Not a very sucessful weekend!

Over the weekend I pulled out my BITX20 and started looking back over its existing problems.

These include the annoying screech when the PTT on the speaker/microphone is pressed and the unlocking of the PA3CNO huff and puff every time PTT is pressed.

For the screech problem I tried adding a FET to switch in and out the microphone and the receive audio amplifier but this didn’t work. I tried adding a diode/resistor combination to bias the receive audio amp off when the TX12 was present but that didn’t work either.

Everything I tried seemed to make no difference as the screech is happening between when the PTT is pressed and when the RX12 turns off. The RX12 and TX12 are switched by a relay on the linear amplifier board and both are filtered by electrolytic capacitors.

In desperation I added a DPDT relay activated by the microphone PTT button and used one half to disconnect the speaker from the receive amplifier as soon as the PTT is pressed. The second half is used to then switch the RX12 TX12 relay on the linear board.

By doing this the speaker is disconnected before the TX12 comes on and the problem has now gone away. I now have a small click when the PTT is released but I can live with that.

The unlocking of the huff and puff was not helped my the relay modification. I added some 100 or 220 uF electrolytic capacitors around the power supply but these made little difference.

My next step was to add a 78L08 voltage regulator in place of the Zener regulator. I can’t buy 78L08 regulators readily in Australia so I used a 78L05 with a voltage divider network to boast the output to 9 volts.

Unfortunately I soldered the regulator in backwards and it didn’t regulate at all. While trying to correct this I had mistakenly left the radio powered and the 78L05 was destroyed. A pop, a red glow and some smoke. This is the first smoke from the BITX20. I don’t have any more 78L05 so it will have to wait.

Last night I went out and drove up to the Yarra Ranges National Park to try and add another park to my Keith Roget Memorial National Parks Award effort with my FT7 and 7Ah battery. I strung a 20m half wave dipole in the trees and called for about 20 minutes with no success. As I was short of time I had to pack up and head back without the much needed QSO.

Next time I will allow more time and try to get a better position. Perhaps the lookout tower at Mt Donna Buang.


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