04-04-2024, 10:13 PM
Hi Ralph,
A 22K resistor instead of the 20K resistor should be no issue, as the error is only 10% I am concerned that the resistance increases as the resistor heats up. The 9000 Ohm resistor, #57 drops voltage from raw B+ to create the plate voltages for the 6A8 and 6K7. Note Cap #9, the 16uF capacitor. This is the filter cap for the main plate for the 6A8, and, through resistors # 23,22 and 47, the triode plate of the 6Q7. Was this replaced? If not, do so. The drawings in the schematic show these resistors of the "dog bone" variety, These almost always are defective, even when they read OK under no load. Are the plate voltages for the 6A8 and 6K7 (Pin 3) within 20% of the documented voltage (220V for the 6A8, 270V for the 6K7)? Is the 6Q7 triode plate voltage near spec (175V)? Is the Osc Plate (Pin 6) of the 6A8 near spec (175V)? With power off, is there a low resistance (less than 2 Ohm) between the osc grid (Pin 5) of the 6A8 and chassis ground?
The screen voltages are low. If the 6Q7 and 6A8 Plate voltages are also low but the 6K7 plate is normal, then the voltage divider formed by resistors 23 and 22 is being loaded down. Change Resistor 23 even though it reads OK. if all that you have is a 22KOhm resistor, don't worry, use it. If the screen voltages (pin 4) for the 6A8 and 6K7 are still low, then pull 1 tube at a time and measure the screen voltage. replace the tube, then pull the other tube. If the voltage goes to normal with one tube in but not the other, then either the tube is shorted, gassy or something in the circuit is loading the tube down. If the voltage is low with either or no tube in place, then check cap #22A. If you did not replace it, do so.
When touching the 6Q7 cap there should be a hum. BTW, I apologize, I forgot to state that placement of the grid lead (called lead dress) is important, especially for the 6Q7, as the grid wire should be either shielded or run as close to the tube shield as possible. I believe that the 6Q7 in this set should have a shield around it, and the 6K7 also should have a shield.
Generally, when touching the grid cap of the 6A8 (the pentagrid convertor), you should receive stations. However, if in your basement with fluorescent, or even worse, LED lighting, your computer, smart TV, etc., you will usually get nothing but noise.
It is possible that the oscillator circuit of the convertor is not working. One way to tell this is to:
-Hold another radio tuned to a midband station in your area near this radio with the radio in question turned off.
-Confirm that you receive a station on the radio being used to test the radio in question.
-Power up the radio in question, set to the broadcast band. Slowly tune from the lowest to the highest frequency on the radio in question. If the radio playing the midband station starts squealing or the station disappears, then the local oscillator of the radio in question is at least oscillating. It may be off frequency but it is oscillating.
As mentioned on my last post, there is a rather complex Band Switch for the 6A8.
If the radio being used to test cannot be tuned to a midband station in the location where you are working on the radio in question, then move to another location. There are times that I must move outside the house with some radios in order to receive anything other than my home AM transmitter.
Most pre-WWII Radios, including this one were designed to be used with an outdoor long wire antenna going to the antenna lead, and the ground lead connected to a cold water pipe or a "made ground" such as a ground rod driven into damp ground. However, if you have strong local stations, a 25ft long wire stretched out even temporarily on the floor (but not touching concrete or tile) should work.
Do you have any other vintage radios, especially pre-WWII or at least pre-1955 radios? do they play in the same environment? I believe that radios with a "ferrite loopstick" (most radios post 1955 and almost all transistor radios) are more resistant to electrically generated noise than plain coil or loop antennas or sets requiring an outdoor antenna.
One last important note:
Often, a device like a radio fails, it could have several symptoms, all caused by the same failed part. However, with an antique radio that was received in a non working condition, there could be several different part failures. The first failure that caused the first owner to stop using the radio and putting it in the attic, basement, barn or worse, and all the other parts that failed when the radio was stored in the rough environment.
Keep us posted, it sounds like you are making progress.
A 22K resistor instead of the 20K resistor should be no issue, as the error is only 10% I am concerned that the resistance increases as the resistor heats up. The 9000 Ohm resistor, #57 drops voltage from raw B+ to create the plate voltages for the 6A8 and 6K7. Note Cap #9, the 16uF capacitor. This is the filter cap for the main plate for the 6A8, and, through resistors # 23,22 and 47, the triode plate of the 6Q7. Was this replaced? If not, do so. The drawings in the schematic show these resistors of the "dog bone" variety, These almost always are defective, even when they read OK under no load. Are the plate voltages for the 6A8 and 6K7 (Pin 3) within 20% of the documented voltage (220V for the 6A8, 270V for the 6K7)? Is the 6Q7 triode plate voltage near spec (175V)? Is the Osc Plate (Pin 6) of the 6A8 near spec (175V)? With power off, is there a low resistance (less than 2 Ohm) between the osc grid (Pin 5) of the 6A8 and chassis ground?
The screen voltages are low. If the 6Q7 and 6A8 Plate voltages are also low but the 6K7 plate is normal, then the voltage divider formed by resistors 23 and 22 is being loaded down. Change Resistor 23 even though it reads OK. if all that you have is a 22KOhm resistor, don't worry, use it. If the screen voltages (pin 4) for the 6A8 and 6K7 are still low, then pull 1 tube at a time and measure the screen voltage. replace the tube, then pull the other tube. If the voltage goes to normal with one tube in but not the other, then either the tube is shorted, gassy or something in the circuit is loading the tube down. If the voltage is low with either or no tube in place, then check cap #22A. If you did not replace it, do so.
When touching the 6Q7 cap there should be a hum. BTW, I apologize, I forgot to state that placement of the grid lead (called lead dress) is important, especially for the 6Q7, as the grid wire should be either shielded or run as close to the tube shield as possible. I believe that the 6Q7 in this set should have a shield around it, and the 6K7 also should have a shield.
Generally, when touching the grid cap of the 6A8 (the pentagrid convertor), you should receive stations. However, if in your basement with fluorescent, or even worse, LED lighting, your computer, smart TV, etc., you will usually get nothing but noise.
It is possible that the oscillator circuit of the convertor is not working. One way to tell this is to:
-Hold another radio tuned to a midband station in your area near this radio with the radio in question turned off.
-Confirm that you receive a station on the radio being used to test the radio in question.
-Power up the radio in question, set to the broadcast band. Slowly tune from the lowest to the highest frequency on the radio in question. If the radio playing the midband station starts squealing or the station disappears, then the local oscillator of the radio in question is at least oscillating. It may be off frequency but it is oscillating.
As mentioned on my last post, there is a rather complex Band Switch for the 6A8.
If the radio being used to test cannot be tuned to a midband station in the location where you are working on the radio in question, then move to another location. There are times that I must move outside the house with some radios in order to receive anything other than my home AM transmitter.
Most pre-WWII Radios, including this one were designed to be used with an outdoor long wire antenna going to the antenna lead, and the ground lead connected to a cold water pipe or a "made ground" such as a ground rod driven into damp ground. However, if you have strong local stations, a 25ft long wire stretched out even temporarily on the floor (but not touching concrete or tile) should work.
Do you have any other vintage radios, especially pre-WWII or at least pre-1955 radios? do they play in the same environment? I believe that radios with a "ferrite loopstick" (most radios post 1955 and almost all transistor radios) are more resistant to electrically generated noise than plain coil or loop antennas or sets requiring an outdoor antenna.
One last important note:
Often, a device like a radio fails, it could have several symptoms, all caused by the same failed part. However, with an antique radio that was received in a non working condition, there could be several different part failures. The first failure that caused the first owner to stop using the radio and putting it in the attic, basement, barn or worse, and all the other parts that failed when the radio was stored in the rough environment.
Keep us posted, it sounds like you are making progress.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
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