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Hum on AM HF receiver
Most of the strong AM signals received on my National HRO are affected by hum.
I put a ceramic capacitor in parallel to each of the 80-type vacuum diodes with no noticeable improvement. Does anyone remember which other cure was proposed to solve the problem? 73 Tony, I0JX |
Hum on AM HF receiver
Most of the strong AM signals received on my National HRO are affected by hum.
I put a ceramic capacitor in parallel to each of the 80-type vacuum diodes with no noticeable improvement. Does anyone remember which other cure was proposed to solve the problem? Before someone gives me a simple answer, I confirm that the electrolytic capacitors were checked to be good and the DC HV is perfectly filtered. No hum in absence of signal. 73 Tony I0JX |
Hum on AM HF receiver
Antonio Vernucci wrote:
Most of the strong AM signals received on my National HRO are affected by hum. I put a ceramic capacitor in parallel to each of the 80-type vacuum diodes with no noticeable improvement. Does anyone remember which other cure was proposed to solve the problem? You sure the power supply filter caps are good? --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
Hum on AM HF receiver
You sure the power supply filter caps are good?
Please read my self-reply. Tony I0JX |
Hum on AM HF receiver
Are you sure the hum isn't real? A lot of solid state
devices trash up the power line. Rectifiers are a big source, lamp dimmers and fluorescent lamps are another. Barring that, look for heater cathode shorts from the detector tube back to the power amplifier tube. -Chuck Antonio Vernucci wrote: You sure the power supply filter caps are good? Please read my self-reply. Tony I0JX |
Hum on AM HF receiver
Antonio Vernucci wrote:
You sure the power supply filter caps are good? Please read my self-reply. If you pull the detector tube out, is there hum? How about the last IF tube? How close to the front end can you get before there is still hum after the tube is pulled out? --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
Hum on AM HF receiver
"Antonio Vernucci" wrote in message ... Most of the strong AM signals received on my National HRO are affected by hum. I put a ceramic capacitor in parallel to each of the 80-type vacuum diodes with no noticeable improvement. Does anyone remember which other cure was proposed to solve the problem? 73 Tony, I0JX Tony I'm assuming you're talking about tuneable hum, where the AC line frequency modulates the received carrier. Does the receiver have AC line bypass caps from each leg of the AC power to the chassis? Have you tried grounding the chassis to a good earth ground to see if the hum modulation level is reduce? If grounding helps, you should check to see that the bypass caps are installed. Also adding ferrite snap on cores on the power may help along with the ground connection. pete k1zjh |
Hum on AM HF receiver
Its MAGNETIC induction from the power transformer into the audio
transformer. -Al Antonio Vernucci wrote: Most of the strong AM signals received on my National HRO are affected by hum. I put a ceramic capacitor in parallel to each of the 80-type vacuum diodes with no noticeable improvement. Does anyone remember which other cure was proposed to solve the problem? 73 Tony, I0JX |
Hum on AM HF receiver
I think its MAGNETIC coupling from the power transformer to the audio
transformer. -Al Antonio Vernucci wrote: Most of the strong AM signals received on my National HRO are affected by hum. I put a ceramic capacitor in parallel to each of the 80-type vacuum diodes with no noticeable improvement. Does anyone remember which other cure was proposed to solve the problem? 73 Tony, I0JX |
Hum on AM HF receiver
"Al Schapira" wrote in message ... I think its MAGNETIC coupling from the power transformer to the audio transformer. -Al Antonio Vernucci wrote: Most of the strong AM signals received on my National HRO are affected by hum. I put a ceramic capacitor in parallel to each of the 80-type vacuum diodes with no noticeable improvement. Does anyone remember which other cure was proposed to solve the problem? 73 Tony, I0JX If its an older HRO, and it has to be with a type 80 rectifier, the power supply is on a separate chassis with no way that hum could be induced into the output transformer. In any case, this would be quite low level hum. Much more likely to be hum modulated onto the carriers due to poor grounding, bypassing, etc. The HRO was always a well designed and well made receiver and should not have this problem. -- --- Richard Knoppow Los Angeles, CA, USA |
Hum on AM HF receiver
On Feb 21, 6:28*pm, "Richard Knoppow" wrote:
"Al Schapira" wrote in message ... I think its MAGNETIC coupling from the power transformer to the audio transformer. -Al Antonio Vernucci wrote: Most of the strong AM signals received on my National HRO are affected by hum. I put a ceramic capacitor in parallel to each of the 80-type vacuum diodes with no noticeable improvement. Does anyone remember which other cure was proposed to solve the problem? 73 Tony, I0JX * * If its an older HRO, and it has to be with a type 80 rectifier, the power supply is on a separate chassis with no way that hum could be induced into the output transformer. In any case, this would be quite low level hum. Much more likely to be hum modulated onto the carriers due to poor grounding, bypassing, etc. The HRO was always a well designed and well made receiver and should not have this problem. -- --- Richard Knoppow Los Angeles, CA, USA While I am not familiar with this particular rcvr, if the pwr supply is on a separate chassis and is connected to the main chassis with a cable, would a bad ground connection in the cable or plugs (if any) cause this? Maybe try running a temporary wire from chassis to chassis to eliminate the possibility. 73 Mike KF6KXG |
Hum on AM HF receiver
"tchrme" wrote in message ... On Feb 21, 6:28 pm, "Richard Knoppow" wrote: "Al Schapira" wrote in message ... I think its MAGNETIC coupling from the power transformer to the audio transformer. -Al Antonio Vernucci wrote: Most of the strong AM signals received on my National HRO are affected by hum. I put a ceramic capacitor in parallel to each of the 80-type vacuum diodes with no noticeable improvement. Does anyone remember which other cure was proposed to solve the problem? 73 Tony, I0JX If its an older HRO, and it has to be with a type 80 rectifier, the power supply is on a separate chassis with no way that hum could be induced into the output transformer. In any case, this would be quite low level hum. Much more likely to be hum modulated onto the carriers due to poor grounding, bypassing, etc. The HRO was always a well designed and well made receiver and should not have this problem. -- --- Richard Knoppow Los Angeles, CA, USA While I am not familiar with this particular rcvr, if the pwr supply is on a separate chassis and is connected to the main chassis with a cable, would a bad ground connection in the cable or plugs (if any) cause this? Maybe try running a temporary wire from chassis to chassis to eliminate the possibility. 73 Mike KF6KXG Its certainly worth a try. I am not sure from the original poster's question if the hum is constant or is present on some stations. Both conditions can be caused by poor filter caps but the second is more likely to come from something else. Strong RF pickup from a poorly grounded supply would be one of the causes. If these are broadcast stations (would mean he has one or more accessory coil sets) another tool would be a simple loop antenna, perhaps two turns of wire around a shoebox sized form. This will give low enough signal strength from local stations to avoid front end overload and will also help to track down modulation coming from sources external to the receiver. And front end overload is certainly another possible cause. -- --- Richard Knoppow Los Angeles, CA, USA |
Hum on AM HF receiver
Al Schapira wrote:
Its MAGNETIC induction from the power transformer into the audio transformer. If this is the case, putting a piece of thick sheet steel between the two transformers will change the noise, for diagnostic purposes. Then you can try and reorient the transformers to reduce coupling. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
Hum on AM HF receiver
Are you sure the hum isn't real? A lot of solid state
devices trash up the power line. Rectifiers are a big source, lamp dimmers and fluorescent lamps are another. Barring that, look for heater cathode shorts from the detector tube back to the power amplifier tube. -Chuck My problem is called "tuneable hum". It has nothing to do with filter capacitors, transformers coupling, cathode to filament leakage, etc. It only shows up on certain stations. It depends on the path followed by RF signal (e.g. if it passes through the rectifiers). Typical cures a - bypass capacitor across the power supply rectifiers - bypasscapacitor between mains and ground. But there still are some stations affected by a considerable hum. I was seeking some suggestions on what to do more. 73 Tony I0JX |
Hum on AM HF receiver
If you pull the detector tube out, is there hum? How about the last IF
tube? How close to the front end can you get before there is still hum after the tube is pulled out? --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." My problem is called "tuneable hum". It has nothing to do with filter capacitors, transformers coupling, cathode to filament leakage, etc. It only shows up on certain stations. It depends on the path followed by RF signal (e.g. if it passes through the rectifiers). Typical cures a - bypass capacitor across the power supply rectifiers - bypasscapacitor between mains and ground. But there still are some stations affected by a considerable hum. I was seeking some suggestions on what to do more. 73 Tony I0JX |
Hum on AM HF receiver
Its MAGNETIC induction from the power transformer into the audio transformer.
-Al In the HRO, the power trasformer is maybe 20 inches away from the audio transformer. My problem is called "tuneable hum". It has nothing to do with filter capacitors, transformers coupling, cathode to filament leakage, etc. It only shows up on certain stations. It depends on the path followed by RF signal (e.g. if it passes through the rectifiers). Typical cures a - bypass capacitor across the power supply rectifiers - bypasscapacitor between mains and ground. But there still are some stations affected by a considerable hum. I was seeking some suggestions on what to do more. 73 Tony I0JX |
Hum on AM HF receiver
"Antonio Vernucci" wrote in message news:47bf2ba9$0$36443$ My problem is called "tuneable hum". It has nothing to do with filter capacitors, transformers coupling, cathode to filament leakage, etc. It only shows up on certain stations. It depends on the path followed by RF signal (e.g. if it passes through the rectifiers). Typical cures a - bypass capacitor across the power supply rectifiers - bypasscapacitor between mains and ground. Tony I0JX Tony Have you tried using an external balanced antenna? Is this problem occuring with a random length antenna wire that is using the AC lines, etc. as the counterpoise for the antenna system? Some more information on the antenna might provide some clues. Tuneable hum shouldn't be a problem when using an external balanced antenna. Pete k1zjh |
Hum on AM HF receiver
"Antonio Vernucci" wrote in message ... Its MAGNETIC induction from the power transformer into the audio transformer. -Al In the HRO, the power trasformer is maybe 20 inches away from the audio transformer. My problem is called "tuneable hum". It has nothing to do with filter capacitors, transformers coupling, cathode to filament leakage, etc. It only shows up on certain stations. It depends on the path followed by RF signal (e.g. if it passes through the rectifiers). Typical cures a - bypass capacitor across the power supply rectifiers - bypasscapacitor between mains and ground. But there still are some stations affected by a considerable hum. I was seeking some suggestions on what to do more. 73 Tony I0JX Tunable hum from the power supply where the filtering is good is found mainly in AC/DC receivers. There are other sources, mainly lack of bypassing especially of the screens of tubes with AVC on them and heater to cathode leakage anywhere in the RF path. In either case the tube or tubes affected can modulate hum onto signals. Since AVC tubes are operating with a non-linear characteristic they can be pretty good modulators. If you have access to a good tube checker, one which will indicate shorts and leakage, test the tubes with it. A tube may test fine for emmision and transconductance but still be leaky. Bypass condensers can sometimes be checked by simply paralleling the cap with a good one but, if its got low enough series resistance this test may not work and only substituting another, known good, cap will do. You are working on a receiver of first class design which did not have this problem designed into it so it must be coming from a defective component. The tubes and by-pass or decoupling capacitors are the most likely. However, I would also do a routine check for correct tube socket voltages and resistance values. These can somtime give you a good clue as to what is wrong. Note that where tubes on AVC are concerned tunable hum can vary with the strength of the signal and with the setting of the RF gain control. This may be another clue. You have not answered my question about getting the same hum on other radios at the same location. There are conditions where the actual signal can be modulated by somthing often high voltage power lines nearby. This is an effect familiar to those with auto radios. -- --- Richard Knoppow Los Angeles, CA, USA |
Hum on AM HF receiver
Tunable hum from the power supply where the filtering is good is found
mainly in AC/DC receivers. There are other sources, mainly lack of bypassing especially of the screens of tubes with AVC on them and heater to cathode leakage anywhere in the RF path. In either case the tube or tubes affected can modulate hum onto signals. Since AVC tubes are operating with a non-linear characteristic they can be pretty good modulators. If you have access to a good tube checker, one which will indicate shorts and leakage, test the tubes with it. A tube may test fine for emmision and transconductance but still be leaky. Bypass condensers can sometimes be checked by simply paralleling the cap with a good one but, if its got low enough series resistance this test may not work and only substituting another, known good, cap will do. You are working on a receiver of first class design which did not have this problem designed into it so it must be coming from a defective component. The tubes and by-pass or decoupling capacitors are the most likely. However, I would also do a routine check for correct tube socket voltages and resistance values. These can somtime give you a good clue as to what is wrong. Note that where tubes on AVC are concerned tunable hum can vary with the strength of the signal and with the setting of the RF gain control. This may be another clue. You have not answered my question about getting the same hum on other radios at the same location. There are conditions where the actual signal can be modulated by somthing often high voltage power lines nearby. This is an effect familiar to those with auto radios. Hi Dick, thanks for useful info. I have to carry more tests on my HRO, including that of connecting my Yagi antenna to it instead of a short indoor piece of wire. I must also try to connect that same piece of wire to another receiver, to see whether there is any difference (actually I have a second HRO to compare). The hum heavily depends on signal strength and on the particular station (there are some with no hum at all), so I would tend not to attribute the cause to internal tube leakage. Your supposition that poor screen / AVC bypassing could be the cause looks interesting and I will do some tests at that regard. However I did not fully understand your argument that putting a capacitor in parallel to an existing one may not a good way to do the test. If I use a low-ESR capacitor, why is the existing one not out of the game? Thanks and 73 Tony I0JX |
Hum on AM HF receiver
"Antonio Vernucci" wrote in message ... Tunable hum from the power supply where the filtering is good is found mainly in AC/DC receivers. There are other sources, mainly lack of bypassing especially of the screens of tubes with AVC on them and heater to cathode leakage anywhere in the RF path. In either case the tube or tubes affected can modulate hum onto signals. Since AVC tubes are operating with a non-linear characteristic they can be pretty good modulators. If you have access to a good tube checker, one which will indicate shorts and leakage, test the tubes with it. A tube may test fine for emmision and transconductance but still be leaky. Bypass condensers can sometimes be checked by simply paralleling the cap with a good one but, if its got low enough series resistance this test may not work and only substituting another, known good, cap will do. You are working on a receiver of first class design which did not have this problem designed into it so it must be coming from a defective component. The tubes and by-pass or decoupling capacitors are the most likely. However, I would also do a routine check for correct tube socket voltages and resistance values. These can somtime give you a good clue as to what is wrong. Note that where tubes on AVC are concerned tunable hum can vary with the strength of the signal and with the setting of the RF gain control. This may be another clue. You have not answered my question about getting the same hum on other radios at the same location. There are conditions where the actual signal can be modulated by somthing often high voltage power lines nearby. This is an effect familiar to those with auto radios. Hi Dick, thanks for useful info. I have to carry more tests on my HRO, including that of connecting my Yagi antenna to it instead of a short indoor piece of wire. I must also try to connect that same piece of wire to another receiver, to see whether there is any difference (actually I have a second HRO to compare). The hum heavily depends on signal strength and on the particular station (there are some with no hum at all), so I would tend not to attribute the cause to internal tube leakage. Your supposition that poor screen / AVC bypassing could be the cause looks interesting and I will do some tests at that regard. However I did not fully understand your argument that putting a capacitor in parallel to an existing one may not a good way to do the test. If I use a low-ESR capacitor, why is the existing one not out of the game? Thanks and 73 Tony I0JX First of all heater to cathode leakage may cause hum modulation which is dependant on the AVC bias voltage so it should not be discounted. While leakage of capacitors is often called equivalent series resistance (ESR) it can also be parallel resistance. For instance its pretty common for bad electrolytic caps to look like dead shorts. The same can happen to paper caps. If the parallel resistance of a capacitor is low paralleling another cap across it will simply put the low resistance across both. So, if one is checking for bad caps its best to actually substitute the cap otherwise you can be mislead. If you don't have a tube checker you can also check by substitution. You can switch the tubes around since the HRO uses the same types in a couple of places. Moving a bad tube should change the symptoms. Since you have another receiver you can do more complete subsitution checking. Check tubes with AVC on them since the varying bias will change the way the tube modulates the signal, assuming its bad. BTW, have you checked to see if the hum changes when you go to manual RF gain control? There may be a clue there if it does. -- --- Richard Knoppow Los Angeles, CA, USA |
Hum on AM HF receiver
On 2/22/08 12:07 PM, in article
, "Antonio Vernucci" wrote: Are you sure the hum isn't real? A lot of solid state devices trash up the power line. Rectifiers are a big source, lamp dimmers and fluorescent lamps are another. Barring that, look for heater cathode shorts from the detector tube back to the power amplifier tube. -Chuck My problem is called "tuneable hum". It has nothing to do with filter capacitors, transformers coupling, cathode to filament leakage, etc. It only shows up on certain stations. It depends on the path followed by RF signal (e.g. if it passes through the rectifiers). Typical cures a - bypass capacitor across the power supply rectifiers - bypasscapacitor between mains and ground. But there still are some stations affected by a considerable hum. I was seeking some suggestions on what to do more. 73 Tony I0JX What happens when you reduce the RF Gain? How about in CW mode? |
Hum on AM HF receiver
Antonio Vernucci wrote:
Are you sure the hum isn't real? A lot of solid state devices trash up the power line. Rectifiers are a big source, lamp dimmers and fluorescent lamps are another. Barring that, look for heater cathode shorts from the detector tube back to the power amplifier tube. -Chuck My problem is called "tuneable hum". It has nothing to do with filter capacitors, transformers coupling, cathode to filament leakage, etc. It only shows up on certain stations. It depends on the path followed by RF signal (e.g. if it passes through the rectifiers). Typical cures a - bypass capacitor across the power supply rectifiers - bypasscapacitor between mains and ground. But there still are some stations affected by a considerable hum. I was seeking some suggestions on what to do more. Tunable hum is easy: most of the time it is caused by either heater-cathode leakage on the AGC, or pentodes in the IF amplifier, or bypass capacitors on the AGC, or IF tubes. Tunable hum is a modulation issue, the small amount of ripple that comes from the heater-cathode leakage is amplified greatly by the AGC circuitry, and AM modulates the IF signal with power-line hum. A little tube swapping very often finds the problem. Start with the AGC/detector tube, and then go to the tubes that are controlled by the AGC line. Make sure that you put any tube that isn't bad back in its original position. If you look at the power supply, you will probably see a multi section electrolytic capacitor that has resistors between the sections. Usually, the AGC tube is powered from the most filtered section of the power supply filter, so if the hum is coming from there, it is usually everywhere. -Chuck |
Hum on AM HF receiver
"Antonio Vernucci" wrote in message ... Are you sure the hum isn't real? A lot of solid state devices trash up the power line. Rectifiers are a big source, lamp dimmers and fluorescent lamps are another. Barring that, look for heater cathode shorts from the detector tube back to the power amplifier tube. -Chuck My problem is called "tuneable hum". It has nothing to do with filter capacitors, transformers coupling, cathode to filament leakage, etc. It only shows up on certain stations. It depends on the path followed by RF signal (e.g. if it passes through the rectifiers). Tony You have been offered several suggestions, and I offered some advice and never heard a reply back to some questions I asked regarding the antenna system and grounding. Tuneable hum can indeed be caused by poor cathode RF bypass caps in the RF and IF stages. The lack of adequate bypass capacitors would allow any RF/IF stage with cathode/filament leakage to be modulated by the AC signal on the filament. No signal, no hum. Pete k1zjh |
Hum on AM HF receiver
I have to carry more tests on my HRO, including that of connecting my Yagi
antenna to it instead of a short indoor piece of wire. Using my HF Yagi antenna instead of the indoor wire all problems completely disappeared and there is no hum at all on any station. The problem however is not due to interfering sources (e.g. lamps, power supplies, etc.) in my house, but to the fact that the RF signal of certain stations (not all of them) follows a path where it gets modulated by alternating current. As a matter of fact, using a simple wire, the ground counterpoise is represented by the house electrical wiring While leakage of capacitors is often called equivalent series resistance (ESR) it can also be parallel resistance. For instance its pretty common for bad electrolytic caps to look like dead shorts. The same can happen to paper caps. If the parallel resistance of a capacitor is low paralleling another cap across it will simply put the low resistance across both. So, if one is checking for bad caps its best to actually substitute the cap otherwise you can be mislead. Still, I am unable to follow your argument. Assume that a screen bypass capacitor is leaky. Its DC resistance typically is in the range of several hundreds kohm (should it be lower, the screen DC voltage would be greatly affected). Having a resistance in parallel to a good capacitor does not impair its bypassing function. Even a low resistance value would not harm (apart from its effect on the DC voltage). In the limit, a zero resistance would turn into a perfect bypass (again, apart from the DC voltage). BTW, have you checked to see if the hum changes when you go to manual RF gain control? There may be a clue there if it does. The hum-to-sound ratio was almost independent of the RF gain control setting (a potentiometer of the tubes cathode). 73 Tony I0JX |
Hum on AM HF receiver
Antonio Vernucci wrote: I have to carry more tests on my HRO, including that of connecting my Yagi antenna to it instead of a short indoor piece of wire. Using my HF Yagi antenna instead of the indoor wire all problems completely disappeared and there is no hum at all on any station. .... BTW, have you checked to see if the hum changes when you go to manual RF gain control? There may be a clue there if it does. The hum-to-sound ratio was almost independent of the RF gain control setting (a potentiometer of the tubes cathode). Both of the above statements indicate that the hum was caused by emissions from your receiver, or your shack. Classically, on a solid state rectified device, the rectifiers don't switch instantly, and hand up for an instant at every zero crossing of the AC waveform. That hang is a 1 usec long short circuit, and emits 60Hz glitches at every crossing. It will radiate 60 Hz modulated broadband hash over a frequency range that includes the AM broadcast band. When you switched to an external antenna, you did two things, one, you got the antenna away from the interfering source, and two, you increased the signal strength of the received stations. Probable sources of such hash are compact fluorescent lamps, regular fluorescent lamps, the power supplies in any test equipment, or soldering irons in your shop, and the radio itself. -Chuck |
Hum on AM HF receiver
On Feb 24, 6:36 am, Chuck Harris wrote:
Antonio Vernucci wrote: I have to carry more tests on my HRO, including that of connecting my Yagi antenna to it instead of a short indoor piece of wire. Using my HF Yagi antenna instead of the indoor wire all problems completely disappeared and there is no hum at all on any station. ... BTW, have you checked to see if the hum changes when you go to manual RF gain control? There may be a clue there if it does. The hum-to-sound ratio was almost independent of the RF gain control setting (a potentiometer of the tubes cathode). Both of the above statements indicate that the hum was caused by emissions from your receiver, or your shack. Classically, on a solid state rectified device, the rectifiers don't switch instantly, and hand up for an instant at every zero crossing of the AC waveform. That hang is a 1 usec long short circuit, and emits 60Hz glitches at every crossing. It will radiate 60 Hz modulated broadband hash over a frequency range that includes the AM broadcast band. When you switched to an external antenna, you did two things, one, you got the antenna away from the interfering source, and two, you increased the signal strength of the received stations. Probable sources of such hash are compact fluorescent lamps, regular fluorescent lamps, the power supplies in any test equipment, or soldering irons in your shop, and the radio itself. -Chuck Tony et al, I have followed this thread with interest as I had a similar problem on a Hammarlund SP-200. Pulling tubes I isolated to the first or second audio or detector. It was only when I measured voltages for each pin on each tube that I began to see the problem. On the first audio I measured the grid and found it to be a bit high |
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