Regarding the two peaks in volume when tuning FM; the very early FM radios used a circuit called slope detector. It works based on the fact that when sitting/tuning on the slopes of the passband curve of a resonant LC circuit with high Q, the amplitude of the signal shifts greatly with even small variations in frequency. Which is why you get away with simple AM detection to demodulate FM. Downside is that you got two volume maxima since you have two slopes left and right of the center frequency. There is a blog article by ALLELCO called "guide to FM slope detection and demodulation technology" lining out some basics which may be a good starting point for further research. And if you compare the mentioned document to your schematic, you will see that the filter L23 A12 along with the diodes in the tube V14A do not form a classic ratio detector, but a dual slope detector, as coil A12 is missing the 3rd winding crucial to the function of the ratio detector, and the diodes in V14A are oriented the same way and not used in opposite polarity like in a ratio detector.
The Garod schematic I am looking at shows a Foster-Seeley descriminator being used as the FM detector. Slope detection was not preferred since it simultaneously detected AM signals, such as impulse noise from arcing.
@@bandersentv Okay, I think I finally got this mystery figured out. I was looking at the schematic for the Canadian version, which uses different reference designators. On the US Sam's schematic, V14, the 6T8 is drawn wrong. This tube contains a triode with 2 diode plates that use the triode's cathode. Also, there is a completely isolated diode from the triode using it's own cathode. In FM, the detector is a Foster-Seeley, in AM, the plate on pin 6 detects the AM signal by shorting the positive half of the 455 kHz to ground. The actual AM take off point is at L37, at the tiepoint of C61 and R72.
Once an Electronics Repair Technician told me that a TV is easier to repair than a Radio. I was surprised to hear that because a TV is a bit more complex than a Radio. When I asked him "Why?" he replied "Built in Oscilloscope."
I have seen tubes such as, 6BQ6 or audio output tubes with those cocoa brown getter spots test just fine in a tube tester, but may behave differently under full power knee test conditions. It does indicate that the tube has had some hard hours on it, but if it works, use it.
Speaking of dark spots on tubes, anyone know the best place to buy 6SH7, 5U4 G and 6BG6 Q in USA? Just started a GE 810 (one owner in very good shape!)
16:49 To add to what was previously said (made sure to read all the comments to avoid repetition), yes, FM is now stereo and when transitioning, one could buy a Multiplex Adapter, some consoles even had this as a factory option like my Körting as evendenced y a propitiatory connector-mount. However, these devices are NLA, sadly. Cheers, Adriel
Hey Bob, excellent job on the restore so far and great to see the raster! As far as the double tuning in the FM reception, it does not have anything to do with the broadcasting being in stereo but using a mono tuner. The stereo signal is modulated with the detected audio at frequencies above the hearing range, specifically between 19 and 57 KHz, and tuning across the RF reception of the radio above and below the center of the station is not affected by the stereo signal RF wise, however, since the modulated signals (mono audio plus the stereo signals) cover a wide bandwidth, quite a wide IF bandpass is required (but not too wide) for there to be good, undistorted audio. This Garod would probably benefit from an alignment to reduce the effect of the double tuning and should improve the clarity of the main, better received tuning point, but as you have observed with aligning other old receivers, it may not completely eliminate it, that may be somewhat in the design of the RF/IF components of the time and/or the trickiness of the alignment of them.
I should also add that even though stereo stations broadcast and require a wide bandwidth for an FM receiver to receive, even old mono FM tuners were designed to pass the same bandwidth, nearly 200 KHz for the channel, because even mono high fidelity sound requires quite a wide bandwidth, and therefore came up with 200 KHz channels (.2 MHz) back from the beginning of the modern FM band. It is quite complex how modulation frequency response affects required channel bandwidth on FM compared to AM, which basically follows the modulation response requirement (a 5 Khz audio frequency response on AM requires a 10 Khz channel- 5 K for the upper and 5 K for the lower sideband).
@@oldradiotvsc9836 The formula for FM occupied bandwidth is 2X peak deviation + the highest audio frequency. For the US FM broadcast band, peak deviation is 75 kHz and the highest audio frequency is 15 kHz = 165 kHz occupied bandwidth for the monaural broadcast. With FM Stereo, there is a 19 kHz subcarrier and a double sideband suppressed carrier at 38 kHz containing the Left - Right signal. These subcarriers have a 15 kHz peak deviation. These subcarriers silently fit within the original Mono signal. Given the stereo subcarriers have a 15 kHz deviation, the signal to noise ratio is noticably poorer then FM mono.
Also, as Steve pointed out, it is using the same slope detector for both AM and FM. While it does work, it results in the two volume peaks. I just shared that portion of the schematic on my community tab.
The test CRT is aluminized. The spot in the center is multiple spot burns not an ion burn. It is a result of having HV applied without deflection. I have seen ion burns on aluminized crts but it is quite rare, and looks like a small box at the center of the screen, maybe 1/4" across. The deflection does deflect ions but only slightly. It was also evidence a gassy tube.
Thanks Bob for the update on the Garod set; very interesting! The set really cleaned up well. A 6AR5 is similar to a 6AQ5. Electrically it is nearly identical. The difference being, the 6AQ5 has the control grid connected to pins 1 and 7, whereas the 6AR5 only connects to pin 1 and pin 7 is open. The 6AR5 is the 7 pin miniature base equivalent to the 6K6 and the 6AQ5 is the 7 pin equivalent of the 6V6. Both these audio output tubes were designed for car radio applications (note the low filament current). The FM problem is due to the IF bandwidth being too narrow. Peak tuning each IF transformer to 10.7 MHz produces a 3 humped response. You will need to stagger tune the IF to produce a 180 kHz 3 dB bandwidth with a flat nose response. The early FM radios were basically a modified AM radio. An RF amp and 1 IF amp was added to boost the receiver gain, then an FM detector was used in place of the AM detector. Many of these early FM radios will even have AGC, which is not needed for FM.
Interesting. It seems that a lot of alignment instructions for early FM radios have us more or less peak tune the IF transformers and not stagger, but as you say, it seems we have to stagger tune them to get the band pass we need. The trouble is that it is difficult to figure out how to stagger tune them just right, since there will be more gain from what is going through the first IF transformers than what is going through the later ones. I would imagine that nearly makes a sweep generator and scope visual alignment much better than a basic signal generator/VTVM one.
@oldradiotvsc9836 Agreed. Sweep Generators were not that common then so, it was common to see a VTVM/signal generator alignment method as well as a sweep generator/oscilloscope method. For the VTVM/signal generator alignment method, they would specify a different alignment frequency for each IF transformer slug.
@@bandersentv I have looked on ebay a few times already and I have never seen one there as for Nebraska it is a bit to far for me (I live in The Netherlands)
Hi Bob. Two questions about this set: 1) I thought that if the H.O.T. was run without drive (gate signal) it would quickly red-plate and burn up? So does putting it in “radio” mode (AM or FM) not only cut the Horizontal drive signal, but the cathode current in the H.O.T. as well? 2) if you’re operating the CRT without the H.V. on the anode cap, isn’t there still about +400 to +600 VDC on the focus grid, and wouldn’t that accelerate a few electrons towards the phosphor? Thanks. I was always the kid who enjoyed looking at the warm glow of the tubes instead of the TV program that was on (unless it was Star Trek, or the Looney Tunes on Saturday mornings)!
Excellent questions. I studied the schematic more closely and realized the mode switch actually cuts filament power to the 5U4 rectifier when in radio/phone mode! That leaves all the tubes and CRT filaments glowing, but there is no plate voltage. All the electrons just get attracted to G2 to complete the circuit. That's how a CRT tester works. No glow on the face while you are testing. I suppose if you connected G2 to the HV anode while using a CRT tester you might get a glowing spot. I'll have to try it sometime.
Regarding the two peaks in volume when tuning FM; the very early FM radios used a circuit called slope detector. It works based on the fact that when sitting/tuning on the slopes of the passband curve of a resonant LC circuit with high Q, the amplitude of the signal shifts greatly with even small variations in frequency. Which is why you get away with simple AM detection to demodulate FM. Downside is that you got two volume maxima since you have two slopes left and right of the center frequency.
There is a blog article by ALLELCO called "guide to FM slope detection and demodulation technology" lining out some basics which may be a good starting point for further research.
And if you compare the mentioned document to your schematic, you will see that the filter L23 A12 along with the diodes in the tube V14A do not form a classic ratio detector, but a dual slope detector, as coil A12 is missing the 3rd winding crucial to the function of the ratio detector, and the diodes in V14A are oriented the same way and not used in opposite polarity like in a ratio detector.
Yes, I see what you mean. Thank you for the detailed response.
What he said lol
The Garod schematic I am looking at shows a Foster-Seeley descriminator being used as the FM detector. Slope detection was not preferred since it simultaneously detected AM signals, such as impulse noise from arcing.
@@billharris6886 Is it the same as the schematic I posted? It's using the same detector for both AM and FM.
@@bandersentv Okay, I think I finally got this mystery figured out. I was looking at the schematic for the Canadian version, which uses different reference designators. On the US Sam's schematic, V14, the 6T8 is drawn wrong. This tube contains a triode with 2 diode plates that use the triode's cathode. Also, there is a completely isolated diode from the triode using it's own cathode. In FM, the detector is a Foster-Seeley, in AM, the plate on pin 6 detects the AM signal by shorting the positive half of the 455 kHz to ground. The actual AM take off point is at L37, at the tiepoint of C61 and R72.
Once an Electronics Repair Technician told me that a TV is easier to repair than a Radio.
I was surprised to hear that because a TV is a bit more complex than a Radio.
When I asked him "Why?" he replied "Built in Oscilloscope."
Never thought of it that way! Good answer! 👍
Always a good thing to see that raster for the first time.
I have seen tubes such as, 6BQ6 or audio output tubes with those cocoa brown getter spots test just fine in a tube tester, but may behave differently under full power knee test conditions. It does indicate that the tube has had some hard hours on it, but if it works, use it.
Speaking of dark spots on tubes, anyone know the best place to buy 6SH7, 5U4 G and 6BG6 Q in USA?
Just started a GE 810 (one owner in very good shape!)
16:49 To add to what was previously said (made sure to read all the comments to avoid repetition), yes, FM is now stereo and when transitioning, one could buy a Multiplex Adapter, some consoles even had this as a factory option like my Körting as evendenced y a propitiatory connector-mount. However, these devices are NLA, sadly.
Cheers,
Adriel
WoopWoop! =3
Hey Bob, excellent job on the restore so far and great to see the raster! As far as the double tuning in the FM reception, it does not have anything to do with the broadcasting being in stereo but using a mono tuner. The stereo signal is modulated with the detected audio at frequencies above the hearing range, specifically between 19 and 57 KHz, and tuning across the RF reception of the radio above and below the center of the station is not affected by the stereo signal RF wise, however, since the modulated signals (mono audio plus the stereo signals) cover a wide bandwidth, quite a wide IF bandpass is required (but not too wide) for there to be good, undistorted audio. This Garod would probably benefit from an alignment to reduce the effect of the double tuning and should improve the clarity of the main, better received tuning point, but as you have observed with aligning other old receivers, it may not completely eliminate it, that may be somewhat in the design of the RF/IF components of the time and/or the trickiness of the alignment of them.
I should also add that even though stereo stations broadcast and require a wide bandwidth for an FM receiver to receive, even old mono FM tuners were designed to pass the same bandwidth, nearly 200 KHz for the channel, because even mono high fidelity sound requires quite a wide bandwidth, and therefore came up with 200 KHz channels (.2 MHz) back from the beginning of the modern FM band. It is quite complex how modulation frequency response affects required channel bandwidth on FM compared to AM, which basically follows the modulation response requirement (a 5 Khz audio frequency response on AM requires a 10 Khz channel- 5 K for the upper and 5 K for the lower sideband).
@@oldradiotvsc9836 The formula for FM occupied bandwidth is 2X peak deviation + the highest audio frequency. For the US FM broadcast band, peak deviation is 75 kHz and the highest audio frequency is 15 kHz = 165 kHz occupied bandwidth for the monaural broadcast. With FM Stereo, there is a 19 kHz subcarrier and a double sideband suppressed carrier at 38 kHz containing the Left - Right signal. These subcarriers have a 15 kHz peak deviation. These subcarriers silently fit within the original Mono signal. Given the stereo subcarriers have a 15 kHz deviation, the signal to noise ratio is noticably poorer then FM mono.
Also, as Steve pointed out, it is using the same slope detector for both AM and FM. While it does work, it results in the two volume peaks. I just shared that portion of the schematic on my community tab.
The test CRT is aluminized. The spot in the center is multiple spot burns not an ion burn. It is a result of having HV applied without deflection. I have seen ion burns on aluminized crts but it is quite rare, and looks like a small box at the center of the screen, maybe 1/4" across. The deflection does deflect ions but only slightly. It was also evidence a gassy tube.
Test CRT is not aluminized.
...that's because ions are a lot heavier than electrons...
Thanks Bob for the update on the Garod set; very interesting! The set really cleaned up well.
A 6AR5 is similar to a 6AQ5. Electrically it is nearly identical. The difference being, the 6AQ5 has the control grid connected to pins 1 and 7, whereas the 6AR5 only connects to pin 1 and pin 7 is open.
The 6AR5 is the 7 pin miniature base equivalent to the 6K6 and the 6AQ5 is the 7 pin equivalent of the 6V6. Both these audio output tubes were designed for car radio applications (note the low filament current).
The FM problem is due to the IF bandwidth being too narrow. Peak tuning each IF transformer to 10.7 MHz produces a 3 humped response. You will need to stagger tune the IF to produce a 180 kHz 3 dB bandwidth with a flat nose response.
The early FM radios were basically a modified AM radio. An RF amp and 1 IF amp was added to boost the receiver gain, then an FM detector was used in place of the AM detector. Many of these early FM radios will even have AGC, which is not needed for FM.
Interesting. It seems that a lot of alignment instructions for early FM radios have us more or less peak tune the IF transformers and not stagger, but as you say, it seems we have to stagger tune them to get the band pass we need. The trouble is that it is difficult to figure out how to stagger tune them just right, since there will be more gain from what is going through the first IF transformers than what is going through the later ones. I would imagine that nearly makes a sweep generator and scope visual alignment much better than a basic signal generator/VTVM one.
@oldradiotvsc9836 Agreed. Sweep Generators were not that common then so, it was common to see a VTVM/signal generator alignment method as well as a sweep generator/oscilloscope method. For the VTVM/signal generator alignment method, they would specify a different alignment frequency for each IF transformer slug.
question
were can I get a few of those high voltage connectors
I have a few scope crt's that use them
I scavenge them from old equipment. Maybe Surplus Sales of Nebraska or eBay has them.
@@bandersentv I have looked on ebay a few times already and I have never seen one there
as for Nebraska it is a bit to far for me (I live in The Netherlands)
Hi Bob. Two questions about this set:
1) I thought that if the H.O.T. was run without drive (gate signal) it would quickly red-plate and burn up? So does putting it in “radio” mode (AM or FM) not only cut the Horizontal drive signal, but the cathode current in the H.O.T. as well?
2) if you’re operating the CRT without the H.V. on the anode cap, isn’t there still about +400 to +600 VDC on the focus grid, and wouldn’t that accelerate a few electrons towards the phosphor?
Thanks. I was always the kid who enjoyed looking at the warm glow of the tubes instead of the TV program that was on (unless it was Star Trek, or the Looney Tunes on Saturday mornings)!
Excellent questions. I studied the schematic more closely and realized the mode switch actually cuts filament power to the 5U4 rectifier when in radio/phone mode! That leaves all the tubes and CRT filaments glowing, but there is no plate voltage.
All the electrons just get attracted to G2 to complete the circuit. That's how a CRT tester works. No glow on the face while you are testing. I suppose if you connected G2 to the HV anode while using a CRT tester you might get a glowing spot. I'll have to try it sometime.