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

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.

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.

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.

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

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

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