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Is transistor faulty or something else amiss

Devnode

Jul 13, 2015
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Hi, all.
I trust you are all doing great. I saw this transmitter circuit at this link,http//www.instructables.com/longrangefmtransmitter. So i wanted to give it a try. I build it on vero board and at first i could not find any ac signal any where but after some time i could trace at up to my second transistor, Q2. However i can find signal at the base and emitter but not at the collector of Q2. i have gone over this countless times looking for any sign of mistake but found none both are 2n3904 and the first one works okay. I am totally at loss as to what the cause can be. Can anyone please help.It is my first time to try radio so i'm a bit out of dept here.
I tried to add a pic or two but it would not upload so i opted for the link. I hope it is okay.
Thank You so much in advance.
Devnode
 

Devnode

Jul 13, 2015
94
Joined
Jul 13, 2015
Messages
94
Hi, all.
I trust you are all doing great. I saw this transmitter circuit at this link,http//www.instructables.com/longrangefmtransmitter. So i wanted to give it a try. I build it on vero board and at first i could not find any ac signal any where but after some time i could trace at up to my second transistor, Q2. However i can find signal at the base and emitter but not at the collector of Q2. i have gone over this countless times looking for any sign of mistake but found none both are 2n3904 and the first one works okay. I am totally at loss as to what the cause can be. Can anyone please help.It is my first time to try radio so i'm a bit out of dept here.
I tried to add a pic or two but it would not upload so i opted for the link. I hope it is okay.
Thank You so much in advance.
Devnode
Your link leads to a 404 :(
Hi. Harald. This is the one https://www.instructables.com/id/The-Ultimate-FM-Transmitter/ . I Am very sorry for wasting your time.
 

Martaine2005

May 12, 2015
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You should really test it out of circuit. Other components can affect the readings.

Martin
 

Devnode

Jul 13, 2015
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You should really test it out of circuit. Other components can affect the readings.

Martin
Hi, Martin. I wiil do so as soon as i get back home. I need to go out now. Duty calls. I will let you know though.
Regards.
 

Harald Kapp

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I tried to add a pic or two but it would not upload
We do have a size limit. Recale your image to a few 100 kB, then it should work.
However i can find signal at the base and emitter but not at the collector of Q2
How are you trying to measure at the collector? The signal there is supposed to be a high frequency FM signal. Your multimeter is very likely not suitable for measuring signals in the frequency range.
 

Devnode

Jul 13, 2015
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We do have a size limit. Recale your image to a few 100 kB, then it should work.

How are you trying to measure at the collector? The signal there is supposed to be a high frequency FM signal. Your multimeter is very likely not suitable for measuring signals in the frequency range.
Hi, Harald. I am using my ( very old) scope for tracing it.
 

Devnode

Jul 13, 2015
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I wiil be off line for maybe an hour or two due to my work. Please dont think that im not responding to your advice but i will be away from internet for some time.
Thankyou so much.
 

Devnode

Jul 13, 2015
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Hi,
Hi, Martin. I wiil do so as soon as i get back home. I need to go out now. Duty calls. I will let you know though.
Regards.
Martin. I did as you suggested and took out the transistor and it tested fine again 0.63 base to emmitter and 0.65 base to collector. I then replace it with a new one in the circuit with the same results. I think ill check the voltages at the various bases, collectors etc and post it tomorrow from work.
Thanks in the meantime to all of you. You are very helpfull.
Regards.
 

Devnode

Jul 13, 2015
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Hi,

Martin. I did as you suggested and took out the transistor and it tested fine again 0.63 base to emmitter and 0.65 base to collector. I then replace it with a new one in the circuit with the same results. I think ill check the voltages at the various bases, collectors etc and post it tomorrow from work.
Thanks in the meantime to all of you. You are very helpful.
Regards.
Hi, Martin. I have noticed that the caps used in the original project are ceramic whereas i used polyester in my project. Do you think it may have caused the problem?
 

Audioguru

Sep 24, 2016
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A multimeter is made to accurately measure the voltage at 50Hz or 60Hz. It works poorly at 1kHz and the FM radio frequency of 100MHz is 100 hundred thousand times higher and a multimeter measures nothing.

That is the same circuit that I fixed 14 years ago:
1) The mic preamp transistor is poorly biased so it is saturated when the battery is new and it is cutoff when the battery voltage has run down a little. I fixed it by biasing the transistor correctly and adding a 5V low-dropout voltage regulator.
2) The radio frequency changes as the battery voltage runs down. So I connected the RF oscillator to the 5V regulator.
3) The radio frequency changes if anything moves towards or away from the antenna. So I added an RF amplifier to isolate the oscillator from capacitance changes at the antenna.
4) The sounds are muffled by de-emphasis in all FM radios so I added pre-emphasis like all FM radio stations have.

I also built it compactly on Veroboard and it works perfectly and sounds perfect.
The 0.1uF (100nF) capacitors are for audio so they should be polyester. The 0.01uF and 4.7pF capacitors are for RF so they should be ceramic. Here is my project:
 

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Audioguru

Sep 24, 2016
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I forgot to say that the original C1 and C2 audio coupling capacitors have such a low value that they cut all low audio frequencies causing a man's voice to sound like a chipmunk. The values of the original R6 and C3 are also wrong and cut high audio frequencies.
Then the sounds heard on an FM radio sound as bad as an old telephone.

Guess what? It is so powerful that it illegally causes interference to normal FM radio stations that your neighbours are listening to.
 

aurelZ

Jun 10, 2019
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Yes this one given by audiogure must work fine!
Only one thing...on your image is 680 ohm emitter resistor and on schematic is 470 ohm
any difference?
 

Audioguru

Sep 24, 2016
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Yes this one given by audiogure must work fine!
Only one thing...on your image is 680 ohm emitter resistor and on schematic is 470 ohm
any difference?
The 680 ohms resistor is the current-limiting resistor (to ground) for an LED I never installed. The 470 ohms emitter resistor R5 is 0.1" to the right of the emitter wire of Q1. C2 and R6 are on the bottom of the circuit board.
 

Devnode

Jul 13, 2015
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A multimeter is made to accurately measure the voltage at 50Hz or 60Hz. It works poorly at 1kHz and the FM radio frequency of 100MHz is 100 hundred thousand times higher and a multimeter measures nothing.

That is the same circuit that I fixed 14 years ago:
1) The mic preamp transistor is poorly biased so it is saturated when the battery is new and it is cutoff when the battery voltage has run down a little. I fixed it by biasing the transistor correctly and adding a 5V low-dropout voltage regulator.
2) The radio frequency changes as the battery voltage runs down. So I connected the RF oscillator to the 5V regulator.
3) The radio frequency changes if anything moves towards or away from the antenna. So I added an RF amplifier to isolate the oscillator from capacitance changes at the antenna.
4) The sounds are muffled by de-emphasis in all FM radios so I added pre-emphasis like all FM radio stations have.

I also built it compactly on Veroboard and it works perfectly and sounds perfect.
The 0.1uF (100nF) capacitors are for audio so they should be polyester. The 0.01uF and 4.7pF capacitors are for RF so they should be ceramic. Here is my project:
Hi, Audioguru. I thought the caps must be changed and i already ordered some ceramics then ill give it another go. Your`s seems to be the real deal though. Can i try it? I mean this time i will find some real help if i got stuck again and i can actually learn more about radio.
 

Devnode

Jul 13, 2015
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Hi, Audioguru. I thought the caps must be changed and i already ordered some ceramics then ill give it another go. Your`s seems to be the real deal though. Can i try it? I mean this time i will find some real help if i got stuck again and i can actually learn more about radio.
Also what diameter is your coils? I see you have ± 10 turns on both.
 

73's de Edd

Aug 21, 2015
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It's for sure that, the one used for the oscillator portion is going to be an IMMENSE 10 improvement over the SWAN's . . . . . Slinky / Mickey M...O...U...S...E coil being utilized.
Both in the respect of mechanical solidity, frequency stability and minimizing any mechanical vibrations microphonic creations.
Now, considering that the perfboard is using .040 holes, that wire gauge would appear to be #18 enamelled and being closewound on a 1/4 in form . . . .OR . . . the companion variable trimmer capacitor specified, would easily be able to accomodate to that value of created inductance .
 
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aurelZ

Jun 10, 2019
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devnode
if Audioguru circuit looks complex to you you can try this one.
princip is the same simpleFMtransmitter.gif
 
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