Author Topic: 5W PLL FM TX Prob- No O/P, no LCD working, only huge voltage drop and 1971 heat  (Read 4492 times)

mohsinbashir64

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Dear Friends

I just reached to the end of the above mentioned transmitter design, pending the 3.20Mhz crystal , two hole ferrite core and the BNC connectors.
I thought to test the transmitter for correctness, used a 3.2768Mhz crystal(temporarily) and ferrite beads (to make the transformer).
On connecting the supply, I find a huge voltage drop (9V to about 1.5V) with a current draw of about 1.2A.
However, no visible work is observed, except that 2SC1971 heats up. The LCD doesnt light up, niether does the duty load.
I check the power jack for short circuits, the Multimeter beeps for a very short while, and later stabilizes.
I have been checking the connections thoroughly, and found no errors.

Please spare a few minutes to help me out. I shall be grateful  :'(

Warm Regards,

Mohsin Bashir

Jan

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The 2SC1971 is probably bad or there's some DC voltage applied to its base pin causing that the transistor is fully opened. Maybe the transformer is connected wrong way.

mohsinbashir64

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thanks for the reply, Jan.
As for the transformer,I connected it as per the pcb(with the thick wire towards the 2sc1971).
Is there any way to check the 2sc1971?
Warm regards,
Mohsin

mohsinbashir64

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Hi Jan,

I removed the 2SC1971 to check the rest of the circuit.
there was no voltage drop without 2SC1971 and the current draw is 0.2A.
The LCD does light up, but all i see is black dots.
Now i ll try to check if the 2SC1971 is bad (with Multimeter-Hfe measurement?).
but as for the LCD thing, what should have gone wrong?

Images posted here for your kind reference.

mohsinbashir64

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could anybody please bother to reply?
Jan I badly need help. please!  :(

mohsinbashir64

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I have successfully completed the transmitter design! thanks Jan :)
the 2sc1971 was bad plus the pic programmer was non functional.