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Posted by: Adam/BD6CR Posted On: 09/30/01 Subject: My mods to pixie 2 and seeking for help to improve Message Posted: I made a PC board according to the web site of WE6W and got bad result at first. I can hear strong AF tones without connecting the antenna and the current consumption is over 150mA (really big for a 9V battery). I do the following steps to eliminate the strong AF tones. 1) connect a 0.01uf in parallel with andio out to ensure that LM386 will not oscillate by itself. 2) connect another 0.01uf in parallel with 10uf power filtering capacitor of LM386 to void RF injection to LM386. 3) connect the third 0.01uf in parallel with power supply input of the board to lower the RF impedence of the battery. The result proved to be effective but I still got very high current. I wonder there is some undesired RF oscillation, so I just cut the power supply line of Q1 and Q2 and insert a 100uH RFC // 120 ohm. It worked and I got a comfortable 100mW output when using 9V power supply, and current is about 40mA. My Q1 and Q2 are not 2N3904 for they are not easy to buy, so I replaced Q1 with 2SC9018 which has a fT of about 800MHz and a hFE of about 120 and replaced Q2 with 2SC8050 which has a fT of about 300MHz and a hFE of about 200. I remember some guy introduced in his web site that 2N3904 are best candidate for the maximum output power. But why? Anybody explain a bit for me? And any other transistors are also good choices? For I really want to obtain over 200mW when using 9V. My another mod to pixie 2 is to add a sidetone circuit using a 5V dc buzzer. JA9MAT also adopt this method but I found in his circuit, the wire of the key will introduce noise when receiving. So, I use a switch to disconnect the key when receiving. I also encountered a problem that I can hardly bear the tx/rx switch clicks in my earphone and they also cause me sending wrong morse code. Sidetone is one way to solve but I still need another way of disconnecting earphone when transmitting. So, I combined the above two mods using a DPDT switch to do the job. Although I have to manually switch betwen TX and RX, I got both good results of TX and RX. Now, when I switch to RX, I can hear signal from my earphone and I can key the sidetone, but it will never transmit, so I can use it as a morse code practising tool. When I switch to TX, I can not hear anything from earphone and when keyed I can hear clear sidetone from buzzer and RF signal transmits to the antenna. Remember to connect a 100uf in parallel with the dc buzzer to get pure tone and the current limiting resistor may be needed to get the buzzer work nicely and securely. 73 de BD6CR, CHINA |
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