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Old 19th December 2011, 03:42 AM   #11
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A pubic hair... that's indeed pretty strange! Very humorous. Thanks for sharing.
Doc
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Old 19th December 2011, 04:37 AM   #12
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Lightning, elephants, pregnant women and high power transmitters do anything they want...

High power TX follies.

A Harris 125,000 Watt frequency agile short wave transmitter. Exotic, finicky. Makes scrapnel faster than any protection circuits can kick in. Problem 1. No signal on a particular wire about six feet long. Examination shows an odd copper color plume on the circuit board at each end. Transmitter had fully vaporized the copper out of the wire without burning the plastic insulation. Problem 2. Walking a tech through a repair over the phone. Read value of X resistor on this board... It's not there. Not there? No, just a big black hole where it should be! (Was a 2 Watt 100K) Problem 3. Power company drops a phase and transmitter kicks back into high voltage switch cabinet... Nothing but a pile of shards inside and a smooth copper coating on inside of entire cabinet.

TX #2 CCA 20,000 Watt FM. 103.5 MHz. My earliest days as a broadcast engineer. On mountain top replacing tube with chief engineer. Look inside tube cavity and see this aluminum strap about 2"x 6" x 1/8" (5cm x 15.25cm x 31.75mm) running from plate lead down to aluminum enclosure that surrounds the tube. I ask chief if tube cavity runs hot? No, runs at ground potential. That's the plate lead, right? Yup. So... That aluminum strap is an INDUCTOR at 103.5MHz? Yup. 9,000 VDC at 14 amps gets put on one end and the other runs at ground potential and it doesn't get real hot? Yup. Okay, this is different.

Same tX, after tube swap. Switch to high power; BANG! About like a quarter stick of dynamite. We go over every square inch of TX, find NOTHING. Bring it up again, it's fine. Go to high power, it's fine. For the next five years... it's fine. Speculation is that it was a sticky contactor did a make before break. Unconfirmed.

TX #3 Gates 10,00o Watt AM 1060KHz. Get a call one Sunday evening from Newsman; There is a white light in the MIDDLE of the 400' (122M) tower where there never was one. Drive down, foggy night. Sure enough. Like a 40W light right in the middle of the tower. Check SWR readings, they're cool. Turn off TX, light dissapears. Turn TX back on, light doesn't come back. Next day go up tower.... Nothing. No arcs. No burn marks. Speculation is wierd damp atmosphere created ball lighning at some node point on tower. Unconfirmed.

TX #4 TOwnsend 45,000 Watt UHF TV transmitter. 491MHz. TX hard crashes. Try to bring it up again: BANG! Another quarter stick of dynamite goes off. Look in surge protector cabinet and an entire bank of 200Watt ceramic surge resistors sit in little pile of shards at bottom of cabinet. Shorted filter cap on HV supply. Order one next day air from Harris and replace the 10 200Watt resistors and everything is fine again.

Same transmitter facility. Storm building outside. I step out to take a leak just as a 1" spark arcs 50' between towers. I step back inside and go out back to relieve myself. Later that night I happen to look over and see a major spark jump from door handle about 3' (1M) to a screen cage around steps leading to basement. I ease around hugging wall and quickly run into screen cage where I spend the night. Don't like lightning IN THE BUILDING!

Doc
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Old 19th December 2011, 04:55 AM   #13
kevinkr is offline kevinkr  United States
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Scary stuff, particularly that last bit.. Glad you survived the night..
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Old 19th December 2011, 10:15 PM   #14
tvrgeek is offline tvrgeek  United States
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We had a printer misbehaving. A big junk-mail monster. Every so often it would quit. NMI kind of dead. We replaced everything. Then we replace the printer. Still, the same problem. My boss was working the problem late at night, and working the customer so we would not get pitched. It stopped. He happened to lean against it. It started. This lead to one of those eureka moments, but those were not the words used. There was nothing wrong with the printer of course, the electricians had failed to connect the earth on the three phase power. As a printer, it would build up several thousand volts of static and that would make the CPU go crazy until it dissipated.

Years earlier, at another company, we had a problem of our tape drive doing a NMI. Z80's were really sensitive. I had to seal every battery UPS in the building to float a phosphor storage scope and a logic analyzer all night to catch it. I used a really short probe to trigger the scope, and a really long one for the traces. This bought me about 5 nanoseconds pre-trigger. I came in in the morning to a Poloroid of the ground jumping up half a volt. This was caused by noise on the overhead power distro duct being induced inside the cabinet because the power AC distro went inside the cabinets before each units supply filters and was capacitance coupled to the signal ground. We fixed it with a very low ESR poly film cap. I had to battery supply the test equipment because the ground glitch caused them to move as well, masking the error. About three weeks work.

I am sure we have all had ones where a cabinet screw would intermittently hit the circuit and the cold solder joint that only failed when the customer was at home.
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Old 19th December 2011, 11:01 PM   #15
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When I was in 1st Infantry Division, division headquarters called up our maintenance battalion to borrow a 100 KW diesel generator. Theirs had shorted out the generator head, the rotary electrical device. Phase to phase was zero ohms. One of our maintenance companies loaned them their generator. Next day, got another call, they needed another 100k generator. They had shorted out the head phase to phase again. There were only 4 other 100 kw generators in the division, all belonging to the other four maintenance companies. I was too low as a staff captain in the battalion maintenance office to rate a jeep ride, so I hitchhiked over to division headquarters about 10 miles to take a look. The guy that gave me a ride turned out to be the Div commander's aide, with a bunch of air support radios in his jeep. I didn't even have a meter. At division headquarters, a major met me and showed me around. They operated about 14 five ton office vans, parked in a circle, with wood platforms behind and plank walkways connecting the vans in a circle. I had slid down one of the army standard stairs from a van in a rainstorm and gotten mud all over my rainsuit so I knew why the walkways. However, it was August, it hadn't rained in weeks. The vans were hooked up to power with many of the standard Army issue lighting kit, a 100' cable with 3 pin connectors on one end and 3 socket connectors on the other end. They were custom connectors, oddly, I didn't recognize them as anything commercial. All van cables were junctioned to a big engineer patch panel at the generator. I could tell that wasn't standard issue. I asked the major why the weird wiring, he said the vans were issued by the Army with a 10 KW gasoline generator with each, and were supposed to be separately powered. The 10 KW design had a "mil-standard" gasoline valve in block motor which had nothing in common with any lawnmower and was known to have a short life between failures. The 100 kw generators had a reliable cummins NHC220 diesel engine. So I forgave them their setup, but we were running out of 100 kw generator heads, Estimated Ship Date on the first one was 3 months ahead. I looked at the grounding on the vans; each was grounded from the body by a 10 ga wire to a 3/8" dia copper coated rod pounded in the ground at least 8' the major said.
A light lit up. It was August, on top of a hill in Kansas, when it hadn't rained in weeks. The grounds weren't hitting the water table. I told the major to run a 10 gauge or fatter wire from ground point to ground point all around the circle, leaving a gap at one point to keep this wire from being a ground loop. He had the engineers do it. I told him it would probably trip a breaker when it did, showing them where the fault was.
I never heard another word, but they quit requesting 100 kw generators from us. A year later, 4 GMC Yukons in military paint and kit were added to the TO&E, one for each captain in the maintenance battalion maintenance office. Being on the TO&E, meant that besides the ones in Kansas we also got four of them put into storage in our wartime stocks in our destination continent. But while I was in, I couldn't even get a military driver's license. The time I drove the 5 ton tractor trailer in convoy, I was totally illegal, despite my Texas all classes all weights chauffer's license.
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Last edited by indianajo; 19th December 2011 at 11:13 PM.
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Old 20th December 2011, 02:23 AM   #16
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OK, another from WAY BACK in 1976. CB radio was all he rage in south central Wisconsin where I worked at a Team Electronics store. Customer bought a CB radio to install in a 1960s Ford Econoline van. Radio came back the next day blowing fuses and I found the reverse polarity protect diode zapped because the customer did the usual bigger fuse ( "because I didn't have the right one" excuse). I replaced the diode, checked the rig and returned it. He came back the next day with the same problem. What??

I went out to the van with a meter and found the truck had positive ground. Yep, the '+' terminal on the battery was NEGATIVE. When the battery was stone cold dead the jumper cables were connected backwards. It had a generator rather than alternator and as you know, lead acid batteries have 2 lead plates and can be charged either direction. The generator also didn't care about polarity as well as wipers, lights etc. I don't remember if it had an AM radio. I told him to fully discharge the battery and charge it correctly this time.

G
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Old 20th December 2011, 02:34 AM   #17
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Here's a couple:
1) Peavey made a 4 way speaker cabinet called the 3020HT. My employer bought 8 of the first ones, which worked out pretty well for a few months until the tweeters started sounding funny. Pull the tweeters, check the diaphragms, they sound fine on the bench. Put them back in, they sound weird. OK, it might be a funky connection on the PCB. Hit all the crossover solder joints with heat, same problem. The crossover caps look good too. Finally I pulled the tweeter shunt inductor from the PCB, and the funny noise stops. What the hell? Grab the magnifying glass, look very carefully at the inductor, and find a tiny burnthrough where one lead crossed the winding before going to the center. It turned out the enamel on the wire wasn't quite up to snuff so the inductor was an intermittent dead short; we ended up replacing that part on all the boxes after alerting Peavey to the problem. They didn't charge us for the new ones, either.

2) The Rhodes division of Fender put out a synth called the Chroma, a fine sounding if ungainly beast. I was repair tech for a music store which sold one of these to a local musician, who had a weird intermittent problem where every once in a while it refused to recognise the keyboard. The thing was FULL of digital 16 pin chips, all plugged into sockets (these days the acres of circuit board would boil down to one FPGA). It worked fine in the shop, but when he went on the road it would lose its lunch once every couple of weeks. Try fixing that, folks. I stared at the thing for hours until I spotted the problem: one of the chips was plugged into the socket slightly wrong, with a corner pin bent out of the socket (of course on the hidden side), but close enough to the metal where it would make contact most of the time. Pull the chip, bend the pin back to true, plug it back in, alles ist gut! The musician wanted to know how much the bill would be as I'd been forever on his machine. "No charge, just get it out of my sight."

Last edited by DSP_Geek; 20th December 2011 at 02:38 AM.
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Old 20th December 2011, 04:53 AM   #18
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A Sony Xplod car amp was having problem with one amp and was going in protect mode.

On comparing around the smd driver transistor, I found a difference in the impedances. Just removed the smd transistor. All components were showing correct. All I found was that there was a leakage between the collector metal and the copper track. Just cut the track and bypassed it with a wire. Problem solved.

Another was a denon tape deck.
One diode was showing good on cold test but was not conducting in the ckt.

Gajanan Phadte
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Old 20th December 2011, 05:44 AM   #19
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For anyone who may own an HP1740A or just 17xx series. You might radically increase its bandwidth and rise time respone by simply wiggling and flexing the coax loop delay line. HP discovered outgassing from the PVC core would gradually insulate the individual strands of the braided sheath around the outside of the coax, especially in large temperature extremes. Other coax suffers from the phenomena, but normal flexing with use negates it by rubbing the individual wires over each other. But the delay line is seldom flexed after instalation. Simple cure from HP bench briefs is just to flex the delay line a bit until scope again meets bandwidth and risetime expectations.

Something to keep in mind regarding signal coax installed in buildings, such as video cables and such.

Doc
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Old 20th December 2011, 06:01 AM   #20
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Another reason to use PTFE insulated coax rather than vinyl, but the accountants would probably disagree. I've seen vinyl insulated speaker wire turn green inside, probably from reactions with the plasticizer or loose PVC monomers.
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