FM to AM converter problem.

[Dick Glennon]

Hi again, I have built the kit from Guy but alas it does not work. It is flattening the batteries in about an hour which are getting warm. Any thoughts on why this is happening would be very much appreciated. Thanks again, Dick.

[David G4EBT]

It sounds as though you have a short somewhere. If it was working correctly (which as you say it isn't), it would only draw a few mA, and the batteries would give many hours of use. The most challenging part of construction is the RDA5807M FM receiver module as it's really an SMD component and not easy to solder.

I'd suggest that you look very closely at all of your soldered joints using a magnifying glass and reflow them as necessary.

The kit with the PCB layout and circuit is here:

https://www.i4cy.com/m0oox/fm2am/

You might wish to seek Guy's advice.

Good luck with it Dick.

[Lloyd 1985]

Hi,

I don't know the device in question, but first things I'd check would be that the battery connections are the right way round, then check for shorts or low resistance across the battery supply, also check any electrolytic caps are in the right way/ not shorted. Also look for solder bridges.

Regards,
Lloyd

Crossed posts! Will look at the link above

[Terry_VK5TM]

I'd also be checking that U1 & U2 are oriented correctly.

For either of those in a socket, try removing them and seeing if the batteries still get warm when powered up.

[David G4EBT]

Originally Posted by Terry_VK5TM

I'd also be checking that U1 & U2 are oriented correctly.

For either of those in a socket, try removing them and seeing if the batteries still get warm when powered up.

U1 is a pre-programmed PIC from the designer - Guy Fernando i4cy and is fitted in an eight pin DIL socket. If damaged, (which it certainly would be if fitted the wrong way round), Guy, (who is a forum member but hasn't visited since August 2021), can supply a replacement for £12.95.

U2 is an RDA5807A FM module - a surface mount device which has solder pads only on the underside. It can't be fitted into a socket as it has no pins and there are no suitable sockets. The solder pads are closer together than a conventional DIL IC. (I'd estimate 2.125mm rather than 2.5mm).

It's a remarkable little module - an FM stereo radio chip that supports worldwide FM bands from 50MHz to 115MHz. There's no doubting that mounting it is the trickiest part of the project, but the saving grace is that if damaged, the current price of an RDA5807A on eBay is £1.19 + £1.20 post. Hence, if it's damaged, it's no drama to replace it.

The underside of the module with the solder pads visible can be seen in pic1 below.

The instructions with the kit give two options for mounting:

1) Solder it like any other SMD component to the pads on the PCB.

To do that is a one-way street - once soldered, it will be next to impossible to remove without damaging it. The edges of the pads are very close together. A great care is needed when soldering to avoid bridging them, and you can't see under the module to see if any contacts have been bridged. You need a fine tipped soldering iron, a good eye and a steady hand.

2) Solder fine wires to the ten pads on the module, and thread the wires through eyelets in the PCB and solder the wires so that if need be, the module can be unsoldered.

The converter originally featured in Summer 2017 BVWS Bulletin, built on stripboard, the layout of which is here:

https://www.i4cy.com/m0oox/fm2am/

I'm not keen on stripboard so I devised a PCB and soon discovered that the only easy way to mount the module was upside down beneath the PCB on Veropins, so the solder pads are visible.

See Pic 2.

Pic 3 shows the top of the PCB I devised.
Pic 4 is the underside of the PCB with the FM module mounted upside down with the solder pads visible.

I guess I made twenty or so at the time, which all worked straight off.

It's basically, and FM/AM 'pantry transmitter' which needs no connection to a radio, so is ideal for live chassis radios. In my location it picks up all the stations that can be heard on an FM portable radio.

There was a lengthy thread back in 2017 which ran to 2020:

https://www.vintage-radio.net/forum/...=137154&page=3

[agardiner]

I have built a few of these and had the same problem. It has always been the FM module which has failed. As David has said, it is very difficult to solder even if you are experienced in surface mount technology, and very easy to damage. It would seem that it is super sensitive to static until in place.

[Dick Glennon]

Hi again, the fm module had already been soldered in by Guy so I did not go near it at all. Hope this may help. Dick.

[David G4EBT]

Hi Dick,

It might be worth disconnecting the positive battery connection and temporarily inserting your multi-meter on the mA range in series to check what current is being drawn. Then remove the PIC from its socket and check the current again. If the current drawn is high with the PIC in place, and just a few mA with it removed, the PIC has been damaged.

If the current is still high with the PIC removed, there's a short somewhere, most likely a solder bridge.

Really, it can only be the PIC, the FM module, or a solder bridge that's causing the excess current.

Has it ever worked, or is it faulty from the outset?

Overall, in a working converter it should only draw a few mA. (It's programmed to switch off after 2 hours use in case its inadvertently left on).

Circuit attached.

All the best with it.