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Author Topic: debugging a small amp  (Read 34139 times)

kin0

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Re: debugging a small amp
« Reply #45 on: July 31, 2011, 03:57:01 PM »
How can I cut the track?

J M Fahey

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Re: debugging a small amp
« Reply #46 on: August 01, 2011, 12:56:39 AM »
Make 2 parallel cuts across it with a sharp X_Acto type knife, the so called box-cutters sold for $1 everywhere.

They must be parallel and separate by 1 mm; then you can lift the thin center slice either by cutting somewhat sideways, or putting tour hot soldering iron tip on it and pulling it away, because the hot tip weakens (a lot) the glue that holds it.
You measure what you have to, and then you lightly scratch both sides of the slit and re-join them with a drop of solder.

kin0

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Re: debugging a small amp
« Reply #47 on: August 08, 2011, 08:55:44 AM »
gets only 0.25V

J M Fahey

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Re: debugging a small amp
« Reply #48 on: August 08, 2011, 12:08:10 PM »
Quote
measure voltage across R14 and R15
I mean what voltage you get across (from end to end) R14 and what you get across R15. 2 values in total.
Even better, post the value to ground (black probe grounded at the battery, red probe measures) on each of R14 and R15 legs, that makes 4 values in total.
Sorry if I wasn't clear.
PS: I guess you cut the approppriate track so the node that joins them is connected to nothing else.


kin0

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Re: debugging a small amp
« Reply #50 on: August 08, 2011, 04:32:43 PM »
Yeah I cut the connection.

The node of the R14/R15 is 0.24
the other part of the R15 is 0
and the other part of the R14 0.9

J M Fahey

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Re: debugging a small amp
« Reply #51 on: August 08, 2011, 05:17:34 PM »
Ok.
Quote
The node of the R14/R15 is 0.24
Now you are talking professionally :tu:
Even if it looks slightly longer or more complex, it ends being the shortest and clearest way.
So R14 itself is getting *only* 0.9V through R13, instead of the almost 9V we expect there.
Why?. Not normal.
Looking at the PCB rather than the schematic, I see a node R13/R14/C11/IC1Pin8.
There's *something" there pulling voltage down; it may be, in no particular order:
1) C11 inverted (upside down)
2) R14/R15 color bands misread, they are 1/10th the expected value (not very likely, but sh*t happens)
3) R13 misread, 10X the expected value (same considerations)
4) IC1 bad/shorted/upside_down, "eating" tons more than it should.
I would pull it (carefully, pampering the solder pads, use solder wick if available or a very good solder sucker)
5) bad connection in the +9V rail: bad soldering, connector, cracked track, you name it.
Ther is no magic nor gremlins, but sometimes an apparently simple problem can drive you crazy.
See that for this very simple one I am suggesting 5 or 6 possibilities, that's why when somebody posts "xxx circuit does not work, what may be happening?" and offers no other clues,  it's impossible to provide a useful answer, the asker must do his part of the job too.
OK, less talk (I), you go measure.
Now you are getting the repair/analyze logic. :tu:

kin0

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Re: debugging a small amp
« Reply #52 on: August 09, 2011, 12:41:15 PM »
The opamp was inversted (Sockets f!!* YEAH  8) ).
Now the node of R14/R15 is 1.7V
the other side of R14 is 9V
and the other side of the R15 is 0V

The opamp (pin 8) is now getting 9V too.

Is it okay now?

joecool85

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Re: debugging a small amp
« Reply #53 on: August 09, 2011, 01:01:43 PM »
Is it okay now?

You tell us, does it work?
Life is what you make it.
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kin0

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Re: debugging a small amp
« Reply #54 on: August 09, 2011, 01:08:20 PM »
I have to connect back the R14/R15 node to the to C12/C2 so I prefer to know first if the voltages should be like that. BTW all the other 4 things are right

J M Fahey

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Re: debugging a small amp
« Reply #55 on: August 09, 2011, 04:27:48 PM »
*If* N(ode)-R14/C11/R13 is 9V, on N-R15/R14/C12 you *must* have half that, since both R14 and R15 are same value, 10K.(check it)
You do not  :(
C12 might be inverted/upside down (make sure negative goes to ground, positive to Vb)
IC1 might be bad after accidental inversion.
Measure Vb , first without IC , then without C12 if necessary.

One of the general-purpose repair systems is:
 Ohm's Law rules; and lets you calculate what you *should* find at different points in the circuit.
If you don't, imagine what might shift those values and check.
As a non-electronic example:
your car engine does not start.
* do you have a spark?
* does gasoline reach the cylinder? if not, why?
* Does it reach the carburetor?
* Does it reach the gas filter?
* is tha gas pump working ?
* any problem with the hoses and conduits ?
* do you have gasoline ?
it's the same.


joecool85

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Re: debugging a small amp
« Reply #56 on: August 10, 2011, 06:41:54 AM »
* Does it reach the carburetor?

You guys still running carbs down there?  We haven't had a mass produced vehicle with a carburettor since 1985 or so.
Life is what you make it.
Still rockin' the Dean Markley K-20X
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J M Fahey

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Re: debugging a small amp
« Reply #57 on: August 10, 2011, 08:02:11 AM »
Not really.
It was just an example of what troubleshooting means, not only in Electronics but anywhere else.
H*ck !!, my Father was an Old style Country Doctor and he used basically the same mindset to "troubleshoot" PEOPLE !!   ;D

Back to cars, you have *any* car in the world available here, thanks to "Globalization".
Only problem is, you pay 2X the international price because of *heavy* taxes >:(

joecool85

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Re: debugging a small amp
« Reply #58 on: August 10, 2011, 08:13:08 AM »
Not really.
It was just an example of what troubleshooting means, not only in Electronics but anywhere else.
H*ck !!, my Father was an Old style Country Doctor and he used basically the same mindset to "troubleshoot" PEOPLE !!   ;D

Back to cars, you have *any* car in the world available here, thanks to "Globalization".
Only problem is, you pay 2X the international price because of *heavy* taxes >:(

Yeah, we get all the "Americanized" versions of things that suck.  Like the Ford Focus until this year was different than the Euro one, now that we have the Euro one it is considered to be the best compact you can get in the US - go figure.
Life is what you make it.
Still rockin' the Dean Markley K-20X
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J M Fahey

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Re: debugging a small amp
« Reply #59 on: August 10, 2011, 12:01:11 PM »
Just checking out of curiosity.
I found Ford Focus US prices from U$16,500 to 22,700 , depending on choices.
Argentine prices run from U$20,900 to 32,400.
Not that much difference as I remembered, now only 50% up.
It used to be 100%, which effectively protected us, so we had a big car industry, with almost a million jobs (in a then 30 million Country).
Now "nice cars are cheaper" .... but that million ex-workers now lives out of a State Help plan, in a slum .... and massively votes for today's Government, so they are encouraged to keep things this way.
Welcome to the Future. :'(

EDIT: just checked Brazil, which aggressively protects its industry.
There the (Argentine made) Ford Focus costs the staggering U$43,200  :o ... but they have the largest Car industry in Latin America, world's fifth largest (surpassed France in 2008), so they must be doing things right.
« Last Edit: August 10, 2011, 12:15:53 PM by J M Fahey »