11-11-2013, 10:05 PM
Well, tonight I tried aligning the green dude on the table and I realized that 1) it does not help whining, AC noise and proper oscilaltion failures where I very quickly fall into the whining and distortion barely having touched the trimmers.
I also noticed that AC noise gets reduced quite a bit when I touch the chassis.
Then I remembered that I am dealing with a separate RF common and chassis, and there should be a cap linking them together for AC. I looked at the sch, there was that cap there, 0.2uF. Then I measured the capacitance (there are no other paths there so it should measure exactly as this cap's capacitance) and instead of expected 0.2uF I had 0.7nF or so, that is - stray.
I started looking for that cap where the assy diagram shows it - it was not there.
So, I connected a 0.22uF cap across and then I saw the noise going down and I was able to align the set.
The sensitivity went up immensely, and now I have my local station received as it should be, with just with a pigtail for the antenna, with volume loud and clear and enough to rip the cone out of the speaker if I let it.
I also checked the SW band and it is working fine, also with just a pigtail for the antenna.
It is playing now quite nicesly as I am writing this.
There is that residual AC noise, but I could see someone soldered an extra piece of wire to lengthen the grid cap wire for 6Q7, and this is the shielded wire so a significant piece of it is no longer shielded, and when I move my hand close it picks up more noise, so I think I might use a piece of a braided shielded wire instead of what is there now.
All in all it is working. I might try to realign the band so the pointer shows exactly what it should....maybe. It is close enough.
I think I might clean both treble and volume pots, there is some scratchies when I rotate them.
I thought of cleaning the tube panels and the band switch but the switch operates quietly and there seems to be all fine with contacts in the tubes.
They are playing "Classic gas"...sounds nice.
I also noticed that AC noise gets reduced quite a bit when I touch the chassis.
Then I remembered that I am dealing with a separate RF common and chassis, and there should be a cap linking them together for AC. I looked at the sch, there was that cap there, 0.2uF. Then I measured the capacitance (there are no other paths there so it should measure exactly as this cap's capacitance) and instead of expected 0.2uF I had 0.7nF or so, that is - stray.
I started looking for that cap where the assy diagram shows it - it was not there.
So, I connected a 0.22uF cap across and then I saw the noise going down and I was able to align the set.
The sensitivity went up immensely, and now I have my local station received as it should be, with just with a pigtail for the antenna, with volume loud and clear and enough to rip the cone out of the speaker if I let it.
I also checked the SW band and it is working fine, also with just a pigtail for the antenna.
It is playing now quite nicesly as I am writing this.
There is that residual AC noise, but I could see someone soldered an extra piece of wire to lengthen the grid cap wire for 6Q7, and this is the shielded wire so a significant piece of it is no longer shielded, and when I move my hand close it picks up more noise, so I think I might use a piece of a braided shielded wire instead of what is there now.
All in all it is working. I might try to realign the band so the pointer shows exactly what it should....maybe. It is close enough.
I think I might clean both treble and volume pots, there is some scratchies when I rotate them.
I thought of cleaning the tube panels and the band switch but the switch operates quietly and there seems to be all fine with contacts in the tubes.
They are playing "Classic gas"...sounds nice.
