Anyone up for some homework? Repair practice guidelines

[tautech]

Looking for some safe and up to date basic repair practice guidelines to post on the Repair board.

Maybe to place in a new thread or https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/msg477909/#msg477909 to give those starting out procedures and pointers for repairs.

The bulk of us need little guidance so why shouldn't we share the knowledge?
Tough one I know, how do you let the unaware loose on equipment/appliance repair?
But how much stuff did we blow up until we were competent enough to back ourselves and really fix stuff? 
Do you remember the pride (or relief) when you finished you first big fix? 

So, what can we do about this?
Will it be a few links or the wisdom of the forum by way of a sentence or two per post extracted to form a good basic set of repair quidelines and that which we can be happy to point newbies to?

What_NZ had a go at this a few weeks ago and to his credit backed out, as it is not a job for one alone. 
https://www.eevblog.com/forum/repair/suggested-for-a-sticky-part-two-comments-or-additions-please/msg470677/#msg470677

Quote from: What_NZ on June 30, 2014, 08:42:11 PM
Actually this is a minefield of issues and best left avoided altogether, it is nearly impossible in a small post to cover every possibility. Maybe someone else may like to tackle it.

Last post from our Dave:
Someone mentioned in another thread doing a video on this, with a general procedure etc. And whilst it would be nice, I suspect it going to be quite a minefield as you suggest. Not to mention a flame magnet 

Had a little crash around on a couple of sites for ideas and to check the quality of advice on offer(epanorama.net/links/repair.html & repairfaq.org/REPAIR/F_tshoot.html#TSHOOT_001)
While there is some OK stuff there, some needs updating(as this might later) to the point I would resist linking it. 

That's not to say a good repair link is not available, I just haven't stumbled on it yet.
Got a good link?

It's no 5 minute job and I wonder if replies/suggestions should be collated into a document that can then be posted, downloaded, uploaded, edited etc.

First...... document format? doc,docx, txt....
Second..... how much information is enough?
Third......Circuit theory NO. A few basic formulae, I think so.
Fourth....Will credit be necessary for your contribution. 

I hope I don't have a tiger by the tail. 

[tautech]

So are we all a bit shy to make a contribution? 

Well as time allows I will start a document by way of "cut & paste" from stuff found online.

Then you can all mark my efforts and some of you will hopefully chime in. 

[tautech]

Most of the basic material that I believe needs be included is in the following pages:

http://www.eevblog.com/wiki/index.php?title=Main_Page
https://www.eevblog.com/forum/suggestions/safety-risks-in-electronics-how-to-avoid-them/
https://www.eevblog.com/forum/beginners/electronics-primers-course-material-and-books/
https://www.eevblog.com/forum/beginners/your-favorite-electonic-introduction-and-reference-material/
https://www.eevblog.com/forum/beginners/the-art-of-troubleshooting-analyzing-cirquits/msg397569/#msg397569

If there are any further threads worthy of study for suitable material, please post them.

Should we try and condense the worthwhile info to put in the Repair board or just link to them?
As there is a PILE of wisdom in the forum, I feel there is little need to look elsewhere.

I have had a suggestion of a flowchart and this seems like a possible solution for the basics.

Any further ideas/suggestions?

[jeremy]

I'll give it a go 

In my experience (I have been involved in teaching at university for many years), there are two key things you need for successful troubleshooting. First, you need tools that you can trust, and you need to regularly check that they are behaving. You also need to know where you can't trust them.

Notes:
- The number of times I have seen students try to measure the current of a circuit with a multimeter that has a blown fuse is astonishing.
- the multimeters that scream at you for having the probes plugged into the wrong sockets are helpful too
- you can't measure uA current properly with a $5 multimeter. You are just lying to yourself

Second, and this kind of a superset of the first, you need to look very carefully at what you are assuming. This is very philosophical, really, really hard and takes a lot of practice. Basically, if you assume anything is ideal or perfect, you're going to need to prove that at some point.

Some experience:

- a student showed me a breadboard with a few op amp circuits chained together. He checked the schematic several times, redid the maths and checked the supply voltage on all the ICs. It was at the point where he was swapping out random components with new ones (note: if you get to this stage, you're doing it wrong!). The problem? One of his single core wires in the signal path had a break in it underneath the insulation. He assumed that the components were at fault, not the wire.

- when I was quite young, I found that unplugging a circuit from power gave me "more squiggles" on the oscilloscope than when it was plugged in. I was using a 1MHz scope which had banana sockets for the probes, and I was using cat 5 cable as the probe. Obviously the problem was the unshielded wires picking up 50Hz mains, but up until then I assumed that oscilloscopes were magical and perfect (big mistake!). There is no such thing as magic

- a student had a circuit that only worked in his bedroom, never in the lab. The circuit had an LDR on it. This one was easy: he always came to the lab in the evening, and the building lighting was not enough to trigger his circuit. He assumed that the LDR was a perfect "light detector".

...I seriously have hundreds of these.

Bonus tip: you get what you pay for. My $400 weller WMRP iron is one of the best investments I have ever made.

Hope this helps.

[wiss]

Step 1: Clean the boards.

Especially valid for old multimeters that are noisy and/or a few hundred ppm off.

[Rerouter]

For very simple circuits simulation toys like falstads circuit simulator can help nut out what the circuit is meant to output for a given input,
http://www.falstad.com/circuit/ (Its a java app, so you need java to run it)

Once again this comes down to knowing the limits of the tools at your disposal, but many a time being able to muck around with set points with it reacting in-front of me has been far more insightful than running spice 30 odd times and not being able to interpret what i am seeing, or not knowing the magic option to introduce a specific glitch to see what happens,

Next up would be not being afraid to pull out the hair dryer or freezer spray, many circuits i have had to repair have been thermal cycling based faults on one or more solder joints, and a quick heat up with a hairdryer was a good way to confirm the fault existed, (It is almost always more insightful to know how to reproduce a fault than how to fix one, as it gives you a starting point),

Beyond this my head loops back to establishing the limitations and ways measuring devices can alter the device being measured, (loading, burden voltage) but all this would detract from the topic at hand, 

[tautech]

All good stuff that can be picked over for a basic repair guide. 

Beyond this my head loops back to establishing the limitations and ways measuring devices can alter the device being measured, (loading, burden voltage) but all this would detract from the topic at hand, 

No it won't.
The basic fact is all measurements will affect the circuit to a greater or lesser degree.
This is stuff that should at least be mentioned and the novice made aware of.

Even worthy of its own thread.