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.

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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.

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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.



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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.