Pititico CW Transceiver Designed By PY2OHH

Play Video about DX Explorer - Improved Pititico CW Transceiver No AM Broadcast Interferences

If you enjoy the video be sure to SUBSCRIBE »

Get $5 of New User Free Credit using PCBWay. Full feature custom PCB prototype services, PCB assembly, SMD stencils and CNC / 3D Printing.

Pititico CW transceiver has a consumption in RX of 1.5mA and a power of about 700mW. With a few modifications and using other transistors, the power can get as high as 1W. Depending on the transistor used, you must keep an eye on it so it will not overheat. I also used 2N3904 transistors with good results. I prefer the TO18-3 ( metal can ) 2N2222A transistor as I can use it with a small heatsink.

Pititico Original Schematic by PY2OHH

THE SCHEMATIC

I found the original schematic designed by PY2OHH long time ago and I always wanted to try it. I built a few versions using various schematics I found online. The original version following the PY2OHH schematic, is in the video presented on the bottom of the article. I liked the original version as well, but I had a lot of AM broadcast band interferences, it needed a low pass filter and I found it really hard to adjust the offset frequency. So I ended up making a few modifications.

The one described here in the article ( Fig. 1 ) it’s close to the original schematic, but with little improvements. In the video I also had some LED’s and a buzzer that I removed later on. I wanted to reduce the power consumption. A big Thank you to Miguel ( PY2OHH ) for helping me understand the circuit and fix some of the issues. One day I hope to have you in my Logbook Miguel. Thanks for all the help.

Fig. 1 – Pititico CW Transceiver Schematic

SCHEMATIC DESCRIPTION

On RX the circuit works as a direct conversion receiver, where the 2N2222 acts as oscillator. The 10K resistor limits the circuit current to 1.5mA. The trimmer capacitor C3 will set your offset frequency. The trimmer capacitor C4, will help you adjust the TX frequency exactly on 7.030MHz. Otherwise the transceiver would transmit around 7.029Mhz. You can also replace them with a fixed value capacitor instead if you want.

In my case for example, having C3 – 33pF and C4 – 10 pF offered me a TX frequency of 7.030MHz and an offset frequency of around 800Hz. I find it a lot easier to use two trimmer capacitors instead and adjust them. It’s not a perfect solution and you may have to go back and forward adjusting them until you get it right. But is a simple and easy solution. This is also the reason why I kept it on a fixed frequency and I am not using a VXO.

In TX the circuit works as a low power oscillator, reaching 1W and currents of 100mA. The transistor supports this current well in CW, but do not keep the CW key activated too long as the transistor will burn out. Another problem is operating without antenna or dummy load, the transistor burns out right away.

The capacitor C7 should be between 120 to 150pF. I changed the value to 330pF increasing the power of the transceiver to 700mW. I also placed a modified Pi Network filter on the output that that improves the harmonics suppression. In the past and in the videos I was using a modified Pi Network filter that acted like a bandpass filter. It was a nice solution, but on transmit a bandpass filter is not a great solution. I tried at that time to remove the AM broadcast band interferences and that was the best I could do back then with the knowledge I had at the time. Now I am a lot happier with the results of the updated version.

To remove most of the AM broadcast band interferences I added C5 together with L2. L2 is important to separate the audio output from the oscillator. With ought L2 the TX frequency and the offset frequency would not work properly. So far it seems that I have very little interferences and I am happy with the result.

PITITICO CW TRANSCEIVER PCB BOARD

Fig. 2 – Pititico CW Transceiver PCB Board

I built my Pititico CW Transceiver in a small wooden box initially using the Manhattan style construction for the circuit. I also made a PCB design that you can use to make PCB boards using the toner transfer method. Unless you want to order the PCB boards from PCBWay as I do as it turns out really nice at the end and looks way better too.

BUILDING THE PITITICO CW TRANSCEIVER

To listen to the audio output, you should use a 300 Ohm phone speaker as Miguel ( PY2OHH ) recommended. It didn’t work for me, maybe I used the wrong speaker. Even better is a high gain audio amplifier. I’m using an LM386 amplifier set to the highest gain possible. If you do not want to use an external amplifier, I would recommend you to try building Pititico II, that has the audio amplifier included in the circuit. Is a lot like a Pixie transceiver, more or less.

Fig. 3 – Pititico CW Transceiver ( older version )

Fig. 4 – Pititico Wooden Enclosure

As you can tell, this is not exactly my prettiest built, but it works perfectly. I wanted to test it before designing the PCB board. I really loved the wooden enclosure I had and I wanted to keep it like this. In the video down below is an older version. It worked fine but I still had AM broadcast band interferences. On the new version I built, I also moved the crystal on the top panel so I can change the crystal for other ( 40m band ) frequencies. Now this transceiver belongs to my friend Andrei – YO6TJJ as I offered it to him as a gift. More like a nice memory between friends.

ADJUSTING THE LOW PASS FILTER

The inductor that I made for the bandpass filter was made on a T37-6 toroid. It has 16 turns of 0.35mm enameled copper wire. Calculated using toroids.info for the value of 1uH it should be 18 turns instead. For me 16 turns offered me the best power output and the filter performs well. To adjust the filter once you finished building the transceiver, simply connect the transceiver to a a power meter and spread or tighten the turns on the toroid for the highest output power. With a 2N2222 transistor with an hfe of about 240 the output power in my case is about 700mW. It depends on the transistor you are using and the amplification factor of the transistor.

Play Video about DX Explorer - Pititico QRP CW Transceiver Designed by PY2OHH

Initially the bandpass filter helped me get rid of most AM broadcast band interferences that I used to have in the older version I built in this video. Now in the latest design with the actual low pass filter the results are much better both with the AM broadcast band interferences and the harmonics suppression.

David ( DL1DN ) from QRP Lifestyle built a modified version for the 10m band, so feel free to do modifications for other bands as well. I hope you like this project as much as I do. Now all I have to do is to learn the Morse code and get on air using CW on this tiny QRP transceiver that I absolutely love. I just need to find the right telephone speaker to really use it as a one transistor transceiver. This should be fun indeed. I may return with an instructional video at some point using the latest design. Sometimes simple things are harder to adjust, especially for beginners.

73, YO6DXE

Check the latest EXCLUSIVE DISCOUNTS offered to the DX Explorer blog readers. Discount codes for various products and services offered by partners and collaborators.