Part 15 type AM transmitter

[Nige from Brum]

Hi all,

Does anyone know which transmitter this is? I acquired it as a swap for one of those dodgy ebay Chinese FM rigs.

It's sold as one of those part 15 compliant/ pantry transmitters. I thought it might be one of the ones from Vintage components like the Spitfire etc. But I dont think it is.
I've tried the PLL switch settings, using the data from Zozo's crane fly transmitter which I am also building at the moment, but to no avail. The chips look to have had the markings removed and the crystal doesn't appear to have a frequency on it.

I have it working on 1440 khz, but want to move it further up the band. I also need to work out what the other 2 banks of 4 dip switches are for. I'm guessing one is for matching the supplied 3 meter aerial, but what's the other one?

Any help or ideas gratefully received.

[Albert H]

Looking at the 8-way DIP switch, assuming the rightmost switch is 1, then 2, 4, 8, etc, you'll see that you have 32 and 128 selected for a total of 160, which, when multiplied by 9 kHz, gives 1440 kHz!

The IC next to the crystal will be a 4060, and the crystal will be either 4608 kHz or 9216 kHz - these frequencies were used in Baud rate generator circuits, so the crystals are cheap and plentiful.

The IC near to the 8-way DIP switch will probably be a 40103 pre-settable divider, and the one in the middle on the left is probably a 4046.

The 4-way DIP switch on the far right selects different coils feeding the aerial, to try to make the "aerial" match at various parts of the MW band.

The IC on the right is probably a 4053, used as a switched-mode modulator. The circuit is probably something like this:

Little AM Rig.gif

Yours looks like it has an extra transistor after the modulator to give it a little more transmit power.

I can't quite work out what the other 4-way DIP is for, but it looks like it might be a further part of the output matching, selecting different capacitors. My circuit uses a variable capacitor at the output. Trimmer capacitors are getting expensive and tweaking with a screwdriver is going to give iffy results, so they've probably made the capacitors switched instead.

[garada]

You have all the information in the link, this is a kit that was sold on Ebay, it is interesting, easy to build and with available components.

https://3v8hb.wordpress.com/2020/02/10/ ... khz-50-mw/

[Albert H]

The circuit you give above omits the modulation level monitor.

[garada]

The circuit you give above omits the modulation level monitor.

Correct, I would implement it taking advantage of the fact that the LM358 is double operational and the small number of components to add.

[Nige from Brum]

Albert and Gerada,

Thank you for the replies.

The circuit diagrams have proven useful as I was wondering why there was the extra set of dip switches.
I've had & built similar transmitters before, but non of them have had switching both in and out of the matching stage.

I still cant figure out the tuning for the PLL though. It doesn't seem to match the usual dip switch settings.

[garada]

Look at the quartz that your emitter mounts and according to its value, use one of these two frequency tables, it is assumed that it should coincide unless it mounts a different value of quartz in which case you would have to calculate.