Friday, October 08, 2010

VHF amp...and the transmiter schematic

I am now in the process of building another version of the 1W VHF amplifier for the FM transceiver, well, in fact, it's still the same version since I didn't managed to get 1W... YET!
Today I made some tests with a 2n2553 and a 2n2866.... it melted solder in one of the base resistors (never I had seen such a thing, normaly resistors burn), a 27 Ohm one, made with 3 different resistor.... but the power was: 1.6mW thats mili! So I still have to learn a little more on VHF amplification... 2 more 2n3553 went belly's always the emiter junction that breaks :)

Anyhow, Pedro - CT5JZX, today sent me a schematic he found for me to have a look and get some ideas! Funny, it's basically the same schematic I use on the transmitting part, that's normal, most parts of my schematic came from Miguel - PY2OHH site.

Here's the said schematic linked from PY2OHH site:

My schematic is only different in the following parts:

Tone encoder: I have a diferent design, but the connection place is the same, also I use the same IC.

VHF oscilator buffer (BFR91 with 100k to base): I have two, one for the PLL and the other for the output amp part. Both connect to the same place via DC decoupling caps.

Oscilator: I don't use the trimer for frequency adjust, I placed a fixed cap after some tests.

Mic amplifier: I don't use an electro mic, so don't need the polarization circuit.

PLL: The same

I am trying to make the final output with 3 2n3553 or 2 and one 2n3866 transistors but if no success I migh just copy Miguel design!

Anyhow, if you need an VHF FM transmiter, this is a design to follow, tested by least!

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