I believe I am having a problem with the AVC section of my Philco 42-321. The set receives local strong stations loudly but other weaker stations at low volumes. I cannot be one hundred percent sure because I do not know much about AVC circuits as they seem to vary from set to set. I believe part number 21 & 22 (0.01 uF cap and 3.3 M Ohm resistor is the AVC and it feeds the control grid for the detector/ audio "pre" amp 7C6 tube. Is this correct? And is there any other possible source of this problem? Does it need an alignment because I have changed all the caps except for one that I am waiting for.
Check for drifted resistors and leaky condensers. AVC usually causes stations to come in too loudly when it fails, though it can do otherwise.
T.
Thanks!
:The AVC consists of parts 19 and 17.
:
:Check for drifted resistors and leaky condensers. AVC usually causes stations to come in too loudly when it fails, though it can do otherwise.
:
:T.
The schematic for the Philco 42-321 shows the IF coming out of transformer 18, and going to the diode plates of the 7C6 tube. Two things happen here. 1, the audio is detected from the IF, and 2, the carrier frequency, 455 Kc is changed to DC. The audio comes out of the bottom of the IF transformer, and goes through 18 C, D, and E, I think, a low pass filter, probably to cut out the 10kc tone from adjoining radio channels. then to the volume control, 20, and to the common ground, with the DC Voltage being blocked by 21. This path, and 19 between the diode plates, provide a ground path for the grids of the tubes the AVC controlls, without it the electrons banging into the grid from the cathode would eventually bias the tube to cut-off, and it would stop working. this is also what your item 22 does for the grid of the 7C6.
The AGC is a negative Voltage produced at the diode plates of the 7C6. It is filtered by item 17, and the resulting Voltage, which is proportional to the strength of the station received, is applied to the grids of the RF amplifier tubes. If you follow the line left from the capacitor, you will see the AVC line stopped by item 2 that lets the RF into the amplifier grid but stops the DC. The amplifiers are called "remote cut off", which means the grids of the tubes are spaced differently in the center than the ends. As the negative Voltage increases, the closser wound grid wires cut off the grid, and the wider spaced wires control the gain. Therefore, the more negative the AVC Voltage, the lower the gain of the RF tubes. Because of the high resistances found in the AVC line, the Voltage must be read by a VTVM or digital meter, a VOM will load it too much. As you observed, everyone who designed an AVC circuit had a different idea how to go about it, but the end result is always the same, strong station, higher AVC, less amplifier gain. A shorted AVC line will result in excellent sensitivity. but distortion on strong stations, so your symptons dont seem to be AVC. However, if you have a high Z input meter, look at the AVC line, the Voltage should change as you tone across stations. Based on the little I know right now, I think antenna circuit problems. Let us know, we will be here.
Lewis
Well that is an excellent explanation. I should be able to figure something out from this. I will see what I can figure out now, thanks.
Edward
::Hello,
::
::I believe I am having a problem with the AVC section of my Philco 42-321. The set receives local strong stations loudly but other weaker stations at low volumes. I cannot be one hundred percent sure because I do not know much about AVC circuits as they seem to vary from set to set. I believe part number 21 & 22 (0.01 uF cap and 3.3 M Ohm resistor is the AVC and it feeds the control grid for the detector/ audio "pre" amp 7C6 tube. Is this correct? And is there any other possible source of this problem? Does it need an alignment because I have changed all the caps except for one that I am waiting for.
:
:The schematic for the Philco 42-321 shows the IF coming out of transformer 18, and going to the diode plates of the 7C6 tube. Two things happen here. 1, the audio is detected from the IF, and 2, the carrier frequency, 455 Kc is changed to DC. The audio comes out of the bottom of the IF transformer, and goes through 18 C, D, and E, I think, a low pass filter, probably to cut out the 10kc tone from adjoining radio channels. then to the volume control, 20, and to the common ground, with the DC Voltage being blocked by 21. This path, and 19 between the diode plates, provide a ground path for the grids of the tubes the AVC controlls, without it the electrons banging into the grid from the cathode would eventually bias the tube to cut-off, and it would stop working. this is also what your item 22 does for the grid of the 7C6.
:The AGC is a negative Voltage produced at the diode plates of the 7C6. It is filtered by item 17, and the resulting Voltage, which is proportional to the strength of the station received, is applied to the grids of the RF amplifier tubes. If you follow the line left from the capacitor, you will see the AVC line stopped by item 2 that lets the RF into the amplifier grid but stops the DC. The amplifiers are called "remote cut off", which means the grids of the tubes are spaced differently in the center than the ends. As the negative Voltage increases, the closser wound grid wires cut off the grid, and the wider spaced wires control the gain. Therefore, the more negative the AVC Voltage, the lower the gain of the RF tubes. Because of the high resistances found in the AVC line, the Voltage must be read by a VTVM or digital meter, a VOM will load it too much. As you observed, everyone who designed an AVC circuit had a different idea how to go about it, but the end result is always the same, strong station, higher AVC, less amplifier gain. A shorted AVC line will result in excellent sensitivity. but distortion on strong stations, so your symptons dont seem to be AVC. However, if you have a high Z input meter, look at the AVC line, the Voltage should change as you tone across stations. Based on the little I know right now, I think antenna circuit problems. Let us know, we will be here.
:
:Lewis
Just a quick comment. AVC reduces signal strength of strong stations but doesn't increase volume on weak stations. Be sure the radio is aligned with IF Transformers peaked.
Norm
:Hi Lewis,
:
:Well that is an excellent explanation. I should be able to figure something out from this. I will see what I can figure out now, thanks.
:
:Edward
:
:::Hello,
:::
:::I believe I am having a problem with the AVC section of my Philco 42-321. The set receives local strong stations loudly but other weaker stations at low volumes. I cannot be one hundred percent sure because I do not know much about AVC circuits as they seem to vary from set to set. I believe part number 21 & 22 (0.01 uF cap and 3.3 M Ohm resistor is the AVC and it feeds the control grid for the detector/ audio "pre" amp 7C6 tube. Is this correct? And is there any other possible source of this problem? Does it need an alignment because I have changed all the caps except for one that I am waiting for.
::
::The schematic for the Philco 42-321 shows the IF coming out of transformer 18, and going to the diode plates of the 7C6 tube. Two things happen here. 1, the audio is detected from the IF, and 2, the carrier frequency, 455 Kc is changed to DC. The audio comes out of the bottom of the IF transformer, and goes through 18 C, D, and E, I think, a low pass filter, probably to cut out the 10kc tone from adjoining radio channels. then to the volume control, 20, and to the common ground, with the DC Voltage being blocked by 21. This path, and 19 between the diode plates, provide a ground path for the grids of the tubes the AVC controlls, without it the electrons banging into the grid from the cathode would eventually bias the tube to cut-off, and it would stop working. this is also what your item 22 does for the grid of the 7C6.
::The AGC is a negative Voltage produced at the diode plates of the 7C6. It is filtered by item 17, and the resulting Voltage, which is proportional to the strength of the station received, is applied to the grids of the RF amplifier tubes. If you follow the line left from the capacitor, you will see the AVC line stopped by item 2 that lets the RF into the amplifier grid but stops the DC. The amplifiers are called "remote cut off", which means the grids of the tubes are spaced differently in the center than the ends. As the negative Voltage increases, the closser wound grid wires cut off the grid, and the wider spaced wires control the gain. Therefore, the more negative the AVC Voltage, the lower the gain of the RF tubes. Because of the high resistances found in the AVC line, the Voltage must be read by a VTVM or digital meter, a VOM will load it too much. As you observed, everyone who designed an AVC circuit had a different idea how to go about it, but the end result is always the same, strong station, higher AVC, less amplifier gain. A shorted AVC line will result in excellent sensitivity. but distortion on strong stations, so your symptons dont seem to be AVC. However, if you have a high Z input meter, look at the AVC line, the Voltage should change as you tone across stations. Based on the little I know right now, I think antenna circuit problems. Let us know, we will be here.
::
::Lewis
I had also considered the IF tuning might be a problem. I am sure after all these years there are many small problems too. It is all good stuff to consider and now time to buzz it out.
Edward
:Hi
:
: Just a quick comment. AVC reduces signal strength of strong stations but doesn't increase volume on weak stations. Be sure the radio is aligned with IF Transformers peaked.
:
:Norm
:
::Hi Lewis,
::
::Well that is an excellent explanation. I should be able to figure something out from this. I will see what I can figure out now, thanks.
::
::Edward
::
::::Hello,
::::
::::I believe I am having a problem with the AVC section of my Philco 42-321. The set receives local strong stations loudly but other weaker stations at low volumes. I cannot be one hundred percent sure because I do not know much about AVC circuits as they seem to vary from set to set. I believe part number 21 & 22 (0.01 uF cap and 3.3 M Ohm resistor is the AVC and it feeds the control grid for the detector/ audio "pre" amp 7C6 tube. Is this correct? And is there any other possible source of this problem? Does it need an alignment because I have changed all the caps except for one that I am waiting for.
:::
:::The schematic for the Philco 42-321 shows the IF coming out of transformer 18, and going to the diode plates of the 7C6 tube. Two things happen here. 1, the audio is detected from the IF, and 2, the carrier frequency, 455 Kc is changed to DC. The audio comes out of the bottom of the IF transformer, and goes through 18 C, D, and E, I think, a low pass filter, probably to cut out the 10kc tone from adjoining radio channels. then to the volume control, 20, and to the common ground, with the DC Voltage being blocked by 21. This path, and 19 between the diode plates, provide a ground path for the grids of the tubes the AVC controlls, without it the electrons banging into the grid from the cathode would eventually bias the tube to cut-off, and it would stop working. this is also what your item 22 does for the grid of the 7C6.
:::The AGC is a negative Voltage produced at the diode plates of the 7C6. It is filtered by item 17, and the resulting Voltage, which is proportional to the strength of the station received, is applied to the grids of the RF amplifier tubes. If you follow the line left from the capacitor, you will see the AVC line stopped by item 2 that lets the RF into the amplifier grid but stops the DC. The amplifiers are called "remote cut off", which means the grids of the tubes are spaced differently in the center than the ends. As the negative Voltage increases, the closser wound grid wires cut off the grid, and the wider spaced wires control the gain. Therefore, the more negative the AVC Voltage, the lower the gain of the RF tubes. Because of the high resistances found in the AVC line, the Voltage must be read by a VTVM or digital meter, a VOM will load it too much. As you observed, everyone who designed an AVC circuit had a different idea how to go about it, but the end result is always the same, strong station, higher AVC, less amplifier gain. A shorted AVC line will result in excellent sensitivity. but distortion on strong stations, so your symptons dont seem to be AVC. However, if you have a high Z input meter, look at the AVC line, the Voltage should change as you tone across stations. Based on the little I know right now, I think antenna circuit problems. Let us know, we will be here.
:::
:::Lewis