|
Grex > Music2 > #182: Too much 'Kool 107' driving you nuts, too? Vent here. |  |
|
| Author |
Message |
| 13 new of 37 responses total. |
n8nxf
|
|
response 25 of 37:
| 
Feb 26 12:25 UTC 1999 |
What about a 1/4 wave open stub tuned to the overpowering station?
Seriously, You may have to go to extremes to solve this problem. Like
wrapping the radio tightly in aluminum foil with only an outlet for the
antenna wire and a filter in the antenna wire to knock down the strong
signal. Ferrite beads would have to be put around any other wires going
to the radio.
If you have ever opened up a radio like the one in question, you will see
that it is noting more than a plastic box with electronics screwed to the
inside of it. The enclosure provides NO shielding at all. Sometimes you
will find a little shielding around the RF section, soldered to the PC
board, but generally not. The standards for FR susceptibly for consumer
electronics are practically nonexistent so many of the radios sold are
very susceptible. The front end of most receivers are also exercises in
minimalism with poor selectivity, sensitivity and rejection figures.
Tower Plaza glows in the dark from all of the RF on the rooftop. You
just don't see it because it never gets dark in downtown Ann Arbor ;-)
However, the equipment up there has to meet pretty stringent regulations
for spectral purity and most stations are pretty particular about making
sure that their signals are clean. I've talked to the technical person
at WIQB too. Several years ago one of their old remote transmitters was
getting into the ARROW's 440 repeater and we were listening to Art Fair
news in the 70 cm amateur band. They were very responsive and I've not
heard them since.
|
russ
|
|
response 26 of 37:
|
Feb 28 04:27 UTC 1999 |
Re #25: A 1/4 wave open stub is just another twist on the
series-resonant trap. The disadvantage is that it's much harder
to adjust; once you've cut it too short, you need to make another one.
It's also quite a bit larger physically, and I wouldn't bet on being
able to coil one up inside the case of a small radio.
I keep thinking about traps which could be built with hardware-store
parts and tuned with something like a 6-32 screw, but I am having
trouble thinking of materials which would be suitable. Copper pipe
suggests itself. The mechanical complexity would go way up, as would
the cost. You'd have a tough time building anything more rugged, though!
I wouldn't worry about shielding the entire radio. This would only
be necessary if the signal was coming in by the speaker leads or
something, and you'd know if that was the problem because the volume
control wouldn't reduce the interfering signal. Blanketing has to hit
before the mixer (the device which converts the 88-108 MHz input signal
to the intermediate frequency of [typically] 10.7 MHz), and the best
way to get a signal to it is via the antenna. Worse comes to worst,
you'd need ferrite beads over the speaker leads and power cord to block
RF from coming in over them.
Oh, I checked Rat Shack for trimmer capacitors. They have nothing of
the sort. I had no time to look at Purchase Radio.
|
devnull
|
|
response 27 of 37:
|
Mar 2 23:52 UTC 1999 |
Actually, I believe that it is possible to have RF coming in through the
speaker wires, and not have the volume control have any effect on such RF.
My boom box has a very quite hum which is constant regardless of the volume
setting; it can be obnoixiously loud sometimes when the volume is turned down
and I'm using headphones. (The hum continues to be just as audible when
the volume is turned all the way down to silence.)
|
danr
|
|
response 28 of 37:
|
Mar 3 00:40 UTC 1999 |
That's not RF. That's 60 Hz AC hum. I'd guess the hum is on the DC power,
which is why it's not affected by the volume control.
|
rcurl
|
|
response 29 of 37:
|
Mar 3 04:49 UTC 1999 |
Actually - probably 120 Hz...bad cap in the power supply?
|
russ
|
|
response 30 of 37:
|
Mar 3 05:00 UTC 1999 |
Re #27: You can probably get rid of the hum by adding more power supply
filtering. If you're interested, enter something in the Science conference.
|
n8nxf
|
|
response 31 of 37:
|
Mar 3 11:21 UTC 1999 |
Get rid of the hum by unplugging the radio and running it on batteries.
It is not uncommon for hum to be caused by magnetic coupling instead of
ripple on the DC.
|
rcurl
|
|
response 32 of 37:
|
Mar 3 16:13 UTC 1999 |
Then one would have the hum in the radio when it was new (and operating
properly). Did it?
|
devnull
|
|
response 33 of 37:
|
Mar 4 10:11 UTC 1999 |
I recognize that the hum is a power supply related issue, not an RF issue.
However, the point I'm trying to make is that you don't have
audio source <--> amp <--> volume control <--> speakers
you have
audio soucre <--> volume control <--> amp <--> speakers
And indeed, if RF is leaking in through the speaker wires, the volume control
won't affect it, if you have this sort of design.
|
n8nxf
|
|
response 34 of 37:
|
Mar 4 12:01 UTC 1999 |
I have never seen
audio source <--> amp <--> volume control <--> speakers
except when a pad is used to balance two or more speakers of an audio
output.
RF doesn't care about connections. RF cares about resonance and / or
proximity.
|
rcurl
|
|
response 35 of 37:
|
Mar 4 15:44 UTC 1999 |
Usually one has audio <-> preamp <-> volume and tone controlls <-> amp <->
speaker (for whatever relevance that has.....).
|
gull
|
|
response 36 of 37:
|
Mar 5 00:49 UTC 1999 |
I've seen amp -> volume control -> speaker, but only for low power (under 1
watt) in things like cheap TVs.
|
russ
|
|
response 37 of 37:
|
Mar 5 05:13 UTC 1999 |
Re #33: Interference coming in through the speaker leads is very
different from "blanketing" interference, which is what other has.
A CB can come in over the the speakers, without affecting the audio
coming from the desired station at all.
|