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FM transmitter Repair

HI guys, I have attempted a repair on an FM transmitter. I replaced TR2,(shorted) VR1,(10.7 V output, VR1 is a SMD L78L09), and R4, it looked overheated, measured OK. I fired it up and then found the output Mosfet (F1)shorted and VR1 with an output of 11.2V. So I replaced the Mosfet and the L78L09. The unit works, output is good. However, the 9 volt regulator again has 11.2 v output and R4 100 ohm resistor is dissipating .76 watts, in 1/2 watt part. The resistance measured with no power from the 9v regulator output to ground is 6.1k ohms. 12.2v input, 11.2v output, ground pin 0v.

I drew a schematic.

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A picture of the board.

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I don't see why the 9V regulator isn't regulating and the 100 resistor gets very hot, it has 8.6V across it. This is the low power mode, the

100 ohm supplies power to the output MOSFET. In high power mode Q2 is turned on to supply power thru the 10 ohm resistor. Any hints are appreciated. I have ran the unit a few hours, and it survived, but I need to find out what else is wrong with it.

Thanks, Mikek

Reply to
amdx
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Is your 12 volts input clean?

Reply to
Tom Biasi

Maybe the 9 volt regulator is shorted input to output.

It is also possiable that the regulator is oscillating. Can you put a scope on it and see ? If so, maybe the capacitors around it need to be replaced to cure the oscillation.

Reply to
Ralph Mowery

Your schematic appears to be incorrect as FB2 is shown shorted. You have not indicated which 100 ohm resistor is R4. If it is the top one, then your FET is probably biased on most of the time.

Reply to
John S

How much DCV you got on osc input?

Reply to
John S

Good eye, thanks. That may not be the only error, I went over it twice, but still.

I labeled R4 in RED.

Thanks, Mikek

Reply to
amdx

I'll look, hopefully this evening. Mikek

Reply to
amdx

No oscillation, It's not shorted. I'm going to make a board to mount the L78L09 for testing. I'll test all three, and make sure they are bad. Thanks, Mikek

Reply to
amdx

It is a little strange the voltage drifts between

0V and 0.62V. Get your hand near and far alters the voltage. But if I use the scope probe, it has just noise in the 20mv range and no DC. Mikek
Reply to
amdx

First things first. Check that the regulator ground connection isn't the victim of a cracked track.

Next, where does the energy in the 100uF capacitor go when the 12V is turned off? Is the 12V simply disconnected when off, or could there be a low impedance to ground when it's off?

Cheers

--
Clive
Reply to
Clive Arthur

Zero Volts on the ground pin, PCB in good condition. I removed the L78L09 and tested it separately, it's bad, as is the one I originally removed. I started a removal of parts to try to Isolate the defect. With the L78L09 removed I put in 9V from a current regulated separate source I removed FB3, Current draw is low, this tells me my two filter caps aren't the problem. Reinstalled FB3. I removed C10, current still high, C10 not the problem. I removed C9, the current dropped low on the 9 volt line. The cap measures 51pf, it's not shorted, just to be sure I installed a

100pf cap, the current went high again. I removed the 100pf cap, to get low current again. I measured the voltages on Q3, B=5.26V, E=1.1V Collector=8.9V. This is not right, but Q3 tests normal with diode function. If I try to measure the voltage on Q3 collector it can trigger the current to go high, forcing a reset of over current on PS. (10Meg input DVM) Reinstalled original 51pf cap.

With power off, the 9 volts is still on. So the only way to watch 100uf discharge is to pull the power plug. All I can say is it discharges quickly.

Thanks, Mikek

Reply to
amdx

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