Got the gen set, no outage.

[CJDave]

[img]/forums/images/icons/tongue.gif[/img] I bought the 94 in December of 1989 thinking that it would last five years before technology or circuitry difficulties rendered it obsolete. It made me a darn good living for thirteen years and it still actually works but it is an orphan now as the company that built it and in fact designed the main tube was sold several years ago. The whole setup; camera, VCR and support gear cost 21,000 and change. This was one of the first IR cameras that did not have to be cryogenically cooled. [img]/forums/images/icons/tongue.gif[/img]

[Pat]

Dave, I'm sure that the level of technology that is represented by that camera is still quite interesting and useful even if the current state of the practice (not even moving up to state of the art) is way ahead of it. There have been vast improvements since then.

I constantly border on future shock. I have a little telephone tech test box I got in trade with some stuff. It is larger than a pocket multimeter, about the size of an old Simson 260. It does a bunch of computer controlled tests and contains a built in TDR (Time Domain Reflectometer.) I need it like I need a hole in the head but don't have anyone around who could use it or I would make them a great deal.

Pat

[CJDave]

[img]/forums/images/icons/crazy.gif[/img] TDR??? Total Digestible Radukomens? [img]/forums/images/icons/wink.gif[/img] I have like NO IDEA what a device like that is used for. The Time Domains that we use here are not reflective so instead of a flashlight, we find them by letting the dog sniff them out. [img]/forums/images/icons/crazy.gif[/img] I have to tell ya, there are times when I feel like a dinosaur around some of this stuff. I'm perfectly at home on my 1952 John Deere Model "A" tractor with the No. 45 front end loader, and the over-center hand clutch. For a fifty-five year old tractor it can move a lot of dirt in an hour, providing it has an experienced John Deere pilot to run it. [img]/forums/images/icons/crazy.gif[/img]

[Pat]

Dave, TDR equipment was in use probably before you were born. What makes having that capability in a small lightweight hand held test set interesting to me is that the equipment used to be huge and heavy.

Transmission lines like twin lead, coax, etc have their natural intrinsic impedance value. You have probably seen and used a balun transformer to convert between 75 ohm coax and 300 ohm twin lead TV wires. (Oh by the way balun stands for BALunced-UNballanced)

If you use a splitter to drive two TV's with an antenna or cable signal the splitter is designed so the single side is a 75 ohm impedance and the other side (double, t ripple, or whatever side) is also 75 ohms at each connection. If you do not match impedances then power transfer is reduced and their is reflected power. Some of the signal bounces off of the discontinuity in impedance and causes reflections (can cause ghosts on your tv) and standing waves.

A short or an open circuit along a transmission line of great length can be a bugger to find. If a pulse of energy is injected into a coax or other transmission line it will propagate donw the line and if there are no discontinuities in the impedance the pulse will not be reflected back, even a little.

We digress again... Tie one end of a rope to a tree. wiggle the rope to make a wave go down the rope to the tree. The wave hits the tree and is reflected back to you. The tree does not match the impedance of the rope.

Dangle a rope over a cliff and do the same as above. The wave travels down the rope and some of the energy is reflected back up the rope. IN the case of the tree the amplitude of the wave at the tree is zero (you don't wave the tree, tree is a "short circuit") in the case of the dangling rope the amplitude is max (open circuit).

Now for your TV cable or other communicatioins transmission line. IF there is a short, an open, or even a discontinuity in impedance due to the cable being partially crushed (stapled too tightly or something) there will be reflections, standing waves and such.

A TDR puts a pulse down the line and detects if there is an echo, timing the two way time of flight of the signal and echo. Knowing the velocity factor of the transmission line you can use the TDR derived time to calculate the distance to the problem. Coax often has a velocity factor ranging from about 0.60 to 0.90 depending on what type cable it is. Velocity factor is the percent of the speed of light that the electrical signal propagates in that particular transmission line.

This will tell a test tech how far down the line there is a problem with the cable. These techniques can be used on cross country power transmission likes. Say a mylar balloon with vacuum deposited aluminum coating from a wedding celebration hits an insulator in a power wire and the wire burns in two. The utility equipment shuts off that wire. You want to go fix it so where is the problem? A TDR will locate the distance to the problem and allow you to efficiently dispatch a repair crew.

Happy New Year!!!!

Pat

[CJDave]

[img]/forums/images/icons/tongue.gif[/img] Pat, I have to say that your explanations are about the best I've ever heard. I had no idea anything like a TDR even existed, much less understood how it could be used, .... until now. [img]/forums/images/icons/smile.gif[/img] A farmer friend had a direct-burial cable carrying three-phase 480V to a remote irrigation pump site and the cable was installed without import backfill in rocky soil. [img]/forums/images/icons/frown.gif[/img] Of course the rocks would occasionally press through and cause one leg of the cable to break. He had just over a quarter mile of cable buried so finding the spot was a huge problem. He figured out that if he set his 8N Ford tractor at one end with a spark plug wire connected to the leg of the cable that showed an open circuit, and then let the tractor just idle, he could drive down the cable alignment with his pickup and with the radio tuned slightly off-station, the pickup radio would pick up the static from the sparking when he got near the trouble spot. This stuff is SO interesting! [img]/forums/images/icons/smile.gif[/img]

[GaryM]

We use a similar device in the aircraft industry, as I'm sure Pat knows. It well tell you how far down the line a broken wire is in a cable bundle after it's been installed in the AC. Saves a lot of time. [img]/forums/images/icons/smile.gif[/img]

[Pat]

Dave, J.C. Whitney and others vend short finders. It is essentially a turn signal flasher and a sensitive meter. You put the flasher in series with the shorted wire and it starts clicking on and off. You move the meter along the wire where it displays needle jerks in time with the rise and fall of the E-M field from the current in the wire (no physical contact needed between the meter and the wire, just get close.) When you pass the short there is no current so the needle quits twitching, pin pointing the short.

Thus finding the short is trivial. Finding an open is a tad tougher. I do admire the "Rube Goldberg" setup you mentioned for finding the broken wire except for the part where the high tension voltage can punch through low voltage insulation and set up the cable for problems.

Pat