08-10-2008, 04:53 PM
Guys, thanks for your help on this.
Steve, I checked the power transformer secondary for shorts to ground. At first it looked like one leg was shorted to ground, but I removed the 7Y4 rectifier and the short opened up. I checked the 7Y4. Nothing is shorted to ground, but it looks like pins 1, 6, and 7 are shorted together. Pin 1 goes to pin 8 through the filament of course, and pin 8 is grounded through the socket wiring. In my other 7Y4 all the pins are open. With that tube installed, there isn't any apparent short of the high voltage winding.
I need to replace the burnt resistors and try again with the "good" 7Y4. By the way, I checked all the tubes and found another shorted one. One of the XXL/7A4 had a 70K ohm short between pins 2 and 6.
Planigan, you are right about the wiring on the positive side of resistor #78 (between 78 and 77). I wasn't clear that I meant the transformer side of the resistor biases the 7B5 through the 470K resistor #69. I checked that wiring and it looks good. The center tap, resistor 78, and the 470K #69 are connected to pin 5 of the 7B5 socket, which is an NC. On your recommendation I checked the other side of resistor #78 and that wiring looks OK too. Ditto the other side of 77, and that is also OK, as far as I can tell.
I'm going to try again with the 1.8K resistor substituting for the field coil, just in case. How much B+ current should I expect through that coil? Less than 100ma?
Steve, I checked the power transformer secondary for shorts to ground. At first it looked like one leg was shorted to ground, but I removed the 7Y4 rectifier and the short opened up. I checked the 7Y4. Nothing is shorted to ground, but it looks like pins 1, 6, and 7 are shorted together. Pin 1 goes to pin 8 through the filament of course, and pin 8 is grounded through the socket wiring. In my other 7Y4 all the pins are open. With that tube installed, there isn't any apparent short of the high voltage winding.
I need to replace the burnt resistors and try again with the "good" 7Y4. By the way, I checked all the tubes and found another shorted one. One of the XXL/7A4 had a 70K ohm short between pins 2 and 6.
Planigan, you are right about the wiring on the positive side of resistor #78 (between 78 and 77). I wasn't clear that I meant the transformer side of the resistor biases the 7B5 through the 470K resistor #69. I checked that wiring and it looks good. The center tap, resistor 78, and the 470K #69 are connected to pin 5 of the 7B5 socket, which is an NC. On your recommendation I checked the other side of resistor #78 and that wiring looks OK too. Ditto the other side of 77, and that is also OK, as far as I can tell.
I'm going to try again with the 1.8K resistor substituting for the field coil, just in case. How much B+ current should I expect through that coil? Less than 100ma?
John Honeycutt
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