1st Dec 2019, 6:28 pm | #61 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,951
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Re: Repair nightmares.
Not necessarily a direct "repair" nightmare, but the tubular chromed-metal legs on the TV/VCR-stands popular a few decades ago were a convenient place to stash contraband. Someone I know who ran a small TV/VCR-rental business in Bristol in the 90s recovered a TV/VCR-and-stand from someone who had not been keeping up the payments.
Stuffed in the stand-legs there were numerous bundles of tightly-rolled banknotes and small sachets of questionably-legal white powder.... |
1st Dec 2019, 7:22 pm | #62 |
Nonode
Join Date: Jun 2010
Location: Spalding, Peterborough, Cambridgeshire, UK.
Posts: 2,851
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Re: Repair nightmares.
The 2N6084. The mainstay of home brew 10W in 40W out amp builders back in the day!
I still have a load of BAX16 (or 13?) diodes that were used for diode tx/rx switching in one such design. ETI maybe?? Rob
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Apprehension creeping like a tube train up your spine - Cymbaline. Film More soundtrack - Pink Floyd |
1st Dec 2019, 7:35 pm | #63 |
Nonode
Join Date: Jun 2010
Location: Spalding, Peterborough, Cambridgeshire, UK.
Posts: 2,851
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Re: Repair nightmares.
Back in late 70s/early 80s, my Pye Telecom days, I well remember occasional intermittent no tx faults on the Reporter MF6AM. Various engineers dumped these sets into Nottingham Depot as they could not observe such a fault. Ron Swain was the man to investigate and repair! He eventually came to the conclusion that it was caused by leaky 4n7 ceramic plate capacitors in the dc direct coupled modulator path that were to blame. Certainly their replacement cured the faults. This problem was escalated to manufacturing in Cambridge. They were not convinced! Ultimately, he was asked if he was absolutely CERTAIN? The reason being, we were told there were over a million such items in stores that would all be scrapped on his word, if he were absolutely certain. He stuck by his decision, (backed by the manager as well) and the problem disappeared from current production. He was vindicated! However, it was a few years in the field before I saw the end of the problem created. At least I knew that a "Blanket replacement" was the solution.
Rob
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Apprehension creeping like a tube train up your spine - Cymbaline. Film More soundtrack - Pink Floyd |
1st Dec 2019, 8:31 pm | #64 |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,184
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Re: Repair nightmares.
I'd hope they were talking about a million capacitors, not sets.
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1st Dec 2019, 8:44 pm | #65 |
Hexode
Join Date: Jun 2011
Location: Glossop, Derbyshire, UK.
Posts: 487
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Re: Repair nightmares.
I had a REMPLOY site that kept blowing power units on a Nortel OPT11 switch board along with the fax and modem power units ( ok for a few weeks then start again ) turned out to be the 3 phase power with no earth in the centre point in the sub station when they turned on a large machine the other 2 phases went down and the office power went up to over 300v
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Any tool can be used as a hammer but a screwdriver makes the best chisel |
1st Dec 2019, 8:56 pm | #66 |
Dekatron
Join Date: Sep 2011
Location: Rugeley, Staffordshire, UK.
Posts: 8,809
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Re: Repair nightmares.
Ha, try working on late 70s/80s hifi equipment. Although it's not always the case, very often you need a degree in taking mechanical assemblies apart (hidden plastic clips etc), be able to unsolder numerous inter-board connecting wires without damage so that they can be resoldered, un-hook impossible to get at ribbon cables from their sockets, measure and unsolder impossible to get at components from PC boards, disassemble chassis in order to (maybe) get at parts that need to be checked and replaced. And so on. Oh, and then double that degree of difficulty if the unit is a cassette deck. Not for the faint hearted, keep well away unless absolutely necessary is my best advice!
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A digital radio is the latest thing, but a vintage wireless is forever.. Last edited by stevehertz; 1st Dec 2019 at 9:07 pm. |
1st Dec 2019, 10:05 pm | #67 |
Pentode
Join Date: Sep 2018
Location: Devon, UK.
Posts: 151
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Re: Repair nightmares.
Here's one from a 'floating madhouse', long ago and far away.
Our hybrid FM broadcast transmitter developed a habit of randomly going off-air. Our Engineer could find no obvious cause. Pressing 'Plate On' would bring it straight back on, until the next failure. All the high power stages, housed in the main cabinet, checked out fine. So he turned to the exciter unit, which produced about 20W of modulated RF for amplification & transmission. Located in a rack a few yards from the main unit, the exciter had a multi-function meter which included RF output power. Our Engineer spent quite some time gazing steadily at this meter, as our beloved rust bucket rolled gently in the sea. He was eventually rewarded with a brief flicker of the needle, accompanied by a 'clunk' as the FM transmitter went off yet again. The exciter's output immediately returned to normal; however, the transmitter remained off-air. The transmitter had no 'recycle' function! Nor was there an 'RF Drive' alarm...this was a pre-production unit. The fault was traced to the exciter's power supply. Wired connections to a power transistor had never been soldered at the factory, just wrapped around the leads. Re-assembly was fun. The transistor was mounted on one of two heatsinks, of the oblong, flat-bottomed type usually fixed to the rear or top of a casing. In the exciter, they were stood on end in the middle of the unit, with the lower ends freestanding in a bowtie-shaped extrusion of about 1mm height in the base of the exciter. A corresponding bowtie-shaped piece of metal sat on the upper ends of the heatsinks. There was a hole in the centre of this endpiece; also a hole in the centre of the extrusion in the exciter's base. Bracing ourselves and this arrangement against the motions of the ship, our Engineer and I had to position the heatsinks right up against the extrusion in the base, then hold them in place whilst gingerly threading a very long bolt through the endpiece, then down between the heatsinks and through the hole down below, then thread a nut onto the end of the bolt and gradually tighten the assembly with the aid of a flat-bladed screwdriver at the top end! This process took about 15-20 minutes. We spoke of the equipment's designers in rather uncomplimentary terms. |
2nd Dec 2019, 3:25 pm | #68 |
Nonode
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,869
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Re: Repair nightmares.
I can well relate to Bill, #55. As the apprentice it was one of my jobs to ensure the field service box of valves were filled up, we covered many makes of TV’s. Yes you have guessed it, after travelling many miles down country lanes to a customers farm only to find the very valve required was not in the box. The engineer was not at all pleased and I got a right bo...... . It didn’t happen again, part of being an apprentice I guess.
John |
2nd Dec 2019, 6:24 pm | #69 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,951
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Re: Repair nightmares.
Another one was a W15FMB where the business-end was mounted in the back of some typically ill-constructed 1970s British car.
Leaking rear-light/window/boot-seals regularly dosed the poor Pye with water, meaning it kept getting 'returns' with water-damage-related issues. Eventually, Malcolm - my boss-at-the-time, and a fan of the red magic-marker - refused further work and wrote across the trouble-ticket "Replace the car not the radio - it's cheaper!" |
2nd Dec 2019, 8:02 pm | #70 |
Octode
Join Date: Jul 2006
Location: Willand, Devon, UK.
Posts: 1,023
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Re: Repair nightmares.
I currently have a fairly modern midi system here that is owned by an old neighbour, all it needs is new belts for the cassette mechanism and the CD drawer drive. I've never had to remove so many screws, and screws of so many different sizes and types to carry out a simple repair. To get where I need to so much has to come apart, and everything is attached to everything else, no outer frame or case. Now I have a pile of electronics and plastic parts on the bench. No nice modular construction here!
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4th Dec 2019, 7:42 am | #71 | ||
Rest in Peace
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
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Re: Repair nightmare's.
Quote:
Mike |
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4th Dec 2019, 7:47 am | #72 |
Rest in Peace
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
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Re: Repair nightmares.
I once dismantled a cooker where the oven had that stopped working, only to eventually find that the timer clock was activated and set to off.
Mike |
4th Dec 2019, 9:12 am | #73 |
Heptode
Join Date: Jul 2009
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 722
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Re: Repair nightmares.
Hi
When I was a young engineer at Mullards Mitcham in the 80s, we put in an energy management system which controlled the temperatures over the whole site. This has distributed control boxes over the whole site controlling steam valves. The PCB in these boxes would fail and we would bring them back to the lab where they worked perfectly - so on the pile of 'spares' they went. But they would fail randomly again and again in different places always working when we tested them. It turned out that because the boxes were mounted high in the ceiling (very tall ceilings!)the temperature up there was high enough to cause marginal 74 monostables to fail. Walking back to the lab cooled them and they again worked! We replaced all of them but the fault continued..... It turned out that the assembly had been done by prototype assembly staff who objected to doing 'tarts work' as there were 30 such boards. As a result, a tantalum had been put in the wrong way round on many boards. These failed on a time basis, of course longer than the time for the monostable!! On demonstrating that system to the head of Philips the software promptly failed and I dropped the leg of my chair on his foot as I got up to get a disc! Loved those days! Cheers James |
4th Dec 2019, 6:59 pm | #74 |
Nonode
Join Date: Jan 2016
Location: Stafford, Staffs. UK.
Posts: 2,529
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Re: Repair nightmares.
Ah yes. One of our fridges failed. Totally dead not even a light but the fuse was intact. Transferred the contents to the other, went off to work and then started taking apart when I got back home. The cable went straight up to the top where the digital control unit lives along with the light (yes the light is actually outside the cavity unusually). Anyhow as I tried to lift the top of, the wiring to the main on/off switch made it hard to remove. Yes the on off switch just above the door that had been knocked and isolated all power!
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4th Dec 2019, 7:32 pm | #75 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,951
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Re: Repair nightmares.
In a previous life I looked after the DNS servers for various organisations; it was occasionally necessary to force-restart the 'named' process on the master server so it would re-read the altered config-files.
For this we used a command-script that would issue the necessary 'kill' instruction across all the different DNS servers. Unfortunately, on one occasion I ran the kill-script without specifying the option that would let the processes restart. And this time I didn't check that the processes had restarted, I just logged-off. Then I went down the pub for a Friday lunchtime drink. It wasn't long before the panicked phone-calls from various service-desks started coming in. |
4th Dec 2019, 9:48 pm | #76 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,171
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Re: Repair nightmares.
Many years ago one of the design teams I was running was developing a 3 SCR controller for a hydraulic pump motor. This had to operate at up to 80V and a 1 min rating of 1000A.
It used a conventional 3 SCR circuit and we used opto SCR's for triggering the main devices. Development went fine and we felt we had a good product. The PCB was to be potted and mounted at the side of the power devices. Initial production runs were commenced and we started getting failure to commutate at about 800A for no apparent reason due to the opto SCR's misfiring. This led to quite a few late nights, we found that if we de-potted the opto's the fault went away. An in depth reading of the opto data sheet revealed that the C-R gate suppression of the device could be varied depending on the dv/dt across the device. This was duly increased and the problem was resolved. 2 things had happened; the units were tested in a hot summer on 72v to reduce some of the heating on the test loads, and the permitivitty of the potting compound was about 3 times that of air, so increasing the coupling across the chip and increasing the dv/dt, leading to false triggering. Ed |
4th Dec 2019, 11:29 pm | #77 | |
Dekatron
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,675
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Re: Repair nightmares.
Quote:
It had already had two visits from a Currys (or was it Comet) engineer who pronounced it "OK now" although he didn't find an actual fault. The problem was gross overheating, on both the small top oven, and the main oven. The penny began to drop when as part of my tests I ran both ovens together, it was by no means perfect, the temperature in both fluctuated wildly, but at least they weren't getting as hot as before. Then my attention began to focus on the physical layout rather than just the electrics in the back of the oven, and guess what? Some bozo at the factory had neatly routed the thermostat capillary tubes, and fitted each of the the phials into the wrong oven's pockets.
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5th Dec 2019, 12:12 am | #78 |
Octode
Join Date: Sep 2017
Location: Stockport, Cheshire, UK.
Posts: 1,999
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Re: Repair nightmares.
I was amused yesterday when a printer at work flashed up a message to tell me to turn it off & on again to sort out a problem.
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Hello IT: Have you Tried Turning It Off & On Again? |
5th Dec 2019, 7:14 am | #79 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,636
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Re: Repair nightmares.
Some cracking stories there... I'd like to slightly expand the theme if I may: what's the best bit of advice as regards repairing/fault finding members have had or whilst fixing something what was a valuable lesson learned?
Mine was something a tutor at college said - "thalt shalt check voltages", a mantra I sometimes turn to when going round in circles or stuck. It's easy to get lost sometimes and chase red herrings down rabbit holes, as DJ says, stopping and going back to square 1 often helps. Another is if totally stumped, tired and ratty is , switch off, walk away and sleep on it but I often find going for a walk helps, there's something about motion that helps cogitation. Lastly it's always worth checking the DUT is plugged in... : ) Andy.
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5th Dec 2019, 9:01 am | #80 |
Hexode
Join Date: Jan 2018
Location: Portsmouth, Hampshire, UK.
Posts: 421
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Re: Repair nightmares.
A few guidelines I was given which still hold water today were (as Andy stated) Check the power rails, ALL of them! Check the clocks are running, check any watchdogs and/or power on reset's. A place I worked at made high end TV studio/OB kit & there were a number of very expensive PCB's that simply didn't work, dead as a doornail, all the basic checks were good. After much head scratching we found it, a locally generated clock from a PLL wasn't working, instead of a 100pF capacitor in the feedback loop there was a 100nF, stopped it dead. As they were 0805 SMD's it was very tricky to prove without removing (this was frowned upon at the time!), I did manage to convince a supervisor to rework one "on the sly" & it burst into life! One measly capacitor stopped a £10,000+ unit from working. As others have said, the intermittent faults are by far the worst.
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The difference between stupidity and genius is that genius has its limits. (Einstein) |