small FM transmitters - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Skywave · 15th Oct 2007, 10:01 pm

Quote:

Originally Posted by plaka78

Tr1= 2N2646

Err, hang on a tick!

Unless I'm having another 'senior moment' here, a 2N2646 is a UNIJUNCTION transistor!

. In the cct. diag., TR1 is clearly drawn as a blocking oscillator using a bipolar (although the 'collector' and 'emitter' electrodes are not clearly identified

. . . . Am I missing something subtle / obvious here?

[Wouldn't be the first time

]

Confused

Al / Skywave

paulsherwin · 15th Oct 2007, 10:06 pm

Quote:

Originally Posted by Skywave

Unless I'm having another 'senior moment' here, a 2N2646 is a UNIJUNCTION transistor!

. Am I missing something subtle / obvious here?

The part no was corrected in post 11 - 2N2369.

Paul

Skywave · 15th Oct 2007, 10:16 pm

Thank you, Paul.

Yes, I did read post #11 & I was a little confused about that as well - since Plaka did not explicitly state that there was an error in his earlier listing. Obviously, a 2N2369 is an appropriate device for TR1, unlike a 2N2646.

However, it's all history now - and I suggest we now leave it at that.

Regards,

Al / Skywave.

plaka78 · 16th Oct 2007, 9:05 am

I would have thought is there was any real problems with harmonics I would have seen some light pattering on my TV via the indoor aerial. I do not have the luxury of a spectrum analyser

. The fact that it does not cause interference to the TV or Radio and that it is low powered I can’t see a problem with it. The results are excellent. I have used this cct with a different audio amp fitted in a small Fidelity
record player for playing 78s. The cct that I have built I have found to be very stable.

Thanks to everyone that has put in there comment’s

Low power Short wave transmitter coming soon that I have built prototype around two 10.7 IF coils. It’s very work well.

Mach One · 16th Oct 2007, 1:45 pm

I have played with such FM modulators in the past and drift has definitely been the main problem. One thing that your circuit seems to be missing is "pre-emphasis". Your high frequencies will be a little dull compared with what they should be.

plaka78 · 16th Oct 2007, 2:40 pm

pre-emphasis is about right and can de altered by changing the value of the audio input capacitor. The one I use sounds very good. As for drift it is very stable.

Skywave · 16th Oct 2007, 5:57 pm

Plaka78 -

What is the current that the final transistor is drawing and what is the power supply voltage to this transistor? I know we are talking (probably) milli-watts here.
And what is the final frequency of the transmitter?

Just curious

Thanks

Al / Skywave

plaka78 · 17th Oct 2007, 8:46 am

Circuit runs on 9V. I will check the current later, although a PP3 seems to last for a very long time

Skywave · 18th Oct 2007, 12:28 am

Quote:

Originally Posted by plaka78

. . . I will check the current later . . .

And the final transmit frequency, please.

Thanks.

Al / Skywave.

plaka78 · 18th Oct 2007, 7:50 pm

The measured current was a constant 1.45ma (very low). As for frequency it should tune across the FM band. I took my cct around to a friend today and he was amazed at the sound quality hence he wants one now

Skywave · 19th Oct 2007, 7:48 am

Thank you, Plaka78.

In my main employment, a need has arisen to design and build a sweeping low-power osc. for 88 - 108 MHz - and now have the relevant info. to make a start.

Al / Skywave

nobby.hall · 19th Oct 2007, 11:26 am

Have you looked at the new part from Silicon Labs ... single chip solution ... Si4713.

Neil

15th Oct 2007, 10:16 pm	#23
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: small FM transmitters Thank you, Paul. Yes, I did read post #11 & I was a little confused about that as well - since Plaka did not explicitly state that there was an error in his earlier listing. Obviously, a 2N2369 is an appropriate device for TR1, unlike a 2N2646. However, it's all history now - and I suggest we now leave it at that. Regards, Al / Skywave.

16th Oct 2007, 9:05 am	#24
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: small FM transmitters I would have thought is there was any real problems with harmonics I would have seen some light pattering on my TV via the indoor aerial. I do not have the luxury of a spectrum analyser. The fact that it does not cause interference to the TV or Radio and that it is low powered I can’t see a problem with it. The results are excellent. I have used this cct with a different audio amp fitted in a small Fidelity record player for playing 78s. The cct that I have built I have found to be very stable. Thanks to everyone that has put in there comment’s Low power Short wave transmitter coming soon that I have built prototype around two 10.7 IF coils. It’s very work well.

16th Oct 2007, 1:45 pm	#25
Mach One Heptode Join Date: Sep 2005 Location: Shoreham-by-Sea, West Sussex, UK. Posts: 611	Re: small FM transmitters I have played with such FM modulators in the past and drift has definitely been the main problem. One thing that your circuit seems to be missing is "pre-emphasis". Your high frequencies will be a little dull compared with what they should be.

16th Oct 2007, 2:40 pm	#26
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: small FM transmitters pre-emphasis is about right and can de altered by changing the value of the audio input capacitor. The one I use sounds very good. As for drift it is very stable.

16th Oct 2007, 5:57 pm	#27
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: small FM transmitters Plaka78 - What is the current that the final transistor is drawing and what is the power supply voltage to this transistor? I know we are talking (probably) milli-watts here. And what is the final frequency of the transmitter? Just curious Thanks Al / Skywave

17th Oct 2007, 8:46 am	#28
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: small FM transmitters Circuit runs on 9V. I will check the current later, although a PP3 seems to last for a very long time

18th Oct 2007, 7:50 pm	#30
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: small FM transmitters The measured current was a constant 1.45ma (very low). As for frequency it should tune across the FM band. I took my cct around to a friend today and he was amazed at the sound quality hence he wants one now

19th Oct 2007, 7:48 am	#31
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: small FM transmitters Thank you, Plaka78. In my main employment, a need has arisen to design and build a sweeping low-power osc. for 88 - 108 MHz - and now have the relevant info. to make a start. Al / Skywave

19th Oct 2007, 11:26 am	#32
nobby.hall Retired Dormant Member Join Date: Oct 2007 Location: Farnham, Surrey Posts: 1	Re: small FM transmitters Have you looked at the new part from Silicon Labs ... single chip solution ... Si4713. Neil