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sabato 8 agosto 2015

Moving an old CB walkie-talkie to 29 MHz AM - Part 2

This is the second post of a short series about converting an old CB walkie-talkie for the 10 meter band (around 29 MHz AM). For those interested, previous post can be found at following link:

Moving an old CB walkie-talkie to 29 MHz AM - Part 1

In present post I will try to describe the simple sequence of operations I did to ensure that my old Inno-Hit RT923 handheld transceiver could work on 29 MHz properly.

First I prepared a small testbench with a 12V power supply, an oscilloscope, a frequency counter, a waveform generator. I built a simple 50 ohms dummy load by using two 100 ohms, 5 watts resistors connected in parallel.

By the current indicator of the power supply, I saw the current consumption was about 25 mA in rx mode, with no signal, raising to some 60 mA with a nicely loud signal at the loudspeaker.

The I started to tweak the tx path. First of all I checked the tx frequency by the frequency counter and I saw it was pretty close to the 29.020 MHz which is the nominal frequency of the tx crystal (the crystal in the local oscillator of the rx is 455 kHz lower). So I decided that there was no real need for adjusting the tx frequency.

Then I went to the oscilloscope and I had a look to the carrier waveform. It looked nicely clean and stable.

I applied a 1 kHz signal to the modulator and tried different resistor values in the ALC circuit (namely R29 and R48) to see if it was possible to raise the RF output power a bit. Shorting R29 proven to give a slight improvement, while changing R48 showed no significant effects, thus in the end I decided to leave it unchanged.

Then I moved to the adjustment of L4 ed L6 in the tx output filter (after the tx power stage). I carefully unscrewed the ferrite cores of both coils to get the maximum output level of the carrier on the oscilloscope, while monitoring the spectral contents of the output by a spectrum analyser.
Here below you can see the results of this step, before and after the operation.

As you can see, the output level of the unmodulated carrier raised from less than 14 dBm to more than 18dBm (on the 50 ohms dummy load). The level of harmonics did not get worst than it was initially.
Final check for the tx was to try to measure the actual modulated output power while whistling into the speaker-microphone. Here below you can see a screenshot taken from the oscilloscope during this test:

Apparently, it looks like there are no signs of overmodulation. Based on measured values (as reported at the bottom of picture above), the PEP power should be slightly above 2 watts, which is fully satisfactory to me.
The power consumption (on the power supply current indicator) was up to 230 mA at 12 VDC (about 2.76 watts). Final transistor T8 was normally hot at peak power, not enough to worry about its safety in normal conditions.

Now a few notes about the rx path: I applied a 29.020 MHz signal to the antenna, modulated by a sine wave at 1 kHz, 100% modulation and adjusted the coils and IF transformers along the rx path for the best AF signal level (measured with the oscilloscope, in the AF section of the receiver).
From the antenna towards the AF amplifier, I adjusted L2 (black core, antenna amplifier), L3 (white core, RF mixer) then T1, T2 and T3 (yellow, blue and pink, respectively 1st, 2nd and 3rd IF transformer).
All went pretty OK and now I'm ready to test my renewed talkie on the air. I'll let you know about my results. See you soon.

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