help with a transmiter project

Hi all i wondered if any one may be able to help, I have built the transmitter here http://snake.contact.bg/ However instead of using the listed 2SC1971 for the final output I have chosen a 2SC1972 for 14W the two transistores have identical characteristics acording to both data sheets, the only diferance being the 2SC1971 has a collector curent of 2A and the 2SC1972 (one I used) has a collector curent of 3.5A.

Anyway the problem i am having is with the resistor R11 Blowing when the variable capacitor C17 is even slightly moved, It almost seems to me as tho to much voltage is getting into the resistor (im not sure) Anyway i spoke to the designer of the transmitter and here is what he recomended

Quote:

Hi

Hmm this sounds strange. I never tested the unit with 2sc1972 so i don't know how it would behave with this transistor, but here are things you should check:

1. C17 - make sure it's good and not shorted (these variable capacitors are weak and tend to short after some time).

2. T14 - 2sc1972 - make sure it's in the correct position and the legs are soldiered correctly. I attached you the datasheets of both 2sc1971 and 2sc1972.

3. If T14 is soldered correctly, then check if it's not dead, these transistors tend to die fast...in my early experiments with the unit i burned like 4 of them in a row ..XD. A dead transistor can possibly be shorted and thus burning R11.

4. I am not sure about the Base-collector resistance value of 2sc1972, can u try with 2sc1971? If it works then we'll know it's not the rest of the unit.

I was wondering if anyone on here in the know, maybe able to shed some more light on it for me.
I would have attached the datasheets for the 2SC1971 and 2SC1972 transistores but this forum dos'nt allow it

You can't simply put in stronger transistor and expect things to work.

Why didn't you put in 300W transistor than? Or 600W?
Do you think it would work?

Even with original transistor this thing is shit, at this power level
a transmitter without PLL should never be used.

Your chances of success are low.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128

	pcs wrote:
	You can't simply put in stronger transistor and expect things to work. Why didn't you put in 300W transistor than? Or 600W? Do you think it would work? Even with original transistor this thing is shit, at this power level a transmitter without PLL should never be used. Your chances of success are low.

I used the higher power transistor because it has the same charicteristics

It also requires more drive and has different input and output impedance.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128

	pcs wrote:
	It also requires more drive and has different input and output impedance.

Ok thanks for the help anyway, apreciate it

Hi pcs I wondered if you could elaberate on, "more drive and has different input and output impedance", a bit more for me please.
What in the data sheets, refer to drive power and input and output impedance.
Maybe you could give me a few pointers

THANKS

Unfortunately this is far from trivial.

In short, with RF transistors there's a lot more
to consider than DC gain and current/voltage rating.

Google S parameters, input/output impedance matching etc.

The worst part of this is that many manufacturers don't even
post the values in datasheets. Sometimes blind experiments
bring success, but often the result is burned silicon, too much
harmonics and instability.

Good luck.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128