06-30-2019, 11:37 PM
Gentlemen:
Thanks to each of you for taking time to reply; much appreciated!
Ron, I agree. The 75 does not influence oscillator operation. I shall re-solder its pins and grid cap when I have time in next day or so. For a period in my working life, I was involved in making picture tubes so am familiar with how pins and caps are physically attached and soldered.
This set does have an AGC, but it does not connect to the oscillator. The 75's dual diodes rectify the signal in the secondary of T-23. A portion of that is returned to the first grid's of both 44 (RF amp) and 39 (IF amp). Thus, if 75 were not working properly, it would for sure cause problems.
Also, I realized after submitting the first post, that I was not paying attention to the dial setting when measuring some DC voltages. AND I did not have the tube shields in place until the end. Six years ago, I recorded a DC voltage of about +15.7 V on top of the RC connected between (pin-2 of T-14) and ground. I then discovered that this voltage varies considerably (15.7 V -> 3.5 V) with both dial setting and presence of shields! That || RC (1500 pf -- 7,500 Ohm) has a time constant of 11 useconds. The oscillator frequency ranges from ~290 -> 1340 kHz (260 kHz IF). Over this range, the AC reactance of that RC ranges from (18 -j365) Ohms to (1 -j80) Ohms meaning it is for practical purposes, a capacitor.
The AC voltage between oscillator cathode and ground would determine the magnitude of the positive feedback, while the DC voltage between cathode and ground would determine the bias point and gain.
I started with 7500, then tried 4000. Will revisit this with smaller incremental changes.
That said, first thing will be to re-solder #75 and re-attach and re-solder loose connection on GC of #77.
Thanks again for your suggestions!!
John
Thanks to each of you for taking time to reply; much appreciated!
Ron, I agree. The 75 does not influence oscillator operation. I shall re-solder its pins and grid cap when I have time in next day or so. For a period in my working life, I was involved in making picture tubes so am familiar with how pins and caps are physically attached and soldered.
This set does have an AGC, but it does not connect to the oscillator. The 75's dual diodes rectify the signal in the secondary of T-23. A portion of that is returned to the first grid's of both 44 (RF amp) and 39 (IF amp). Thus, if 75 were not working properly, it would for sure cause problems.
Also, I realized after submitting the first post, that I was not paying attention to the dial setting when measuring some DC voltages. AND I did not have the tube shields in place until the end. Six years ago, I recorded a DC voltage of about +15.7 V on top of the RC connected between (pin-2 of T-14) and ground. I then discovered that this voltage varies considerably (15.7 V -> 3.5 V) with both dial setting and presence of shields! That || RC (1500 pf -- 7,500 Ohm) has a time constant of 11 useconds. The oscillator frequency ranges from ~290 -> 1340 kHz (260 kHz IF). Over this range, the AC reactance of that RC ranges from (18 -j365) Ohms to (1 -j80) Ohms meaning it is for practical purposes, a capacitor.
The AC voltage between oscillator cathode and ground would determine the magnitude of the positive feedback, while the DC voltage between cathode and ground would determine the bias point and gain.
I started with 7500, then tried 4000. Will revisit this with smaller incremental changes.
That said, first thing will be to re-solder #75 and re-attach and re-solder loose connection on GC of #77.
Thanks again for your suggestions!!
John
