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Old 6th Dec 2019, 8:32 am   #101
FrankB
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Default Re: Repair nightmares.

I was asked to repair an amp that the customer had replaced the speaker in.
looking at the schematic, I noticed the OEM had been electromagnetic, and the replacement PM.
The amp needed a total re-cap and I did that and replaced the field coil that was missing with a nice choke I had.
Amp worked fantastic on the bench, but when replaced into the cabinet, howled with feedback like a monster.
I fought it for 2 weeks. Then looked close at the setup again & realized (finally) that the PM speaker was influencing the tubes in the chassis, as the OEM had been a shielded speaker.
I sure felt dumb on not catching that earlier.

The next one was back in the early 70's. Went on a house call and worked on an old lady's B&W TV set. Replacing the vertical osc. & output tubes, general cleanup, etc. and it looked fantastic.

"The picture isn't as good as it was before" she kept saying.

No amount of adjusting would make her happy, when she suddenly said "There, that picture that was on the set is the one I want, that's how it looked before".
Well, I had been adjusting the vert. hold, and the pix was rolling slowly.

AHA! I thought, thinking back to an old TV servicing article I had read.
So I let the picture roll forward & back and had her pick out which picture she liked the best. After about 5 minutes she found a picture she liked that "Looked just like it did before". I left the vert locked in and she was happy.
A whole hour wasted, on a psychological problem with her.

But she was happy and I got paid. Sometimes you gotta wonder......

(BTW The article I had read was a tech in the Eastern U.S. who had the exact type of customer and he got the bright idea of rolling the picture to let the customer pick out the one they liked the best. Originally I got a good laugh out of it, but that article saved my bacon on that job. Without it, she would have never been satisfied.

She was the same customer that had to have "Round wire from the antenna, as the flat wire flattens out the picture". Apparently some "TV repairman" had told her that.
A Darwin Award for her.
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Old 6th Dec 2019, 10:56 am   #102
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Default Re: Repair nightmares.

Something that I wasn't really involved with but the carpet cleaner at work had failed and one of the maintenance guys had a quick look at it. Used a volt stick to check that voltage was getting to everything, which it was, but still no joy. So we got manufacturer to repair it. Turns out in was the neutral connection that had failed, somewhere in the main cable iirc.
Another fault with the same machine was a faulty suction motor (it has 2) and this resulted in little to no suction, it became obvious to me that air/suction was leaking out via the unused suction motor. All that was required was to tie a plastic bag around the second suction motors inlet port in the dirty water tank and all was well.
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Old 6th Dec 2019, 11:39 am   #103
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Default Re: Repair nightmares.

Worst nightmare used be the statement,"my husband's had a look at it, he is a pit electrician"!
You may be can guess I worked in the Mexborough area and Askern, both mining areas.
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Old 6th Dec 2019, 11:41 am   #104
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Default Re: Repair nightmares.

"I've tightened all the loose screws".
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Old 6th Dec 2019, 12:06 pm   #105
Leon Crampin
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Default Re: Repair nightmares.

One of the hardest faults I had to diagnose was on my own Bosch dishwasher, which is now about 18 years old. It is the first of the type where programme selection is made using buttons on the top edge of the door - previous models had a mechanical timer.

After years of proper operation, the machine would sometimes wash very badly, this being due to an insufficient fill of water. The water level detection is done from an air pressure chamber with a diaphragm, operating a standard V3 microswitch at mains potential. All pretty standard stuff.

The switch is open when water is needed and closes briefly at half fill, when the microprocessor turns on the wash motor, which lowers the water level, briefly opening the switch again. Filling then continues until the switch finally closes - machine full.

This machine was not accepting any more water after the first switch closure, giving a very low water level and a poor wash. Spending ages with a Fluke and getting not entirely convincing readings on the dead side of the switch (checked separately) I happened to brush the back of my hand on the bundled wiring loom. Sitting on a stone floor - I got a distinct tingle.

Long story short, Bosch use bio-degradable wire in these machines which has porous insulation. In a warm damp situation, there is electrical leakage between the wires in the loom. Because the water level input interface to the microprocessor is stupidly high impedance, there was enough leakage current to trip and hold the processor at "water full" status on the first operation of the switch. I later confirmed with the Fluke (Zin = 10 Meg) that there was up to 20V on the dead side of the open switch - obviously more without a meter connected.

The fix - fit a 100k 2W (to get the voltage rating) resistor from the dead side of the switch to neutral, to shunt the leakage current. Instant cure.

Coming from an automotive background, I'm amazed at the sheer incompetence of this design. Finding the fault took me a while though; one normally takes PVC wire for granted.

Leon.
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Old 6th Dec 2019, 1:07 pm   #106
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Default Re: Repair nightmares.

I had a Hotpoint washer dryer which would regularly trip the RCD in the CU during the spin cycle. At that time the CU had real fuses.

After much AVOing and Meggering with the power off I decided to inspect the wring harness. It turned out that during the spin cycle the harness was being stretched. As a result a "bin bag" cable tie with a conductive wire centre was cutting into a wire and also contacting the machine's chassis. I replaced all the cable ties with proper plastic ones and had no more trouble.

No doubt a bean counter had decided that bin bag ties were cheaper than plastic ones and would probably last until the machine was out of warranty.
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Old 6th Dec 2019, 5:48 pm   #107
duncanlowe
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Default Re: Repair nightmares.

Customers fault descriptions:

'It's gone funny. '

I have now trained the closest of my family to desist from this description. I think questioning whether 'it' was doing standup or sitcom got the message across.
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Old 6th Dec 2019, 6:02 pm   #108
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Default Re: Repair nightmares.

Another is arrive on site at 9am to be told ‘it’s a nothing job, you’ll only be here 10 minutes ‘, you know your going to be there well into the afternoon!

“We need someone out today”, usually on a Friday,.....when did it fail?, Tuesday morning!

A failure of planning on your part does not constitute an emergency on mine.
Regards, Alan.
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Old 6th Dec 2019, 6:14 pm   #109
Maarten
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Default Re: Repair nightmares.

Quote:
Originally Posted by stuarth View Post
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.
Even a trained technician can struggle with reading and understanding the terrible schematics that B&O published. I've had to redraw circuits to understand them.

However, chances are that this phantom twiddeler can't even read a well drawn schematic.
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Old 6th Dec 2019, 6:19 pm   #110
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Default Re: Repair nightmares.

I generally avoided doing consumer TV/VCR-type stuff myself but occasionally helped-out with the workload for a local TV-guy.
One job was to swap a faulty VCR for a customer. Unplug old one, plug in new one, test, get jobsheet signed - 20 minutes.
Next day the customer complained that "it's not recording properly". I returned, tested, everything fine. More interrogation of the customer revealed that they were used to BBC1 being on preset1, BBC2 being on preset2 and ITV being on preset3, whereas everyone I know has always had BBC1 on 1, ITV on 2, BBC2 on 3 and so that was how I'd set it.

Another time I had a complaint that "all it records is CEEFAX". They'd wanted to record an afternoon showing of a movie that started at 3PM, and had set the timer to start at 3:00. At 3 in the morning all the BBC were broadcasting was "Pages from CEEFAX" which the VCR duly recorded.
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Old 6th Dec 2019, 6:31 pm   #111
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Default Re: Repair nightmares.

Quote:
Originally Posted by Maarten View Post
Quote:
Originally Posted by stuarth View Post
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.
Even a trained technician can struggle with reading and understanding the terrible schematics that B&O published. I've had to redraw circuits to understand them.

However, chances are that this phantom twiddeler can't even read a well drawn schematic.
I was given a very nice AIM Databridge (LCR) meter because it was not able to be fixed. The real fault was nothing more than bad contacts on the EPROM socket (which I replaced by a turned-pin one). The so-called repairer had found that certain ICs in the display driver area did not test as he thought they should and had replaced them with 74C90 counters. Not surprising the originals didn't test as 74C90s or that the instrument worked not-at-all after he'd replaced them. They should have been 74C906 and 74C907 drivers.

Mind you I was working on a piece of specialised datacoms test gear the other week and had found that the wrong IC had been fitted at the factory. A 40174 (6 bit latch) rather than a 4017 (/10 counter). What is surprising is that this only affected one function of the instrument _and it was one that was not checked in the self-test or most diagnostics_. So the thing passed the tests, it even worked for most uses.
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Old 6th Dec 2019, 6:32 pm   #112
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Default Re: Repair nightmares.

Quote:
Originally Posted by G6Tanuki View Post
I generally avoided doing consumer TV/VCR-type stuff myself but occasionally helped-out with the workload for a local TV-guy.
One job was to swap a faulty VCR for a customer. Unplug old one, plug in new one, test, get jobsheet signed - 20 minutes.
Next day the customer complained that "it's not recording properly". I returned, tested, everything fine. More interrogation of the customer revealed that they were used to BBC1 being on preset1, BBC2 being on preset2 and ITV being on preset3, whereas everyone I know has always had BBC1 on 1, ITV on 2, BBC2 on 3 and so that was how I'd set it.
Back in the analogue TV days I never knew anybody who didn't tune the channels in BBC1, BBC2, ITV, Channel 4 (and then Channel 5 when it came on-air) order. Was that unusual?
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Old 6th Dec 2019, 6:36 pm   #113
Maarten
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Default Re: Repair nightmares.

Quote:
Originally Posted by Leon Crampin View Post
One of the hardest faults I had to diagnose was on my own Bosch dishwasher, which is now about 18 years old.
That could also be one of those pyromaniacs. Please make sure the relays and their solder connctions are in order.
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Old 6th Dec 2019, 6:42 pm   #114
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Default Re: Repair nightmares.

In the analog days BBC1 having been the first to come on-air got slot-1, when ITV came on-air it naturally got the second slot. I can remember setting up the 'biscuits' on the old mechanical tuners this way, so to change channels only involved one 'click' either way.

When BBC2 came on, it got 'logical position 3' on the selector-mechanism (whether that was rotary or push-button) - this logic carried over to TVs/VCRs with electronic tuning.

Another confusion - would you put the "VCR" channel on preset 0 or 9?? I always put it on preset 0.
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Old 6th Dec 2019, 6:47 pm   #115
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Default Re: Repair nightmares.

Quote:
Originally Posted by TonyDuell View Post
I never knew anybody who didn't tune the channels in BBC1, BBC2, ITV, Channel 4 (and then Channel 5 when it came on-air) order. Was that unusual?
No, that was absolutely the standard, and even today when you tune a Freeview TV it will usually, by default, put the first five channels in that order. I've never heard of anyone using Tanuki's scheme, maybe it was a local convention?
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Old 6th Dec 2019, 7:07 pm   #116
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Default Re: Repair nightmares.

Before I retired, I worked as a mostly factory based customer service engineer repairing tx & rx from anywhere worldwide. I can't remember the country but I had a rx returned for a performance issue. Picking it off the "in" rack, it rattled. It didn't look damaged at all either. Removing the top cover revealed the problem. These had a perforated case smps module in the rear corner. The rattle was caused by 2 quite large high voltage electrolytics wrapped up in insulation tape with the connecting wires going into the smps via a ventilation slot. These just dangled inside the main chassis and were wired to the pcb where the original capacitors had been removed from.
This was the first job before fixing the original fault.
On another note, I used to often get a fault description of "unserviceable", "u/s" or "faulty" written in the fault description area of the large fault return label as used by RAF or RN.
This is complex gear I am talking about. Sometimes just a single module returned that was part of a complex system. Needless to say, quite often it was returned to customer as "no fault found"! Which was the truth!
Rob
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Old 6th Dec 2019, 7:07 pm   #117
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Default Re: Repair nightmares.

Many years ago I was told about an old lady who, on hearing crackling and squealing sounds coming from her radio speaker attempted to cure it by squirting oil through the grille. Well, laugh as you may, there's at least a logic here given that a squirt of oil usually cures squeaks and squeals in most mechanical devices. Seemingly all that was lacking was the dear old lady's understanding that this was an electronic radio not a door hinge..
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Old 6th Dec 2019, 7:25 pm   #118
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Default Re: Repair nightmares.

Several years ago we started getting occasional nuisance tripping of the main rcd. Sometimes switching my pc on caused it. Another time, starting the microwave.
It was a devil to find. Turned out that our immersion heater that had been switched off for at least 30 years ( on a switch in the airing cupboard) had a low resistance to earth. This was only found after removing most circuits from the fusebox in the garage. The fact we had never used it meant it was completely overlooked. I can only assume the neutral of the element that was not isolated by the switch caused the unbalance just below tripping threshold. Our switching on of other item tipped the balance and caused the nuisance trip.
Rob
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Old 6th Dec 2019, 7:31 pm   #119
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Default Re: Repair nightmares.

"Squirting oil" in the previous post reminds me of some farm 2 way radios, Philips M294E to be exact, they were mostly surface mount pcb, 170Mhz.
One cauliflower cutting gang foreman had the bright idea of squirting half a can of wd40 into the volume control slot to stop it crackling! This was the thread easing type, not our switch cleaner! Result, one radio written off as it would never hold correct alignment again.
Rob
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Old 6th Dec 2019, 8:01 pm   #120
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Default Re: Repair nightmares.

Domestic wiring - specially when it's been extensively modified over the years - can be a real pain to fault-find/repair on, specially when you discover things like 'borrowed neutrals' between upstairs and downstairs lights and you're having the panel replaced by one with RCBOs.

It can take some time to work out the foibles of the wiring in an old house; I've come across a crazy bit of wiring in a pair of adjoining bedrooms where the switch in one bedroom controlled the lights in the other!

Also, someone I know bought a house that suffered from random RCD 'trips'; in his investigations he found a switch-panel in an outbuilding which fed an outgoing SWA that went to who-knew-where. Suspecting this might be in part associated with the trips, he turned the switch off. Didn't fix the trips, but being distracted by other things forgot to turn the switch back on again.

A week later his paddock flooded with sewage. The 'mystery' switch was powering a pump in the septic-tank.
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