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Old 30th Nov 2019, 7:17 am   #41
Radio Wrangler
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Default Re: Repair nightmares.

Downstairs in the PCB shop at HP South Queensferry was a very large multi-headed numerically controlled drill. It had four heads and each head drilled holes in a stack of sheets of raw PCB material. It was about the size of a transit van. Control was from a separate cabinet with a paper tape reader.

It wasn't reliable. Every so often it would go silly and while drills were through the boards, the servos would shoot everything off to one corner, breaking the drills and writing off a dozen boards. People like me suffered erratic delays to our prototype boards, manufacturing were even more upset as the service firm was called out.

Repair after repair failed to make any difference. Eventually, the towel was chucked in and a new machine of a different make was ordered. The new beastie had only three heads, but boy was it fast. Air turbine powered drill heads and the X-Y table ran on air bearings on a huge granite slab. This Excellon drill proved reliable.

The old machine was put up for sale as "not working"

I heard that it was bought by a man who dealt in these machines and that when he turned up with a lorry for he thing, he asked where the earth strap was.... said it was a big 6" wide copper braid strap which went from the control cabinet to the main machine "Without it, the servos will shoot off to one corner....."

Looks like the aftermarket dealer knew more about the product than the manufacturer's service organisation did.Buying a different make was definitely the right thing to do.

So, a repair-from-hell story from the customer's viewpoint.

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Old 30th Nov 2019, 10:42 am   #42
Malcolm G6ANZ
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Default Re: Repair nightmares.

On one of the clinical linacs I worked on the machine shut itself down every time the beam was turned on. This meant a 15 wait as timers ran out. We spent a long time trying to sort it, changing boards thyratrons etc. Eventually I found that when the beam on button was pressed a certain relay was activated this was a chassis mounted relay and not on any of the boards that had been changed. As usual it had a protection diode actrss the coil. This diode was short citcuit, so when powered the supply line was seeing a direct short and shut itself down, so shutting down the whole machine.
Bear in mine that the manual was about 150 A3 sheets.
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Old 30th Nov 2019, 11:15 am   #43
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Default Re: Repair nightmares.

Back in the 60's I made a call to an old lady's house to attend to KB portable,a KV003.The complaint was that there was an intermittant flash on the screen accompanied with a 'tick' on sound.I checked for eht discharge and found nothing and the fault cleared.Over many weeks I attended to the same call and sometimes the fault occurred and sometimes it didn't.I even had the set in the workshop for extended soak testing and it never showed.After returning the set one Saturday morning whilst wating for the set to warm up I was conscious of the time and just as the picture appeared I glanced at the clock and there was a 'tick'.Eureka, quite by chance I saw that the flash and 'tick' happened when the pendulum on the grandfather clock reached the end of its travel and hit the spring.I cannot remember what the metals involved were but here was the source of the interference.Once I had shown this to the lady she was happy as the clock was a retirement gift to her late husband and she asked me to go no further as she felt his presence.I suppose the intermittant nature of the problem was caused by atmospheric conditions but in all that time I had never noticed the regularity of the 'tick'.Peter.
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Old 30th Nov 2019, 11:42 am   #44
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Default Re: Repair nightmares.

One of the most difficult repairs I had was on one of my beloved 2465B scopes a while back. I published it, just in case anyone else got caught out.

What happened was the trace intermittently deflected downward and then recovered randomly and it got more frequent with time.

This one was a real stretch to repair, the main board required removal from the scope. Normally I can repair most things in a day or less, but this one pushed out to one week and I was beginning to wonder if I could repair it, I refused to give up. Later I found that a professional repairer in the USA had a stack of main boards they were never able to repair, so they just put them aside.

It turned out to be a tin whisker on some track-work, which was about the only place in the entire scope without a good conformal coating on the pcb and nearby component pins.

The story, with photos, is here:

http://worldphaco.com/uploads/TEKTRO...K_LEAKAGE..pdf


More recently I had an LG computer video monitor in my workshop that suddenly developed an intermittent fault, where the menu appeared on its own, almost as if a Ghost had pushed the menu button and half of the picture, split down the vertical mid-line changed contrast.

Luckily I was wise to what must be happening, in that two completely unrelated parts of the circuit were malfunctioning at exactly the same time intermittently. Sure enough it was a tin whisker bridging two of the finely spaced pins on the processor engine IC. The board was labelled "Pb free". If you see boards like this examine them under a microscope.

So I've wised up now to the vagaries of tin whisker faults and I'm ready for the next one. Though given the number of AF11X transistors I have replaced in the past 40 years, I should have been better prepared for this nightmare and I must be a slow learner.

As I mentioned in the article, the usual methodology of fault finding is to understand the circuit and therefore formulate a likely location for the fault that fits the observed signs of the fault. (it is no different in Medicine, looking at neurological signs & symptoms of a defect and pinpointing the anatomical location of the lesion, provided you understand the anatomy and functions).

However, tin whiskers create entirely new circuit sub-systems by linking them together, tipping the usual fault analysis methodology on its head. At least when it happens, if the fault seems "impossible" or "bizarre" it is a clue that a whisker might be involved.
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Old 30th Nov 2019, 11:48 am   #45
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Default Re: Repair nightmares.

I recall building a test rig in the 70s that included a multi 7-segment LED display. These devices were usually red in the mid 70s. For some reason I made the display pcb and fitted a coloured translucent filter before I had tested the pcb, having prototyped it all with a 'rat's nest' beforehand.
At the time we were all enthralled with the Sinclair Cambridge calculators, and I had just splashed out a week's wages on a Sinclair Oxford Scientific. This had some sort of purple filter over the red LED display giving a cool effect. I selected a green filter for my rig display thinking that it would give a cool purple display. The rig worked but the display was blank... I spent a lot of time, and called in the assistance of other electronic techs, demonstrating that the multiplexed displays were taking current but not emitting light. Eventually I removed the filter and felt the asses ears growing from the top of my head.
At the time we had a peg board on the workshop wall where errors were displayed - the pcbs with relays or transistors on tinned wire legs twisted to reverse the connection due to mirror-image pinouts being laid out by the pcb draughtsperson. My filter ended up on there !
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Old 30th Nov 2019, 12:31 pm   #46
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Default Re: Repair nightmares.

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Originally Posted by bluepilot View Post
Reminds me of the story of the microwave link that dropped out for a few minutes every morning...
As a CCTV technician at one time, I have a variation on this. We had a riverside site with one camera on the opposite side of the river, with both video coming back from it and RS232 telemetry / control going to it via a microwave link.

There was a problem, which ought to have been forseen, when big ships (passenger ferries) pulled in - the picture from the other side of the river would vanish only to return when the ship pulled away.

The real mystery was that sometimes, when the ship pulled away and the link was restored, the remote camera could no longer be controlled.

Key to this was the realisation that the telemetry receiver was not only controlled via RS232 but was also initially programmed via RS232 as well.

The links in both directions were FM, so in the absence of a signal the receiver at the camera end was producing typical FM no-signal 'roar' which was then being converted into a continuous stream of random RS232 'data', some portion of which would eventually manage to 'stun' the telemetry receiver by reprogramming it. The link was eventually moved upriver but in the meantime we added a 'squelch' control to disconnect the data feed to the telemetry receiver under no-signal conditions, and that fixed the loss of control problems.
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Old 30th Nov 2019, 5:01 pm   #47
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Default Re: Repair nightmares.

The lid on vs. lid off posts reminded me of this.

When my daughter was very young she had her favourite cartoon VHS tape that she played in our Hitachi top loader. The tape cassette was unusual in that it was made of red plastic rather than the usual black.

(Some of you will be ahead of me at this point).

It played fine most of the time, but on a bright sunny day with the curtains open the play key popped straight back up.

Because my day job at the time was repairing VHS machines I quickly realised that the sunlight was passing through the translucent case and falling on the tape-end sensors, which were normally only illuminated by an incandescent lamp in the centre of the cassette through one or other of the transparent leaders.

The less well informed probably thought the thing was haunted.
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Old 30th Nov 2019, 6:46 pm   #48
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Default Re: Repair nightmares.

Quote:
Originally Posted by BulgingCap View Post
I recall building a test rig in the 70s that included a multi 7-segment LED display. These devices were usually red in the mid 70s. For some reason I made the display pcb and fitted a coloured translucent filter before I had tested the pcb, having prototyped it all with a 'rat's nest' beforehand.
At the time we were all enthralled with the Sinclair Cambridge calculators, and I had just splashed out a week's wages on a Sinclair Oxford Scientific. This had some sort of purple filter over the red LED display giving a cool effect. I selected a green filter for my rig display thinking that it would give a cool purple display. The rig worked but the display was blank... I spent a lot of time, and called in the assistance of other electronic techs, demonstrating that the multiplexed displays were taking current but not emitting light. Eventually I removed the filter and felt the asses ears growing from the top of my head.
At the time we had a peg board on the workshop wall where errors were displayed - the pcbs with relays or transistors on tinned wire legs twisted to reverse the connection due to mirror-image pinouts being laid out by the pcb draughtsperson. My filter ended up on there !
BC
Now this brings me to my early days as a trainee. One of the experienced guys was building something with an LCD (bought from RS components) with a microcontroller. He spent hours if not days on it, and couldn't get anything out of the display. I don't know why, but he went out to his car and fetched his polarised sunglasses, and suddenly he could see the display wasn't blank. RS had sold some displays with the polariser missing. Not the first gotcha from RS, who had also sold some electrolytics withe the polarity marking back to front. As my boss described it you got 'the sound of amps' (Gnnnnnn) followed by a big bang and a shower of foil.
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Old 30th Nov 2019, 8:36 pm   #49
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Default Re: Repair nightmares.

One of my techs asked me to take over a repair, the unit had been bouncing back and forth with no fault found; the customer complained that it constantly crashed.

The said unit was a Digitech PDS3500 MIDI pedal, memorising midi programme changes etc.

I telephoned the customer and had a long chat with him about the problem, and he agreed to come in the next day and show me the fault, which he could recreate at will.

The customer arrived, and after a brief chat, he set up the Atari Mega4, along with a rack of MIDI equipment, along with the PDS3500.

When I send my MIDI data to the PDS3500, it crashes. He did so, and it did indeed crash.

What is in the file I asked. Just the data for one of our sets, about 5MB


I pointed to the specifications in the PDS3500 owners manual. It clearly states the unit has 256KB of RAM, if you send 5MB to it, it will crash.


If you produce another file just for the PDS3500, just containing the preset changes you need the PDS3500 to control, and miss out all the note data, it will be fine.

20 minutes later, all was well, and the customer left a happy bunny.

But, none of this would have been necessary if he had read the instructions.


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Old 30th Nov 2019, 10:39 pm   #50
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Default Re: Repair nightmares.

I've done the re-assemble then wonder why it's not working only to open it up and find that you've missed a connector. Not to bad on some devices but professional keyboards do have a lot of screws.
The blank but backlit screen on my GEM S2 was my most annoying case of this, primarily as I'd only opened it up to clean the plastic in front of the screen and it's also relatively rare compared to it's competition at the time (but polyphonic aftertouch!).

Also if you put PIC microcontrollers in the wrong way round they get hot and don't work anymore, don't ask me how I know that. Plus when they are of the picAXE variety more expensive to replace.
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Old 30th Nov 2019, 11:18 pm   #51
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Default Re: Repair nightmares.

Something else I've just remembered.
A nice looking CD recorder/player, one of the big Japanese makes, I've forgotten which. Stated fault was "Won't record"

Played normal CDs OK, but when you put a new blank CDR in and tried to record the display displayed an error message which was verbose, in that it was more than a code, but it did not give you a clue as to what the "fault" turned out to be.

I was nearly going to condemn it as BER and return it to the customer unfixed.

The Internet gave me the answer, (I think I had dial-up at the time) a forum post mentioned "Music CDs" should be used, and sure enough you had to use special disks where you paid a premium, presumably to placate the holders of any copyrights you might breach.
Who would want such a restriction when you could use a computer with a CDR drive?
And why didn't the error say something useful like "Insert Music CD"?

The owner was unaware of this, there was no fault.
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Old 30th Nov 2019, 11:27 pm   #52
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Default Re: Repair nightmares.

Then there are the repairs that are nightmares not because they are difficult, but because they are easy.
Like the 200 mile round-trip to a site, just to press the reset button on a Panasonic telephone system. The service desk should have told them to try that.
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Old 1st Dec 2019, 12:25 am   #53
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Default Re: Repair nightmares.

Last week, I got a call from my local community library that half the building was in darkness.

I went and had a look at everything that I could and everything seemed in order.

Because this was Council property, I couldn’t open up the fuse boxes, so the electrician got called out.

He promptly flicked a 32A switch and everything was on.

Why didn’t I spot that you ask? The reason, the switch was mounted back to front so that when you thought it was in the on position it was actually off (and vice versa)

Apparently the Council knew about it but wouldn’t spend the money fitting it the correct way because the building was about to be demolished.

The building is not going to be demolished and there is a note next to the switch to tell people which way it should point.
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Old 1st Dec 2019, 2:11 am   #54
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Default Re: Repair nightmares.

Also if you put PIC microcontrollers in the wrong way round they get hot and don't work anymore, don't ask me how I know that. Plus when they are of the picAXE variety more expensive to replace.

YES I know about that!!!
Its actually called PICK axe murder :0

Usually occurs after you have done it ten times testing, and "remember clearly" what goes where!!

I smile, but feel for you.

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Old 1st Dec 2019, 2:24 am   #55
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Default Re: Repair nightmares.

That comes under self inflicted, as does driving 70 miles to a site and leaving your tools at base and once arriving at a remote site without the key. Who me?
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Old 1st Dec 2019, 9:33 am   #56
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Default Re: Repair nightmares.

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"Music CDs" should be used, and sure enough you had to use special disks where you paid a premium, presumably to placate the holders of any copyrights you might breach.
You reminded me....I've heard of this. I think it was in the early days of recordable CDs and you are right.....the recording companies were worried about copyrights and artists revenue so special CDs for audio were produced that had some sort of code on them that 'told' the corresponding recorder that it was OK to record.... and yes they were more expensive than a normal CDR. I think these were only ever made as 'write once'.

I had completely forgotten about this and remember now when I was at Philips in the Tech Advice department we had queries on this and had to explain about the difference between CDR and 'Music CDs' (or 'Audio only') as some manufacturers called them.....'but they are much more expensive' was the reply!

I have no idea when any of this changed or even if it ever did since I did all my audio recordings on a PC. I don't even know if 'Music CDs' still exist.

At least the Philips recorders displayed an appropriate message if the wrong disc was inserted......something like 'Please use and an Audio CD' or similar.
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Old 1st Dec 2019, 12:46 pm   #57
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Default Re: Repair nightmares.

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"Music CDs" should be used, and sure enough you had to use special disks where you paid a premium, presumably to placate the holders of any copyrights you might breach.
Yes, I remember them. The idea was indeed to pay the copyright holders in the same way that you are supposed to pay a fee for playing (almost) any music in public. I don't know whether they still do but it was common for inspectors from the Performing Rights Society (?) to visit pubs. If they found a juke box or radio playing they would force landlords to buy a licence on the spot (assuming they didn't already have one) or threaten them with being prosecuted. I remember back in the sixties that sometimes when you bought a tape recorder they would offer you a licence to record music as well.
The CDs had some sort of code on them which was supposed to be recognised by the CD recorder. The whole idea was rapidly forgotten when people worked out how to record music CDs on a PC. Presumably the CD recorder only checked the code on record and not playback.
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Old 1st Dec 2019, 4:34 pm   #58
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Default Re: Repair nightmares.

I can confirm that the home audio CD recorder decks needed special Audio blank CD-Rs or CD-RWs. It was another rip off for the consumer, forcing people to buy blank media which included a private tax for the music industry irrespective if you were to use the medium for your own compositions (such as myself and friends did) or dubs of vinyl or tapes you had already paid for (again, as I did.) To add insult to injury, most of those discs stopped being readable after only a few years.

As we know, the internet, mp3 and PC burners gave the music business its come-uppance and stopped that little racket!

I recently found a bag from Richer sounds with a pile of blank Audio CD-R /RW, the receipt was from 2002 I think. I must dig out the old Philips and see if they are usable.
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Old 1st Dec 2019, 5:11 pm   #59
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It was another rip off for the consumer, forcing people to buy blank media which included a private tax for the music industry
In the case of compact cassettes they didn't get away with it because the manufacturers threatened to sell only pre-recorded cassettes of music for which no royalties were due. Of course you could have recorded over if you wanted. If that had happened then I guess that something like the national anthem played backwards on a penny whistle would have been top of the pops every week
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Old 1st Dec 2019, 6:12 pm   #60
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Default Re: Repair nightmares.

I had to sort out the "reassemble after repair and now it doesn't work" issue for a bench-technician. He'd 'forgotten' the detail in the manual which said that there were several different-length screws used to retain the PA screening-can, and had managed to put one of the longest screws in a hole where there should have been one of the shortest.

The over-long screw had hit one of the PA output-tuning-capacitors, shorting it to earth and resulting in the untimely death of a pair of Motorola's finest 2N6084 transistors - whose replacement-cost was about equal to a day of the offender's wages.

I had to pacify the customer who had to wait another few days until we could source another pair of replacement output-trannies - I didn't actually charge for either the original repair or the 'repair of the repair' to keep him happy.

Let's just say that the offending technician didn't get a christmas-bonus, or a re-hire later that year....
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