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SX-190 buzz in audio
My SX-190 has buzzed at a level just loud enough to be annoying since
I got it a couple years ago (minus a lot of time when it wasn't working at all) even in standby. One of the things I did to the radio is replace the electrolytic caps, including the main power supply filter. I used the same 2200 uF the schematic called for, but the buzz is still there. Does anyone with one of these radios have a real quiet set, or do they all buzz like this? Would putting caps across the rectifiers reduce the buzz, or do I need to go to a quieter external supply? Or should I do something to the audio amp? One more thing- where can I get a set of tuning tools that includes the hex drivers for IF can slugs? Thanks, MR P.S. Download 100+ pages of SX/AX-190 documentation in an indexed .pdf file he http://www.rehorst.com/mrehorst/Alli...Info-Oct02.pdf |
Mark Rehorst wrote:
My SX-190 has buzzed at a level just loud enough to be annoying quiet set, or do they all buzz like this? Would putting caps across the rectifiers reduce the buzz, or do I need to go to a quieter external supply? Or should I do something to the audio amp? I had a buzzy set of that vintage, not an SX-190, and it turned out the power xfmr was bad. Sounded just like power line noise and was worse with an external antenna. Go figure. One more thing- where can I get a set of tuning tools that includes the hex drivers for IF can slugs? Radio Shack? -Bill |
Mark Rehorst wrote:
My SX-190 has buzzed at a level just loud enough to be annoying quiet set, or do they all buzz like this? Would putting caps across the rectifiers reduce the buzz, or do I need to go to a quieter external supply? Or should I do something to the audio amp? I had a buzzy set of that vintage, not an SX-190, and it turned out the power xfmr was bad. Sounded just like power line noise and was worse with an external antenna. Go figure. One more thing- where can I get a set of tuning tools that includes the hex drivers for IF can slugs? Radio Shack? -Bill |
On Fri, 31 Oct 2003 00:00:33 -0400, --exray-- wrote:
I had a buzzy set of that vintage, not an SX-190, and it turned out the power xfmr was bad. Sounded just like power line noise and was worse with an external antenna. Go figure. Was that mechanical buzzing of the transformer, or was there some electrical problem? My radio buzzes through the speaker. One more thing- where can I get a set of tuning tools that includes the hex drivers for IF can slugs? Radio Shack? Really? I'll check it out. Thanks! -Bill |
On Fri, 31 Oct 2003 00:00:33 -0400, --exray-- wrote:
I had a buzzy set of that vintage, not an SX-190, and it turned out the power xfmr was bad. Sounded just like power line noise and was worse with an external antenna. Go figure. Was that mechanical buzzing of the transformer, or was there some electrical problem? My radio buzzes through the speaker. One more thing- where can I get a set of tuning tools that includes the hex drivers for IF can slugs? Radio Shack? Really? I'll check it out. Thanks! -Bill |
Mark Rehorst wrote:
On Fri, 31 Oct 2003 00:00:33 -0400, --exray-- wrote: I had a buzzy set of that vintage, not an SX-190, and it turned out the power xfmr was bad. Sounded just like power line noise and was worse with an external antenna. Go figure. Was that mechanical buzzing of the transformer, or was there some electrical problem? My radio buzzes through the speaker. No, this was electrical buzzing just like receiving power line noise. mechanically it was quiet. -Bill |
Mark Rehorst wrote:
On Fri, 31 Oct 2003 00:00:33 -0400, --exray-- wrote: I had a buzzy set of that vintage, not an SX-190, and it turned out the power xfmr was bad. Sounded just like power line noise and was worse with an external antenna. Go figure. Was that mechanical buzzing of the transformer, or was there some electrical problem? My radio buzzes through the speaker. No, this was electrical buzzing just like receiving power line noise. mechanically it was quiet. -Bill |
Mark Rehorst wrote:
My SX-190 has buzzed at a level just loud enough to be annoying since I got it a couple years ago (minus a lot of time when it wasn't working at all) even in standby. One of the things I did to the radio is replace the electrolytic caps, including the main power supply filter. I used the same 2200 uF the schematic called for, but the buzz is still there. Does anyone with one of these radios have a real quiet set, or do they all buzz like this? Would putting caps across the rectifiers reduce the buzz, or do I need to go to a quieter external supply? Or should I do something to the audio amp? If it is a buzz, it is probably not a cap. If it is a hum, it is probably a cap. What stage is the buzz from? If you pull the mixer tube, does it still buzz? If you pull the IF stages? What if you pull the detector? At what stage is the buzz getting into the signal? One more thing- where can I get a set of tuning tools that includes the hex drivers for IF can slugs? Cain Electronics in Hampton, VA still has TV tuning sets. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
Mark Rehorst wrote:
My SX-190 has buzzed at a level just loud enough to be annoying since I got it a couple years ago (minus a lot of time when it wasn't working at all) even in standby. One of the things I did to the radio is replace the electrolytic caps, including the main power supply filter. I used the same 2200 uF the schematic called for, but the buzz is still there. Does anyone with one of these radios have a real quiet set, or do they all buzz like this? Would putting caps across the rectifiers reduce the buzz, or do I need to go to a quieter external supply? Or should I do something to the audio amp? If it is a buzz, it is probably not a cap. If it is a hum, it is probably a cap. What stage is the buzz from? If you pull the mixer tube, does it still buzz? If you pull the IF stages? What if you pull the detector? At what stage is the buzz getting into the signal? One more thing- where can I get a set of tuning tools that includes the hex drivers for IF can slugs? Cain Electronics in Hampton, VA still has TV tuning sets. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
In article , --exray-- writes:
Was that mechanical buzzing of the transformer, or was there some electrical problem? My radio buzzes through the speaker. No, this was electrical buzzing just like receiving power line noise. mechanically it was quiet. Any idea of what was wrong with the power trans inside, or did you just replace it and throw it away? Maybe the shielding between the windings was kaput. I have a cheap AM/FM clock radio that sounds like light-dimmer power line noise even on FM. I've tried tacking caps all over the place, to no effect. The DC supply is clean on a scope. You have to wonder how such a sound gets into FM -- it sounds exactly like the sort of noise that sends you reching for the ANL switch on a real radio. --Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
In article , --exray-- writes:
Was that mechanical buzzing of the transformer, or was there some electrical problem? My radio buzzes through the speaker. No, this was electrical buzzing just like receiving power line noise. mechanically it was quiet. Any idea of what was wrong with the power trans inside, or did you just replace it and throw it away? Maybe the shielding between the windings was kaput. I have a cheap AM/FM clock radio that sounds like light-dimmer power line noise even on FM. I've tried tacking caps all over the place, to no effect. The DC supply is clean on a scope. You have to wonder how such a sound gets into FM -- it sounds exactly like the sort of noise that sends you reching for the ANL switch on a real radio. --Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
Mike Knudsen wrote:
I have a cheap AM/FM clock radio that sounds like light-dimmer power line noise even on FM. I've tried tacking caps all over the place, to no effect. The DC supply is clean on a scope. You have to wonder how such a sound gets into FM -- it sounds exactly like the sort of noise that sends you reching for the ANL switch on a real radio. --Mike K. A lot of that kind of noise comes from solid state rectifiers in the powersupply. The HV rectifiers don't switch off quickly enough, and essentially short out the supply for an instant around the zero crossing point of the input sinewave. This always sounds like a very raucus 60Hz (half wave rect.), or 120Hz (full wave rect.) buzz, and it comes in thru some RF or IF stage in the receiver. This can be solved one of two ways: 1) add a series resistor to each each diode to limit how much current can flow, 100 ohms, or some such. (Note, one for each diode!) 2) switch to fast recovery diodes. As to why cheapy FM radios respond to this kind of AM noise, it is all in the use of a ratio detector vs a limiter/discriminator. Ratio detectors are pretty good at eliminating AM, but limiter/discriminator stages are very good at eliminating AM. Ratio detectors are basically simpler, and as such much cheaper. -Chuck Harris, WA3UQV |
Mike Knudsen wrote:
I have a cheap AM/FM clock radio that sounds like light-dimmer power line noise even on FM. I've tried tacking caps all over the place, to no effect. The DC supply is clean on a scope. You have to wonder how such a sound gets into FM -- it sounds exactly like the sort of noise that sends you reching for the ANL switch on a real radio. --Mike K. A lot of that kind of noise comes from solid state rectifiers in the powersupply. The HV rectifiers don't switch off quickly enough, and essentially short out the supply for an instant around the zero crossing point of the input sinewave. This always sounds like a very raucus 60Hz (half wave rect.), or 120Hz (full wave rect.) buzz, and it comes in thru some RF or IF stage in the receiver. This can be solved one of two ways: 1) add a series resistor to each each diode to limit how much current can flow, 100 ohms, or some such. (Note, one for each diode!) 2) switch to fast recovery diodes. As to why cheapy FM radios respond to this kind of AM noise, it is all in the use of a ratio detector vs a limiter/discriminator. Ratio detectors are pretty good at eliminating AM, but limiter/discriminator stages are very good at eliminating AM. Ratio detectors are basically simpler, and as such much cheaper. -Chuck Harris, WA3UQV |
Mike Knudsen wrote:
In article , --exray-- writes: Was that mechanical buzzing of the transformer, or was there some electrical problem? My radio buzzes through the speaker. No, this was electrical buzzing just like receiving power line noise. mechanically it was quiet. Any idea of what was wrong with the power trans inside, or did you just replace it and throw it away? Maybe the shielding between the windings was kaput. Insulation breakdown. I have a cheap AM/FM clock radio that sounds like light-dimmer power line noise even on FM. I've tried tacking caps all over the place, to no effect. The DC supply is clean on a scope. You have to wonder how such a sound gets into FM -- it sounds exactly like the sort of noise that sends you reching for the ANL switch on a real radio. --Mike K. It has a vacuum fluorescent display, doesn't it? Disconnect the display and it'll be quiet. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
Mike Knudsen wrote:
In article , --exray-- writes: Was that mechanical buzzing of the transformer, or was there some electrical problem? My radio buzzes through the speaker. No, this was electrical buzzing just like receiving power line noise. mechanically it was quiet. Any idea of what was wrong with the power trans inside, or did you just replace it and throw it away? Maybe the shielding between the windings was kaput. Insulation breakdown. I have a cheap AM/FM clock radio that sounds like light-dimmer power line noise even on FM. I've tried tacking caps all over the place, to no effect. The DC supply is clean on a scope. You have to wonder how such a sound gets into FM -- it sounds exactly like the sort of noise that sends you reching for the ANL switch on a real radio. --Mike K. It has a vacuum fluorescent display, doesn't it? Disconnect the display and it'll be quiet. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
In article , Chuck Harris
writes: A lot of that kind of noise comes from solid state rectifiers in the powersupply. But why would the SX-190 suddenly develop this problem, unless it's something else? --Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
In article , Chuck Harris
writes: A lot of that kind of noise comes from solid state rectifiers in the powersupply. But why would the SX-190 suddenly develop this problem, unless it's something else? --Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
In article , Chuck Harris
writes: This can be solved one of two ways: 1) add a series resistor to each each diode to limit how much current can flow, 100 ohms, or some such. (Note, one for each diode!) Would shunt caps help? What if it's a bridge unit and I can't get in series? Replace with discrete diodes and Rs? 2) switch to fast recovery diodes. How would these be specified in a catalog? "Fast recovery power diodes"? Thanks, Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
In article , Chuck Harris
writes: This can be solved one of two ways: 1) add a series resistor to each each diode to limit how much current can flow, 100 ohms, or some such. (Note, one for each diode!) Would shunt caps help? What if it's a bridge unit and I can't get in series? Replace with discrete diodes and Rs? 2) switch to fast recovery diodes. How would these be specified in a catalog? "Fast recovery power diodes"? Thanks, Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
In article , Chuck Harris
writes: As to why cheapy FM radios respond to this kind of AM noise, it is all in the use of a ratio detector vs a limiter/discriminator. Ratio detectors are pretty good at eliminating AM, but limiter/discriminator stages are very good at eliminating AM. Ratio detectors are basically simpler, and as such much cheaper. Right, and I am amazed that the top-of-line Heathkit AR-15 FM receiver used a ratio detector. Supposedly they can sound just as good, but I've always associated ratio dets with "cheap." --Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
In article , Chuck Harris
writes: As to why cheapy FM radios respond to this kind of AM noise, it is all in the use of a ratio detector vs a limiter/discriminator. Ratio detectors are pretty good at eliminating AM, but limiter/discriminator stages are very good at eliminating AM. Ratio detectors are basically simpler, and as such much cheaper. Right, and I am amazed that the top-of-line Heathkit AR-15 FM receiver used a ratio detector. Supposedly they can sound just as good, but I've always associated ratio dets with "cheap." --Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
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Mike Knudsen wrote:
2) switch to fast recovery diodes. How would these be specified in a catalog? "Fast recovery power diodes"? Thanks, Mike K. Yes. A number like 1N5407 rings a bell but I'm saying that on memory. Actually I've never seen a case where the diodes caused any audible noise in a regular ole radio although I hear this recommendation often. Tunable hum at times maybe. If this set is making a racket like powerline noise as opposed to a 60 cycle hum it pretty much has to be some leakage in the AC components...plug, switch, xfmr, etc. Remotely possible is some sort of weird rf oscillation. Should be easy to sort between the two by powering it up on DC. -BM |
Mike Knudsen wrote:
2) switch to fast recovery diodes. How would these be specified in a catalog? "Fast recovery power diodes"? Thanks, Mike K. Yes. A number like 1N5407 rings a bell but I'm saying that on memory. Actually I've never seen a case where the diodes caused any audible noise in a regular ole radio although I hear this recommendation often. Tunable hum at times maybe. If this set is making a racket like powerline noise as opposed to a 60 cycle hum it pretty much has to be some leakage in the AC components...plug, switch, xfmr, etc. Remotely possible is some sort of weird rf oscillation. Should be easy to sort between the two by powering it up on DC. -BM |
Moonman wrote:
In article , (Scott Dorsey) wrote: One more thing- where can I get a set of tuning tools that includes the hex drivers for IF can slugs? Cain Electronics in Hampton, VA still has TV tuning sets. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." Can someone please, please explain to me what you're talking about here. Of course I know that IF cans have holes in the top through which to turn the ferrite slugs inside to screw them up or down for tuning purposes. What I don't know is where to get the tuning wands for old radios (or even if they are any different from those for new radios, like I presume I might buy at Radio ****). I also don't know why when the query is made about finding tuning tools for "IF can slugs" (obviously talking about radios), the response is the contact an outfit that "still has *TV* tuning sets". I'm about to start working on three old ('30s and '40s) radios that I bought on eBay, and I don't have ANY tuning slugs. I also have some old boatanchors to work on. Where should I get my set of tuning tools? A somewhat ignorant person Most alignment tool sets will have a variety of sizes included...everything from the tiny hex common in later vintage oriental gear to the larger hex size found in older equipment. The alignment tool set at RS has (had) 6 or so double-ended tools. If none are found, Antique Electronic Supply has a wide variety of them in sets and single. http://www.tubesandmore.com -Bill |
Hi Mike,
I was posting that reply to the side topic of the cheapy AM/FM that buzzed like a bee. I don't have a schematic or other info on the SX190. Is is vacuum tube rectifiers, or selenium? If it is selenium, they tend to get noisy when they go bad. It could be they are arcing internally (or breaking over) when they are under the stress of the peak inverse voltage. -Chuck, WA3UQV Mike Knudsen wrote: In article , Chuck Harris writes: A lot of that kind of noise comes from solid state rectifiers in the powersupply. But why would the SX-190 suddenly develop this problem, unless it's something else? --Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
Hi Mike,
I was posting that reply to the side topic of the cheapy AM/FM that buzzed like a bee. I don't have a schematic or other info on the SX190. Is is vacuum tube rectifiers, or selenium? If it is selenium, they tend to get noisy when they go bad. It could be they are arcing internally (or breaking over) when they are under the stress of the peak inverse voltage. -Chuck, WA3UQV Mike Knudsen wrote: In article , Chuck Harris writes: A lot of that kind of noise comes from solid state rectifiers in the powersupply. But why would the SX-190 suddenly develop this problem, unless it's something else? --Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
For fast recovery diodes and bridges, look in your mouser catalog under
rectron, or ST. They both make gobs and gobs of the beasts. If the unit was ok, and now suddenly buzzes, it might mean that the bridge has one diode that is breaking down. First try replacing the bridge with most anything silicon with at least 2x the HV as a PIV. If that doesn't work, then look at the cap that goes from the power line to the chassis. These are supposed to keep power line noise out of the insides of the chassis. They also take a beating, and are usually cheap wax paper caps. Another thing you can do is run the antenna of another radio as a coax and a 1-2 inch loop of a few turns. bring this loop around to various parts of the SX, and listen for the noise. If your SX can hear it, another receiver should also. -Chuck Mike Knudsen wrote: In article , Chuck Harris writes: This can be solved one of two ways: 1) add a series resistor to each each diode to limit how much current can flow, 100 ohms, or some such. (Note, one for each diode!) Would shunt caps help? What if it's a bridge unit and I can't get in series? Replace with discrete diodes and Rs? 2) switch to fast recovery diodes. How would these be specified in a catalog? "Fast recovery power diodes"? Thanks, Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
For fast recovery diodes and bridges, look in your mouser catalog under
rectron, or ST. They both make gobs and gobs of the beasts. If the unit was ok, and now suddenly buzzes, it might mean that the bridge has one diode that is breaking down. First try replacing the bridge with most anything silicon with at least 2x the HV as a PIV. If that doesn't work, then look at the cap that goes from the power line to the chassis. These are supposed to keep power line noise out of the insides of the chassis. They also take a beating, and are usually cheap wax paper caps. Another thing you can do is run the antenna of another radio as a coax and a 1-2 inch loop of a few turns. bring this loop around to various parts of the SX, and listen for the noise. If your SX can hear it, another receiver should also. -Chuck Mike Knudsen wrote: In article , Chuck Harris writes: This can be solved one of two ways: 1) add a series resistor to each each diode to limit how much current can flow, 100 ohms, or some such. (Note, one for each diode!) Would shunt caps help? What if it's a bridge unit and I can't get in series? Replace with discrete diodes and Rs? 2) switch to fast recovery diodes. How would these be specified in a catalog? "Fast recovery power diodes"? Thanks, Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
In article , Chuck Harris
writes: I don't have a schematic or other info on the SX190. Is is vacuum tube rectifiers, or selenium? It's an all sandy-state set sold by Allied/Radio Shack. Actually a very nice receiver, with linear tuning and a superb preselector. Rectifiers wold be silicon diodes. --Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
In article , Chuck Harris
writes: I don't have a schematic or other info on the SX190. Is is vacuum tube rectifiers, or selenium? It's an all sandy-state set sold by Allied/Radio Shack. Actually a very nice receiver, with linear tuning and a superb preselector. Rectifiers wold be silicon diodes. --Mike K. Oscar loves trash, but hates Spam! Delete him to reply to me. |
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Mark Rehorst wrote:
On 01 Nov 2003 03:35:10 GMT, r (Mike Knudsen) wrote: But why would the SX-190 suddenly develop this problem, unless it's something else? --Mike K. It didn't develop the problem suddenly. It has buzzed since the day I got it a couple years ago. Now that much bigger problems are solved, I am looking at the more minor problems, such as buzzing audio. The literature (magazine reviews, etc.) on the radio has some vague references to audio hum or buzz, so I think this behavior is normal for the radio. Why they didn't just fix the problem before releasing it onto the market I can't say. The audio amp buzzes at all times, and it is steady. It even buzzes when the radio is switched to standby. I believe there is some noise in the power supply lines and that the audio amp stage has poor power supply rejection, so the buzz gets to the speaker. I will check the supply line and the audio output with a scope and see if the noise looks the same. Then I need to figure out how to kill the noise. A brute force solution would be to use a modern IC audio amp that has a reasonable amount of power supply rejection. But I would rather try to quiet the source of the noise than to make such a severe modification to the radio. The noise may have some effect on RF performance as well as the audio, so it would be best to kill it. Thanks for all the input so far... MR Could you elaborate a bit more on what it sounds like? Is it an AC buzz or an AC hum? Is it 60 cycles? Is the power supply a plain-jane bridge or is it a switching type? -Bill |
Mark Rehorst wrote:
On 01 Nov 2003 03:35:10 GMT, r (Mike Knudsen) wrote: But why would the SX-190 suddenly develop this problem, unless it's something else? --Mike K. It didn't develop the problem suddenly. It has buzzed since the day I got it a couple years ago. Now that much bigger problems are solved, I am looking at the more minor problems, such as buzzing audio. The literature (magazine reviews, etc.) on the radio has some vague references to audio hum or buzz, so I think this behavior is normal for the radio. Why they didn't just fix the problem before releasing it onto the market I can't say. The audio amp buzzes at all times, and it is steady. It even buzzes when the radio is switched to standby. I believe there is some noise in the power supply lines and that the audio amp stage has poor power supply rejection, so the buzz gets to the speaker. I will check the supply line and the audio output with a scope and see if the noise looks the same. Then I need to figure out how to kill the noise. A brute force solution would be to use a modern IC audio amp that has a reasonable amount of power supply rejection. But I would rather try to quiet the source of the noise than to make such a severe modification to the radio. The noise may have some effect on RF performance as well as the audio, so it would be best to kill it. Thanks for all the input so far... MR Could you elaborate a bit more on what it sounds like? Is it an AC buzz or an AC hum? Is it 60 cycles? Is the power supply a plain-jane bridge or is it a switching type? -Bill |
On Sat, 01 Nov 2003 16:31:45 -0400, --exray-- wrote:
Could you elaborate a bit more on what it sounds like? Is it an AC buzz or an AC hum? Is it 60 cycles? Is the power supply a plain-jane bridge or is it a switching type? -Bill It is a buzz. Connecting a scope in parallel with the speaker reveals a very complex waveform that is in sync with the 120 Hz power supply ripple. A mod article in 73 magazine suggested raising the feedback capacitance to flatten the audio response and reduce hum and noise. I tried it and it had no audible effect on the buzz. I also tried putting capacitance in parallel with the rectifiers. Again, no joy. I tried replacing the rectifiers with Schottky types. No change. I am going to try adding a 3 terminal regulator to supply the audio stage from a regulated source instead of the unregulated power it now uses. If that works, maybe I'll replace the audio board with a regulated power supply and IC power amp... MR |
On Sat, 01 Nov 2003 16:31:45 -0400, --exray-- wrote:
Could you elaborate a bit more on what it sounds like? Is it an AC buzz or an AC hum? Is it 60 cycles? Is the power supply a plain-jane bridge or is it a switching type? -Bill It is a buzz. Connecting a scope in parallel with the speaker reveals a very complex waveform that is in sync with the 120 Hz power supply ripple. A mod article in 73 magazine suggested raising the feedback capacitance to flatten the audio response and reduce hum and noise. I tried it and it had no audible effect on the buzz. I also tried putting capacitance in parallel with the rectifiers. Again, no joy. I tried replacing the rectifiers with Schottky types. No change. I am going to try adding a 3 terminal regulator to supply the audio stage from a regulated source instead of the unregulated power it now uses. If that works, maybe I'll replace the audio board with a regulated power supply and IC power amp... MR |
Mark Rehorst wrote:
On Sat, 01 Nov 2003 16:31:45 -0400, --exray-- wrote: Could you elaborate a bit more on what it sounds like? Is it an AC buzz or an AC hum? Is it 60 cycles? Is the power supply a plain-jane bridge or is it a switching type? -Bill It is a buzz. Connecting a scope in parallel with the speaker reveals a very complex waveform that is in sync with the 120 Hz power supply ripple. A mod article in 73 magazine suggested raising the feedback capacitance to flatten the audio response and reduce hum and noise. I tried it and it had no audible effect on the buzz. I also tried putting capacitance in parallel with the rectifiers. Again, no joy. I tried replacing the rectifiers with Schottky types. No change. I am going to try adding a 3 terminal regulator to supply the audio stage from a regulated source instead of the unregulated power it now uses. If that works, maybe I'll replace the audio board with a regulated power supply and IC power amp... MR Are you getting a lot of harmonics of the 120Hz?. Maybe a small value cap like .05 across the filter caps would be worth a try. If you redo the audio stage what about the 'modulation' of other stages? -Bill |
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