First I have to say that I can receive Navtex service in another way but it would be fun to make a dedicated receiver, so from the 500Khz to 4Mhz converter board I started to design an 490 and 518Khz receiver. First tests were with an low frequency oscillator near the desired frequencies ense a direct conversion receiver. The first idea was to divide a higher frequency cristal to get the oscilator but after some tests there was to much noise for pratical use.
I put my brains to a good use and came up with a nice idea using junk box cristals. If I use a 4.200Mhz cristal mixed with an 3.686 I would get 514Mhz that is 4Khz from one of navtex frequencies... 518Khz.
Problem is I get signal inversion and the decoder I am using (JavaNavtex) has no provision for that. Another issue is that I am limited to only one frequency.
Decided to make a vfo around 3168Khz (3686-518).
After some trial and error on the coil I got to the frequency... powered on the computer and there was a transmission going on....
There is one NE602 working as an audio preamplifier, you just use the input and output connecting the oscilator pin 6 to vcc. It's more expensive than using an 741 or TL series but thak's less components around (just 2 input caps if you connect from another NE602)
I managed to copy some text but the oscilator was still drifiting so had to move constantly the indicator, anyhow I got something..... now the problem is to get a stable vfo so I can let the receiver work stand alone. I am thinking in some form of pll but that adds some complexity. To be seen.