03-11-2020, 06:16 PM
Hi.
I made up a frequency changer today with a single 1J42A. The RF was derived from a ferrite rod with a medium wave coil and I used the tuning winding fed directly to one grid of the 1J24B, initial testing of this part was done with the anode and G2 fed from HT with their own respective load resistors, the G2 was decoupled with a 1nf. Good signals were seen on the anode. Next was to make up a local oscillator. I've been told in the past that using the second grid of the 1J42A won't work properly in this type of circuit. Me being me decided to try it and see what would transpire. A suitable coil was found in a couple of boxes of oscillator coils I got from Ed Dinning. The wiring was simple with one winding of the coil between G2 and HT and its secondary to the second G2 via a 100pf cap and the earthy end grounded. At this point I was using a resistor as anode load. Immediately on applying power the oscillator ran.
So next I cobbled in a RS midget IF transformer. After some rough alignment the RF carrier was seen on the scope of my local on the output of the IF transformer, also the two adjacent stations were seen when tuned, tracking though is bad at the moment and this needs some work. So we can see the 1J42A will work as a frequency changer. Next was to try it into the TA7642 and I was able to drive a set of headphones from it. Tuning is pin sharp and obviously there is no bleed between the local and the next closest station.
Next is to get the tracking right an then transfer this to a more permanent construction method than it all over the bench.
I'll post a diagram of the FC later on.
I made up a frequency changer today with a single 1J42A. The RF was derived from a ferrite rod with a medium wave coil and I used the tuning winding fed directly to one grid of the 1J24B, initial testing of this part was done with the anode and G2 fed from HT with their own respective load resistors, the G2 was decoupled with a 1nf. Good signals were seen on the anode. Next was to make up a local oscillator. I've been told in the past that using the second grid of the 1J42A won't work properly in this type of circuit. Me being me decided to try it and see what would transpire. A suitable coil was found in a couple of boxes of oscillator coils I got from Ed Dinning. The wiring was simple with one winding of the coil between G2 and HT and its secondary to the second G2 via a 100pf cap and the earthy end grounded. At this point I was using a resistor as anode load. Immediately on applying power the oscillator ran.
So next I cobbled in a RS midget IF transformer. After some rough alignment the RF carrier was seen on the scope of my local on the output of the IF transformer, also the two adjacent stations were seen when tuned, tracking though is bad at the moment and this needs some work. So we can see the 1J42A will work as a frequency changer. Next was to try it into the TA7642 and I was able to drive a set of headphones from it. Tuning is pin sharp and obviously there is no bleed between the local and the next closest station.
Next is to get the tracking right an then transfer this to a more permanent construction method than it all over the bench.
I'll post a diagram of the FC later on.
