09-26-2013, 11:48 AM
Just got this radio yesterday, been playing with it, replacing parts as needed. I've found several modifications that were made in the past, things like a .001 cap across the primary of the mixer transformer for the AM band, and another from the plate of the BFO to ground.. sorta defeated the purpose of having the BFO in there at all.
Now to what I need some help with: Despite my best efforts, I cannot get the antenna input stage to track above 1100 or so. It's good to that point, but gets farther off the farther I go up the dial. 1400 tunes at about 1550 (looking at the signal on my scope), and it won't catch 1500 at all, still too much capacitance. I've replaced the cap between chassis and B-, that didn't change a thing. Taking that .001 out of the mixer definately gave me better gain (and the ability to actually tune the mixer input!). Everything else looks OK, just can't get that last bit of tracking.
The other problem is that something is going into some sort of parasitic oscillation.. in the LO or mixer stage... I can't tell which (two separate tubes), but if I put my finger on the LO section of the tuner to load it, then let go, the parasitics go away for a while. Guess I could try replacing the 1R5 and see if that helps, but somehow I doubt it will. So, if anyone has any pointers, or has dealt with these problems on this radio, please let me know.
Now to what I need some help with: Despite my best efforts, I cannot get the antenna input stage to track above 1100 or so. It's good to that point, but gets farther off the farther I go up the dial. 1400 tunes at about 1550 (looking at the signal on my scope), and it won't catch 1500 at all, still too much capacitance. I've replaced the cap between chassis and B-, that didn't change a thing. Taking that .001 out of the mixer definately gave me better gain (and the ability to actually tune the mixer input!). Everything else looks OK, just can't get that last bit of tracking.
The other problem is that something is going into some sort of parasitic oscillation.. in the LO or mixer stage... I can't tell which (two separate tubes), but if I put my finger on the LO section of the tuner to load it, then let go, the parasitics go away for a while. Guess I could try replacing the 1R5 and see if that helps, but somehow I doubt it will. So, if anyone has any pointers, or has dealt with these problems on this radio, please let me know.
