Thoughts on this AM Transmitter

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shorty
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Re: Thoughts on this AM Transmitter

Post by shorty » Wed Feb 07, 2018 12:59 pm

I came across this vfo 1.8mhz 10w am transmitter online while trying to find a 1994 copy of practical wireless.

http://www.robkalmeijer.nl/techniek/ele ... index.html

My knowlwdge of am transmitters isn't great, it looks like it could be used in the medium wave band with a few adjustments, move the oscillator frequency down 200Khz to the top of the mw broadcast band, bypass the 741 mic preamp with line audio into the tda2030 audio amp, the mosquito also uses the tda2030, is this a standard am mw rig layout.
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yellowbeard
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Re: Thoughts on this AM Transmitter

Post by yellowbeard » Wed Feb 07, 2018 5:38 pm

What practical wireless are you looking for specifically? I probably have it here if it's from 1994... :whistle

shorty
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Re: Thoughts on this AM Transmitter

Post by shorty » Wed Feb 07, 2018 6:09 pm

@Yellowbeard, I don't know the month, it's the one with a low power crystal controlled 1.8Mhz transmitter/receiver, i built the transmitter section back in the day with a 1.602Mhz crystal, the oscillator was a bc548 followed by two bd139's.

I found the small transmitter the other day it as 1994 on the pcb, i started looking for my copy of practical wireless but it's mia, gone to the landfill in the sky probably. I'd be greatful if you have a copy of the article, thanks Yellowbeard.

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yellowbeard
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Re: Thoughts on this AM Transmitter

Post by yellowbeard » Wed Feb 07, 2018 6:29 pm

PM sent...

shorty
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Re: Thoughts on this AM Transmitter

Post by shorty » Wed Feb 07, 2018 6:54 pm

Thanks yellowbeard, thats the one, i'll have a play to see if still works...THANKS

Albert H
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Re: Thoughts on this AM Transmitter

Post by Albert H » Thu Feb 08, 2018 1:05 am

That's the PW "Chatterbox". It's meant to deliver about 10 Watts on 160m amateur band.

There are several glaring omissions from the circuit:

There's no output lowpass filter. There will be plenty of harmonics!

The modulation transformer had to be hand wound on a ferrite former. This had a frequency response similar to an old telephone with a carbon microphone! It might be good enough for speech on the amateur band, but it'll sound horrible for music.

It uses a VFO. This might be OK for short amateur contacts, but stability won't be sufficient for broadcast use.

There's no filtering of the audio bandwidth. This probably wouldn't matter with the crappy modulation transformer, but will cause problems if you find a good one!

Bearing in mind the above criticisms:

Adding a lowpass filter to the output will just take two toroidal coils and some capacitors.

A mains transformer with a tapped secondary can be put to use for a pretty good modulation transformer. At this power level, I found a 40 VA toroidal with a centre-tapped secondary. I used one end of the secondary connected to the supply, the other end connected to the top of the output RFC, with the audio fed from the TDA2040 through a big capacitor (2200µF) to the centre tap. The primary side of the transformer fed a couple of back-to-back LEDs (through resistors). One of the LEDs was on the front panel as a crude level indicator, and the other illuminated a small LDR across the modulation level pot. This gave reasonable compression!

The VFO can be turned into a VCO with the addition of a couple of varicaps. The sample of the frequency can be taken from the output of the buffer stages, and fed to a simple CMOS synthesiser. I sometimes used a PLL02 (CB synthesiser chip) because I had loads of them, but I also used the 145106 PLL IC as well (http://f5ad.free.fr/Docs_ATV/MC145106.pdf) with great results.

I replaced the microphone preamp with a two stage, gyrator-based audio lowpass filter, using a couple of quad op-amps.
"Why is my rig humming?"
"Because it doesn't know the words!"
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