FM to AM converter - variation on a theme!
 

I noticed last week, sometime, a post about a FM/AM converter to give some new life to older AM only radios.

I thought this was a really good idea, but missed out on getting one from the member concerned.

So I thought I do my take on in. I have ordered some of the FM modules and was going to control it with an Arduino I have lying around. But these modules are going to take a while from China! In the mean time I have another unit I have been using a project over the last few months, so I decided to adapt the AM modulator/transmitter from the design published and attach this gizmo to it!

It’s a small unit that does:

Operating voltage 6-15v (10-15 ma)
Has built in FM receiver
Has a built in MP3 player (using either a micro SD card or a USB memory stick)
Has an AUX input
And has a bluetooth module built in, ideal for iPhone !
And all for under £4.00

So I breadboarded the transmitter side, added a connecter and external PSU (it was also working from 6 AA batteries with no problems). I had to change the circuit around a bit - but not to much.

It works a treat! Ive added a few photos, of the soon to be finished (ie put in a box!) version.

Derek

Re: FM to AM converter - variation on a theme!
 

I should like to see the schematic of that modulator. It looks about as simple as you can get, and as such will have a degree of FM imparted to the signal as well as the intended AM.

I also have a pantry transmitter project in mind for when I get "a round tuit".

I have been very impressed with the AM sound from my 405 line TV modulator which operates around 50MHz. The oscillator is free-running, as is yours, but drift, even at this low VHF frequency is not a problem, and I can listen to the carrier on a communication receiver with the BFO switched on and the note is pure with near 100% modulation applied.

I see no reason why it wouldn't work at 1MHz with the appropriate tank circuit and a PA stage to put out a few miliwatts.
Uses an MC1374, the lower half of this cct

http://www.earlytelevision.org/405_modulator.html

Re: FM to AM converter - variation on a theme!
 

This was the original article: http://www.i4cy.com/m0oox/fm2am/

I just built the transmitter part.

All I really did was adjust the 220pf caps to 150pf in order to move the frequency along the scale, we have a local radio station very close to where the original was set for. I wired the am_disable to ground so it's on all the time. I used a different transistor (I had a bc457 in by box of bits). I also had to tweak the value of the input resistor to get a slightly better volume level.

Derek

Re: FM to AM converter - variation on a theme!
 

Hi.

That looks a very useful and versatile project to build. Any chance of seeing the details of the bluetooth unit and the various interconnections to the AM modulator?

Regards
Symon

Re: FM to AM converter - variation on a theme!
 

I can describe the changes I made (as above I teased a few of the values but that was optional you don't need to). First if you follow the link to the original article you will find the circuit diagram.

I built everything to the right of R1 470k (including the resistor; I later changed the value to a 100k as the volume was a bit low)

Next I permanently connected R2 1k to ground (-ve line from battery), so as to enable the transmitter all the time.

Next I changed the values of C4/C5 to 150pf to change the frequency, as the original values just happened to be very close to a local station. You may not need to do this step.

Next I added a 100uf cap and a 470ohm resister across the +ve/-ve ( for the cap) and the resistor in series withe the +ve. This reduces the voltage to the circuit and reduces the ripple (and hum) caused by the low quality plug-in-the-wall PSU I used. This power supply was 9v unregulated and was also used to power the Bluetooth/radio/mp3 module.

Next connected the provided short leads like this;

2 GND wires, one -ve power the other audio ground to the -ve per on the PSU where in connected to the board I made.

1 +ve (9-12volt required) to the PSU +ve direct from the transformer. So the same power supply is used for both the transmitter and the module.

Left+Right audio connected to R1 as Audio input to the transmitter.

Thats it. The module is so simple it does not even come with a connection diagram, it's printed on the PCB! Attached photo. Sorry is not a very good photo but you can see the silk print showing the connections and the pre-supplied connection plugs / leads

The unit is control by an IR remote, but will auto play MP3 etc when you insert a USB/card in it.

Derek

Re: FM to AM converter - variation on a theme!
 

What a great idea Derek!

As you say, those little units are so cheap - even from a UK supplier, only £7.00 or so with free P&P. I might have a go at one, if only out of interest. Taking into account the component value changes that you made, and modifications to the circuit, I've attached a sketch which I think incorporates all of the changes that you made if I've understood correctly.

Presumably, for convenience, both the modulator section and the FM unit could be powered from an internal PP3 battery rather than a 'wall wart' unless the current consumption of the FM unit is too high to make that realistic?

I notice that the remote control will switch the FM unit itself on an off, but I imagine that the only way to switch both the modulator and FM unit jointly, would be by an on-off switch on the modulator to cut the power supply to both units?

Incidentally, what's the length and height of the FM unit please Derek?

Re: FM to AM converter - variation on a theme!
 

I have one of the FM/MP3 modules, and I seem to remember it pulls about 20mA, so a PP3 isn't practical for long term use. It's fine with a spare wall wart of any reasonable voltage, as it has a regulator to provide 5V. I bought it to play MP3s on the hifi system, but it's redundant for that now, so I may try adding the MW oscillator if I can find the right inductors in the junk box. Presumably the 150uH inductor is basically a MW aerial coil. I can measure a few coils using my Chinese component analyser.

Re: FM to AM converter - variation on a theme!
 

There might be an issue with the FM modules +ve feed via that 470 ohm resistor in your schematic..

Lawrence.

Re: FM to AM converter - variation on a theme!
 

Hello,

Thanks for drawing that out for me, it's not quite right I've attached a JPG.

The FM unit I have measures (front panel dimensions) 105mm(W) 25mm(H) 30mm(D). The PCB is approximately 70mm(W) 25mm(D)

I've been using this for a week or so now, sometimes all day sometimes just for an hour. It seem really stable and the AM frequency does not drift. The only thing I can fault it with is that the auto-tune for the FM channels does not seem to have an edit facility to get rid of the false positives so it detects 25 presets, but only 5-6 are actually stations!

The 150uh is one of the miniature chokes (looks like a 1/4w resistor), so is the 1mh

Ignore the black line in the middle of the JPG! it's just the PSU connections that should be different.

Further info, I did run the unit from 8xAAA batteries (most are capable of 500-700mha, so at around 25ma they should last around 24 hours continuous use)

I decided the PP3 would not keep in running long enough. And it the end the PSU option was far better for me.

Derek

Re: FM to AM converter - variation on a theme!
 

Does that FM MP3 unit have a built in regulator, if so is it under control in terms of on/off? If so, would it power the AM osc as well? Just a thought.

Lawrence.

Re: FM to AM converter - variation on a theme!
 

Just had a close look,

It does have a surface mount regulator 7805, so yes you could solder a wire to that to provide 5 volts for the AM transmitter.

It's not controlled unfortunately, the hole module has no power "button" as such not even on the remote control. I suspect it's because it is sold as an auto-motive device normally so presumable controlled by the car electronics? who knows.

I may get the soldering iron out later and see if I can attach a wire to it.


Derek

Re: FM to AM converter - variation on a theme!
 

Thanks Lawrence and Derek for your helpful feedback.

Derek has also clarified the point that you made Lawrence and I've modified the sketch accordingly. Given the consumption of the FM module it clearly makes more sense to power the converter from a 'wall wart' rather than batteries. Hence, I've shown a concentric power socket, and have also added a reverse Voltage protection diode as a precaution in case a wall wart which has the centre-pin negative gets inadvertently plugged in, but the diode could be omitted if felt unnecessary.

Hopefully, the circuit now looks OK.

Derek has made the point that he only changed C4 & C5 from 220pF to 150pF to move the AM frequency higher up the MW band away from an interfering station.

Re: FM to AM converter - variation on a theme!
 

Thank you David for the circuit diagram, that is very clear. When time permits, I'll be building this circuit.

Regards
Symon.

Re: FM to AM converter - variation on a theme!
 

Hi David / Derek
Is there any problems with frequency drift with this little AM TX.
I built a Minimod as an experiment but have put it aside as it suffered rather from frequency drift.
So after an hour or so you would need to retune the radio. I was using a Synthesized radio so I don't think it was that wandering off tune.
One other radio I was using was also synthesized but in multiples of 9kHz so that was not helpful at all when the Minimod TX drifted.
I tried to use quality components and I tried various types of cap in the tuned circuits including mica and ceramic, some drifted more than others.

Mike

Re: FM to AM converter - variation on a theme!
 

hi,

This AM transmitter seems to be very stable and does not drift significantly. I think in part this is due to the tank circuit using a choke type coil that's encapsulated. Also it uses emitter coupling to the aerial. The frequency remains stable even with voltage change in the supply. The only thing that seems to effect the frequency is if you grab the aerial, even then not much change.

Derek

Re: FM to AM converter - variation on a theme!
 

I've had no problems of drift with the Mini-Mod or the BVWS FM/AM converter and the audio from both is excellent, leastways to my ears. Howard ('Half a Mullard') visited me recently to see a Mini Mod in action and was impressed enough to get on with building one. I've built about ten Mini Mods using the PCB I designed (based on Ian's original layout), and about twenty FM/AM converters/boards on the PCB I designed for that project. The feedback I've had from recipients has been very favourable.

Re: FM to AM converter - variation on a theme!
 

I can endorse David's comments on both the Minimod I have built both.
The 2 Minimods that I built were on PCBs supplied by David. The FM/AM converters I built one on a PCB also supplied by David .The other one was supplied ready built by David.

Dave
M0HBV

Re: FM to AM converter - variation on a theme!
 

I've built two MiniMods on home-designed perf-board and Veroboard layouts and have never noticed any frequency drift, but mine are used with conventionally-tuned radios.

Re: FM to AM converter - variation on a theme!
 

I've had a doodle around to create a PCB layout using the digital display FM unit, based on the circuit at post #13 above, using Derek's ('coopzone') adaptation to merge the FM unit with the modulator part of the circuit of the BVWS Summer 2017 FM/AM Converter design. I've checked it carefully for errors, but that doesn't mean there aren't any, so I've added it here for interest, rather than for use at this stage.

As soon as I obtain an FM module, I'll build it and report back. (I'm not sure if the FM unit needs an aerial or has one incorporated).

Meanwhile, any feedback would be welcome.

Pic 1 is the circuit
Pic 2 is the component layout on the PCB
Pic 3 is the PCB artwork (reversed) for positive UV resist board, and a negative mask for the dry film UV process.
The positive mask can also be used with the 'iron-on' laser printer technique. If the image as shown is printed off and ironed on, it will come out the correct way around.
Pic 4 is the type of PCB terminal blocks that I'll use.

Hope that might be of interest.

Re: FM to AM converter - variation on a theme!
 

Just a heads up in case anyone is confused: David has drawn the layout for a BC184L transistor, which has a bce leadout. This will be fine for any general purpose type with the 'Japanese standard' leadout, but not for the BC547 used by the OP (or for a bog standard BC184) as this has a cbe leadout.