Do you have an antenna connected to your radio? If an antenna is connected and it doesn't bring up the signal something may be wrong between antenna and top cap of your 6L7. Sometimes an antenna coil is open.
Norm
Working on a Grunow Teledial 12 or 1541 - I notice that when I touch or get near the grid/top connection of the 6L7 1st detector tube - the signal and volume increases substantially of a tuned in station. I'm curious as to what causes this and how do I get the volume/signal to stay that loud all the time?
yes I have an antenna connected. The top of the 6L7 is connected right to the tuning condenser - but I'll look at the coil. Thanks
:Hi Dave
:
: Do you have an antenna connected to your radio? If an antenna is connected and it doesn't bring up the signal something may be wrong between antenna and top cap of your 6L7. Sometimes an antenna coil is open.
:
:Norm
:
:Working on a Grunow Teledial 12 or 1541 - I notice that when I touch or get near the grid/top connection of the 6L7 1st detector tube - the signal and volume increases substantially of a tuned in station. I'm curious as to what causes this and how do I get the volume/signal to stay that loud all the time?
However, if the set receives strong stations across the dial, and it is also capable of pulling in weak ones with precision, then your set is working properly. Bands A, B, and C of your set utilize the AVC (automatic volume control) circuit to bias the grid of the 6L7 tube. This circuit throws the 6L7 grid, and any other grid of any other RF tube that is connected to this circuit, more negative when stronger stations are received. This way maximum sensitivity is utilized by your receiver for weak stations, and then the receiver's sensitivity is reduced for strong local stations, which both reduces distortion in the RF stages, as they do not otherwise have a volume control and would distort if too strong a station was received, and also keeps you from having to adjust the volume control for each station. A further benefit of this circuit is that when listening to distant stations, that tend to fade and flutter, the automatic volume control circuit reduces this problem to some extent.
One way to tell whether the automatic volume control is what is causing your interesting phenomenon is to tune in a very distant station. Gently touch the 6L7 grid and the chassis at the same time. Receiver volume should not increase. It may actually decrease because you are cancelling out part of the signal through your hand. Full receiver sensitivity is being used for this distant station. The grid is no more negative than the chassis, and touching it does not change its charge much. If you tune in a strong local station, though, and touch the grid, the station will get louder because the grid was originally somewhat negative due to the AVC voltage being applied to it. Touching the grid terminal leaks some of this negative voltage to ground, thereby increasing the receiver's sensitivity. You do not want the receiver to do this with local stations, however. It should normally play them at a somewhat reduced volume.
Your set is quite the set, too. It has some pretty amazing circuits. It also has an Automatic Frequency Control, or AFC, which is normally only used in FM receivers. What it does is assist in reducing station de-tuning, especially where this is critical with short wave.
At any rate, make sure your receiver has a very good antenna attached to it. Make sure it is well aligned, too, using a good quality signal generator of known working condition. After this is all done, if the set receives stations extremely well, as it should with a set of this high quality, and you find that local stations get louder when you touch the grid lead, then you can assume that this is normal AVC operation. If ALL stations get louder when you touch the grid terminal, then you need to examine your front end and make sure that it is in proper working order.
Thomas
:Hi Dave
:
: Do you have an antenna connected to your radio? If an antenna is connected and it doesn't bring up the signal something may be wrong between antenna and top cap of your 6L7. Sometimes an antenna coil is open.
:
:Norm
:
:Working on a Grunow Teledial 12 or 1541 - I notice that when I touch or get near the grid/top connection of the 6L7 1st detector tube - the signal and volume increases substantially of a tuned in station. I'm curious as to what causes this and how do I get the volume/signal to stay that loud all the time?
Just a note: for perfect performance of the AVC circuit (and for that matter, any other circuit within the radio), make sure that the condensers have absolutely no leakage. Your multi-meter, on its most sensitive resistance checker setting (the one that you can move the meter with your hands), should read exactly where it started after the condenser has charged up. If it lingers at anything less than infinity (10 million ohms or less), then the condenser is leaky, and will ruin AVC operation. Any leakage in the AVC condensers will leak off some of the negative AVC voltage, and the receiver will distort somewhat on strong local stations.
Thomas
:If your radio is performing normally without you touching the grid cap of the 1st detector, what you are hearing when you touch it and the signal gets louder is the action of the automatic volume control being cancelled out. Now, before you assume this, what Norm said is also very true. If your radio is not properly connected to an antenna or the primary RF section is not aligned properly for each band, or if there is a faulty component in the primary RF section, touching the grid terminal of the 6L7 tube will increase volume because your body will be sending a direct signal to the terminal that is stronger than the signal being received through the primary RF section. Be sure that this section is aligned properly. Also, a very good antenna for your radio is the V-Doublet antenna. You can find a very good example in the General Electric section of the Resources section of this web page. Use 300 ohm flat television lead in cable, found at Radio Shack, for the lead in wire.
:
:However, if the set receives strong stations across the dial, and it is also capable of pulling in weak ones with precision, then your set is working properly. Bands A, B, and C of your set utilize the AVC (automatic volume control) circuit to bias the grid of the 6L7 tube. This circuit throws the 6L7 grid, and any other grid of any other RF tube that is connected to this circuit, more negative when stronger stations are received. This way maximum sensitivity is utilized by your receiver for weak stations, and then the receiver's sensitivity is reduced for strong local stations, which both reduces distortion in the RF stages, as they do not otherwise have a volume control and would distort if too strong a station was received, and also keeps you from having to adjust the volume control for each station. A further benefit of this circuit is that when listening to distant stations, that tend to fade and flutter, the automatic volume control circuit reduces this problem to some extent.
:
:One way to tell whether the automatic volume control is what is causing your interesting phenomenon is to tune in a very distant station. Gently touch the 6L7 grid and the chassis at the same time. Receiver volume should not increase. It may actually decrease because you are cancelling out part of the signal through your hand. Full receiver sensitivity is being used for this distant station. The grid is no more negative than the chassis, and touching it does not change its charge much. If you tune in a strong local station, though, and touch the grid, the station will get louder because the grid was originally somewhat negative due to the AVC voltage being applied to it. Touching the grid terminal leaks some of this negative voltage to ground, thereby increasing the receiver's sensitivity. You do not want the receiver to do this with local stations, however. It should normally play them at a somewhat reduced volume.
:
:Your set is quite the set, too. It has some pretty amazing circuits. It also has an Automatic Frequency Control, or AFC, which is normally only used in FM receivers. What it does is assist in reducing station de-tuning, especially where this is critical with short wave.
:
:At any rate, make sure your receiver has a very good antenna attached to it. Make sure it is well aligned, too, using a good quality signal generator of known working condition. After this is all done, if the set receives stations extremely well, as it should with a set of this high quality, and you find that local stations get louder when you touch the grid lead, then you can assume that this is normal AVC operation. If ALL stations get louder when you touch the grid terminal, then you need to examine your front end and make sure that it is in proper working order.
:
:Thomas
:
::Hi Dave
::
:: Do you have an antenna connected to your radio? If an antenna is connected and it doesn't bring up the signal something may be wrong between antenna and top cap of your 6L7. Sometimes an antenna coil is open.
::
::Norm
::
::Working on a Grunow Teledial 12 or 1541 - I notice that when I touch or get near the grid/top connection of the 6L7 1st detector tube - the signal and volume increases substantially of a tuned in station. I'm curious as to what causes this and how do I get the volume/signal to stay that loud all the time?
thanks for the info. Not finished with your suggestion below, but I did find that one half of the discriminator transformer in the AFC circuit has no continuity. I pulled it thinking it may have been one of the brittle wires etc - but to no avail. The center tapped half shows continuity (10 ohm total/5 ohm to CT) the secondary so-to-speak side is open. Any ideas on rectifying this?
At any rate, the test I was asking you to perform was rather simple in nature--just seeing if only local stations got louder when the grid terminal was touched, or if both local and distant stations got louder. This simply says whether your phenomenon is due to normal AVC operation (the former circumstance), or if the primary RF circuits are out of alignment or your antenna lacks proper size (latter condition).
Good luck with the discriminator in the AFC circuit. That really is quite the radio. If it is adjusted properly, according to the original literature, you should be pretty amazed at its sound quality.
Thomas
:Thomas,
:
:thanks for the info. Not finished with your suggestion below, but I did find that one half of the discriminator transformer in the AFC circuit has no continuity. I pulled it thinking it may have been one of the brittle wires etc - but to no avail. The center tapped half shows continuity (10 ohm total/5 ohm to CT) the secondary so-to-speak side is open. Any ideas on rectifying this?
:Thomas,
:
:thanks for the info. Not finished with your suggestion below, but I did find that one half of the discriminator transformer in the AFC circuit has no continuity. I pulled it thinking it may have been one of the brittle wires etc - but to no avail. The center tapped half shows continuity (10 ohm total/5 ohm to CT) the secondary so-to-speak side is open. Any ideas on rectifying this?
T.D.
:Well, you can open it up and try to find the break. Often the break will be in a lead running to one of the terminals, though on occasion the break can be down in the coil, which can be hard to repair. The AFC circuit should have no affect on receiver sensitivity, though. All it does is prevent receiver frequency drift. Do not confuse this circuit with the AVC circuit, though it is good that you found this problem. I may be wrong about this, but from what I can see in the circuit diagram, the output of the AFC circuit (plate of 6J7 AFC control) only goes to the ultra high frequency band oscillator coil. I think this circuit is only used for that band. I may be wrong. If you get this circuit working properly, you may find that it affects other bands, but I don't think it does. When AFC is turned on, it should help keep stations locked on frequency for the band it affects. You may notice this as you tune over the dial with this circuit on. Stations will tend to be "pulled" wider than their normal positions. If you can imagine this as something kind of tugging on each end of a station and making it wider. It seems this way, at least. Since the AFC circuit is trying to keep the receiver in tune with a certain station, as you tune away from that station it will tend to follow you for a moment until you have tuned away far enough, and then it will pop out of place. Depending on your AFC circuit, things may or may not seem this way. All in all, though, this circuit should add some stability to your ultra high frequency short wave band.
:
:At any rate, the test I was asking you to perform was rather simple in nature--just seeing if only local stations got louder when the grid terminal was touched, or if both local and distant stations got louder. This simply says whether your phenomenon is due to normal AVC operation (the former circumstance), or if the primary RF circuits are out of alignment or your antenna lacks proper size (latter condition).
:
:Good luck with the discriminator in the AFC circuit. That really is quite the radio. If it is adjusted properly, according to the original literature, you should be pretty amazed at its sound quality.
:
:Thomas
:
::Thomas,
::
::thanks for the info. Not finished with your suggestion below, but I did find that one half of the discriminator transformer in the AFC circuit has no continuity. I pulled it thinking it may have been one of the brittle wires etc - but to no avail. The center tapped half shows continuity (10 ohm total/5 ohm to CT) the secondary so-to-speak side is open. Any ideas on rectifying this?
:
::Thomas,
::
::thanks for the info. Not finished with your suggestion below, but I did find that one half of the discriminator transformer in the AFC circuit has no continuity. I pulled it thinking it may have been one of the brittle wires etc - but to no avail. The center tapped half shows continuity (10 ohm total/5 ohm to CT) the secondary so-to-speak side is open. Any ideas on rectifying this?
I tried shorting the path of the 50k resistor to the chasis on a weak and a loud station - both results are the same - all audio was practically lost. Very Similar to when the "buttons" on the teledial are depressed to silence the stations. Since I had the problems with the open discriminator AFC transformer - I removed it temporarily and that circuit is now open until I figure out a replacement - not sure if that changes what you described I should do. I did notice that the original phenomena that I spoke of - stations getting louder - happens on the P-A (police - aircraft band) when I get close or touch the grid of the 6L7, I don't notice this on the Broadcast AM band. But I did notice that on AM - getting close to the grid on the 6k7 RF amp - I do get a louder signal. Maybe we're back to the whole doublet antenna? Not sure as this is my first run in to a doublet being needed (and I am only using a single wire).
The reason why some bands may get louder when you touch the grid of the 6L7 and not others is because each band is fine tuned separately. Each band has its own RF coil that must be aligned with a signal generator according to the original literature for this radio. If the RF front end is not aligned for each band, each band may not be working as efficiently as it should.
As for shorting the 50K to ground, you are supposed to short AFTER the 1MEG to ground, not before it. If you short before the 1MEG (at the 50K), you will be shorting out the audio signal just like the interstation silencer does. The general trend for this circuit diagram (and most circuit diagrams) is to work left to right, but the AVC circuit, since it takes its signal down at the audio end and sends it back to the RF sections, works right to left. This is also true with most superheterodyne circuit diagrams. So, that said, finding the *after* spot on the 1MEG resistor would be to the *left* of the 1MEG resistor. To the left of the 1MEG resistor is the beginning of your AVC circuit, and it continues on as I originally stated with the resistors and condensers I originally stated.
Good luck!
T.D.
:Thomas,
:
:I tried shorting the path of the 50k resistor to the chasis on a weak and a loud station - both results are the same - all audio was practically lost. Very Similar to when the "buttons" on the teledial are depressed to silence the stations. Since I had the problems with the open discriminator AFC transformer - I removed it temporarily and that circuit is now open until I figure out a replacement - not sure if that changes what you described I should do. I did notice that the original phenomena that I spoke of - stations getting louder - happens on the P-A (police - aircraft band) when I get close or touch the grid of the 6L7, I don't notice this on the Broadcast AM band. But I did notice that on AM - getting close to the grid on the 6k7 RF amp - I do get a louder signal. Maybe we're back to the whole doublet antenna? Not sure as this is my first run in to a doublet being needed (and I am only using a single wire).
Also, another thing concerning that antenna connection. I do not know how it is worded on your receiver, but as it is worded in the diagram, I can see how some confusion could result. The terminal marked ANTENNA is the terminal you want to use for your long wire. If you were to wire your long wire to the DOUBLET terminal, your long wire would only work for the A and D bands. The only time that the DOUBLET terminal should be used is if you use a doublet antenna. In that case, each half of the doublet would be wired to the two terminals, one terminal per half. Make sure that your long wire antenna is wired to the ANTENNA terminal, and in the case that the terminal is not marked clearly on your receiver, follow the wiring to the switch to confirm which terminal is what. If you have your long wire attached to the wrong terminal, this could be what is causing your trouble.
As you can see in the schematic, when switched to the A band, both the Antenna and Doublet terminals are connected to the high end of the A band coil (regular AM). When the D band is selected, the first RF amplifier is skipped, and a coil, which connects directly to the grid of the 6L7 tube, is connected to both the Antenna and Doublet terminals. As stated, when these two bands are selected, it does not matter which terminal your antenna is wired to. Both terminals feed the appropriate band coil in the appropriate conventional antenna place. With the B and C bands, however, the lower half of each primary RF coil is wired to the chassis, which is in turn wired to the Doublet terminal (so that either ground or the doublet serves as that half of the antenna), and the top half of each coil is connected only to the Antenna terminal. If your long wire was connected to the Doublet terminal, for the B and C bands you would be grounding it instead of connecting it to the proper coils. This would make those bands perform poorly.
I did try across both as a matter of fact. One silences and the other doesn't seem to do much - on either a weak or strong station. Not sure if there may be another issue in here - possibly the AVC not working I'd say (as my strong stations are STRONG and the weak ones are weak and I have to move around the volume quite a bit).
:Thomas,
:
:I did try across both as a matter of fact. One silences and the other doesn't seem to do much - on either a weak or strong station. Not sure if there may be another issue in here - possibly the AVC not working I'd say (as my strong stations are STRONG and the weak ones are weak and I have to move around the volume quite a bit).
Finally found it! Yep - just as you said (sorry I was looking in another place as the radio has had some modifications done in the past which I want to rectify). The strong station became louder and the quiet ones stayed the same - though I guess I expected the quiet ones to be louder and the strong one to possibly be softer - would this be a result of possibly the 1 meg resitor you mentioned (if out of tolerance?)? I have recapped all electrolytics and wax/paper ones - replaced some high wattage resistors that were burned or open. I haven't gone thru all the rest though. I am fighting a replacement speaker and field coil that only is 2k ohms with a 2k ohm 25W resistor in parallel (4.4k original). This is sort of throwing off my voltage divider and sending about 260V/107V to the tubes vs 260/90V - so I'm using my variac to compensate somewhat at this point. When I got the set it had 2 5y3 tubes in place of where the 5z4's are. So I replaced with 5z4's as I am trying to re-tube it with all the original metal cased tubes. I notice both of these run kind of hot and my transformer has more warmth to it than I'd like. Any thoughts?
A note about parts: condensers are what short out and draw too much current. Resistors usually drift up in resistance, so they do not usually increase current draw as they age. Some radios, however, have wire wound power resistors encased in a metal housing (the kind with multiple taps on them). On occasion these manage to form shorts and leaks to the chassis. To test one of these units, disconnect all wiring to it and test for leakage to the chassis between each terminal. Unless the diagram says that the resistor is connected to its metal case at some point, it should not be. Also, any chokes or transformers or field coils that are mounted the chassis must not have leaks.
What are the modifications to the set? Let me know what they are and I will let you know if you must change them for the sake of the power supply or not. If they are minor audio or RF alterations, this should not affect your power supply much. I make alterations to the audio section of my radios all the time, depending on how harsh the audio is. I put in a special kind of negative feedback system that I came up with myself. It makes the bass notes fuller and richer, and makes the treble crisper with crisper highs, instead of the brute force sound some radios have. This is usually a modification I make to my phonographs where I'll be playing my 78s a lot and want nice fidelity to the sound. With AM radio, having full definition treble is actually slightly undesirable because you don't really want all the carrier frequency sound and all that. Anyway, so yeah, let me know what those modifications are.
As for the AVC circuit, that is exactly how it is supposed to operate. Notice that your volume control isn't until after all of those RF stages. If you receive a strong local station, it will overdrive the RF stages and make them distort. You are not able to control this because there is no volume control prior to the RF stages. Still, when a distant station is received, you want to employ full sensitivity of those stages. The automatic volume control circuit automatically controls the sensitivity of these stages for ease of operation. On very early radios, the volume control was placed at the beginning, on the antenna circuit. You had to turn the volume up way high in order to find the distant stations. Then, when you ran across a local one, it would blare. This is obnoxious and troublesome. This circuit helps keep all the stations at a more average volume so that you don't have to adjust the volume all the time. Furthermore, when receiving distant stations that drift and flutter, this circuit reduces those effects to some extent, making your listening experience a more pleasurable one. It helps stabilize the volume of the station being received. To some extent this circuit also reduces interferance, as it lowers receiver sensitivity whenever crashes of static occurr, and then increases the sensitivity again when they are gone. It does not do this much, though, as it is designed to be somewhat sluggish and respond comparratively slowly compared to a crash of static for the following reason: The only drawback of the AVC circuit is that you will notice that when a symphony is played over the radio (if any symphonies are ever played over AM anymore. ...seems like it's all just propaganda talk these days), whenever the symphony gets really quiet, and then crashes into a loud segment, the dynamics of the quiet to loud are diminished to some extent, ruining the dynamics of the music. You will also notice this when listening to Glenn Miller's "Tuxedo Junction." If you own the record, you know that the quiet to loud segments are much more dramatic on the record than over AM radio. Crosley tried to reduce this situation somewhat with their special expansionator tube (I think that is what it's called....something like that). The AVC circuit is designed to respond quickly enough to somewhat reduce flutter and fade, but not so quickly as to ruin all the dynamics of a musical piece. Still, it does ruin the dynamics to some extent. Music over AM radio seems somewhat like a "wall" of music, as some of the volume dynamics are missing. At any rate, that is exactly how your AVC circuit should work. Normally it reduces the volume of local stations, so when you short it out, they'll get louder. This will not happen for distant stations because the AVC circuit is not sending any negative bias to the RF tubes. They are working at full efficiency to bring in that distant station.
At any rate, now that you have discovered your AVC circuit and how it operates, I want you to still examine your radio for the original fault you came on here for. Check to see what touching the grid lead of the 6F7 tube does. Does it simply perform the same task performed by you shorting out the AVC circuit? If so, all you are doing when the volume gets louder when you touch the grid terminal, is draining out the negative voltage in the AVC circuit. However, if, when you touch this grid lead on DISTANT stations, the music still gets louder, then you know you have a different problem because, as you remember, the AVC circuit only makes local stations quieter, and shorting it out will only make local stations get louder. If distant stations get louder when you touch the grid lead of the 6A7, what you want to do is first make sure that your antenna wire is connected to the Antenna terminal and not the Doublet terminal. Then, if this is satisfied, take a good signal generator of known condition (properly callibrated), and align the various RF circuits prior to the 6F7 tube for each band. These circuits in the primary RF amplifier circuit must be aligned in order for them to work at their maximum efficiency.
Good luck and be thorough. If it helps when I type these long things, print them out. I often write a lot of detail that needs to be gone over in completion.
Take care,
Thomas
:Thomas...
:
:Finally found it! Yep - just as you said (sorry I was looking in another place as the radio has had some modifications done in the past which I want to rectify). The strong station became louder and the quiet ones stayed the same - though I guess I expected the quiet ones to be louder and the strong one to possibly be softer - would this be a result of possibly the 1 meg resitor you mentioned (if out of tolerance?)? I have recapped all electrolytics and wax/paper ones - replaced some high wattage resistors that were burned or open. I haven't gone thru all the rest though. I am fighting a replacement speaker and field coil that only is 2k ohms with a 2k ohm 25W resistor in parallel (4.4k original). This is sort of throwing off my voltage divider and sending about 260V/107V to the tubes vs 260/90V - so I'm using my variac to compensate somewhat at this point. When I got the set it had 2 5y3 tubes in place of where the 5z4's are. So I replaced with 5z4's as I am trying to re-tube it with all the original metal cased tubes. I notice both of these run kind of hot and my transformer has more warmth to it than I'd like. Any thoughts?
have come along way with this unit - everything seems to be working well except the AFC as I see little difference when on or off. I noticed that on the 6k7 AFC amp, I cannot get 4v to appear where denoted on the Riders manual. The cap was replaced and the resistor checks for approx. 570 ohms. I did have to replace the discriminator transformer with another NOS Miller model (which I hope will work as the leads were similiar and was for 455 KC's). It seems at most I can get 1/2 volts to 1 volt to appear. Bad tube possibly?
Thomas
:Thomas,
:
:have come along way with this unit - everything seems to be working well except the AFC as I see little difference when on or off. I noticed that on the 6k7 AFC amp, I cannot get 4v to appear where denoted on the Riders manual. The cap was replaced and the resistor checks for approx. 570 ohms. I did have to replace the discriminator transformer with another NOS Miller model (which I hope will work as the leads were similiar and was for 455 KC's). It seems at most I can get 1/2 volts to 1 volt to appear. Bad tube possibly?
ANother question is with the UltraWave band. I have been trying to tune this circuit; however, I cannot find a resonance anywhere from 18 to 70 KC's on the dial to ping with my signal generator. I experimented by moving the detector coil in and out of the solder lug as described in the Rider's manual - anyother suggestions on getting this band inline (though I'm sure there's not too much out there in AM mode)?