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Old 4th Jun 2017, 7:16 pm   #21
turretslug
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Default Re: Intermittent faults...

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Originally Posted by Refugee View Post
On going through the AC side I found that the mains voltage selector was full right up with char coal.
The previous owner before the one I was fixing it for should have been able to trace it easily from the smell. They didn't and it was down to me to hide a chocolate block under the transformer.
Never liked voltage selectors- and the AR88's is one of the more horrid examples.
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Old 5th Jun 2017, 12:25 am   #22
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Default Re: Intermittent faults...

It just does not have the rating it needs for the heavy transformer in the set.
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Old 5th Jun 2017, 1:08 pm   #23
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Default Re: Intermittent faults...

Phrase "intermittant faults" brings back fond memories of days in an old hut at Stone, doing what we called "Bask Testing " on transmission kit. Or back in a repeater station looking for the elusive loss of signal. It was always a dry joint on the frame, and I found the best way of finding them was to get a low level tone ( something like -30 ) sent at the 0 point ,and chase with headphones. A normal level tone ,too often, "wet" the dry joint till the next time.
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Old 5th Jun 2017, 3:17 pm   #24
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Default Re: Intermittent faults...

Nothing like as complex as the preceeding faults but I had this old Bakelite Avo (7 or 8 - I forget which) that mostly read fine but every now and again it would give you half the value of the voltage/resistance/amperage it was measuring. I just about tore my hair out trying to figure it out and was reluctantly coming to the conclusion the meter movement was faulty. Then I realised the fault disappeared when present if the meter was resting face down on it's terminals - quick examination with a magnifying glass and sure enough the positive terminal had a hairline almost invisible crack in the solder and the faintest indication of movement - few moments with a high wattage soldering iron remaking the joint and it has been as right as rain ever since!!
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Old 5th Jun 2017, 6:17 pm   #25
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Default Re: Intermittent faults...

I have one on my Teac A3440, just once in a while the speed starts to go down, not quickly but over a few seconds. Then just as the tape is about to stop it starts off again at the right speed which is normally set to 15 i.p.s.. I am patiently waiting for the fault to become continuous, it could be anything from the servo control, loss of capstan motor voltage or drive from its darlington control transistors. I do not think a slipping belt would produce this fault so just have to be patient. The machine is very large and heavy so not the sort of thing to be able to get to quickly. Other times it will play for hours without the fault showing. All intermittent faults can be a devil to trace unless the fault comes on and stays. :-(
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Old 5th Jun 2017, 6:42 pm   #26
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Default Re: Intermittent faults...

My best one is a ship with a dodgy camera signal, the firm sent up a couple of fitters three times, still the same. My boss asked (pleaded) me to go up to Scotland and have a look, taxi to Heathrow, business class to Edinburgh, taxi to ship. The fault (found in a few minutes) was a half screwed in connector, it was a 38999 mil spec. job, they have a red line on them, which, if visible means it's not fully home.

All in all a very nice (sunny) day of posh travel and fish and chips (cooked in dripping) at the harbour side.
 
Old 5th Jun 2017, 8:29 pm   #27
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Default Re: Intermittent faults...

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....Then I realised the fault disappeared when present if the meter was resting face down on it's terminals ...
Just wondering how you viewed the scale when it was face down, a mere 'terminals height' above the surface it's resting on (c10mm?) ?! Or for that matter, what drove you to make that rather obscure kind of test?!
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Old 5th Jun 2017, 8:31 pm   #28
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Default Re: Intermittent faults...

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My best one is a ship with a dodgy camera signal, the firm sent up a couple of fitters three times, still the same. My boss asked (pleaded) me to go up to Scotland and have a look, taxi to Heathrow, business class to Edinburgh, taxi to ship. The fault (found in a few minutes) was a half screwed in connector, it was a 38999 mil spec. job, they have a red line on them, which, if visible means it's not fully home.

All in all a very nice (sunny) day of posh travel and fish and chips (cooked in dripping) at the harbour side.
That's always fun: I recall having to fly out to Darmstadt [with mandatory taxis and overnight stops both ways in en-route plush hotels] and type-in a mere three lines of Cisco IOS to fix a problem which I could have solved from home in ten minutes via a SSH session.

But they wanted me physically on-site to type the magic Voodoo-That-Only-I-do.

[]Hint: weekday night-time in Darmstadt really lacks any kind of party-vibe[]
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Old 6th Jun 2017, 1:41 pm   #29
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Default Re: Intermittent faults...

The most recent one for me was a friend's guitar tuner. It was hit and miss whether it worked at the start of a gig, though it was nearly always working by the end

Anyway, it turned out that an SMD chip wasn't properly soldered to the board, several pins just resting on the pads.

I'm not set up for SMD but I managed to resolder all the pins by winding some copper wire on the end of the bit. With the help of a magnifier I could just about fix it without bridging any pins.
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Old 6th Jun 2017, 4:31 pm   #30
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Default Re: Intermittent faults...

In a previous job I was called out late at night to a control room where the data handling system for calls was causing major problems by crashing occasionally. Of course it never happened during a normal day shift, always waiting till the most awkward time. I went and rebooted it, babysat for an hour, drinking tea, as you do, then started on the forty mile drive home. You guessed it, half way home, the phone rang, it happened again, and again, and again. Eventually it was traced to a cracked track in a backplane, a problem I have seen before on rack mounted plug in assemblies. I think it's due to flexing when the boards are inserted. Why it would never happen while I was there I will never know. Plenty of instances where radios etc have been removed from vehicles faulty, only to perform perfectly in the workshop, heated up, cooled down, banged, held upside down, sat on, etc etc. Return them to service and within half an hour smugly fail right in front of you. AARRGHH!!!
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Old 6th Jun 2017, 5:39 pm   #31
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Default Re: Intermittent faults...

Another computer-related one I dealt with was a leased-line [the old analog "EPS" Post-Office type of 4-wire for those who remember such things] used for data-transmission between a satellite site near Guildford and its 'master' in Manchester.

Periodically it would go down for a minute or so then the fault would clear and it'd run fine for an hour, two days, a week - anyone's guess!

BT were stumped - they put loggers on both ends but - as we all know, "To Observe is to Interfere" and the problem never showed itself! About all they could tell us was that the fault was "close to the B-end equipment".

After much frustration I eventually identified it. The two-pair cable ran from the modem rack, under the computer-room floor and down to the frame-room. When the cable had been installed it had got snagged on one of the screw-jacks used to support the false floor. When someone heavy stood - or a heavily laden trolley of tapes was wheeled - on the relevant false-floor tile it rocked slightly, pressed the supporting screjwack down and caused a short between one TX core and one RX core - resulting in massive crosstalk and the line dropping-out. Remove the weight, the short disappeared, fault cleared.

Why did it never show up when the BT test-gear was on the circuit? Well, the test-gear was on its own - lightweight - trolley that stood next to the modem-rack, blocking the aisle to other traffic whilst not itself being heavy enough to trigger the fault!
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Old 6th Jun 2017, 10:50 pm   #32
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Default Re: Intermittent faults...

I recall an intemittent fault with s car stereo, which, in principle, was similar to the problem describe by G6Tanuki. A local garage asked me to check why a Blaupunkt stereo wasn't working on one channel. When I reported that it was all OK on my test bench, they said 'We've tried another radio in the car and that works OK. Turns out that they'd fitted a low power (2 x 6Watt) basic stereo, whereas the one that belonged in the car was a high power (2 or 4 x 25 watt). Stereos of this type have their outputs floating at half supply (about 6.5vDC) and obviously none of the speaker wires must br commoned or connected to chassis. What had happened was that a screw in the vehicle's trim panel had cut into one speaker lead, earthing it to the car's chassis. By chance it was the negative speaker lead that was earthed this way, so the low power stereo worked perfectly well, as it's speaker negative terminals were earthed anyway, whereas the high powere one had one of it's outputs shorted to ground via the errant screw. Rerouting and insulating the affected wire solved the problem!
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Old 7th Jun 2017, 11:24 am   #33
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Default Re: Intermittent faults...

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Quote:
Originally Posted by avocollector View Post
....Then I realised the fault disappeared when present if the meter was resting face down on it's terminals ...
Just wondering how you viewed the scale when it was face down, a mere 'terminals height' above the surface it's resting on (c10mm?) ?! Or for that matter, what drove you to make that rather obscure kind of test?!
I had it face down on the desk propped up at the meter end resting on it's terminals at the other and a mirror at 45 degrees so I could see the meter movement while probing/wiggling things around the back. Eventually I noticed when I lifted it up slightly the fault appeared and when I put it down it vanished - had to be the terminals!!

As for why - sheer desperation EVERYTHING checked out find according to the circuit diagram etc. but the fault still appeared from time to time! So I figured if I could see the meter when probing it and the fault appeared I might stand a chance.

Of course I'd resoldered just about every other joint in the meter thinking of dry/ctracked - but not the termianls
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Old 7th Jun 2017, 11:54 am   #34
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Default Re: Intermittent faults...

One of the most amusing was told to me by my mate Kenny - an ex BT Lineman.

A young lad kept finding broken/chewed-up O/H lines out on a site in rural Gloucestershire. These took some finding - until he got the hang of inputting a tone and hanging out of his van with an old tranny.

Anyway - he couldn't understand this ... "It never happens when I'm there ...". He told Kenny about this ... who advised "Lock that gate and fix the fences/barbed wire!". The Lad replied "What? !!!".

To wit Kenny counselled: "It's those poachers and their shotguns again" !!!!!!!
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Old 7th Jun 2017, 11:57 am   #35
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Default Re: Intermittent faults...

Extremely common and not just poachers.
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