I am working on a GE G97 console radio which has a separate AFC tube, 6J5, and an AFC on/off switch (see Riders 9-47). My experience with AFC (or AGC) has always been a feedback loop from the 2nd detector to the RF and IF stages. I have never come across an entire stage like this before. The tube and the socket voltages appear OK but the AFC does not seem to work too well. Can someone explain how this works?
Thanks, Nick

AFC and AVC or AGC are not the same things. AGC and AVC stand for automatic gain or volume control (same thing pretty much). AFC stands for automatic frequency control. The automatic gain or volume control simply takes the signal and rectifies it so that the amplitude (which, in the unrectified signal, is both positive and negative...rectification allows you to have only one polarity) can be 'seen' by the other circuits. The amplitude of the signal is a voltage, and, when the RF information is filtered out, it becomes simply a fairly steady voltage that only varies with signal strength variation. It is used to control the bias of the RF tubes.

AFC, which is automatic frequency control, keeps your receiver locked onto a signal. It prevents drift due to changes in the components as they warm up. The circuitry can be quite simple, or can be quite elaborate. One elaborate method of frequency control is similar to the detector in an FM radio. There are variations, but it can have two windings, one that is tuned slightly above the desired frequency, and one that is tuned slightly below that frequency. Deviations in the frequency throw the circuit out of balance. When the circuit is in balance, voltages created in the two coils cancel out. When out of balance, one coil produces more voltage than the other, and a difference voltage results (the difference between the two voltages). This voltage can be used to control the local oscillator so that the frequency drift is corrected.

I can't see your schematic on this web page, so I can't really analyze the circuit. Such a circuit really isn't -necessary-, especially at regular broadcast frequencies, though it may be desirable at short wave or FM frequencies. If you wish to enjoy the 'modern' conveniences of what sounds like a thrillingly elaborate radio of the day, you will want all of the circuitry to function as it should. Electric tuning and all of those other extras can only make the experience even better. Who needs a computer and wi-fi when you can be in the future with a General Electric radio? As wonderful as modern technologies are, nothing is more thrilling than elaborate radio circuitry and a properly working motorized tuner. Just press a button, and there's your station! (Philco Mystery Controls are a real thrill, too.)

Thomas

Thomas,

Some manufacturers in around part of the 1930s produced some AM broadcast radios utilizing AFC similar to the FM AFC. One that comes to mind is the General Household & Utilities (Grigsby-Grunow) 12B chassis. The radio used a Teledial setup, kind of like a big telephone dial that could be used to quickly select a station. Pushing one of the station buttons would mute the audio and momentarily disable the AFC while one moved the dial assembly like dialing a telephone. Each of the buttons could be adjusted for a different station. It was possible that the station would not be exactly on but very close to the station, close enough that the AFC would pull the station right on to the station carrier signal thus not having to fine tune with the station tuning knob.

Radiodoc
**********

:AFC and AVC or AGC are not the same things. AGC and AVC stand for automatic gain or volume control (same thing pretty much). AFC stands for automatic frequency control. The automatic gain or volume control simply takes the signal and rectifies it so that the amplitude (which, in the unrectified signal, is both positive and negative...rectification allows you to have only one polarity) can be 'seen' by the other circuits. The amplitude of the signal is a voltage, and, when the RF information is filtered out, it becomes simply a fairly steady voltage that only varies with signal strength variation. It is used to control the bias of the RF tubes.
:
:AFC, which is automatic frequency control, keeps your receiver locked onto a signal. It prevents drift due to changes in the components as they warm up. The circuitry can be quite simple, or can be quite elaborate. One elaborate method of frequency control is similar to the detector in an FM radio. There are variations, but it can have two windings, one that is tuned slightly above the desired frequency, and one that is tuned slightly below that frequency. Deviations in the frequency throw the circuit out of balance. When the circuit is in balance, voltages created in the two coils cancel out. When out of balance, one coil produces more voltage than the other, and a difference voltage results (the difference between the two voltages). This voltage can be used to control the local oscillator so that the frequency drift is corrected.
:
:I can't see your schematic on this web page, so I can't really analyze the circuit. Such a circuit really isn't -necessary-, especially at regular broadcast frequencies, though it may be desirable at short wave or FM frequencies. If you wish to enjoy the 'modern' conveniences of what sounds like a thrillingly elaborate radio of the day, you will want all of the circuitry to function as it should. Electric tuning and all of those other extras can only make the experience even better. Who needs a computer and wi-fi when you can be in the future with a General Electric radio? As wonderful as modern technologies are, nothing is more thrilling than elaborate radio circuitry and a properly working motorized tuner. Just press a button, and there's your station! (Philco Mystery Controls are a real thrill, too.)
:
:
:Thomas

Thomas/Radiodoc,
You guys are priceless!! Haven't run in to the situation first posted, but hope to retain your invaluable info if I ever do. Thanks

marv

:Thomas,
:
:Some manufacturers in around part of the 1930s produced some AM broadcast radios utilizing AFC similar to the FM AFC. One that comes to mind is the General Household & Utilities (Grigsby-Grunow) 12B chassis. The radio used a Teledial setup, kind of like a big telephone dial that could be used to quickly select a station. Pushing one of the station buttons would mute the audio and momentarily disable the AFC while one moved the dial assembly like dialing a telephone. Each of the buttons could be adjusted for a different station. It was possible that the station would not be exactly on but very close to the station, close enough that the AFC would pull the station right on to the station carrier signal thus not having to fine tune with the station tuning knob.
:
:Radiodoc
:**********
:
:
::AFC and AVC or AGC are not the same things. AGC and AVC stand for automatic gain or volume control (same thing pretty much). AFC stands for automatic frequency control. The automatic gain or volume control simply takes the signal and rectifies it so that the amplitude (which, in the unrectified signal, is both positive and negative...rectification allows you to have only one polarity) can be 'seen' by the other circuits. The amplitude of the signal is a voltage, and, when the RF information is filtered out, it becomes simply a fairly steady voltage that only varies with signal strength variation. It is used to control the bias of the RF tubes.
::
::AFC, which is automatic frequency control, keeps your receiver locked onto a signal. It prevents drift due to changes in the components as they warm up. The circuitry can be quite simple, or can be quite elaborate. One elaborate method of frequency control is similar to the detector in an FM radio. There are variations, but it can have two windings, one that is tuned slightly above the desired frequency, and one that is tuned slightly below that frequency. Deviations in the frequency throw the circuit out of balance. When the circuit is in balance, voltages created in the two coils cancel out. When out of balance, one coil produces more voltage than the other, and a difference voltage results (the difference between the two voltages). This voltage can be used to control the local oscillator so that the frequency drift is corrected.
::
::I can't see your schematic on this web page, so I can't really analyze the circuit. Such a circuit really isn't -necessary-, especially at regular broadcast frequencies, though it may be desirable at short wave or FM frequencies. If you wish to enjoy the 'modern' conveniences of what sounds like a thrillingly elaborate radio of the day, you will want all of the circuitry to function as it should. Electric tuning and all of those other extras can only make the experience even better. Who needs a computer and wi-fi when you can be in the future with a General Electric radio? As wonderful as modern technologies are, nothing is more thrilling than elaborate radio circuitry and a properly working motorized tuner. Just press a button, and there's your station! (Philco Mystery Controls are a real thrill, too.)
::
::
::Thomas

:I am working on a GE G97 console radio which has a separate AFC tube, 6J5, and an AFC on/off switch (see Riders 9-47). My experience with AFC (or AGC) has always been a feedback loop from the 2nd detector to the RF and IF stages. I have never come across an entire stage like this before. The tube and the socket voltages appear OK but the AFC does not seem to work too well. Can someone explain how this works?
:Thanks, Nick

Hi Nick. Is this the same GE 97 that had the 200Vdc on the 6A8 grid? I've been wondering what you found to be the culprit.

::I am working on a GE G97 console radio which has a separate AFC tube, 6J5, and an AFC on/off switch (see Riders 9-47). My experience with AFC (or AGC) has always been a feedback loop from the 2nd detector to the RF and IF stages. I have never come across an entire stage like this before. The tube and the socket voltages appear OK but the AFC does not seem to work too well. Can someone explain how this works?
::Thanks, Nick
:
Yes Robert, this is the same beast! Several radio club members (njarc.org) insisted it was due to a shorted RF coil (T4). I wasn't convinced because the windings were not overlapping, but I felt I had to remove it to prove or disprove the shorting theory. After disconnecting the 11 or so wires I had to map out the connections as the schematic did not provide that information. The coil turned out to be in good condition with no shorts.
I reconnected the wires one stage at a time and found the high voltage was coming from the band switch (contact 19 in S1). This is not connected to any other part of the circuit so it remains a mystery where it is coming from. By leaving this connection out of the circuit I was able to get the set to work on the AM band; the other positions (contact 21/22) are only used for shortwave reception. The band switch is buried under the push button assembly and has 24 contacts connected to three coils and PB assm. and I don't have the courage or energy to attack this problem. I will live with AM only and forget about the short wave unless I can find someone to do this for me.
Thanks for everyones input on this set. In spite of the frustrations, it is a very interesting chassis, as are many of these old relics.
Nick S
:Hi Nick. Is this the same GE 97 that had the 200Vdc on the 6A8 grid? I've been wondering what you found to be the culprit.