22-07-2018, 04:23 PM
I have now got the TRF radio I was working on to a state worth a mention. My intention at some point is to build a working superhet receiver and much of what I have done is with this aim in mind. That is one reason why I have used multiple stages giving (loosely) a superhet appearance / partial design. As such this design is more for learning and test purposes rather than a radio for general use, but it does work to the extent that a good selection of stations can be received with quality good enough for general listening (and does include my favourite Radio Caroline 
). It is not my intention to make major changes now it is somewhat complete, but to use it and see how it goes.
The photos show some early development, predominantly with different detector circuits (diode and infinite impedance). The infinite impedance detector was found to be better for signal strength so I proceeded with it. Shown is the block diagram of the final circuit. It will be seen that there is effectively two RF gain controls as well as volume. The first RF amp (EF183) I have provided with a screening can, although, I have not found in practice, that this makes any difference. The other two RF amps do not have cans although I could probably make similar in the way I do for the rod pentodes if required. The tuning capacitor has 3 sections, one for each RF amplifier. At this point I have just used a standard numbered knob for operation, although I will maybe look more closely at this in the future. The audio stage has two valves, the output being single ended. The audio output stage was predominantly copied from an internet design for this type of valve, although using the RS Components supplied output transformer that I had to hand.
In operation I find the radio very stable. It is easy to overdo things with RF gain and the two controls do effectively inter-react quite a bit making things tricky at times. I have read a number of times in the past that a TRF set can be tricky to use, which I am finding, but, as I gain experience, it is getting easier. Without an aerial there is some reception, but I have found that an external aerial improves things greatly, and is needed for meaningful use. I have used my frame aerial. This is tuned, and although is the best input I have found, particularly for selectivity, it is easy to overload the input so complicating control. Input is via PL259 / SO239 connector, principally because that I use with my comms receiver. Another thing I have found useful, particularly when using a wire aerial, is an earth connection. The earth I use is not ideal as I am upstairs. It consists of a 14SWG copper wire strapped to a water pipe in the airing cupboard and across to my room under the carpet on the landing. I have found it of some use in the past, but seems to work wonders with this radio as regards noise. Selectivity in general seems reasonable making the geared drive on the tuning capacitor fairly essential. The audio stage is a little lacking, and could do with more gain, although is sufficient for the small room use in which I use it.
Tracy
). It is not my intention to make major changes now it is somewhat complete, but to use it and see how it goes.The photos show some early development, predominantly with different detector circuits (diode and infinite impedance). The infinite impedance detector was found to be better for signal strength so I proceeded with it. Shown is the block diagram of the final circuit. It will be seen that there is effectively two RF gain controls as well as volume. The first RF amp (EF183) I have provided with a screening can, although, I have not found in practice, that this makes any difference. The other two RF amps do not have cans although I could probably make similar in the way I do for the rod pentodes if required. The tuning capacitor has 3 sections, one for each RF amplifier. At this point I have just used a standard numbered knob for operation, although I will maybe look more closely at this in the future. The audio stage has two valves, the output being single ended. The audio output stage was predominantly copied from an internet design for this type of valve, although using the RS Components supplied output transformer that I had to hand.
In operation I find the radio very stable. It is easy to overdo things with RF gain and the two controls do effectively inter-react quite a bit making things tricky at times. I have read a number of times in the past that a TRF set can be tricky to use, which I am finding, but, as I gain experience, it is getting easier. Without an aerial there is some reception, but I have found that an external aerial improves things greatly, and is needed for meaningful use. I have used my frame aerial. This is tuned, and although is the best input I have found, particularly for selectivity, it is easy to overload the input so complicating control. Input is via PL259 / SO239 connector, principally because that I use with my comms receiver. Another thing I have found useful, particularly when using a wire aerial, is an earth connection. The earth I use is not ideal as I am upstairs. It consists of a 14SWG copper wire strapped to a water pipe in the airing cupboard and across to my room under the carpet on the landing. I have found it of some use in the past, but seems to work wonders with this radio as regards noise. Selectivity in general seems reasonable making the geared drive on the tuning capacitor fairly essential. The audio stage is a little lacking, and could do with more gain, although is sufficient for the small room use in which I use it.
Tracy
