Home |
Search |
Today's Posts |
|
#1
![]() |
|||
|
|||
![]()
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. |
#2
![]() |
|||
|
|||
![]()
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 |
#3
![]() |
|||
|
|||
![]()
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. |
#4
![]() |
|||
|
|||
![]()
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. |
#5
![]() |
|||
|
|||
![]()
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. |
#6
![]() |
|||
|
|||
![]()
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. |
#7
![]() |
|||
|
|||
![]()
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. |
#8
![]() |
|||
|
|||
![]() |
#10
![]() |
|||
|
|||
![]()
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 |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
QUESTION & REQ: Real Audio Radio Recorder with Schedule/Timer | Antenna | |||
SX-117 ANL causes Low Audio | Boatanchors | |||
SX-117 ANL causes Low Audio | Boatanchors | |||
FA 4 lots of 1930s boxed NOS audio and receiver tubes | Boatanchors | |||
Automatic RF noise cancellation and audio noise measurement | Antenna |