Broadcast band receiver mod is on hold

I kept the idea of making the SSB receiver by modifying a broadcast band receiver on hold for now.  Initial testing shows unacceptable levels of noise from the radio circuit and the frequency instability in the built in tuner. The tuner jumps few hundred kHz per second. To make anything useful first of all the tuner must be re wired to some crystal based or PLL based tuner. I will try to do the tuner modification first when I manage some time.

Design Of A Code practice OSC

I have just started to learn CW and realized that a code practice OSCILLATOR is very useful. Using simple 555 astable oscillator was initial idea but it generates square wave and not pleasant to hear. I took help from my ham friend VU3BMT and he suggested a modified version of twin T oscillator originally introduced at k4icy CPO and more information at Twin-T Audio Oscillator, here and here. I build the version modified by VU3BMT and here is the drawing from his notebook

This design worked remarkably nice. Clean Sine wave from Oscillator in the first power up attempt, but since I did not have the transistors 2N4403 2N4401 and 2N7000 so I used 2N3904 2N3906 and BS170. Replacing transistor did not seem to alter circuit performance. My circuit looks like below

 Output from oscillator only without final power amplifier

Construction in Manhattan style

Test of frequency and gain of power amplifier

Relay driver using a vacuum tube

This project uses a vacuum tube in a relay driver circuit. Actually the tube makes the circuit complex and a relay driver can be many times simpler than that, but my target was to use the vacuum tube and to demonstrate that a vacuum tube can be used in low voltage circuit and can be used to drive a relay, though the presence of vacuum tube makes unnecessarily complex. Main moto of this experiment to have fun. Relay is energized when S2 is turned on. The circuit diagram is taken directly from my notebook page so it is not properly edited. I could not find some time to make proper nice looking  drawing in Kicad.This circuit was tested in a breadboard.

RSSI device for CXA1019S Single chip radio

The RSSI device is built around the famous LED bargraph driver IC LM3914. The radio IC CXA1019S when run from 3V supply generates a variable voltage of 0.23V to 3V in its METER pin (pin 20). In this LM3914 circuit R2 and R1 was so chosen that the all the LED are OFF when METER pin at 0.23v and all LED are ON when METER pin at 3V. Now this design is functional, but it has a drawback, this I found out later when the circuit was actually constructed. The radio IC generates 3V in its METER pin when there is no received signal and 0.23V when there is full receive signal and anything in between 0.23V-3V when signal is fluctuating. That means the relation between received signal strength and METER pin voltage is inverse. Due to this the RSSI indicator shows all LED ON when there is no signal and all LED OFF when there is strong signal. This is kind of weird, but I got used to this. Another problem with this circuit is that each LED consumes around 5mA of current, when all LED glowing the total current of RSSI circuit exceeds 50mA. For an RSSI indicator the power consumption of 50mA is too much. The workaround is to use a series resistor to each LED to reduce the LED current to 1mA per LED. This will make max power consumption to approx 10mA peak. Final value of R1=12K and R2=20K linear POT.

This is my first post,

I am working on a old SW radio receiver. Plan is to make it capable to receive CW and SSB. This original RF board includes a Sony make CXA1019S IC that is the heart of this radio. I have plan to add a input preselector, RSSI device, BFO, Digital Frequency display, and completely re-design systems for LO and RF Frontend. This is just a plan. So far I have done the Digital frequency display and the RSSI device. Detailed construction of these 2 will be posted here soon.