Author Topic: I just looked and learned  (Read 4064 times)

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Offline M0BSW

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I just looked and learned
« on: December 17, 2012, 04:50:41 pm »
I just watched Dave video on the scope repair , I followed his method, and found the fault on a bench multimeter I had been given a scrapper, it turn out to be a diode shorted, I probed it accross the component and it showed a short , so I lifted a leg of it and still short, replaced it and bingo it powered up  :phew:
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Offline M0BSW

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Re: I just looked and learned
« Reply #1 on: December 17, 2012, 06:17:36 pm »
 As I'm learning , there seems that there is a procedure to fault finding, guess this is something learned over the years, for me I think it's pure luck ,at the moment if I happen stumble over the fault. Then I suppose there is the reason why it developed the fault in the first place, and is the fault still really there. oh there's lots to learn, it must be nice if you are given lots of broken electronics and fix them all, even better if you get paid for it. :palm:
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Online EEVblog

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Re: I just looked and learned
« Reply #2 on: December 17, 2012, 09:36:00 pm »
Well done!
Yes, it help if you do repair every day.
I haven't done repair for a day job for more than 20 years.
So all I usually got since then is debugging my own designs, production failures and stuff like that, which can require a totally different procedure to general product repair.

Dave.
 

Offline rsjsouza

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Re: I just looked and learned
« Reply #3 on: December 17, 2012, 10:43:48 pm »
I failed and succeeded to repair a large number of electrical/electronic equipment since my childhood days (after I used to destroy all my electric toys to see what was inside), and never got paid in cash for the work... Only the sense of fulfillment and a lot of money saved that would otherwise go to the repairman.

Just like everything that involves practice, you are right in saying that it takes some time to get a sense of the size and source of the problems when they present themselves. You are also right to say that luck is involved, as I learned over the years that most failures have an easy repair: blown fuses, faulty or rusted power switches, broken or failing power cords, bad or scratchy contacts in rheostats, etc.

Obviously that over time you feel braver and start venturing yourself inside of the equipment's housings. In this case I still found that most of the failures are easily diagnosed with a voltmeter as they are usually related to the power supply circuitry - to have an idea, I still have some old radio books that mention the majority of issues can be diagnosed with a simple 120V light bulb probing voltages inside the tube radios! :)

However, as Dave's video shows, the process of pinpoint the culprit is a different story, as the symptom may reveal itself in a specific part of the circuit but the faulty device is in a complete separate area. Performing a careful trace of the signals will get closer to the source, but sometimes it really becomes a matter of chance and patience. I recall having an amplifier that was doing a loud pop in one of the channels, and some signal tracing revealed an entire board full of "candidates" that needed replacement (old capacitors). The fix? Replaced all of them (~15) after trying to do a one-by-one replace-test (the pop was really loud!).

The most disheartening situations happen when you diagnose, find a faulty device and, when replacing it, it fails again. That shows the problem is being caused by a third element and the diagnosis is a lot harder. These cases are way more involving and they usually involve not only an analysis of the schematics but also a round robin process of replacing the faulty device and trying to trace the signals during blowup - in these cases the ohmmeter helps as it can identify short circuits where they shouldn't exist (just like the video). Another great way is the "blow-whatever-is-in-the-way" method used in the video (which works wonders but you have to be careful not to blow the PCB).

Another disheartening scenario is when the faulty device is replaced and another problem shows up. This is more common when the original issue is located in the power supply (or some high power circuitry) and, during the failure, another device gets blown by overvoltage or overcurrent.

The only scenario where you are 100% reliant on luck is when the issue is intermittent. In this scenario absolutely all bets are off; you have to try your best to find a way to make the problem reveal itself consistently, but you need to be prepared to never find the actual source of the problem.

Suggestion: keep finding nice equipment around the house to fix or even from friends and family - be careful with these last two: either get whatever they would throw them away anyways or only promise you will "take a look". If you fix some, you will find yourself invited to stay for dinner or to a nice and cold beer... (at least that is what happened during my high school and university years! :) )
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Online EEVblog

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Re: I just looked and learned
« Reply #4 on: December 17, 2012, 11:03:49 pm »
However, as Dave's video shows, the process of pinpoint the culprit is a different story, as the symptom may reveal itself in a specific part of the circuit but the faulty device is in a complete separate area. Performing a careful trace of the signals will get closer to the source, but sometimes it really becomes a matter of chance and patience.

Yes, I've found countless short using the multimeter hunting technique. But in this instance it just didn't work. And it looks like the fault is not in the usual suspects of the bypass capacitors or components. But you have to go down that route first because that's the best bet.
Sometimes you just don't get lucky, and that's usually on a Friday afternoon, or when you have to meet a critical deadline  |O

Dave.
 

Offline FenderBender

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Re: I just looked and learned
« Reply #5 on: December 17, 2012, 11:46:25 pm »
I failed and succeeded to repair a large number of electrical/electronic equipment since my childhood days (after I used to destroy all my electric toys to see what was inside)....

Oh...you too?

OT: It definitely seems like some sort of experience thing to me. Though I'm sure I'd run across the problem sooner or later, experience does help to point you in the right direction.
 

Offline nadona

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Re: I just looked and learned
« Reply #6 on: December 18, 2012, 02:35:46 am »
As I'm learning , there seems that there is a procedure to fault finding, guess this is something learned over the years, for me I think it's pure luck ,at the moment if I happen stumble over the fault. Then I suppose there is the reason why it developed the fault in the first place, and is the fault still really there. oh there's lots to learn, it must be nice if you are given lots of broken electronics and fix them all, even better if you get paid for it. :palm:

Congratulations!
Ha-ha-ha. That's good, too!
 

Offline ptricks

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Re: I just looked and learned
« Reply #7 on: December 18, 2012, 12:35:21 pm »
I've done several hundred repairs over the time I worked for a warranty repair center, often without schematics. The best tools to have and they are cheap is a curve tracer and a signal injector .  With the curve tracer you can check parts like transistors without removing and they are not bad either at viewing capacitor issues. Signal injector sends a set frequency, I like a 60hz sine wave , into the circuitry at any point you want that can then be viewed on an oscilloscope to see how the signal is being changed based on the components under test.  For audio gear a lm386 small powered amp can be used to listen to the signals that are present when the equipment is powered. I used a test amp that was protected from over voltage so you really could probe anywhere.

One method of troubleshooting is start with the power supplies and move on from there. Often even problems that do not appear to be power supply related can cause issues and it is easy to check so why not.
Another thing that is done quite a bit is to isolate parts of circuits by their functions. Sometimes you can lift legs of components to isolate that circuit, other times an xacto or other sharp blade is used to cut a trace, soldering it when the repair is complete.
 

Offline M0BSW

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Re: I just looked and learned
« Reply #8 on: December 18, 2012, 04:44:48 pm »
I know your all busy , Thank You all for giving up your time answering my questions, quite strange all this for me people think because I've been a radio amateur some several Years, I know all this stuff, Yes when it comes to antenna's , filters, I do, but this stuff is much more interesting. I'm trying  not to run before I can walk with it. I'm sure most people interested in electronics have  a bucket full of things not repaired, I built a curver Tracer, from the design of Alan WE2AEW, I like this and I have tested all the parts I own, fasinating piece of equipment, at the moment I'm having a go at a power supply variable, as the curver tracer was a complete success, and it's very nice to know your all out there with advice to.
 :) Paul
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