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Old 28th Nov 2019, 7:26 pm   #21
mole42uk
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Default Re: Repair nightmares.

Mine’s a 1980’s synthesiser of the musical variety - it was repaired, sent back the customer who subsequently reported that it wasn’t quite right. Back it came and seemed to be okay until I decided that the main PCB needed a clean. As I’ve done many times before, it went into the dishwasher, came out sparkling but refused to work thereafter …
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Old 28th Nov 2019, 8:19 pm   #22
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Default Re: Repair nightmares.

Always think strange when faced with seemingly-inconsistent 'repair-fails'.

Mid-80s through to the mid-90s I managed 30 or so sites using X.25 networking and PADs used to connect RS232/V24 serial-port terminals [DEC VT220, Lear-Siegler ADM3A, Tektronix 4010/4014] to various academic/research computer-sites.

At one site - sometime early-morning - the PAD would lock-up. There was seemingly no obvious pattern, apart from it always being weekdays and early-morning. The first-line guys were perplexed - so they called me in.

I had RF-sniffing loggers and - to cut the story short - one of the temporary staff at the site had a partner who was a taxi-driver. After dropping his missus off he'd fire up his radio and call the office for the first-call-of-the-day - precisely while parked under the twin-twisted-pair overhead cable linking the 'shed' back to the main building.
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Old 28th Nov 2019, 8:28 pm   #23
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Default Re: Repair nightmares.

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Originally Posted by G6Tanuki View Post
After dropping his missus off he'd fire up his radio and call the office for the first-call-of-the-day - precisely while parked under the twin-twisted-pair overhead cable linking the 'shed' back to the main building.
Reminds me of the story of the microwave link that dropped out for a few minutes every morning and evening but otherwise worked perfectly. The engineers could find nothing wrong with it. Then they realized that the beam was just brushing a hilltop. Every morning a farmer drove his cows to pasture and they walked through the beam. Every evening he drove them back home again. Raising the dishes a few feet solved the problem.
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Old 28th Nov 2019, 10:02 pm   #24
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Default Re: Repair nightmares.

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Originally Posted by duncanlowe View Post
when you take the lid off to investigate the problem goes away, and only comes back after the lid is back on.
Had that one at work! Can’t say much about the device, but basically it is triggered by a pulse and then a delay before it outputs 2 of its own, this particular device had passed its first test, then its pre lid final test, but after the lid was stuck on (with conductive silicone) and the unit put through its final test it failed, because the 2 pulses had come together and become one! So I pulled the lid off, which isn’t easy, and checked it again, and the fault was still there, left it overnight without the lid and next morning the fault had mysteriously gone away! It passed all the tests again, so the lid was stuck on again... damn thing failed again after that!!! So off came the lid again, and this time I did a bit more prodding about, warming the board made the pulses slowly come back to normal, I thought maybe it was heat sensitive, but once cooled it was still working, and it kept working! So on went the lid again, and guess what? It stopped working again!!! It was becoming annoying at this point, ripped it open again, and more prodding and poking, I discovered breathing on the PCB would make the fault reappear, then warming it with hot air would clear it. I then decided to replace the SMD resistor that sets the timing between the pulses, and that’s where the problem was, not the resistor itself, but some strange brown stuff in underneath it, which must have been slightly hygroscopic, and when the lid was glued on it must have generated some moisture that was absorbed by the goo and made it more conductive. I took all the parts from the PCB in that area and cleaned it up, replaced the parts and the problem was gone. I ended up keeping that unit as a test standard in the end!

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Old 28th Nov 2019, 11:52 pm   #25
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Default Re: Repair nightmares.

While not a repair as such, someone in the same lesson as one of my school friends was bench testing a circuit he had just made on a copperboard, & had the good idea to hold it steady in a vice.

When power was applied there was a fireworks display of blown components as the vice short circuited the entire assembly, much to the annoyance of the teacher taking he lesson.
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Old 29th Nov 2019, 1:41 am   #26
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Default Re: Repair nightmares.

When I was an apprentice running a flip top Murphy on its side and the oil filled lopty exploded sending oil everywhare , I cannot remember the orignal fault.

Things were so diffrent then.
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Old 29th Nov 2019, 1:48 am   #27
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Default Re: Repair nightmares.

I have always found laptops to be nightmares.
They are the ultimate rubber job.
They bounce like those "super balls" you used to be able to buy in the early 1970s.
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Old 29th Nov 2019, 2:20 am   #28
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Default Re: Repair nightmares.

Post 24 reminds me of a manufacturer's service advice on a popular, worldwide-selling audio amp which had a mostly SMD main board. The glue dots under each and every component had, with age, turned conductive.
Rather than remove the parts, the high impedance areas were modified to pass slightly more current to mask the problem in critical areas. It's lucky I was privy to the service advice otherwise I'd have been stumped on several occasions, as the faults would usually be intermittent.
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Old 29th Nov 2019, 8:01 am   #29
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Default Re: Repair nightmares.

I've come across the test lead snafu, I use a homebuilt dummy load with 3 BNC's on each channel, one for a scope, one for a DMM etc, and on several occasions I've had either low OP from an amp or no OP. I chucked several BNC leads before sussing the problem. The resistors sit on one big heatsink, which is a tunnel type with a big fan on one end, this is mains powered and had an earth crimped to the body of the HS, there must have been an earth loop.

I've tried to fix a few Iphone's, getting the minute screws back in afterwards is highly problematic, they also seem to have been crossed with Mexican jumping beans, often leaping several feet off the bench. I've spent many happy hours searching the floor whilst being berated by my daughter "Is my phone done yet". Needless to say the Iphone had less screws in it everytime I put it back together.

Talking of screws why do you always have at least two left over after you take something apart no matter how careful you are?

A.
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Old 29th Nov 2019, 8:04 am   #30
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Default Re: Repair nightmares.

Quote:
Originally Posted by Diabolical Artificer View Post

Has anyone had a similar repair job that made you look like an idiot or had you reaching for the whiskey bottle?

Andy.
Of course, everybody involved in electronics servicing has had this problem. It is due to the many permutations and vagaries of faults that can occur with electronic equipment.

In fact, after all the intermittent and frankly diabolical faults I have had to repair in a lifetime, I'm surprised I'm not curled up under a tree in a local public park clutching a brown paper bag with a whisky bottle in it. So you are in good company.
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Old 29th Nov 2019, 11:00 am   #31
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Default Re: Repair nightmares.

Quote:
Originally Posted by mickm3for View Post
When i was an apprentice running a flip top Murphy on its side and the oil filled lopty exploded sending oil everywhare , i cannot remember the orignal fault,
things were so diffrent then
Quite frankly, I'm surprised that you remember ANYTHING after an event like that mate!
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Old 29th Nov 2019, 11:10 am   #32
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Default Re: Repair nightmares.

Repairs are relatively easy and straight-forward...
.
.
.
.
compared to some customers.

Some other customers were wonderful, though.

I suspect we saw a similar spectrum of humanity at dad's garage as the radio/TV trade had to deal with.

David
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Old 29th Nov 2019, 4:19 pm   #33
G6Tanuki
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Default Re: Repair nightmares.

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Repairs are relatively easy and straight-forward...
.
.
.
.
compared to some customers.
Oh yes! My real hate when doing field-service calls were when - after you'd fixed the fault, got the customer to sign the jobsheet - they'd casually say "Oh, while you're here, can you have a look at.....?"

which was often either on something utterly unrelated to what our support-contract covered (often on equipment supported by my competitors - or not on any sort of contract at all) or if it _was_ something covered under contract it would be of a nature and complexity several times greater/needing much more time than the original fault-call.

One particular taxi/courier company was really bad at this: you'd go to head-office to replace an intermittent base-station microphone - jobsheet-time 30 minutes+travel - then the "While you're here" would be to look at one of their drivers' radios. Of course you'd been listening to the base-station radio and the drivers chatting while you'd been doing the initial job - and knew that the driver in question was at least 20 miles away.
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Old 29th Nov 2019, 4:54 pm   #34
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Default Re: Repair nightmares.

I had a silly one a few weeks back at work. One of the 3 phase pump motors kept tripping its overload and blowing the black phase fuse that also protected it. After using a current clamp and meggering it (insulation 680K, so not brilliant), it still intermittently blew the black phase fuse I decided that the motor was at fault and changed it over dropping it on my thumb in the process!! After replacing the motor it did not work at all and I found out that the replacement motor that was on our shelf was also faulty (O/C windings) and a third motor was tested ok and fitted.

I came back to it later only to find the fuse had blown again. After a lot of head scratching I discovered that someone in the past had wired up an old engine cooling heater to that phase between the fuse and the 3 phase pump contactor (which was not on any circuit diagram and buried in the bottom of a control cabinet) which had a broken element which was causing the black phase to blow in the first place. This was causing the pump when it started to try and run on 2 phases, stall and then trip its overload.

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Old 29th Nov 2019, 5:56 pm   #35
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Default Re: Repair nightmares.

Out of interest I did once attempt to repair one of my Daughter's cast off ipods. By the time it was unglued it was apparent that it wasn't designed for repair, so it wasn't and no longer exists.
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Old 29th Nov 2019, 7:34 pm   #36
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Default Re: Repair nightmares.

Quote:
Originally Posted by duncanlowe View Post
Now it's funny no-one has talked about a problem that is common to may types of equipment. Oxygen starvation. It causes all manner of odd faults. However, universally, when you take the lid off to investigate the problem goes away, and only comes back after the lid is back on, proving my point.
I wouldn't call it a nightmare repair, but I have had equipment that only worked when the cover was opened, the problem was a UV-EPROM which refused to work in the dark.

David
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Old 29th Nov 2019, 8:11 pm   #37
G6Tanuki
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Default Re: Repair nightmares.

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I wouldn't call it a nightmare repair, but I have had equipment that only worked when the cover was opened, the problem was a UV-EPROM which refused to work in the dark.

David
Ah yes - I recall an early-60s battery/mains radio I was once asked to fix - apart from the booked noisy volume-control which was easy to solve, when tested it also had massive hum when tuned into a station.

Instinctively, I swapped the smoothing-electrolytic in the power supply. No difference! A scope on the power-rails showed an acceptable ripple level. I was starting to think about leakage between the windings in the mains-transformer when I noticed the flaking black paint on the glass case of the front-end transistor [OC44]. Yes, it was playing the phototransistor-game and the fluorescent-tube bench-light was wobbulating the local oscillator. A bottle of Tipp-Ex came to the rescue.

I'd quoted a price based on half an hour to replace the volume-control; I'd now spent at least 2.5 hours on it. No profit there!
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Old 29th Nov 2019, 9:53 pm   #38
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Default Re: Repair nightmares.

Quote:
Originally Posted by factory View Post
Quote:
Originally Posted by duncanlowe View Post
Now it's funny no-one has talked about a problem that is common to may types of equipment. Oxygen starvation. It causes all manner of odd faults. However, universally, when you take the lid off to investigate the problem goes away, and only comes back after the lid is back on, proving my point.
I wouldn't call it a nightmare repair, but I have had equipment that only worked when the cover was opened, the problem was a UV-EPROM which refused to work in the dark.

David
Funny, my experience is with gear that is averse to light.
Thermistors and EPROMs score highly.
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Old 29th Nov 2019, 10:32 pm   #39
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Default Re: Repair nightmares.

My worst has been when toroidal transformers first became available. I had made a "stonkin" guitar amp using 4 KT88's. It worked superbly. About three weeks after I sold it, it returned. SHORT circuit primary winding inside toroid. Air freight new transformer from Sydney. Fit transformer. everything working perfectly. bench test, doing everything I could think of to "blow it up". Nothing worked. Delivered amp back to customer. Four days later I get an ( angry) phone call. Same thing, it doesnt work. I drive 140 kliks out to the farm where the amp is used. Customer leads me to a shed some few hundred yards from the house? Why is the amp out here? "Missus cant stand the noise".
Inside the shed is an ANCIENT Lister/Petter diesel generator. Two black wires lead from the generator to the amp, via an uninsulated power socket. No earth!! No earth on the generator!! I ask the customer to start the generator. It runs well with its 50 years of leaked oil everywhere. BUT its doing about 1300 revs, NOT 1500 revs. I speed the generator up, take amp back and unwind the toroid. It has an insulation arc that has completely cut the wire in two about half way along the primary. Order another toroid.

That amp only cost me $1300 to repair. I origionally made $800 profit.

Joe
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Old 30th Nov 2019, 12:07 am   #40
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Default Re: Repair nightmares.

My father told me a story about a bag sealing machine in the 1960s when they were very special indeed.
They got it working perfectly in the workshop with the new safety sensor to stop you putting a hand in.
Come demo day and the covers went on and the thing tried to seal the reps hand into a plastic bag.
They had handled the board too much and the paint was chipped on the OC series transistors.
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