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Author Topic: debugging a small amp  (Read 34142 times)

joecool85

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Re: debugging a small amp
« Reply #60 on: August 10, 2011, 12:50:11 PM »
I hear you Juan.  Almost everything sold here is made in China, Taiwan or Japan in that order.  And the US Gov't is going to try and make it worse by getting rid of the tariff on shoes from Vietnam which will effectively push New Balance (still employing 1,000 US workers) out of the US entirely.  There use to be a HUGE shoe industry here in Maine and now it is all but gone.
Life is what you make it.
Still rockin' the Dean Markley K-20X
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kin0

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Re: debugging a small amp
« Reply #61 on: August 12, 2011, 01:19:36 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.


I took out the  IC and the voltage at the node of R14/R15 is still 1.7V. I don't see any point in getting out the C12 because it's not connected and it's in the right direction. However there is a problem with the resistor - their color band shows 10k (Brown-Black-Orange-Gold) but while multimeter I measured -13k to -17K (it was jumping all the time). Should I put a new pair in?

J M Fahey

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Re: debugging a small amp
« Reply #62 on: August 12, 2011, 02:44:18 PM »
Quote
I don't see any point in getting out the C12 because it's not connected and it's in the right direction.
Well, *is* it connected or is it not? ;)
Just humor me and pull it, then re-measure.
Also lift *one* end of R14 and R15 and measure them "in the air"
Also follow any copper track that's connected to the Node-R14-R15 , it might be connected (by error) to something else, by a solder drop, a too carbonized residue, whatever.
Current just doesn't dissapear in the air, there must be a path somewhere.
If you have 9V across 2 equal resistors in series, you must have 1/2 that across each of them.
You do not?
Then put your Sherlock Holmes cap on, grab your trusty looking glass and start searching for the truth.
The pipe is not necessary. ;D

kin0

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Re: debugging a small amp
« Reply #63 on: August 13, 2011, 01:01:38 PM »
Pulled the cap out and the voltage on the R14/R15 node is 3.12V  ;D and the voltage on the other part of the R15 (the one that goes to ground) is still 0V. Oh, and I lifted the both R4 and R15 legs and both resistors are exactly 10K. And the traces don't connect to anything that they don't have too

J M Fahey

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Re: debugging a small amp
« Reply #64 on: August 13, 2011, 11:55:19 PM »
OK. Node R14-R15 3.15V .................. Fine.
Node R15-Ground 0V ........................ Fine.
And "the other end" of R14 ? :trouble :trouble :trouble

kin0

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Re: debugging a small amp
« Reply #65 on: August 14, 2011, 01:57:16 AM »
still the same 9v...

J M Fahey

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Re: debugging a small amp
« Reply #66 on: August 14, 2011, 02:07:02 AM »
OK, lift the R15 leg that goes to ground, and re measure voltage to ground at the R14-R15 node and on the other side of R14. (2 values)

kin0

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Re: debugging a small amp
« Reply #67 on: August 14, 2011, 07:48:02 AM »
oh I just connected R15 back to its place  xP

J M Fahey

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Re: debugging a small amp
« Reply #68 on: August 14, 2011, 09:19:17 AM »
And the voltages would be ? .........

kin0

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Re: debugging a small amp
« Reply #69 on: August 14, 2011, 10:13:54 AM »
I was checking the ohms and read your post 10 minuets later. I'll check it then I'm back home

kin0

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Re: debugging a small amp
« Reply #70 on: August 15, 2011, 05:42:08 AM »
Okay

R14/R15 node is 2.25V
the other side of the R14 is 9.12V and the other side of the R15 is 2.01V

J M Fahey

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Re: debugging a small amp
« Reply #71 on: August 15, 2011, 11:59:20 AM »
Quote
and the other side of the R15 is 2.01V
8 posts above you said "the other side of R15 is 0 V"
Yes, I asked you to lift a leg to measure it resistance "in the air" but then you 
said "just connected R15 back to its place".
Which one of those contradicting ones is the correct answer?
Besides that, at the R14/R15 node you *must* have 1/2 the voltage you have across both in series.
MUST, period.
*If* you do not, then SOMETHING ELSE IS CONDUCTING CURRENT TO GROUND.
You have the board, you check it, with good light and a loupe, if necessary.
If the board is homemade, you may have improperly etched copper connecting what it shouldn't or small slivers of copper when you perforated it or carbonized flux acting as an unexpected resistor or a blob of solder touching what it should not.
You have the board, you find it.
We can't go on without solving that.
I assume the IC is not there, is it?
Just for you to understand this: solder two new, free 10K resistors "in the air", forget the board, and connect free ends to a 9V battery.
What do you measure across each resistor?
OK: you should measure the same onboard.
If you do not, (so far you don't), find why.
Good luck.

kin0

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Re: debugging a small amp
« Reply #72 on: August 15, 2011, 03:38:13 PM »
In the last post I meant that then the R15 was in air the part which was connected to node of R14/15 was 2.12V and the other part which was in the air was 2V. I'll try to find the problem and post if there will be any improvement.

phatt

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Re: debugging a small amp
« Reply #73 on: August 15, 2011, 06:49:51 PM »
Have you checked your DMM leads lately?

Have you checked the battery in the DMM?
They can do strange things to your readings when battery is low.

Phil.

J M Fahey

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Re: debugging a small amp
« Reply #74 on: August 15, 2011, 07:13:12 PM »
Fine. As you can see  you have a 10K resistor connected to +9V, its other end connected to another resistor "in the air" so it has a gazillion ohms (open circuit) SO: what's pulling your voltage down?
That's what you have to find.
Do not discard *anything*.
Maybe you say "the track ends in an IC socket , there's no IC in there so the current can't leak there .... "
Wrong.
How do I know?
Well , really I don't, let's check.
What I can't say is "no, everything's right, there's no problem there".
Good luck.