Radiophile valve transmitter

[Semir_DE]

1575kHz and 1584kHz are compliant channel frequencies and transmissions in either of these channels would cause an unwelcome beat.

Hi Terry,

you are absolutely right. Using odd frequencies is not an option even under the current conditions. I have a distribution system here to feed off the air signals to my collection of old radios. This consists of a distribution amplifier that uses an LT 1227 video amplifier chip. The circuit has two inputs:

One is from a mini whip active antenna that I have placed in our garden a bit away from the house. This will pick up quite a few stations on LW, MW and SW at night time, many actually from the UK but also from Spain Italy and eastern Europe.

The other input is for modulators, this has a level adjustment possibility so I can mix in just the right amount of my AM TX signals with the off air signals. Since hardly any frequency is completely quiet I need to hit a channel frequency spot on in order to avoid annoying beats. I now use DDS generators with AD9833 driven by Arduinos that I had introduced here a while ago as frequency references. These are very precise and can generate any frequency I need.

Currently I am using 1080kc, 1260kc and 1440kc. The 1260kc one is still using a ceramic resonator but has not exhibited any significant drift since I set it up a  year ago. Since one of my favorite internet stations "KABL 960" broadcasts in the US on 960kc I originally had set up a TX here to re-broadcast this program on 960kc. This turned out to be  a problem at night when other stations on 963kc and 954kc started interfering with this non channel frequency causing strong beats. So I had to move that program to 1080kc...

The distribution system makes my own frequencies blend in very nicely with the rest of the received regular broadcasts. If it is of any interest here I would be happy to  open a new thread and introduce the circuit.  I have 3 AM TX circuits running here 24/7 in AM stereo. I had started out with AM Mono TXes but later upgraded them to AM stereo when I got some AM stereo receivers from the US. AM stereo has an audio bandwidth of 10kc and on a good tuner can sound almost like FM.

Two of the TXes redistribute DAB+ broadcasts the third is connected to a headless Raspberry Pi running "Volumio" this latter one can either play music from my own collection or be tuned to an internet radio station. Did I mention I especially like "KABL" from San Francisco  ...

cheers

Semir

[Semir_DE]

Apart from these two valve projects I also have built an AM TX sing a TBA520 PAL decoder chip as a balanced modulator and oscillator.

The TBA520 can also be used as a PAL encoder. Labgear made a colour bar generator that used it.

Yeah I know, those chips can be quite useful. In the 1980s I had built a PAL pattern generator using about 50 TTL chips.
 The sync generator in that project was the one published in Television magazine in the late 70s.

For the PAL encoder I used the successor of the TBA520 the TBA990. When driven with a low carrier signal level this worked very well. This was way before dedicated ICs like the Sony CX22017 had become available to me as a hobbyist...