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Electronics => Repair => Topic started by: John Heath on May 21, 2016, 05:36:31 pm

Title: trouble shooting techniques for LED flashlights , tech vs PHD
Post by: John Heath on May 21, 2016, 05:36:31 pm
LED flash light has a resistor between battery and LED that has come lose.

Tech , solder resistor back.

PHD in electrical engineering.

Exhaustive tests have been made of photon emitting source , carbon current restrictive device and Volta pile energy source. We feel we are on the edge of a break through so a formal request is made for additional funding. 

The message I want to coney from an old fart that has been there done that is do not over think a problem. Before trouble shooting place a hand on the PC board to feel for hot spots. Also the eyes are powerful tool for discoloration , water damage , cold solder. Tap a PC board for intermittent connections. More often than not this is all that is required. Trouble shooting , Scopes and diagrams should be a last resort when all else fails.

If it does come to trouble shooting , scopes and diagrams then rule number one " do not think ". 12 years of education just to walk and chew gum in life is a strong indicator we are not the sharpest tool in the shed. With " do not think " in mind religiously measure power supplies , fuses , low value resistors and any solid state device that is on a heat sink. If the temptation to think is overwhelming then take a break to build up will power. If nothing comes from the religious measurement algorithm then you are ethically bound to phone the costumer and inform them that there is a potential danger thinking therefore the original estimate must be renegotiated.
Title: Re: trouble shooting techniques for LED flashlights , tech vs PHD
Post by: CatalinaWOW on May 21, 2016, 05:40:19 pm
I agree with the first paragraph of advice, not the second.  The first paragraph is the result of thinking.  Thinking without action is pointless, which is exactly what overthinking is.
Title: Re: trouble shooting techniques for LED flashlights , tech vs PHD
Post by: sync on May 21, 2016, 06:18:26 pm
I'm not in the professional repair business. I'm only a hobbyist. But I have repaired a lot of devices. Usually I try to analyze the fault before starting the repair. That means thinking about it. The actual troubleshooting procedure then can differ. Two examples:

First, a HiFi amplifier with died at a party playing too long, too loud music. Experience tells me that most likely the output transistors are short. Measure them and found the problem. Then checking the driver transistors too.

Second, a DMM (HP 34401A) with works except AC  and frequency modes. Before I had open it, I studied the schematic and analyzed the AC measuring path. Then I measured supply voltages of the AC path input amplifier. Bingo! +15V is missing. It was a shorted decoupling cap.
Title: Re: trouble shooting techniques for LED flashlights , tech vs PHD
Post by: John Heath on May 21, 2016, 07:38:33 pm
I hear you and agree. Of course you have to think. However when it comes to paying the rent in the service business thinking is disaster. A lesson I learned the hard way. I will give a classic example. A supply comers in for repair on Monday.  I trouble shoot this and that for hours and come to the conclusion that ECR of the main filter condenser is high and change it. I have just lost 100 dollars for one 50 dollar power supply that took all day with a turn key operating cost of 150 dollars.  You see the problem. However on Tuesday another power supply comes in.  I can see the main condenser filter is round on top just like yesterdays high ECR condenser. I change the condenser without thinking and it works so 50 dollars is made in 30 minutes not 8 hours. Now we are making money. It is experience that makes money not theory. Theory is for the weekend as a hobby but not for business.
Title: Re: trouble shooting techniques for LED flashlights , tech vs PHD
Post by: 3db on May 21, 2016, 10:20:20 pm
I think it's both theory and experience.

The fault you described shouldn't have taken hours.
You had observed that the cap looked dodgy.
A a few measurements would have pointed you further in that direction.
Put another cap across it and things get better.
Change cap, all done.

I do think it's very important to try and understand why something failed.
Often things get repaired but the underlying fault is still there, and of course it then fails again.

Obviously this is just my opinion based on the comment you posted and is not intended as a
criticism how you dealt with that repair.

3DB   :D

Title: Re: trouble shooting techniques for LED flashlights , tech vs PHD
Post by: tggzzz on May 21, 2016, 10:31:11 pm
Contra-example: the ready, fire, aim repair technique.

We've all come across service technicians that don't know what's wrong, so they confidently tell the customer "X needs replacing", where X can be anything from a battery to an operating system. And after the expensive replacement has been done, the fault is still there - because the fault hadn't been diagnosed and they were blindly flailing.

Clearly there's a balance that isn't evident in the OP's first post.
Title: Re: trouble shooting techniques for LED flashlights , tech vs PHD
Post by: BMack on May 22, 2016, 07:45:11 am
I have a terrible habit of overthinking. It's rare that people bring stuff into our shop that is easy but sometimes I miss the easy stuff by thinking it's going to be like everything else. I recently had an issue where after plugging in the unit and flipping on the switch the LED would come on and I'd start troubleshooting. Then afterward I'd unplug it and flip the switch off. I did this several times, everything seems to be working but I'm unsure which component is failing to send the signal to turn on the unit and I cannot get a schematic. At some point I get distracted after plugging in the unit, I turn back and the LED is on but the switch is off...turns out the LED always comes on after plugging the unit in, it just happened to take the same time as me plugging it in and flipping the switch. Switch wasn't working so I shorted the connector the switch goes to and the unit turns on. :palm:
Title: Re: trouble shooting techniques for LED flashlights , tech vs PHD
Post by: John Heath on May 24, 2016, 02:01:02 am
Do not be hard on your self. I could see anyone making that mistake. I once had a I.P. ping problem for a video board. I could ping the sign with my lap top but she could not from a distance. I then used my lap top to ping her and it worked? How can I ping both the sign and her yet she can not ping the sign ? I turned to a friend with hands in the air. He said turn the sign off. Clever , I knew where he was going. I  turned the power off to the sign then asked her if she could now ping the sign. She said yes , its okay now. I told her we are not out of the woods yet until we find out what printer or computer is using the same I.P. address as the sign. The point here is the benefits of team work. I call it exchanging dogs. You have 2 dogs on the floor. I have 2 dogs on the floor . We exchange repairs. More often than not you will fix my problems easy and I yours.  Not sure why it is like that. Perhaps taking a fresh new look at a problem from different shoes.