversed charging polarity?
The battery is now slowly discharging on its own.

[wraper]

Oh dear. How come it's working anyway?

Most likely works like a diode between what's supposed to be C and E.

[naujoks]

So the battery was indeed plugged in the wrong way.
I plugged it in the right way and let it charge for a bit and just tested the handset. It's working fine.
Is there any reason why I should connect the transitor in the right way, given that it's actually all working right now? Or will that part fail again in the near future?

[wraper]

So the battery was indeed plugged in the wrong way.
I plugged it in the right way and let it charge for a bit and just tested the handset. It's working fine.
Is there any reason why I should connect the transitor in the right way, given that it's actually all working right now? Or will that part fail again in the near future?

You should re-solder it properly. It may look working properly on a first glance but it's not. For example there could be larger than normal  voltage powering something, or say power won't be removed when it should. And if it's used for battery charging, it may destroy the battery.

[naujoks]

Ok. I put it in properly.
It's still working, and the PSU is now drawing only 0.1A, which is a good sign too, I guess.
Thank you for your help!