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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 26th Mar 2019, 2:33 pm   #81
60 oldjohn
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Default Re: FM to AM Converter?

I intend fitting one of these FM to AM converter boards inside a car radio (Radiomobile 4220) c1950. Should I include circuit protection against voltage spikes? The converter board needs 3v at 22ma sourced from the radio 12v pos earth.


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Old 25th Jul 2020, 3:29 pm   #82
__i4cy__
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Default FM to AM Radio Converter

Not everyone likes to construct on veroboard as it can be rather fiddly. So finally I’ve got around to creating a new PCB for the FM to AM Radio Converter (hopefully living up to David’s earlier popular design).

If anyone is interested more details can be found at …
http://www.i4cy.com/m0oox/fm2am/
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Old 29th Jul 2020, 6:46 pm   #83
David G4EBT
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Default Re: FM to AM Converter?

Excellent Guy - great to see this thread woken from it's slumbers - well done on your efforts!

The converter has been a deservedly popular project (81 posts and 14,000 views of this thread). The PCB will make it easier to construct and lessening the chance of errors. The RDA5807M module is a little challenging to solder, but is surprisingly robust, and at 3.50 for five post free from UK suppliers on eBay, it's not going to break the bank if any mishaps occur.

Especially useful to provide FM on live-chassis radios such as the ubiquitous Bush DAC90A, to which the direct connection of other external devices to provide FM cannot be safely achieved.

Every success with it.
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Old 1st Aug 2020, 9:39 am   #84
__i4cy__
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Default Re: FM to AM Converter?

The project had gone a little quiet of late, and with all this spare time I decided to design a PCB. Originally, the plan was to follow up with a PCB depending on the project’s interest. Thankfully David kindly solved this with his popular board design, while adding to the success of the project.

The RDA5807M FM receiver module is the only tricky part to solder to veroboard, especially because of its metric pitch, but is somewhat simplified when using a PCB.

The new red PCB is double sided, and has all the frills you get with a professionally manufactured board, including a silkscreen, solder resist, and tinned pads. The only change I’ve made to the circuit is Q1 which was originally a BC184L. As you know this transistor has pin-out variations depending on the part number suffix (BC184, BC184C have different pin-outs). As this caused problems, Q1 has now been replaced with a 2N3904 which is more widely available, cheaper and does not suffer from pin-out variations. (In fact any general purpose low power NPN transistor with Ft > 100MHz and Hfe > 100 should suffice for Q1.)

All else remains the same, including the PIC firmware.

Guy
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Old 1st Aug 2020, 1:10 pm   #85
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Default Re: FM to AM Converter?

Inspired by the recent renaissance of this thread I decided to have a go, I had ordered the PIC from Guy ages ago. I have just finished populating a PCB I knocked up yesterday.

Current drawn is 23mA @ 3,1V, I can scope the data pins (2 and 3) from the PIC and have pulses when the button is pressed. The oscillator is around 860kHz, adjustable by about +- 10kHz, but I have no AM audio output. I have removed the RDA module and will try another one as it is a bit fiddly to fit and I may well have damaged it in the process.

What level of audio should I expect from the output (Pin 7) of the RDA module?
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Old 1st Aug 2020, 1:31 pm   #86
60 oldjohn
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Default Re: FM to AM Converter?

Hi Eddie, a bit late now but you could have used pin 8 instead of pin 7, I dare say a blob of solder to short the two pads should have worked especially if connection to 7 was damaged.

John.
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Old 1st Aug 2020, 2:15 pm   #87
__i4cy__
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Default Re: FM to AM Converter?

Quote:
Originally Posted by eddie_ce View Post
What level of audio should I expect from the output (Pin 7) of the RDA module?
The output is headphone level, so around 300mV p-p.

As the firmware sets the RDA5807M module to mono, you could indeed try using pin-8 instead of pin-7 as John has suggested. But I wouldn’t advise shorting these two pins together.
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