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Old 5th Dec 2019, 9:11 am   #81
kalee20
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Default Re: Repair nightmares.

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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
I'd say it is: Never take anything for granted.

This applies especially when fault-finding something which has been built and has never worked, but also when fault-finding something which has worked once and now doesn't.
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Old 5th Dec 2019, 11:13 am   #82
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Default Re: Repair nightmares.

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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
Don't be intimidated by cct diagrams. They may seem intimidating in their complexity, but you normally only need to focus on one area relevant to the fault you're chasing. A bit like using a dictionary or a map.
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Old 5th Dec 2019, 11:54 am   #83
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Default Re: Repair nightmares.

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Quote:
Originally Posted by Diabolical Artificer View Post
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
Don't be intimidated by cct diagrams. They may seem intimidating in their comkplexity, but you normally only need to focus on one area relevant to the fault you're chasing. A bit like using a dictionary or a map.
Good point, and as one who has often experienced that problem, I will remember that. However, it's not always easy to do that, especially with more modern equipment when 'an area' is connected to, and affected by numerous other paths and signals, not just series connected 'strings' of circuitry as you tend to get in an old radio.

Yours, slightly less intimidated from Rugeley.
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Old 5th Dec 2019, 12:59 pm   #84
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Default Re: Repair nightmares.

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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?
I have three:

{1} Don't believe the customer's description of the fault - always verify it for yourself. I had several instances over the years where a tech believed the customer's fault-description and began fault-finding only to discover after fruitless hours with the datascope that the customer's original fault-description was rather wide of the mark.

{2} If you're the Nth person to be investigating a fault, always check any rectification work carried out by the previous (N-1) techies. I've seen what was originally a single fault had become multiple faults after several different people had each made their own attempt at fixing it.

{3} Don't crack under pressure: when you've got a major service/piece of equipment down and customers/senior-manager-types/CEOs are continually asking how much longer it will take before normal service is resumed, tell them - politely - that their continual interruptions are actually delaying you from fixing their problem and that it'll be fixed much quicker if they just leave you to get on with it. "When I'm giving you a progress-report I'm not working on the problem!".
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Old 5th Dec 2019, 1:32 pm   #85
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Default Re: Repair nightmares.

(Most of the fault-tracing I do is component-level repair on classic computers, so not all of this will apply to vintage radios, but much will)

1) Check the power supplies first. The unit can't work without the correct power.

2) If you find a fault, correct it, even if it seems to make things worse. The unit was not designed to work with faults.

3) Randomly swapping parts will get you nowhere fast.

4) You can't fix a fault that is not there. That's why intermittent faults are such a pain. You can only fix them and know they are fixed if they show up on the bench.

5) Faultfinding consists of finding out why the unit is not working. The circuit diagram (etc) will tell you what it should be doing. Test gear will tell you what it actually is doing. You then need to work out what could cause it to be doing what it is doing.
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Old 5th Dec 2019, 2:08 pm   #86
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Default Re: Repair nightmares.

I agree with the comments and if I may add one , always, if available, get the service manual with updates out or at least the circuit diagram . I have spent far too much time on a fault simply because I didn’t have the current service manual.
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Old 5th Dec 2019, 2:39 pm   #87
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Default Re: Repair nightmares.

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4) You can't fix a fault that is not there. That's why intermittent faults are such a pain. You can only fix them and know they are fixed if they show up on the bench.
Yes, but when I was a test engineer it was our job to make the fault appear. We did this by mechanical means, that is by bending or tapping the PCB, waggling and tapping components and sockets etc, and by alternatively applying heat (hair dryer) or cooling the area or component using freezer spray. That usually worked. When it didn't and a large amount of time had already been expended testing the PCB, then it would have to be scrapped. You can't spend days fault finding what was then (early 70s) an expensive £30 PCB. I still use these methods today on complex 70s and 80s hifi equipment that have intermittent faults. The last one was a small transistor that honked like hell when tapped. In fact, even if a fault is not intermittent, if I'm struggling I will often spend ten minutes tapping components and sockets on boards to see if anything responds, and yes they have on many occasions. It's not scientific, but for average Joe Electronics like me, it helps me get by and I've fixed hundreds of vintage equipments from basic valve radios to complex video recorders.
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Old 5th Dec 2019, 3:13 pm   #88
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Default Re: Repair nightmares.

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Don't believe the customer's description of the fault -
Problem reported: On/Off switch broken
Investigation finds On/Off switch to be OK. Customer explains that the TV can't be turned on. Further investigation reveals mains dropper burned out.
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Old 5th Dec 2019, 3:14 pm   #89
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Default Re: Repair nightmares.

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3) Randomly swapping parts will get you nowhere fast.
There used to be a joke satirising the approach by the field-service* technicians employed by a large-and-now-defunct British computer-company, along the lines of "if his car had a flat tyre he'd try to fix it by jacking all four corners and swapping the wheels round - if that didn't fix it he'd change the battery".

*or "Field Circus" as they were often described.
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Old 5th Dec 2019, 3:43 pm   #90
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Default Re: Repair nightmares.

Sets which have been “got at” can be interesting.

I once had to fix one of those B&O linear tracking record decks. It had a confusing array of symptoms, but a diagnosis based on “recently soldered joints” revealed several transistors the wrong way round, npn where pnp should have been etc. The final elusive fault was that the circuitry to detect the position of the linear tracking carriage wouldn’t work properly at some parts of its travels. It used a strip of clear plastic with various black lines as it’s position sensor. It had been swapped for a different version from another deck type!

The same fiddler once fixed an active speaker with a burned out power amp in record time. We were amazed, even more so when the owner was delighted with the repair, and didn’t come back. Then we tried to use a stock version of the speaker in the audio demo room.....

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Old 5th Dec 2019, 3:46 pm   #91
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Default Re: Repair nightmares.

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Don't believe the customer's description of the fault - always verify it for yourself.
I saw a classic example of this in a company I worked for some years ago. A colleague of mine had a problem with his mobile phone (a Motorola Timeport, if anyone remembers those). It had crackly audio. I heard it, and it sounded like an intermittent connection to the earpiece or microphone. Our office manager sent the phone off for repair, but described the fault as 'poor reception', rather putting two and two together and making five, and entirely misunderstanding how a digital GSM phone works.

The phone came back with a test sheet showing a comprehensive evaluation of its RF performance, which was of course well within spec. It still crackled. I think he got a new one.

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Old 5th Dec 2019, 4:06 pm   #92
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Default Re: Repair nightmares.

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Originally Posted by Diabolical Artificer View Post
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?
  • The first question you ask should always be: Has it ever worked before? That can make a difference.
  • It's usually easier to repair something that doesn't work at all than something that sort of nearly works: The deader, the better.
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Old 5th Dec 2019, 4:59 pm   #93
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Default Re: Repair nightmares.

One of the other nightmares was what I called the "all hands on deck" syndrome: I had a couple of clients where the response to a failure was for their management to call-out _everyone_ they had in their emergency-callout list, whatever the time of day or night.

So I'd turn up at a site along with a couple of BT linesmen, some people from the power-company, the UPS/generator technician and an ISP technician. Often we would then all wait - sometimes for hours - before the customer's keyholder arrived so we could all get into the building. In these cases the 'problem' was invariably something either so catastrophic - several-inches-and-rising-fast of snow-melt-water in the false-floor void - that none of the assembled team could do anything about it, or it would turn out to be something simple like the remote-controller cabinet for the radios had been unplugged back at head-office. [yes, both of those being real examples].
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Old 5th Dec 2019, 6:14 pm   #94
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Default Re: Repair nightmares.

Quote:
Originally Posted by G6Tanuki View Post
Quote:
Originally Posted by TonyDuell View Post
3) Randomly swapping parts will get you nowhere fast.
There used to be a joke satirising the approach by the field-service* technicians employed by a large-and-now-defunct British computer-company, along the lines of "if his car had a flat tyre he'd try to fix it by jacking all four corners and swapping the wheels round - if that didn't fix it he'd change the battery".

*or "Field Circus" as they were often described.
I heard that one about DEC (Digital Equipment Corporation), who were American. Their 'Field Service' was always called 'Field Circus' or 'Failed Circus'.

Anyway the version I head was :

How do you spot the Field Servoid with a flat tyre ?
He's swapping out all the tyres to find out which one is flat.
How do you spot the Field Servoid who has run out of petrol?
He's swapping out all the tyres to find one which one is flat
What does the Field Servoid do if he can't find the flat tyre?
He replaces the alternator.

From my experience, both observing them with machines on official service contract and later from running machines at home that had been on official service contracts, that is very near the truth!
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Old 5th Dec 2019, 6:55 pm   #95
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Default Re: Repair nightmares.

Here's one I've not seen in anyones list:

Never believe your instruments.

Now before anyone jumps on it, I'm referring to the habit people have of believing what they see even if it makes no sense. Your instrument might be faulty, it may not be connected where you think it is, and so on.

I spend a lot of time at work trying to reproduce faults that can't be reproduced. This is largely beause many designers / manufacturers assume they have understood the problem, and then we get asked to test the fix. It pains me how often I have to remind people that if we can't reproduce the problem with the part without 'the fix' then what will testing the fix prove? Just to prove the point I had a problem that I found I could induce with a power dip a very precise time after the device was told to go into sleep mode. When I say precise, I mean between 950mS and 955mS, and then not every time, about one in forty. Anyway, yesterday I demonstrated to the designers in person, that their fix had made no difference at all!
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Old 5th Dec 2019, 7:22 pm   #96
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Default Re: Repair nightmares.

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I heard that one about DEC (Digital Equipment Corporation), who were American. Their 'Field Service' was always called 'Field Circus' or 'Failed Circus'.
The |D|I|G|I|T|A|L| engineers I dealt with supporting MINC-11 stuff and the VAX11/785 were always OK; the ones sent out to fix faults on our MicroVAX-II fleet were sometimes not-so-hot.

You know you've got problems when the 'engineer' looks at the console error-messages and asks "What operating-system is this running?"
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Old 5th Dec 2019, 9:04 pm   #97
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Default Re: Repair nightmares.

At the museum where I volunteer, we have a display that activates when a visitor stands in front of it. It is a reverse illuminated mirror that, when illuminated from the rear, becomes clear and a picture of a Sargent major barks orders at you. The system works well but the mirror remains illuminated at all time and it loses its effect.
The company that supplied the gear sent its "maintenance engineer" to look at it. He was complete with a set of test lamps and a 17th edition badge. When he was scratching his head it was suggest that he removed the feed from the lamp relay box to the lamps, his response was that he was not allowed to do that!
That was 2 years ago and it still remains faulty.

Ed
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Old 6th Dec 2019, 12:13 am   #98
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Default Re: Repair nightmares.

"The set is completely dead" (But it has a light illuminated on the front)

"It's only the picture valve" (The set is all transistor)

"It can't be the aerial, it's only been up 6 months" (Where's the logic in that?)

"Can you clean the inside of the screen please" (It's a 22inch colour CRT with normal "push through" presentation.

That last one got me into trouble. The set was close to a window, it was a bright sunny day and I could just about perceive some streak marks that the lady had made with the polishing cloth or whatever she used. She was clearly a house proud woman, the house was immaculate.

She was not having it that the blemish was on the outside of the glass so I asked for a cloth and a drop of Fairy Liquid.

This is where I made my mistake, I cleaned and polished half of the screen to prove my point.

"Are you saying my house is dirty?" she said as she threw me out.
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Old 6th Dec 2019, 1:41 am   #99
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Default Re: Repair nightmares.

People have often said to me "My radio/record player/tape recorder/guitar amp has blown up." Having wised up to this terminology, I now always ask "Did it actually go bang?" Usually the reply will be along the lines of "No, you know what I mean, it just stopped working." WELL ITS NOT BLOWN UP THEN HAS IT!! Sorry...

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Old 6th Dec 2019, 2:03 am   #100
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Default Re: Repair nightmares.

Agree with never believing the customer's 'diagnosis'. Things like:
"It just needs a new switch"
"It's the condensor or the coil"
"It's a wire off"
"It's got to be something simple" (usually when half the SMPS is charred)
"It's just a faulty fuse" (one of my favourite clangers from the terminally thick).
(For TVs, no matter what the fault ): "It's the tube"

...and of course they'd fix it themselves, but haven't the time
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