Short wave MC1496 Hybrid pantry TX

[murphyv310]

Hi.

I've been trying to make a good quality pantry transmitter to feed my radio sets. I had set on using short wave and tried many approaches, all worked well enough but some distorted at 70% modulation or would FM the carrier.
When looking for an IC for another member here I found a drawer full of the ubiquitous MC1496 is P and N guise. I look at the datasheet I decided to have a go with the chip and a 15.060Mhz crystal. After many attempts using veroboard in the past I was reluctant to try it as I've been told in the past it's very lossy at RF.
A new LCR meter that arrived a few days ago was used to test the capacitance between adjacent tracks which only read between 2 & 5pf so I thought I'd give it a go. I made up an oscillator first using the crystal and a 2N2222A. The result was good with 500mv output from the 15.060Mhz Xtal. Next was to make up the modulator. The layout follows the circuit loosely and again I was rewarded with good results, some changes were made to the datasheet circuit, I replaced the gain setting resistor between pins 2 & 3 with a 390 ohm for more gain, I also added an RF transformer using toroid construction as loading pin 6 or 12 produced distortion. Results at this stage were very good. Adding any more than a metre length of wire for an antenna gradually brought in distortion and range was not good, in fact directly feeding a radio wasn't great either. So obviously an RF buffer was called for, initially I tried a bipolar buffer in different configurations, they worked but the waveform had deteriorated, things improved using a FET in the first stage but It still wasn't as good as I hoped. Next I tried a 1J18B which was a total cure. The only disadvantage of this approach is you need a high voltage source for the valve. The results are first class and I have to say I'm really pleased with the way this works. Signal level with a few feet of wire is good with range around 15 meters It can just be heard 30 meters away on a sensitive radio but its very weak, reducing the HT to the Rod Pentode reduces range if needs be.
On my circuit C1 is 270pf, C2, 100pf, C3 27pf, C4 22pf, Cx 56 pf at 15mhz but as much as 22nf on long wave.

Feel free anyone to use this circuit or share but I've already had my circuits appear elsewhere and whoever has done this has accredited the design to themselves which is rather annoying, if sharing please ask.

https://youtu.be/e2-fON-QTYQ

Thread also on GVR

[Graham G3ZVT]

I use a pair of the similar MC1374 chips in my Band I TV modulator. The AM audio sounds more like FM broadcast quality, and the sound oscillator is free running at 48.25MHz

On my to-do list is to try the same circuit tuned to medium-wave.

[Al (astral highway)]

Nice one, Trev. Good skills working this on veroboard and a nice layout and narrative on YouTube. I like the use of the rod pentode as your buffer and it’s all nice an low profile.

Cheers