Fault Finding with Minimal Documentation

[Single_Perf]

I was wondering if anybody had some Tips, Tricks or insight into the best method or procedures on Fault Finding without a Schematic or similar documentation. I am quite new in my workplace and havn't yet been shown these areas. Alot of our gear we hire out does not have any schematics available for it (probaly simply because manufacturer wont release it). And I have been finding it quite difficult.
Some of the equipment ranges from battery chargers, portable video monitors and other video related products some quite complex...
So any wisdom shared on this would be greatly appreciated. 

[Kiriakos-GR]

Interesting question .  

I will give an easy answer ...
It can be 10 people having the same problem , the solution will found from the one who is trained at searching.

In order to do that , you must know at list three foreign languages as I do , and to not be afraid to knock every door , that could possibly hides even a small piece of information ...

Even using Google properly , its an art.

I had recently one story of one damaged battery charger , the device was fine only one transistor was blown in the input (220V) the identification of it, was not possible .
In this PCB was only two major parts , this transistor and one IC , started searching for the IC , and found what type was the Transistor , because those two, always works in pairs .

I found the solution in just seven minutes..  by searching the web.

[dengorius]

Find the problem you must.
Do or do not, there is no try!

     Sorry mate, couldn't resist


And back in topic, the only (probably useless) tip i can give you is this: try to reconstruct the schematic from the pcb. Break the circuit down into logical block and troubleshoot them one at a time. But i think what helps the most in situations like these is experience.

[djsb]

Hi,
Please refer to my post here

https://www.eevblog.com/forum/index.php?topic=392.0

HTH.

David

[RayJones]

Always check the power supplies first.

If a switch mode, check especially for high ESR caps, or the real obvious bulging variety.

You'll be amazed at how many consumer devices you can fix with those simple steps.

[Neilm]

The first thing I do when fault finding a board is a visual check.
1) It can give you some oversight into the PCB as a system.
2) It allows you identify those bits of circuit that belong together (i.e. this bit is the input protection, that is measurement and the like. See Dave when he takes mulit-meters to bits)
3) It allows you to spot that large black bit around an IC which shows that the "magic smoke" has escaped.

[Kiriakos-GR]

The visual inspection are good as indication, up to a point.

It could even become a trap , if it is not combined with true knowledge, about the circuitry that you deal with.

The common usual mistake , are to blindly exchange damaged looking parts,
and the true fault to be actually else-ware,
the upcome are that new parts will fail too.

Thats why the old Greek freelancers technicians (80s) , they was had as standard 24 hours continuous  testing, before one repair considered as an successful one.

At 90s we had full documentation about most devices ,  voltage check points & oscilloscope curves check points ....  was our weapons to be more productive , and minimize the testing period.

In our times , industry managed to turn us as totally blind ones.
And up to this point , I would say , that I call as "expert" troubleshooter, the one who has manage to repair things , with the use of less sophisticated diagnostic methods + instinct + determination.        

[Anders]

Since you’ve got a number of similar products you should document some test points on a working unit. If you’ve got the opportunity and time just make your own test points if the unit’s doesn’t have any already marked and make note of voltages and curve forms. Remember that different settings on the unit might generate different values at the test points, make notes of the settings.
On way to do this (like I sometimes do) is to take a (or as many you need) picture of the circuit and then doodle on the values from the test points to your picture using Photoshop or similar. Also remember to update your documentation once you find the standard issues for each product like marking the “suspects” on the picture and so on!
Good luck mate!

[mikeselectricstuff]

Always check the power supplies first.

If a switch mode, check especially for high ESR caps, or the real obvious bulging variety.

You'll be amazed at how many consumer devices you can fix with those simple steps.

An extremely common failure mode on consumer SMPSs is a leaky electrolytic in the startup circuit. The typical symptom is not turning back on after a power outage or being unplugged - sometimes mis-diagnosed as being 'damaged by power surge' etc.
Look for a low-voltage electrolytic on the high voltage side of the supply and replace it, making sure you use a 105 degree rated cap. Don't bother measuring,  just replace it regardless. Of the various VCRs etc. I've fixed for friends/relatives & my own stuff, I'd say 50% have been this problem