Strangest electronics repairs

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Over the years, I've encountered some truly bizarre electronics problems. I'm certain that others have their own stories to tell as well. Starting this thread as a place to talk about them.

I'll start it off.

One day at a division of Litton in silicon valley the TFT VLF comparator (that we used to dial in all our frequency time bases at the time) lost lock. An old HP 117A showed good WWVB field strength on a separate antenna. Looking at the TFT a test at the antenna connection showed the proper 15VDC supply output to power the active FET antenna element on the roof. Heading up to the roof I measured mere millivolts at the connection to the antenna. Okay, I'm thinking shorted coax.... Dropping down a floor to a utility closet I found a BNC barrel connection splice in wire laying aside and air conditioning duct. Removing supply side I measure 15VDC. What? Remove output side and get no low ohms readings on antenna side. Looking at barrel it is simply black inside. Measure barrel and I get 10 ohms center to outside... Problem turned out to be a VERY conductive fungus had grown in BNC barrel due to condensation from leaning against cold air duct. Scrubbed inside of male ends with triclorethane and replaced center F-F barrel with a new one and heat shrunk tube around it to keep humidity out. Problem solved. A danged conductive fungus! The ten ohm part really impressed me.

Doc
 
So... no takers on the conductive fungus. Okay, I've got more.

North Atlantic Phase Angle meter. Exotic box. Basically read the difference in gain and phase between a refference and a test signal anywhere between 10Hz and 100KHz. Only this one is absolutely in spec on everything EXCEPT exactly at 100Hz. There it's just out. What the? Scratched my head a long time on that one. Finally found a phase lock frequency range control IC was skipping over an entire band that should have been there. The phase lock on the range below and above was good enough to ALMOST cover the adjacent range, except at the very end of their lock ranges. 100Hz should have been mid band on a range, but it was alternating between the range below and above. Replaced the IC and all was well.
Doc
 
Sony used a glue to hold the fiberglass insulator to the bottom of packaged LCR filters used heavily in the BetaCam SP VTR lines. After 20-30 _years_ it becomes conductive and shorts out parts of the filter. After removing the module from the board it can be cleaned off and re-installed without any glue and once again work correctly. I've 'repaired' dozens of these.

They also use that glue to hold rework wires in place after they mod a board with trace cut(s).

 
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That's the stuff I was looking for (referring to your post, not the glue.)

Sony has other little tricks that will trip you up. On virtually all their consumer CRT and older projection bigscreen sets their PWM power supply is about the only one that goes UP in frequency when it reaches regulated voltage. They use a DC swamping coil on their transformers to regulate output. Don't know if their studio monitors did the same... as I never had one with a PS failure.
Doc
 
Worst thing I ever fixed was a GR impedance comparator. To make a 40 hour troubleshooting story short, there was a cracked solder joint on the grid of a tube. Signals passed the capacitance of the crack with no problem, so the instrument sort of worked, but there was a slight phase shift that prevented calibrating it. One touch of the soldering iron fixed it, but figuring out where to touch was a nightmare.
 
Ooh I still shiver when I think about working on some of Magnavox's early computer monitors with just out Ecofriendly low lead solder. Bittle as heck, a lot of functions still done with discretes. So oodles of tiny suface mount parts held with brittle solder then they mount the flyback right on the motherboard so the slightest jar breaks... something really hard to find! That's when I concluded -again- that consumer electronics was not for me.
Doc
 
Okay, keeping it going:

A mitsubitshi (sp?) 13 inch color CRT TV. Works great if you power it up cold, but if you turn it off then right back on it emits a horrible buzzing and doesn't light up CRT.... Okay from receiver module signal gets split off to sound circuit and the detected video goes off to a video Jungle sircuit that splits video into H-drive, V-drive and Red, Green and Blue signals. Using an issolation transformer I start probing the jungle. WHAT? Unless fully powered down (or like on a power bump) the jungle chip gets confused and switches the Vertical drive out the HORIZONTAL drive pin! (Basically sending 30hz to flyback). Wasn't a characteristic of all jungle chips, just the one in that set. A new jungle chip cured it. Still got that jungle chip in my junk box.
Doc
 
I bought my first color TV from a friendly master sergeant in the Army, but while watching Star Trek the Movie right after the nuclear explosion, or nova, or whatever it was, all the colors went out but red! Stayed that way, too. I didn't have time or resources to fix it in the middle of Kansas, so I gave up TV until I got out. In Jeffersonville, I buy the Sam's Photofact, pull the case off, and start to debug. Zowww!!! it shocked me silly. Turns out the entire rent house was wired upside down, black and little blade were neutral, white and big blade were hot. Thus the steel TV chassis was hot, too. Wasn't even a steel box outside the house at the meter, just a big ball of electrical tape under the eaves for each of the two connections (no third). The fuse box was the other side of the house from the service entrance. After I found another house to buy, the landlady offered it to me for $11000. Sorry, the asbestos siding was a real barrier besides the stupid wiring. Never did fix that TV, finally gave up on TV's, something about 200 electrolytic capacitors and no working scope.
 
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I worked on medical equipment for 25 years and seen some crazy stuff

The #1 weirdest (and most entertaining) was a DVD player in a tent hospital in Iraq. They used the player for movies so patients can take their mind off things as the stretchers moved through the Emergency Room. The Australian medic asked me if I could fix DVD players, sure--I'll give it a shot. Had a CT scanner to work on but gave it a minute, threw in a regular music CD which played so I told him I swing through again at 9 PM.

That night I pulled the cover off the DVD player, loaded in DVD and watched the focus scenario as it attempted to read the disc. The laser moved up, moved down then slightly oscillated as it attempted to focus. Figured there was enough dust on the sensor (Iraq has plenty of dust) but the player kept the same problem with slight oscillation of the laser. Pulled the transport out, looked at it with a magnifying glass and found the problem. There was a hair stuck between the laser and the transport. A very short, very curly hair that acted as a counter spring. Removed the hair and it worked fine, the movies were back on. I attempted to sneak out....

The Australian medic was back and very happy that the DVD player was working and called everyone in. He asked me what the problem was and I told him problems with laser focusing. For some reason, he asked me why so I told him about the very short, very curly hair that acted like a spring. Attempted to sneak out again....

He said "Can't blame any of us, nobody in my section has very short, very curly hair!" Uhhhhh....not exactly true....uhhhhh. Hmmm, so I put it this way "In Texas they call them "short hairs" sooo...." The Americans in the audience smiled but Australians don't know what "short hairs" slang converted to in the Queen's English. I attempted to leave again...

The Australian medic was steadfast in none of his medics had short hair. So I basically told him that I was not implying cranial hair--more along the lines of body hair that is located in other areas of the body. Ya know, it's known to be very curly? His eyes lit up and he yelled "Oh, you talking about pubes, Mate!" The entire section roared with laughter, the patients were laughing and it became quite the scene.

Since the world now knew (tent walls don't stop sound) I told him "I'm not sure what kind of movies you watch, but invite me next time"....then I made my escape. In the end, it was a good thing--those patients needed something to laugh about in such a depressing place.
 
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
 
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.
 
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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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.

 
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."
 
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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
 
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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