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Old 29th Jul 2022, 3:18 pm   #1
Tim
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Default I知 an idiot!

I finally got round to looking at my Heathkit IG18 Audio generator a week or so ago.
Not working, fault traced to power supply. Duff series pass transistors, and although the smoothing caps had been replaced they were still ancient. Whip out caps and transistor. So far so good( this is where the idiot part comes in). Fit new caps, 470uF 63 volt. Even managed to get them the right way round. Quick look at transistor specs. BFX85 should be OK as a replacement. Fitted transistor from several I had sorted, lying on the bench.
Now sort of worked, but HT(?) now low at about 30 volts. Generator appeared to work though. Zenner voltage wrong, with base of transistor more or less at collector. Change zenner resistor and make up a new 43 volt zenner from a 10 and a 33volt. Still the same. 10 volts across the 10 volt zenner, but weirdness across the other.
Tim the idiot shorts out the 10 volt zenner( on the bottom, -ve end) to see if the 33 volt one is working, and BANG. Switch off quick!
Now what? Out with the transistor to check. It’s a 2N5415, which had the circuit called for a PNP transistor ( rather than an NPN)would have been a good choice.
Changed both zenners again and zenner resistor. Check voltage with transistor removed. Spot on 44 volts. Whack replacement tested BFX85 transistor in, but notice sparks as I fit leads into PCP holes. Presumably from 3x 470uf caps at 50 or so volts.
Uh-oh. Murphy is obviously looking over my shoulder, so checked transistor to find I had zapped it, and was dead short in all directions. Goodness knows what would have happened if I had fitted that in!
So I metaphorically poked Murphy in the eye, discharged caps via 47 ohm resistor to common, checked a used 2N2119 and fitted that, with a clip on heat sink. Switch back on with everything crossed and working at last! Correct voltages and mere millivolts of ripple.
It just goes to show we all make a fool of ourselves sometimes!

I was a little surprised that it worked at all with a PNP transistor. Perhaps the transistor was breaking down, and slowing current to flow.
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Last edited by Tim; 29th Jul 2022 at 3:25 pm. Reason: I’m an idiot.
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Old 29th Jul 2022, 3:32 pm   #2
G6Tanuki
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Default Re: I知 an idiot!

We all make occasional silly mistakes;

I once absent-mindedly fitted a bipolar transistor in place of one of the tiddly little plastic-encapsulated 7805 three-legged voltage regulators.

There was smoke.
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Old 29th Jul 2022, 4:03 pm   #3
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Default Re: I知 an idiot!

I once blew all 4 filaments in a battery set by shorting out a voltage regulator with a meter probe. We've all done similar stupid things.
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Old 29th Jul 2022, 4:12 pm   #4
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Default Re: I知 an idiot!

I remember years ago I blew up a telly by connecting the earth of my scope probe to one of the heatsinks when I was a bit tired and not thinking properly, yes it turned out to be a live chassis set! Didn’t make that mistake again… although I do have an isolation transformer now, and better access to service information!

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Old 29th Jul 2022, 4:21 pm   #5
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Default Re: I知 an idiot!

I did that too, Lloyd by hanging my soldering iron on the chassis of the G11 - and in the customer's house! In my defence the mains switch had shorted live contacts unknown to me.
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Old 29th Jul 2022, 4:50 pm   #6
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Is there anyone here who does/did practical design or repair work who has _NOT_ made a silly mistake?

I managed to feed 12V up a transputer link back in the day and killed a rather expensive processor as a result.

I disabled the over-current trip on a DEC H754 regulator brick in attempt to find why it was giving no output. The reason turned out to be the crowbar firing at the wrong time, without the overcurrent trip, the short provided by the crowbar blew transistors right off the board.

And many more..
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Old 29th Jul 2022, 5:02 pm   #7
Dickie
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Default Re: I知 an idiot!

I wired up a DPST mains switch 90 deg out, as it were, such that turning it on shorted the mains out. You can imagine......
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Old 29th Jul 2022, 6:47 pm   #8
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Default Re: I知 an idiot!

Quote:
Originally Posted by Dickie View Post
I wired up a DPST mains switch 90 deg out, as it were, such that turning it on shorted the mains out. You can imagine......
I have seen that done by a boss who used to sometimes come to production to "help".
It left the guy who plugged it in really scared.
There was a 13 amp fuse in the plug. Only the ceramic tube with about half of one of the end caps remained.
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Old 29th Jul 2022, 6:51 pm   #9
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Quote:
Originally Posted by TonyDuell View Post
Is there anyone here who does/did practical design or repair work who has _NOT_ made a silly mistake?
I doubt it!
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Old 29th Jul 2022, 7:58 pm   #10
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Default Re: I知 an idiot!

Oh, the list. One quite memorable one is working on a camera flashgun. They have quite a big capacitor charged to a fair old voltage to supply the flash tube. I got a bit of a bite a couple of times, so decided to discharge the cap. The flashgun survived (I still have it somewhere) but the tip of the screwdriver didn't. Good lesson in source impedance!
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Old 29th Jul 2022, 8:00 pm   #11
duncanlowe
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Quote:
Originally Posted by locknut View Post
Quote:
Originally Posted by TonyDuell View Post
Is there anyone here who does/did practical design or repair work who has _NOT_ made a silly mistake?
I doubt it!
There are probably a few, but if there are, they are liars!
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Old 29th Jul 2022, 8:27 pm   #12
Alistair D
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Default Re: I知 an idiot!

Quote:
Originally Posted by Welsh Anorak View Post
I did that too, Lloyd by hanging my soldering iron on the chassis of the G11 - and in the customer's house! In my defence the mains switch had shorted live contacts unknown to me.
Snap - almost - In my case I had gone to re-solder the pins of the field output chip before condemning it.

The flash was burned into my retina for hours.

I presume your G11 was like mine in that the on/off switch was part of the control drawer.

Al
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Old 29th Jul 2022, 8:47 pm   #13
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Default Re: I知 an idiot!

Made plenty of silly mistakes over the years, but my favourite story is about a colleague years ago, who got a shock while working on a large 32" widescreen TV set. As he rapidly pulled his hand out of the set he hit the neck of the tube and broke it (the tube that is). Expensive!
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Old 29th Jul 2022, 8:56 pm   #14
locknut
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Quote:
Originally Posted by duncanlowe View Post
Oh, the list. One quite memorable one is working on a camera flashgun. They have quite a big capacitor charged to a fair old voltage to supply the flash tube. I got a bit of a bite a couple of times
Yes, they do make you jump!

I've had a few little burn holes in the finger ends from accidentally touching primary side capacitors on switched mode jobbies. They don't look much, but they do sting a bit for a few days
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Old 29th Jul 2022, 9:05 pm   #15
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Default Re: I知 an idiot!

Another classic one is from about 30 years ago, when I had a workshop at my mum’s. I was working on a portable TV, and had put the plug in my pocket for safety. I think my mum knocked at the door, and I got up to answer it. With the plug still in my pocket!I managed to drag the TV off the bench, and unfortunately my AVO8 which was attached by test leads st the time. I can’t remember what happened to the TV( so it was probably OK)but that particular AVO was never the same.
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Old 29th Jul 2022, 9:25 pm   #16
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Default Re: I知 an idiot!

I could give personal examples but it would take people all night to read....

"Mistakes I made bucket loads, but then again, far too many to mention" should suffice
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Old 29th Jul 2022, 9:42 pm   #17
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Default Re: I知 an idiot!

There is a saying, can稚 remember it completely but it goes along the lines of The person who said they have never made a mistake never made or fixed anything

I think we have most if not all made mistakes and Cruising marine sums up mine very well.
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Old 29th Jul 2022, 10:15 pm   #18
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Default Re: I知 an idiot!

Quote:
Originally Posted by Dickie View Post
I wired up a DPST mains switch 90 deg out, as it were, such that turning it on shorted the mains out. You can imagine......
Snap!!

I should have known better as I wasn't exactly a noobie at the time but the four switch tags were arranged in a square and I obviously didn't interpret the markings correctly......
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Old 29th Jul 2022, 10:53 pm   #19
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Default Re: I知 an idiot!

Many yars ago, when I worked on fixing TV's I fixed this one. HMV colour set worth LOTS of money.
Had a convergence fault. The convergence board was one that "folded " up out of the chassis so that you could see picture while adjusting the numerous coils and pots. It was a LARGE board with probably 30 transistors on. So convergence had really gone wonky. Found a transistor that had gone leaky. Replaced that but now I had to tweak the peaking coil to get the picture square. Without thinking, pick up my thin METAL screwdriver and shove it into the coil. Thats weird!! the picture is square. Remove the screwdriver its trapezoid again. Reinsert the screwdriver and twiddle the ferrite tweaking screw to the opposite direction. Whats that smoke ? Its the peaking coil RF heating the screwdriver AND the coil. Bang goes the coil. I pull the screwdriver out of the coil and get a lovely set of stripes from the gnurling on the screwdriver handle. At the same time the transistors, now connected to a smoking shorted coil went off like a string of chinese firecrackers. I ducked as bits of plastic flew everywhere and had another three techs hiding and running. The boss is bright white with fright. I couldnt hide that one !!!. I wasnt fired either. BUT every tech there had a "go " at rebuilding that TV. NOBODY could fix it.
Worst case of egg on face ever. I felt about 1" tall, and considering the other techs were self taught and I had a degree, all the more so.

Joe
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Old 30th Jul 2022, 1:23 am   #20
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Default Re: I知 an idiot!

I readily admit to doing more stupid things than I can remember. It is nothing to be ashamed about.

I'll bet, like a lot of posters here, the number of non stupid things they (I) have done far outweighs the daft stuff.

Let's not make too much of it, everyone has 'sub optimal' days!

Steve.
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