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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.

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

Hello there, after quite a long pause I write again on this blog to describe my recent experience of moving an old CB walkie-talkie to 29 MHz AM in the 10 meter band.
I had found a nice pair of Inno-Hit RT923 talkies for 15 euros, while visiting a junk dealer shop near home, looking for something cheap and unusual for my personal collection of useless things. They looked in a very good condition and I though that probably there were still working too.
In the next few days, I sold the better-looking one on an on-line auction site for 10 euros and I decided to try moving the remaining talkie on 10 meters, 29 MHz AM. Here below a couple of photographs of the "star" of this post.

The Inno-Hit is a 3 channels, 2 watts, crystal-controlled handheld CB transceiver. The receiver is a single-conversion superetherodyne with 455 kHz IF. The tx frequency and the local oscillator frequency in the rx are controlled by two different crystals. The LO crystal is of course 455 kHz below the actual rx frequency.
The same rig was sold with several brands and some differencies (mainly cosmetic); for example Royce 1-402, Brilliant BT-1232, Finetone TNC-203, MacDonald CE-420, Universum BSG-3271 and maybe others. All those rigs came from the mid '70s and had a very robust steel case, with a small opening door on the front to access the battery pack. Some of them had external connectors for an external antenna, earphone, power supply or battery charger. Usually they were sold with only one channel actually equipped with crystals (for CB channel 11).

The astonishing successes of my good friend Franco I5KAP on 10 meters AM, with even very old and simple QRP rigs, inspired me.

A visit to Franco (who live 10 km or so from my QTH) convinced me to go ahead with my plan. Franco was very kind and helped me in deciding which modifications would have made sense to try.
First decision, was to leave the transceiver as much as possible in its original box and appearance. Given the very limited space inside the case, the only sensible modifications to do were to add external connectors for the antenna and power supply (or external battery) and of course to buy a new pair of crystals (for the rx and tx oscillators) for the 29 MHz AM band.
The crystals were ordered to a factory here in the north of Italy (unfortunately, they costed about 25 euros each, including taxes and shipment). Then I bought for 2 euros each some schematic diagrams of the walkie-talkies I mentioned above (in particular, those which had the antenna and power supply connectors) and I proceeded with my work.
I identified the better points in the circuit were to connect the external antenna and power supply, I prepared the small removable plastic panel that was inserted on the right side of the case (like in photographs below) and quickly checked that all was OK, both mechanically and electrically.

Now it was simply a matter of setting a small measurement bench and do some tweaking on the tx path and the rx path to ensure that the talkie could work properly at the new frequency. This will be the subject of next post. Thank you for reading.