Small low power MW transmitter - UK Vintage Radio Repair and Restoration Discussion Forum

plaka78 · 29th Sep 2007, 12:26 pm

I have made this from recycled and surplus bits. The base is made from a CD case, the tuning knobs are Milk bottle tops. The results are excellent and it works with a range of about 8 metres of the ferrite rod aerial. The circuit is powered by a PP3 9V battery.

Darren-UK · 29th Sep 2007, 1:50 pm

That's a neat little job, well done. Good thinking with those bottle-tops too. We've had a discussion on the use of ferrite rods for transmission purposes in the forums quite recently and the general opinion was that performance is fairly poor. Have you tried any other type of antenna, just out of curiosity ?

(I would mention, as a directive to all members, that forum rules Section F must be observed when discussing transmission equipment......to prevent this thread drifting onto prohibited topics).

Mullard · 29th Sep 2007, 5:48 pm

Any chance of a circuit diagram?
cheers,
John

Paul Stenning · 29th Sep 2007, 5:59 pm

The neat thing with a ferrite rod, if it can get enough range, is that it is contained in the case with the rest of the unit. No trailing bits of wire etc.

plaka78 · 29th Sep 2007, 6:06 pm

Reference to Darren Question I have tried a metre of wire for the Aerial
And it increases the range by a factor of about three.

Reference to Johns Question I will draw out the diagram and post it. It may not be of the best design, I am not a design engineer but I have enough knowledge to strings things together.

Tony

plaka78 · 29th Sep 2007, 6:11 pm

Reference to Pauls Question.

Its has enough range on the ferrite Rod for room use. The only trailing lead is the input lead. If I had a jack socket handy I would have fitted to the front of the unit.

Paul Stenning · 29th Sep 2007, 6:30 pm

Out of interest have you measured the current consumption from the battery? Or have you run it long enough to know roughly what the battery life is?

plaka78 · 29th Sep 2007, 6:52 pm

The current is 19ma

Paul Stenning · 29th Sep 2007, 11:04 pm

About what a trannie radio at moderate volume would consume then. That's excellent.

I was worried that it might be running somewhat higher (because of the larger metal can transistor) which would flatten batteries too quickly.

plaka78 · 30th Sep 2007, 10:00 am

The only reason that I used the large transistor (BFY50) Because there were some on old scrap cct boards were I work

plaka78 · 2nd Oct 2007, 6:30 pm

CCT Diagram

This cct was made from recycled components and the transistors I used were at hand
TX
R1=120K
R2=47R
R3=1K
R4=220R
C1=0.1 µF
C2=22pF
VC1 and VC2= Tuning Caps(MW/LW radio types)
L1= Ferrite rod with original MW coil
TR1= 2N2645
TR2=BFY50

Mod
R5=68R
R6=68R
R7=220R
R8=5K to 50K Pot
R9=47K
R10=4.7K Value may need changing to get 4.5V at TR3 collector
R11=4.7K
R12=180R
R13=390K
R14=100K Pot
R15=4.7K
R16=200R
R17=2.7K
C3=0.47 µF
C4=0.47 µF
C5=0.47 µF
C6=22 µF
TR3,4,5=BC548

Tr1 is the main oscillator tune via VC1, Any 6 Pin Osc coil will work (Red Toko or older big ones used in all sixties radios)
R2 is not decoupled (give a little bit of negative feedback to give clean sinewave) Tr2 is the TX output (Class C) R3 limits how negative the base goes. L1 VC2 (series Tuned cct ) shunt fed via R4. I found this gave very good results (clean sinewave at the junction of L1 and VC2. Wire aerial can be added by connecting to junction L1 and VC2 via 22pF Cap
TR3 is a basic amp, R8 Modulation Level, Tr4 is the second dc coupled to TR5. R14( Modulator offset) is adjusted to get 4.5V at TR5 emitter .
It may be necessary to connect a small valve Capacitor across R9(try 150pF)

paulsherwin · 2nd Oct 2007, 6:46 pm

Have you tried any other transistors in place of the 2N2645 and BFY50? From the circuit, it looks as if most general purpose Si transistors should work (BC548 even

)

I only ask because I don't think I have a 2N2645 in the junk box. Looks to be a nice simple design - in fact I suspect you could lose one of the amplification stages in the modulator.

Paul

plaka78 · 2nd Oct 2007, 7:15 pm

Most transistors will work in the Osc section and any RF transistors in the output. although BFY50s work really well at RF and are easy to source.

Tony

Mullard · 2nd Oct 2007, 9:32 pm

Many thanks for the circuit. I wll give it a try.

John

Hunts smoothing bomb · 16th Oct 2007, 7:31 pm

Just given the oscillator section a try with some odd components in the junk box,
Found a coil that works but didn't have a variable cap so used a 450pf fixed one instead.

I tried both a BC107 and a BFY51 in the place of the 2N2645 with good results.

connected to the scope I have a kink on the top of the sign wave that I cannot get rid of.
also there was enough power coming off just the oscillator section to cause a hetrodyne on Gold that I happened to be listening to at the time!!!

The experimenting goes on!!

Cheers
Lee

plaka78 · 17th Oct 2007, 8:43 am

Try a 100pf cap and check again and note the frequency.

arjoll · 21st Oct 2007, 8:00 am

Just been toying with the idea of building one of these, but replacing the oscillator section with an old PAL crystal (4.433619) divided by three with a 4017. This shoud give me around 1478 kHz, not quite a standard 9 kHz channel (1476 is close!) but far enough away from any local stations (closest is Rhema on 1404). I've got a few old crystals from some dead VCR's I've stripped.

Can anyone think of any reasons that this wouldn't work?

Andy Day · 21st Oct 2007, 7:15 pm

And what is the audio quality? As good as most other stations received on AM?

29th Sep 2007, 12:26 pm	#1
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Small low power MW transmitter I have made this from recycled and surplus bits. The base is made from a CD case, the tuning knobs are Milk bottle tops. The results are excellent and it works with a range of about 8 metres of the ferrite rod aerial. The circuit is powered by a PP3 9V battery. Attached Thumbnails

29th Sep 2007, 1:50 pm	#2
Darren-UK Retired Dormant Member Join Date: Apr 2003 Location: Blackpool, Lancashire, UK. Posts: 4,061	Re: Small low power MW transmitter That's a neat little job, well done. Good thinking with those bottle-tops too. We've had a discussion on the use of ferrite rods for transmission purposes in the forums quite recently and the general opinion was that performance is fairly poor. Have you tried any other type of antenna, just out of curiosity ? (I would mention, as a directive to all members, that forum rules Section F must be observed when discussing transmission equipment......to prevent this thread drifting onto prohibited topics).

29th Sep 2007, 5:48 pm	#3
Mullard Pentode Join Date: Jul 2007 Location: Clophill, Bedfordshire, UK. Posts: 127	Re: Small low power MW transmitter Any chance of a circuit diagram? cheers, John

29th Sep 2007, 5:59 pm	#4
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,071	Re: Small low power MW transmitter The neat thing with a ferrite rod, if it can get enough range, is that it is contained in the case with the rest of the unit. No trailing bits of wire etc. __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

29th Sep 2007, 6:06 pm	#5
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Small low power MW transmitter Reference to Darren Question I have tried a metre of wire for the Aerial And it increases the range by a factor of about three. Reference to Johns Question I will draw out the diagram and post it. It may not be of the best design, I am not a design engineer but I have enough knowledge to strings things together. Tony

29th Sep 2007, 6:11 pm	#6
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Small low power MW transmitter Reference to Pauls Question. Its has enough range on the ferrite Rod for room use. The only trailing lead is the input lead. If I had a jack socket handy I would have fitted to the front of the unit.

29th Sep 2007, 6:30 pm	#7
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,071	Re: Small low power MW transmitter Out of interest have you measured the current consumption from the battery? Or have you run it long enough to know roughly what the battery life is? __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

29th Sep 2007, 6:52 pm	#8
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Small low power MW transmitter The current is 19ma

29th Sep 2007, 11:04 pm	#9
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,071	Re: Small low power MW transmitter About what a trannie radio at moderate volume would consume then. That's excellent. I was worried that it might be running somewhat higher (because of the larger metal can transistor) which would flatten batteries too quickly. __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

30th Sep 2007, 10:00 am	#10
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Small low power MW transmitter The only reason that I used the large transistor (BFY50) Because there were some on old scrap cct boards were I work

2nd Oct 2007, 6:30 pm	#11
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Small low power MW transmitter CCT Diagram This cct was made from recycled components and the transistors I used were at hand TX R1=120K R2=47R R3=1K R4=220R C1=0.1 µF C2=22pF VC1 and VC2= Tuning Caps(MW/LW radio types) L1= Ferrite rod with original MW coil TR1= 2N2645 TR2=BFY50 Mod R5=68R R6=68R R7=220R R8=5K to 50K Pot R9=47K R10=4.7K Value may need changing to get 4.5V at TR3 collector R11=4.7K R12=180R R13=390K R14=100K Pot R15=4.7K R16=200R R17=2.7K C3=0.47 µF C4=0.47 µF C5=0.47 µF C6=22 µF TR3,4,5=BC548 Tr1 is the main oscillator tune via VC1, Any 6 Pin Osc coil will work (Red Toko or older big ones used in all sixties radios) R2 is not decoupled (give a little bit of negative feedback to give clean sinewave) Tr2 is the TX output (Class C) R3 limits how negative the base goes. L1 VC2 (series Tuned cct ) shunt fed via R4. I found this gave very good results (clean sinewave at the junction of L1 and VC2. Wire aerial can be added by connecting to junction L1 and VC2 via 22pF Cap TR3 is a basic amp, R8 Modulation Level, Tr4 is the second dc coupled to TR5. R14( Modulator offset) is adjusted to get 4.5V at TR5 emitter . It may be necessary to connect a small valve Capacitor across R9(try 150pF) Attached Thumbnails

2nd Oct 2007, 6:46 pm	#12
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,936	Re: Small low power MW transmitter Have you tried any other transistors in place of the 2N2645 and BFY50? From the circuit, it looks as if most general purpose Si transistors should work (BC548 even ) I only ask because I don't think I have a 2N2645 in the junk box. Looks to be a nice simple design - in fact I suspect you could lose one of the amplification stages in the modulator. Paul

2nd Oct 2007, 7:15 pm	#13
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Small low power MW transmitter Most transistors will work in the Osc section and any RF transistors in the output. although BFY50s work really well at RF and are easy to source. Tony

2nd Oct 2007, 9:32 pm	#14
Mullard Pentode Join Date: Jul 2007 Location: Clophill, Bedfordshire, UK. Posts: 127	Re: Small low power MW transmitter Many thanks for the circuit. I wll give it a try. John

16th Oct 2007, 7:31 pm	#15
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Small low power MW transmitter Just given the oscillator section a try with some odd components in the junk box, Found a coil that works but didn't have a variable cap so used a 450pf fixed one instead. I tried both a BC107 and a BFY51 in the place of the 2N2645 with good results. connected to the scope I have a kink on the top of the sign wave that I cannot get rid of. also there was enough power coming off just the oscillator section to cause a hetrodyne on Gold that I happened to be listening to at the time!!! The experimenting goes on!! Cheers Lee __________________ Lee Last edited by Hunts smoothing bomb; 16th Oct 2007 at 7:34 pm. Reason: Spelling

17th Oct 2007, 8:43 am	#16
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Small low power MW transmitter Try a 100pf cap and check again and note the frequency.

21st Oct 2007, 8:00 am	#17
arjoll Dekatron Join Date: May 2006 Location: Invercargill, New Zealand Posts: 3,457	Re: Small low power MW transmitter Just been toying with the idea of building one of these, but replacing the oscillator section with an old PAL crystal (4.433619) divided by three with a 4017. This shoud give me around 1478 kHz, not quite a standard 9 kHz channel (1476 is close!) but far enough away from any local stations (closest is Rhema on 1404). I've got a few old crystals from some dead VCR's I've stripped. Can anyone think of any reasons that this wouldn't work?

21st Oct 2007, 7:15 pm	#18
Andy Day Pentode Join Date: Aug 2005 Location: Hemel Hempstead, Hertfordshire, UK. Posts: 154	Re: Small low power MW transmitter And what is the audio quality? As good as most other stations received on AM? __________________ The older I get, the faster I was!