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help with a transmiter project
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nay27uk
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Post help with a transmiter project Reply with quote
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
Mon Jun 08, 2009 11:15 am View user's profile Send private message Visit poster's website
pcs
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Post Reply with quote
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.

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Marko - PCS Electronics
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Turn your PC into a FM radio station!
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Mon Jun 08, 2009 12:31 pm View user's profile Send private message Visit poster's website
nay27uk
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Post Reply with quote
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
Mon Jun 08, 2009 6:00 pm View user's profile Send private message Visit poster's website
pcs
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Post Reply with quote
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
Mon Jun 08, 2009 6:17 pm View user's profile Send private message Visit poster's website
nay27uk
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Post Reply with quote
pcs wrote:
It also requires more drive and has different input and output impedance.


Ok thanks for the help anyway, apreciate it
Mon Jun 08, 2009 10:08 pm View user's profile Send private message Visit poster's website
nay27uk
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Post Reply with quote
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
Tue Jun 09, 2009 10:46 am View user's profile Send private message Visit poster's website
pcs
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Post Reply with quote
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
Tue Jun 09, 2009 11:04 am View user's profile Send private message Visit poster's website
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