Pres. jackson 2sc2166 self distruction

[mr.alex]

Hallooo..

I've a new problem...

A friend of mine have gave me a Pr.Jcks to fix (1st version 2sc2166 + mrf477 or 2sc1969).

I have find the TR503 - 2sc2166- broken, so i have tried to replace this transistor:

0: disconnect PCB - 834
1: replace 2sc2166, check the 2sc1969 seems ok .
2: check the bias current for 2sc2166 about 10ma (+/- 5ma).
3: check the bias current for 2sc1969 about 50ma. (+/- 5ma).

Go to switch ON, and "Puff" after 60 seconds the power goes to 6watt down to 1watt..
And check again 2sc2166 and it's broken again for the 2nd time.

What i'm wrong !?

Regards
AleX

[zodiac]

The 1969 has a different pin out to the 477 and you must cross the emitter and collector over to get it to work.
If you use a 2SC1945 as a replacement for the 477 they will fit without having to change the pins over but you will only get about 10w from it.
The bias for the 2166 should be 50ma and the same for the 477.
Have you checked the thermal diode on the 2166.

[mr.alex]

Yes..My friend, i have already checked the pinout connection for the 2sc1969 and it's correct,
also for the thermal diode seems ok.

Today, in my mind is born a new question :

Why have you don't checked the capacitors between driver and final stage !?

And, what about the bias current for the 2sc1969 and 2sc2166 ?!
Reading the service manual, 2sc2166 need a bias of 50ma, 2sc1969 or mrf477 80ma, is this correct ?!


What do you think about it !?

Regards
Alex (italy)

ps: sorry for my not strong english.

[zodiac]

Alex,
Your not using a 477, you have a 1969 in there, If you adjust the bias to 80mA it will probably overdrive the final (1969) you are using, there should be no more than 0.7v at the base of the final and driver, with a dead key on side band and the mic gain set at zero.
If you think the capacitor is at fault then change it for a new one to eliminate it. To check the thermal diodes you need to de-solder one end from the board and test it with your meter set to diode test.
Dave.

[mr.alex]

Ok..Thank you for your support.

I will let you know as soon as possible,
depend by my free time after work .

Tell me just one think:

It's correct to use bias 50ma (or less as 30ma) for the final transistor as 2sc1969,
and 15ma for the driver as 2sc2166 !?..what do you think.

Regards
AleX

[zodiac]

I would set them both at 50mA, then check to make sure there is no more than 0.7v at the base pin of both, as I said above.
Dave.

[mr.alex]

Before to start, just one consideration..

If a put a "Digital tester" to check Base and Emitter for 0.7 volts,
i suppose that this measure will be alterd by the internal resistance of my D.T .

May you give me, please, some good advice to read this "load" between Base and Emitter of 2sc2166 !?

Thanks .
AleX

[zodiac]

Alex,
You only check the voltage at the base of the driver and the final one then the other, with the black probe of your meter to the PSU's negative connection.
With a dead key in side band mode there should be no more than 0.7v.

[mr.alex]

Driver...Dead again for the 2nd time.

Some Details:

After change driver 2sc2166 ..

driver bias 10ma 0.7volt Base/Emitter
final bias 35ma 0.7volt Base/Emitter

Next advice, please ?!

Regards
AleX

[zodiac]

It sounds like there is a short from the collector to the chassis of the 2166.
Make sure there is no connection between the two, do a continuity test from the tab of the driver to the chassis just to eliminate that possibility.

[mr.alex]

It sounds like there is a short from the collector to the chassis of the 2166.
Make sure there is no connection between the two, do a continuity test from the tab of the driver to the chassis just to eliminate that possibility.

Already Checked ...it's isolated

It sound strange...cause for the first 30 seconds it works ok,
and the final 2sc1969 it's still ok, more or less i will change 2sc1969 with 2sc1945,
what do you think !?

Next check!?

..please, help..

AleX

[zodiac]

Alex,
It's no good banging your head on the wall, you should be hitting the radio with a bigger hammer LOL.
You say it only lasts about 30 seconds before it blows again but in what mode are you testing it in.
I would have thought with only 10mA at the driver it shouldn't have done any damage at all.
Dave.

[mr.alex]

During the bias current setting on 50w dummy load ..i've used USB with DEAD KEY.

After that, i re-insert again the pcb, change to AM and push PTT without modulation..
..And you know the rest of story.

No one have some idea about this ?!
What can i do !?

AleX

ps: Thanks again for your patience !!

[mr.alex]

Last news

I have restored this RTX with MRF477 instead of 2sc1945 (pin to pin similar)..just to try.

The bias has been adjusted correctly..50ma on 2sc2166, 80ma on MRF477,
Now the max power out is 3watt and there is no selfdistruction for Tr.driver and Tr.final,
so i have changed the couple capacitor between Pre.Driver -> Tr.Driver -> Tr.Final .

I'm demoralized about this situation, i have not seen
nothing like this problem on a RTX before.

I don't want to bring this RTX into bad assitance center.

Any ideas !?

73+
......

[hackindave]

I know this sounds super simple, but are you sure that the mounting hardware is not shorting to ground. 50ma is what your bias should be.

[hackindave]

Yes..My friend, i have already checked the pinout connection for the 2sc1969 and it's correct,
also for the thermal diode seems ok.

Today, in my mind is born a new question :

Why have you don't checked the capacitors between driver and final stage !?

And, what about the bias current for the 2sc1969 and 2sc2166 ?!
Reading the service manual, 2sc2166 need a bias of 50ma, 2sc1969 or mrf477 80ma, is this correct ?!


What do you think about it !?

Regards
Alex (italy)


Seems to me the 1969 is only 50 ma on bias. Try setting it there and see what happens. 80ms might be driving it too hard

ps: sorry for my not strong english.