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Power Supply Hazard Warning
John VK3ZRX has reported to his local
QNEWS Coordinator the following tale which has a very important message for all
During a very `colourful' incident at the QTH a near disaster occurred.
While it did not threaten either life or property as he was present at the time he has prepared a warning for other hams.
The shack suddenly filled with copious black smoke which stopped after shutting every thing down. The challenge was then to determine the source and to clear the smell out of the shack.
By `nose' elimination the faulty piece of equipment was located by the characteristic smell. After the unit was dismantled the offending device was identified. A rather large black mark was clearly visible on the printed circuit board - in the vicinity of the mains supply entry.
Culprit: -Capacitor - brand: - AEE - 'Miniprint'
Description: radial lead capacitor encapsulated in a clear or slightly amber coloured resin, with silver label inside resin.
Label has brand, type, and other info. The edge that is opposite side to leads is rounded.
Problem: Although rated for 250 VAC service, the capacitors are unreliable in this role. Typically the encapsulation splits, allowing moisture to enter the capacitor element leading to eventual catastrophic failure of the capacitor.
A few years ago there was a spate of domestic fires caused by 'MISTRAL' box fans - this type of capacitor was the culprit.
(Although not stated in the press, I am certain this is correct. I had one of these fans which failed with large amounts of black smoke.
One of the AEE caps in the thyristor-based motor speed control unit had failed.)
I have now seen several failures of this type capacitor and I have also found several unused ones in the junk box with cracked encapsulation.
Solution: Get rid of them!
If you have any in the junk box, a much better place for them is the bin.
I think these caps predated the 'X' and 'Y' mains rating system -
I have only ever found them in 'X' type applications (i.e. from line to neutral). If used as a 'Y' cap (i.e. line or neutral to ground) they would be especially hazardous.
Further Activity: - As most amateurs are inherently bower birds and plenty of older equipment can be found in many shacks it would be advisable that any equipment of Australian manufacture should be carefully inspected and any of the offending capacitors replaced with a more modern device.
Moral: - When purchasing older equipment bear in mind that while the unit is cheaper than a new one it bears the potential to become a `time bomb' that can cause havoc in the future -
Peter Ellis VK1KEP
QNEWS April 18 2004
How to sidestep EMI
Do you wonder about computers that are supposedly EMI compliant, but still put out heaps of RF interference? How do they do it and still comply with the regulations?
The answer was revealed on the VK-VHF mailing list . Many computer designers, who tend to design down to a price and don't care much about the interference consequences of their products, have resorted to some ingenious trickery to maintain compliance.
With a stable oscillator you get a hard line spectrum that may exceed EMC limits. However if you randomly frequency modulate the clock frequency you can reduce EMI readings by as much as 20 decibels. Total RF output remains high, but it's spread over a range of frequencies.
Thus the regulations are complied with, but the computer puts out as much interference as ever. As Chas VK3BRZ pithily wondered, do the design engineers concerned wash themselves properly, or just spread the dirt around thin enough that it's not so noticeable?!
Related reading: "Why digital engineers don't believe in EMC"
Amateur Radio Exams
Please check the WIA web site for exam and course information
Always check with the clubs for the latest information
Where else are LIPD's to be
The 70 centimetre band is not the only place where small transmitters such as so-called LIPD'S or "low interference potential devices" can be found.
An ACA brochure called "Spectrum Opportunities for Short-Range Radio-communications" lists many more.
You'd be amazed at the sorts of applications to which low powered transmitters have been put.
Possibly the most common examples are, Remote controlled car alarm systems and Garage door openers. These operate near 304, 345 and 433 MHz. Personal and Duress alarms, with medical and safety applications operate around similar frequencies as well as above 900 MHz.
Fancy some radio foxhunting? Well, animal tracking devices use 151 and 173.5 MHz. And if you're into tracking Whales and/or Sharks, you need to go down to the 48 to 49 MHz band for this.
What about locating lost joggers? Lost? We can only assume that they are - why else would anyone go round and round the same block a dozen times if they're not lost? No matter, 77.4 MHz is the place to be for athlete location systems.
What is termed "Auditory assistance" is found near 3 MHz, around 42 MHz, the FM broadcast band, and around 2.4 GHz.
These are generally the systems where people put on wireless headphones for spoken information at museum exhibits and the like. The 2.4 GHz segment is also shared with other applications, such as barcode readers.
Listening to cordless phones is illegal, yet Baby-monitors are not. You can find these around 35 MHz, although we have also heard them near 27 MHz.
Cordless phones, by the way, use a variety of frequencies, such as 1.7/40, 30/39, 915, 1900 and 2400 MHz. Cheap two way radios can be found near 27 MHz, with 55, 152 and 433 MHz also in use.
Radio controlled models use a variety of frequencies, such as 27, 29, 36, 41 and 433 MHz. And somewhat bigger models, trains to be exact, use frequencies near 900 MHz for tracking purposes.
Perhaps someone is bugging you? Well, listen for yourself by tuning around 39.5, 88 to 108, 174 to 230, 520 to 820 and 915 to 928 MHz. You'll hear a feedback howl in the receiver if your room is being monitored.
We already know about those pesky crane controllers near 433 MHz and an alternative frequency for them is near 472 MHz.
Keeping tabs on crims is easy if you tune to around 314.2 MHz, that's
the "Home detention" monitoring device frequency.
Have you ever thought about communicating with your bodily implants? No, well, apparently 262 kHz is the place to try for 'implantable medical
Biomedical telemetry however also uses various VHF and UHF frequencies including the 70cm LIPD band. We just hope no one is using a 70cm. hand-held in the vicinity!
Moving up, we see the Gigahertz segments used for applications such as distance and speed measurement, cruise control systems, radar fluid measurements and handheld data terminals.
But we've barely scratched the surface. We could go into e-tags, product security tags, computer networks, underground communications, wireless weather stations, video surveillance and more.
It's amazing what has a transmitter or receiver attached to them these days.
Interesting microwave frequencies
Is there really anything to hear above 1300 MHz? Murray
VK2KGM says yes, and has submitted the following item on the S-band, which is
between 2 and 4 GHz.
Some interesting S-Band (2-4 GHz) frequencies to watch are the News Helicopters - and other terrestrial links just above the Amateur Band (2.40 GHz to 2.45 GHz) in the broadcast link band.
Equipment needed includes an old Galaxy MDS grid dish and down converter (which has a Local Oscillator on about 1500 MHz or so), an old FM satellite TV receiver and a TV set. The sound is on a 7.0 MHz sub-carrier - you can set your satellite receiver for this. Many people who have disused Galaxy systems on their roofs are happy to give them to you for taking the trouble of getting them down.
Murray built a down-converter kit from Minikits, but found that many of the old down-converters also work OK.
The list of TV Outside Broadcast S-Band link frequencies Australia wide are:-
ABC - 2477 MHz & 2596 MHz
Seven Network - 2505 MHz & 2624 MHz
Nine Network - 2633 MHz & 2652 MHz
Ten Network - 2561 MHz & 2678 MHz
These are licensed for 20 Watts maximum input into their antennas.
And thanks to Murray VK2KGM for this rundown of microwave activity.
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