UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - FM to AM converter

David G4EBT · 30th Nov 2018, 6:30 pm

Just to update this thread before it goes into hibernation, I tweaked the earlier layout slightly, have built an AM modulator PCB and have obtained one of the little Chinese FM modules of the type that Derek - 'Coopzone' used. The modulator works fine, though I had to change C4 & C5 to 220pF to lower the frequency slightly away from an interfering station. The modulator comes out on 950 kHz (315 Metres).

One slight peculiarity is that the sine wave output has a slight kink in it (crossover distortion?).

We're not aiming for hi-fi 'audiophoolery' perfection with something as basic as this, but I'd be interested in any thoughts that anyone might have as to the possible cause.

Pic 1 shows the modulator circuit, update 19 Nov.
Pic 2 shows the updated layout.
Pic 3 shows the built modulator PCB.
Pic 4 is the positive and negative artwork for the PCB if it's of any use to anyone.
Pic 5 shows the slight distortion of the sine wave output of the modulator board.

The little digital FM unit comes with no instruction as to how to use it, and some may argue that it's so simple that it ought not to require instructions. When I press 'scan', it brings up 'P1' scans, and when it finds a station, I assumed it would stop on that station, and maybe you then have to hold the 'scan' button down to store the station as programme #1. However, in a second or two, it simply moves to the next available station, then in another second or two, the next, and the next...so I've yet to discover how to store stations for each programme position. (I think it goers up to 30 programme positions, but of course on most channels there's no station - just 'white noise').

I assume that when I've worked out how to store stations, they can be selected on the remote using the the numerical buttons.

When I've got it working, I'll have to decide how to house it. (Surely not yet another comb-jointed box!).

Incidentally, the FM module has what appears to be a rudimentary antenna printed on the PCB.
There's also a wire to which to connect and external antenna. I guess a 40cm length of wire as with the original converter would be adequate.

Hope that's of interest.

30th Nov 2018, 6:30 pm	#45
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,763	Re: FM to AM converter - variation on a theme! Just to update this thread before it goes into hibernation, I tweaked the earlier layout slightly, have built an AM modulator PCB and have obtained one of the little Chinese FM modules of the type that Derek - 'Coopzone' used. The modulator works fine, though I had to change C4 & C5 to 220pF to lower the frequency slightly away from an interfering station. The modulator comes out on 950 kHz (315 Metres). One slight peculiarity is that the sine wave output has a slight kink in it (crossover distortion?). We're not aiming for hi-fi 'audiophoolery' perfection with something as basic as this, but I'd be interested in any thoughts that anyone might have as to the possible cause. Pic 1 shows the modulator circuit, update 19 Nov. Pic 2 shows the updated layout. Pic 3 shows the built modulator PCB. Pic 4 is the positive and negative artwork for the PCB if it's of any use to anyone. Pic 5 shows the slight distortion of the sine wave output of the modulator board. The little digital FM unit comes with no instruction as to how to use it, and some may argue that it's so simple that it ought not to require instructions. When I press 'scan', it brings up 'P1' scans, and when it finds a station, I assumed it would stop on that station, and maybe you then have to hold the 'scan' button down to store the station as programme #1. However, in a second or two, it simply moves to the next available station, then in another second or two, the next, and the next...so I've yet to discover how to store stations for each programme position. (I think it goers up to 30 programme positions, but of course on most channels there's no station - just 'white noise'). I assume that when I've worked out how to store stations, they can be selected on the remote using the the numerical buttons. When I've got it working, I'll have to decide how to house it. (Surely not yet another comb-jointed box!). Incidentally, the FM module has what appears to be a rudimentary antenna printed on the PCB. There's also a wire to which to connect and external antenna. I guess a 40cm length of wire as with the original converter would be adequate. Hope that's of interest. Attached Thumbnails __________________ David. BVWS Member. G-QRP Club member 1339.