Nuts & Volts - February 2006

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out the presentation on the CRT face. By this, I mean a kind of subtle wavy action in the picture appearing to run vertically. It is most noticeable with text but can be detected in any kind of graphic along the borders of a given object.

I have consulted “Troubleshooting and Repairing Computer Monitors” by Stephen J. Bigelow. The closest I can deduce from consulting this publication is the capacitors in one of the power supplies are aging and will need replacement. I was considering opening the monitor's case and "shotgun" replace all large capacitors in sight, but this will probably be a fool's errand.

Another possibility mentioned in the book is the shielding around the video cable may be nicked or kinked, but the cable appears intact. Can anyone offer some other ideas or a better rationale for isolating the problem?

"Squiggle" or wavering of a CRT image is often caused by magnetic interference. I have seen a cheap "wall wart" (plug-in power brick) power supply cause noticeable interference from over four feet away. Make sure all transformers and motors (and fluorescent lamp ballasts) are far away from your monitor. I also once tracked down a very peculiar case of the monitor jitters to the fluorescent ceiling light wiring in the room downstairs - it took a call to the building electrician to fix it.

Robert Zusman, Scottsdale, AZ

[#12053 - December 2005]

I have a control arm that moves up and down, pulling a cable. I would like to place in a remote site, a series of LEDs that indicate the position of the arm. How can I convert the movement of the cable or arm to activate the LEDs in relation to the arm/cable?

#¹ It is difficult to design instrumentation for such an application without more definition of the nature of the motion and the dimensions involved, i.e., Is the motion of the control arm an arc or is it linear? Are we talking about inches of movement

Okay, let’s try this one again.

Editor

[#12052 - December 2005]

I have some 4559 aircraft landing lights, they are 28 volt 600 watt. I was hoping to use them for some concert lighting I do. But, I haven't come up with a way to power them, being that I need a 28 volt 21. 5 amp power supply (I assume AC or DC will work).

I tried an off-the-shelf standard home dimmer, carefully setting it (low) to 12V-15V, but it just blew the bulb.

I work in the theatrical lighting industry and we use these lamps all the time. They are a PAR 64 (paraboic reflector) lamp and should be used in a suitable housing (Par Can) since they get very hot, very quickly. These are very narrow spot type lamps and throw an amazing amount of light. The way we use them is to make a "series 4-fer", i.e., wire four of them in series and then use conventional 110-120 volt dimming systems for them. You might want to set your dimmers for these to only go to about 90% of full voltage to keep from blowing these bulbs. One other thing I like to do when wiring these and when constructing the series harnesses is to provide a small LED at each lamp wired in such a way as to light the LED only when the lamp in question fails to complete the circuit. Just put an LED at each point in the circuit where failure to complete the circuit causes the LED to light. This makes troubleshooting a breeze when you lose a lamp. This is not absolutely necessary for the operation of these lights. Very often, since this is a series circuit, over-voltage will blow all lamps on the 4-fer, so regulating the voltage to an absolute maximum of 112 volts is imperative (28V x 4 = 112V). A standard 2.4KW theatrical dimmer is capable of handling one string of these ACLs (600W x 4 = 2.4KW). Setting the profile of a theatrical electronic dimmer to 90% gives the following: 120V x . 90 = 108V. So when you "go to full," you

are actually staying below the blowout threshold. If you need the Par Cans for these, I would recommend eBay — search for UL approved PAR 64 fixtures and you will be all set. These fixtures usually come with a porcelain base for other types of PAR lamps; the lamps you have use screw terminals. The bases can be removed easily and I would recommend crimping on uninsulated fork or ring stakons of the proper size for the screws (#8) and for the wire (usually #16 or #14).

David Shepherd New York, NY

daveshep@earthlink.net

I just read the reply to the question about running an aircraft landing light at home and found it hard to believe it made it past the editors.

The first part is way off base. The reason the lamp blew is that a dimmer doesn't actually control the voltage but, in effect, the duty cycle of 170V peak pulses. To the dimmer, the cold filament in that lamp will appear as a nearly direct short and I would expect it to blow the triac in the dimmer, as well as the lamp.

The second part of the reply is just plain wrong. The lamp in question is incandescent. While in its original application it would be run on DC, it is not a "DC only lamp" but will work equally well on AC.

I can offer a viable solution: modify an old microwave oven transformer. These are readily available in junked ovens and are often rated as high as 1KW. To convert one, simply cut away the old (and dangerous!) high voltage winding without damaging the primary and carefully wind enough turns of #10 insulated wire around the core to achieve the desired output voltage. It will take some experimentation, but if you wind a couple turns around it, apply power, and measure the output voltage of your new winding, you will know the approximate voltage per turn and can calculate the necessary number of turns accordingly. Good luck!