Author Topic: EEVblog #710 - Intercom System Repair  (Read 21903 times)

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Offline digital

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Re: EEVblog #710 - Intercom System Repair
« Reply #50 on: February 02, 2015, 08:39:45 am »
Dave you did not waste your time at all you made a very interesting video and I am sure it was enjoyed by many forum members,in a commercial environment the end result would be the same as yours not worth the time.Cheers
 

Offline Gribo

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Re: EEVblog #710 - Intercom System Repair
« Reply #51 on: February 02, 2015, 09:37:28 am »
Great video  :-+
Even though there is no rise time specified in the datasheet (page 14), I would try to patch a POR device (the TPS3809 can tolerate 5V).
I am available for freelance work.
 

Offline bookaboo

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Re: EEVblog #710 - Intercom System Repair
« Reply #52 on: February 02, 2015, 11:01:33 am »
Looks very much like brown out caused by an unstable or slowly ramping up power supply causing the micro to lock up. I remember a product line at my previous employer displaying just this characteristic, the PIC54 or PIC55 would work fine until the battery was discharged and recharged from a certain level, we were using a standard RC and diode reset circuit no amount of messing with these component values would solve the problem!
When you had the issue of "it works one time in 5" I bet if you leave it longer and let the caps discharge fully you get a higher % success rate, quick switching on/off just dips the 5v rail slowly.

We fitted one of the MCP voltage supervisors and all was well again.
http://ww1.microchip.com/downloads/en/AppNotes/00686a.pdf
A faster solution might be to do a little 555 circuit to give the PS all time to settle.


P.S. Keep the repair videos coming, great for those of us into repairs but also a great resource for designers as there is so much to learn from seeing what's gone wrong in other products.
« Last Edit: February 02, 2015, 11:03:47 am by bookaboo »
 

Offline Pasky

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Re: EEVblog #710 - Intercom System Repair
« Reply #53 on: February 02, 2015, 03:26:22 pm »
I really enjoyed this video as it shows me how a professional debugs a problem.  It was quite a learning experience and it's also great to know I'm not alone in making an ass of myself when I think I've solved the problem only to see it's far worse than I thought.   :-+

I'd love to see more videos like these.
 

Offline robrenz

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Re: EEVblog #710 - Intercom System Repair
« Reply #54 on: February 02, 2015, 03:45:13 pm »
Excellent video  :clap:  Very educational for my level  :-[  The fact that you did not totally fix it is irrelevant to its diagnostic educational value

Offline Araho

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Re: EEVblog #710 - Intercom System Repair
« Reply #55 on: February 02, 2015, 05:22:57 pm »
Haven't bothered reading the entire thread for suggestions (and neither have I watched the last 10 minutes of the video yet), but couldn't the symptoms of the Reset-line be caused by a partially dead cap on the line?

Say, if the cap in the RC-network there has failed almost short, it will pull the line low unless you can counter it with a low enough R on top (ie. by putting another resistor in parallel as Dave did) to make it a voltage divider again. Or am I just thinking bullshit now?  ;D  O0
 

Offline parbro

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Re: EEVblog #710 - Intercom System Repair
« Reply #56 on: February 02, 2015, 05:33:39 pm »
Dave removed the cap on the reset line but it didn't seem to make a difference.
 

Offline Araho

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Re: EEVblog #710 - Intercom System Repair
« Reply #57 on: February 02, 2015, 05:34:36 pm »
Ah, okay, didn't catch that bit.
 

Offline German_EE

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Re: EEVblog #710 - Intercom System Repair
« Reply #58 on: February 02, 2015, 06:03:41 pm »
"It is highly likely this thing is screwed and I will have wasted a lot of time for nothing. "

This is not true as the time was not wasted, we all got to see inside something new and the blog members learned some more about fault finding.
Should you find yourself in a chronically leaking boat, energy devoted to changing vessels is likely to be more productive than energy devoted to patching leaks.

Warren Buffett
 

Offline donmr

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Re: EEVblog #710 - Intercom System Repair
« Reply #59 on: February 02, 2015, 09:18:52 pm »
Could it be that the traces that power the MCU are faulty in some way and the MCU is actually getting powered via the RESET pin?

I have seen this happen with other designs/parts.
 

Offline donmr

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Re: EEVblog #710 - Intercom System Repair
« Reply #60 on: February 02, 2015, 09:22:32 pm »
You probably did this but I don't recall seeing it on the video:

What I recall was that the "reset" trace would not go high even after you lifted the MCU pin.
Was it still 12k to GND?  I'd lift the cap to make sure it wasn't bad.  Then find whatever was
pulling that trace low.
 

Offline Stonent

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Re: EEVblog #710 - Intercom System Repair
« Reply #61 on: February 02, 2015, 11:27:20 pm »
"It is highly likely this thing is screwed and I will have wasted a lot of time for nothing. "

This is not true as the time was not wasted, we all got to see inside something new and the blog members learned some more about fault finding.

When the checks stop coming from google, that's when you've wasted your time.
The larger the government, the smaller the citizen.
 

Offline EEVblog

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Re: EEVblog #710 - Intercom System Repair
« Reply #62 on: February 02, 2015, 11:34:07 pm »
"It is highly likely this thing is screwed and I will have wasted a lot of time for nothing. "
This is not true as the time was not wasted, we all got to see inside something new and the blog members learned some more about fault finding.

Yes, I meant if I continue with it from this point.
 

Offline mswhin63

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Re: EEVblog #710 - Intercom System Repair
« Reply #63 on: February 03, 2015, 01:36:39 am »
Great video Dave, not sure if you saw my YouTube comment but have you tried to contact AIPHONE (not the contractor) to find out if the new models are interchangeable.

Someone on twitter pointed out their product page that very specifically states the GT is not backward compatible with the GF.

No worries, I don't have a twit ter account :)   >:D
.
 

Offline Anks

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Re: EEVblog #710 - Intercom System Repair
« Reply #64 on: February 03, 2015, 01:55:03 am »
seen this before. Used some form of averaging mode and not seen the wobbly rail. The chip as a arsey reset and the wobbly line is the problem. It's unlikely that the latch up as caused any issues and the chip just needs its reset invoking correctly. Unless you let the latch up catch for long it won't do any harm that's what's the sot packaged is there to prevent. You had the current limit set conservatively for most the tests.
 

Offline RupertGo

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Re: EEVblog #710 - Intercom System Repair
« Reply #65 on: February 03, 2015, 02:59:10 pm »
I dunno. One of the things you pick up when you fix things regularly is when something should be marked as NFR - Not For Resuscitation. There are the clear faults where a component or physical thing has failed; not always quick to find, but unambiguous when you do and unambiguously fixed thereafter. There are the clear beyond economic repair units, where the damage is too widespread, faults have cascaded, or parts are unobtainable/too expensive. And then there are the heartsinks, which sort of limp on after you've sort of fixed the problem, but you're not quite sure that you have and the behaviour isn't solid enough to let you be confident that you fully understand what's going on. When you factor in your time and effort - and in particular, what else you could be doing instead - that final category of repairs gets old really quickly,

Nobody likes walking away from something where there's a chance that one more push will ensure victory and if you're a hobbyist who enjoys the chase then these things can be kept going for a long time (I spent more than a year on a mammothly complicated colour dual-standard buggered TV chassis when I was a kid; it was great fun and I learned a lot. It never worked.) Later in life, not so much.

I guess everyone who follows EEVBlog does so because they have a passion for electronics, whether or not it's their job as well. Sorting out hard problems can be addictive (I remmeber a really good column called In Your Workshop in one of the UK TV technical magazines, which specialised in that.)

Sometimes, though, you have to look at a quivering patient on the table, flip the switch and throw the sheet over its head. There's always another.



 

Offline station240

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Re: EEVblog #710 - Intercom System Repair
« Reply #66 on: February 04, 2015, 11:40:57 am »
Forget the SCR/power circuit, does it work properly if powered directly off a 5V supply ?
I'm sure you could get a proper 24 to 5V module that would fix inside that casing.

I'd also try putting that SOT reset chip back into the circuit, using flying leads. See if that part is fine, and the power/RC components were the issue.
 


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