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Old 12th Nov 2023, 7:51 pm   #1
G6Tanuki
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Default Accidents will happen...

Recently, I was reworking a nice big ceramic coil along with a little airspaced capacitor as an antenna matcher for my longwire on 3.5MHz. It's near-as-makes-no-difference a halfwave, so it presents a high impedance and there's lots-of-volts but not-so-many-amps going up the spout.

After putting the 'thing' in a box, and wiring it up, I just couldn't get it to peak like it did when it was a lashup on the bench.

Then I saw a spark! And Smoke!

Turns out that in my packaging-and-boxing it up, I'd accidentally dropped a splash of solder across a couple of turns of the coil. Meaning there was a shorted-two-turns in the middle of the inductor and the inductor was not providing the requisite reactance, the few hundred Watts of carrier preferring to whizz round in the shorted-turns and heat them until the solder-splash melted itself.

A wasted afternoon of perplexity. But now with the solder-splash evicted I can get a 1:1 SWR.
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Old 12th Nov 2023, 8:06 pm   #2
Cruisin Marine
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Default Re: Accidents will happen...

It happens to us all! We are impatiert beasts come near completion of any project.

All's well that ends well
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Old 13th Nov 2023, 1:44 pm   #3
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Default Re: Accidents will happen...

When l first started in TV repairs l was told l needed a 'scope. ln fact the other engineer in the workshop said "you can take away all my tools, but don't take away my 'scope"

So having bought my Hameg 203-6 l set to a frame fault on a TV. l slipped with the probe and the top blew off the IC and hit the ceiling.

"Well", said the other engineer, "at least it has a definite fault on it now"

Confidence shattered, l didn't use the 'scope again for ages.
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Old 13th Nov 2023, 2:00 pm   #4
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Default Re: Accidents will happen...

Quote:
Originally Posted by lightning View Post
When l first started in TV repairs l was told l needed a 'scope. ln fact the other engineer in the workshop said "you can take away all my tools, but don't take away my 'scope".
The only person who would say that is one who is either new themselves, or has no deductive reasoning skills. A vast number of faults can be traced without power even being applied; a large number can be traced with simple tools like a multimeter or signal injection. The remaining tricky ones, which IMHO are the least common of all, involve a scope, where waveforms need to be monitored, etc.
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Old 13th Nov 2023, 2:15 pm   #5
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Default Re: Accidents will happen...

Dropping the back of some ancient KB monochrome tv after I'd fixed it, thusly knocking the end off the tube.

Oh how I larfed.
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Old 13th Nov 2023, 2:31 pm   #6
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Default Re: Accidents will happen...

Thankfully in this case no permanent harm was caused [my transmitter's high-SWR protection did its thing rather well, though when the solder-splash started arcing I was a bit concerned whether the power-management in the PA was going to have a fast enough response time]. Matched pairs of MRF476 output-transistors are not easy to come by.

I guess the learning-moment here is "check everything for solder splashes". In times past I had a similar problem with a radio that used push-button switches for band selection; while doing a rebuild, a single short strand of wire escaped from the end of a piece of flex I was preparing and, unnoticed, found its way into the switch. The fault it caused took me a day to
fix - after that I used one of the wire-strippers that catch the piece of insulation [and any nicked strands of the wire].
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Old 14th Nov 2023, 12:41 am   #7
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Default Re: Accidents will happen...

Quote:
Originally Posted by DrStrangelove View Post
Dropping the back of some ancient KB monochrome tv after I'd fixed it, thusly knocking the end off the tube.

Oh how I larfed.
Been there, done that, got the tee shirt.(although, in my case, it was a Philips set)
The worst part was dealing with the supercilious snot at the Philips parts counter.
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Old 14th Nov 2023, 11:08 am   #8
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Default Re: Accidents will happen...

Buying an unused untested GEC KT66 on eBay and, before putting it in the tester, running round the pins with my DMM leads just to check for any cold shorts. I was holding the valve and both leads and, inevitably, the valve slipped and fell vertically down onto a hard chipboard floor. It landed top first.

Thank heavens the glass didn't break. But now the heater was open-circuit (I could see little bits of its insulation loose inside the envelope) . This was years ago, when KT66s were a bit cheaper than they are now, but still it spoilt my day. Live and learn though. I now hold fragile things above the workbench which generally has several layers of old bath towel as padding on it.

Cheers,

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Old 14th Nov 2023, 2:08 pm   #9
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Default Re: Accidents will happen...

I’d just completed an output transistor replacement in a Quad 405-2 amplifier and checked that it was giving its rated output etc when I decided just to check a few voltages on the PCB. You can guess the next bit….

Well the test probe slipped and provided a new set of faults to diagnose and rectify. Not just one fault of course, but a set of faults because they don’t come alone in a directly coupled amplifier!

Martin
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Old 14th Nov 2023, 2:32 pm   #10
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Default Re: Accidents will happen...

Forgetting to 'normalise' the bench PSU, whereby it was feeding 70v into a 50v cap isn't a thing I'll repeat. I noticed a slight bulge in the 6,800 cap out of the corner of my eye...by the time I registered what was happening, bang! It made an awful mess (and I doubt it was good for my ears, either).
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Old 15th Nov 2023, 12:37 am   #11
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Default Re: Accidents will happen...

One of my best was diligently replacing the big electrolytics on a 24-volt supply. It was a warm, late Spring day, & I was a bit too relaxed.
Plugged the supply in and "bang" ----suddenly it was "snowing" in the Tx site "Speech input room".

Yep! a -24v supply!
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Old 15th Nov 2023, 12:51 am   #12
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Default Re: Accidents will happen...

Quote:
Originally Posted by lightning View Post
When l first started in TV repairs l was told l needed a 'scope. ln fact the other engineer in the workshop said "you can take away all my tools, but don't take away my 'scope"

So having bought my Hameg 203-6 l set to a frame fault on a TV. l slipped with the probe and the top blew off the IC and hit the ceiling.

"Well", said the other engineer, "at least it has a definite fault on it now"

Confidence shattered, l didn't use the 'scope again for ages.
It was hardly the 'scopes fault.

When I spent a lot of time fixing Picture Monitors at a TV Studio, I hardly used my DMM.

The 'scope can be used for a quick check of DC power supplies, check signal outputs from the sync separator, see whether DC supplies derived from an overwind on the EHT transformer are really DC. (A DMM on "DC" often reads "just a little low" when the filter cap on such supplies is dead), apart from more esoteric things.

Not work, but my dear old Mum in Law's TV had a vertically collapsed scan.
The Fluke 77 said the +150v supply was about 110v, so "a bit low".
I borrowed a 'scope from work, which showed a bunch of line rate pulses!

Relaced the cap & M.I.L had her TV back.
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Old 15th Nov 2023, 1:04 am   #13
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Default Re: Accidents will happen...

Quote:
Originally Posted by ben View Post
Quote:
Originally Posted by lightning View Post
When l first started in TV repairs l was told l needed a 'scope. ln fact the other engineer in the workshop said "you can take away all my tools, but don't take away my 'scope".
The only person who would say that is one who is either new themselves, or has no deductive reasoning skills. A vast number of faults can be traced without power even being applied; a large number can be traced with simple tools like a multimeter or signal injection. The remaining tricky ones, which IMHO are the least common of all, involve a scope, where waveforms need to be monitored, etc.
Forty plus years of faultfinding brings me to respectfully disagree with you.

"Signal injection" is at least as complex as monitoring with a 'scope & DMMs lie to you when confronted by odd waveforms.
As I pointed out above, a 'scope can perform all of the common tests you would use a DMM for except resistance measuring.

I'll pit my "deductive reasoning" against anyone's & I can't see the sense in deliberately "blindfolding" yourself.
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Old 15th Nov 2023, 1:14 am   #14
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Default Re: Accidents will happen...

Quote:
Originally Posted by Oldmadham View Post
I can't see the sense in deliberately "blindfolding" yourself.
Nor can I.
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Old 15th Nov 2023, 2:25 am   #15
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Default Re: Accidents will happen...

Just to be clear. I'm not saying 'scopes are not useful. I have one for the purpose. Just that the idea that you cannot repair TVs without one is totally untrue.
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Old 15th Nov 2023, 9:11 am   #16
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Default Re: Accidents will happen...

Nothing does everything. The value of scopes and multimeters are clearer in terms of what each cannot do.

A scope is a general purpose instrument, but it doesn't measure resistance, capacitance, inductance or ESR. Its voltage measuring capability is of low resolution and only modest accuracy. It really comes into it own when there are instability problems and there there are 'interesting' waveforms. In TV terms, think colour decoders, teletext, VCRs.

A multimeter is a general purpose instrument but it doesn't measure fast-changing things or ESR. It's misleading when waveforms are anything other than sines and DC. It comes into its own for resistance checks, continuity checks and where you want to know a voltage with some degree of precision.

Even with both a scope and a multimeter, there is still value in ESR meters, component bridges and such like.

For teaching someone electronics, scopes are good because they can show beginners the waveforms you've been talking about. They can see things in action. A lot can be done with a multimeter given some imagination and a fair amount of experience. But scopes make it easier. Easier is important with beginners. Yes, you have to teach them how to drive a scope, but that's not difficult.

We live in an age awash with surplus test equipment. I've given away a number of scopes over the last few years. Some decent instruments turn up at multimeter prices. You no longer have places like old TV repair shops which might have an old Telequipment scope in a cupboard, to be brought out only in very special circumstances. There's a lot more choice.

Scopes are no-longer quite so special.

Using one as your first-stab instrument can yield clues that a meter wouldn't show... bursts of oscillation, ringing, etc.

If you can look at a problem from more than one direction, it's easier for something to click and give you a solution.

Scopes and multimters are NOT in competition. THey are complementary.

David
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Old 15th Nov 2023, 10:08 pm   #17
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Default Re: Accidents will happen...

Well l've just dropped a clanger today!

JVC 49" TV in for backlight change. Horrible job, screen stuck down with silicone, rear reflector glued to the metal back, LED bars stuck down with glue.
Tiny connectors on the LED bars that wouldn't come off, two broke and l had to repair them.

Got all that done, screen back in, then one of the long thin PCB's caught on my shirt and it ripped one of the ribbon cables off.

That was it, set is scrap, two hours and £40 backlight kit wasted
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Old 18th Nov 2023, 9:29 pm   #18
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Default Re: Accidents will happen...

That reminds me of the time I was struggling to remove a cover that was clipped together and ripped a ribbon cable socket off the PCB, taking the tracks with it. I only needed to open it to replace a broken co-ax socket. Thankfully the unit was mine.

I have also let the air into a CRT on a nice Grundig TV as I was putting the cover on after repairing it. Oops
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Old 19th Nov 2023, 3:01 pm   #19
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Default Re: Accidents will happen...

As an apprentice TV engineer, it was my job to clean the faulty set prior to the engineer setting to work. We used to have a number of PAM 600 mono tv's which were a 17" short neck. The tuner side "Right looking at it" I think. Had a plastic escutcheon to cover the big hole. This could get quite mucky, One was particularly yukky, so to remove the caked grime I use "Serviceol" which, while removing the caked grime, also melted the plastic. ARGHHHHH.. no spare either... I got a right telling off for that one. But lived to tell the tale
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Old 19th Nov 2023, 4:35 pm   #20
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Default Re: Accidents will happen...

One of my 'moments' involved the time I wass doing a memory upgrade to a rather expensive Cisco GSR for a client.

There were a bunch of screws affixing screening-covers and the like. Some short, some long. I didn't realise this, and the long ones fitted where the short ones should have gone bored their way catastrophically through a multi-layer PCB.
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