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Lessons learned : funny but serious
Lessons learned : funny but serious
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Old 5th November 2017, 08:52 AM   #1
RhythMick is offline RhythMick  United Kingdom
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Join Date: Oct 2015
Default Lessons learned : funny but serious

Sharing for fun, but also it's always worth learning from others' mistakes.

I built my brother a pair of monoblock power amps (he did the woodwork for the frames). I took them down to install and spend a lovely couple of days listening.

At my house the mains floats around 245-250, so I wired the primary of the mains transformers for 250, ie -10 to 240. His house is about 235 to 240 so I changed the -10 out for the 0, carefully checking wiring colours against the spec. I then measured the heater voltages and all was spot on.

At least that's what happened with the left amp. With the right I mentally relaxed, after all it's just "do the same again", right ? I also allowed myself to get distracted by our conversation. Instead of changing the -10 for the 0, I changed the 240 for the 0. So primary only had 10v worth of turns. And I didn't check against spec.

Power up was impressive. Fortunately the primary fuse (500mA) did it's job and protected everything else, but I'd never imagined it could hit the ceiling with enough force to chip the plasterwork (amp was upside-down).

Lessons learned ...

1. Confidence is 1 step short of failure or worse
2. Always design in protection against failures
3. Always check the wiring against spec
4. Fuses can fail spectacularly.
5. Don't be over the amp at switch on. Imagine if that fuse had gone in my eye at that speed!

Last edited by RhythMick; 5th November 2017 at 09:08 AM.
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Old 5th November 2017, 04:37 PM   #2
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
Something I drill into my trainees: "Never think up reasons not to check something."

Your story is a perfect example.
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Old 15th November 2017, 12:13 AM   #3
MAAC0 is offline MAAC0  Portugal
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I remember a friend of mine barely getting blind when a opening a bottle a cork hit his eyes. moral of story, One can never be carefully careful.
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Old 15th November 2017, 02:27 AM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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Join Date: Apr 2008
Location: Carlisle, England
Its a bit like old engineering adage "measure twice, cut once"

I am very careful building up amps. I read the parts list component value twice to make sure then read the component drawer twice to ensure the right component goes in the right holes.

Confidence can be a killer.
In my apprentice days in class I built a valve pre amp.
I turned it off as it wasn't working and touched the circuit and got a belt off the HV rail.
My tutor said I had to discharge the main capacitor.
So next time I discharged the main capacitor and touched the circuit and got a belt again. I had forgotten to turn it off.

Things hitting the ceiling reminds of another apprentice who put his electrolytic cap in circuit backwards. As he powered his circuit up there was a loud bang followed by the capacitor bouncing off the ceiling.

Don't stand over the amp is also good advice. When I got into SMPS I was very much a beginner and got to the point where I hid behind a chair and plugged it in.
Had a few exploding mosfet's before I got my act together.
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Last edited by nigelwright7557; 15th November 2017 at 02:31 AM.
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Old 16th November 2017, 01:33 PM   #5
RhythMick is offline RhythMick  United Kingdom
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Thanks Nigel. This is one of those cases where I hope that other beginners can learn from my mistake.
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