07-20-2013, 02:22 PM
Hey All,
Turns out the “mystery” cap is indeed a 3000 pf. I acquired a cap meter and it measured 3003 pf. But I installed a new one anyway since I ordered one. Unfortunately it still has a bad oscillation – motorboating really.
So I thought maybe since the B+ was 20v higher than the schematic, it might be the problem. So I put 470 ohms in series with the field coil and dropped B+ to within a couple volts of the schematic reference. Did me absolutely no good.
So I thought maybe the 2 volts pp was too much ripple (really only 2%) so I added filtering to bring it down to less than 1 volt pp. Did me absolutely no good.
So I was poking filter caps everywhere in the circuit, but places it stopped the oscillation either killed the IF or the audio. Until I poked a vacant pin on the 2nd IF toob. Pin 5 had no connection, but just touching it with a screwdriver killed the oscillation. Looked in the toob manual and pin 5 on the 7B7 is labled IS – not NC. Looked up IS and found that it is an Internal Shield. So I just connected pin 5 to B- and presto – motorboating is gone. And I had already added a toob shield to that toob.
Did the alignment and it now works – on local stations. Even the shortwave works.
Turns out the “mystery” cap is indeed a 3000 pf. I acquired a cap meter and it measured 3003 pf. But I installed a new one anyway since I ordered one. Unfortunately it still has a bad oscillation – motorboating really.
So I thought maybe since the B+ was 20v higher than the schematic, it might be the problem. So I put 470 ohms in series with the field coil and dropped B+ to within a couple volts of the schematic reference. Did me absolutely no good.
So I thought maybe the 2 volts pp was too much ripple (really only 2%) so I added filtering to bring it down to less than 1 volt pp. Did me absolutely no good.
So I was poking filter caps everywhere in the circuit, but places it stopped the oscillation either killed the IF or the audio. Until I poked a vacant pin on the 2nd IF toob. Pin 5 had no connection, but just touching it with a screwdriver killed the oscillation. Looked in the toob manual and pin 5 on the 7B7 is labled IS – not NC. Looked up IS and found that it is an Internal Shield. So I just connected pin 5 to B- and presto – motorboating is gone. And I had already added a toob shield to that toob.
Did the alignment and it now works – on local stations. Even the shortwave works.
