Need help troubleshooting Oatley K281 kit - diyAudio
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Old 25th August 2013, 04:45 AM   #1
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Default Need help troubleshooting Oatley K281 kit

Hi, Ok so I've built this Oatleyelectronics K281 amp and it doesn't go and I can't figure out why. Other than the light coming on, It seems completely dead.
I have checked all the solder joints and resistors, everything looks ok. there was some confusion over the resistors because the parts list was not a list of the parts supplied for some reason. But the parts generally matched what was printed on the pcb. I have absolutely no idea what to do from here. I don't want to throw it away.
Help!!
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Old 27th August 2013, 09:05 AM   #2
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Ok. The schematic shows the same values that were provided. So I'm confident that I have all the right parts in the right places. I have been over everything with a multimeter to check values and continuity. I have no shorts. Branko at Oatleyelectronics told me to check that I have 12V at the junction of R6 and R7, and also on pin 4.
I do have 12V at the R6 R7 junction, but absolutely nothing at all on pin 4.
So I have absolutely no Idea what to do now. I seem to have signal and correct voltage getting to the LM1875, but nothing coming out whatsoever.
Does anyone think that maybe I have a faulty chip? I can't think of anything else.
Please help!!
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Old 27th August 2013, 10:22 AM   #3
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I would like to help but I have no schematic. Can you email me the schematic and then I will know where to advise.
jon@jonsnell.co.uk
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Old 27th August 2013, 11:12 AM   #4
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Quote:
Originally Posted by JonSnell Electronic View Post
I would like to help but I have no schematic. Can you email me the schematic and then I will know where to advise.
jon@jonsnell.co.uk
Thanks, any help would be great.
I have a paper schematic. I'll scan it at work and email it as a pdf.
Thanks heaps.
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Old 27th August 2013, 12:58 PM   #5
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On the output IC you must have;
1/ 12v, 2/ 12v, 3/ 0v, 4/ 12v, 5/ 24v.
If pin 1 + 2 are correct pin 4 should be.
Don't get bogged down in the circuit. Take it one step at a time and separate, in your mind, each part.
There should be 12v across the large capacitor that feeds the loud speaker.
With a speaker connected and at switch on you should hear a thump as the large capacitor charges up.
Your soldering looks OK.
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Last edited by JonSnell Electronic; 27th August 2013 at 12:58 PM. Reason: More info added
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Old 27th August 2013, 01:13 PM   #6
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Quote:
Originally Posted by JonSnell Electronic View Post
On the output IC you must have;
1/ 12v, 2/ 12v, 3/ 0v, 4/ 12v, 5/ 24v.
If pin 1 + 2 are correct pin 4 should be.
Don't get bogged down in the circuit. Take it one step at a time and separate, in your mind, each part.
There should be 12v across the large capacitor that feeds the loud speaker.
With a speaker connected and at switch on you should hear a thump as the large capacitor charges up.
Your soldering looks OK.
Thanks, you probably got multiple emails from me. They kept bouncing back so I kept trying. I'm surprised you got it.
I'll measure it again tomorrow to make sure but I'm pretty sure I'm getting all those voltages except pin 4. I haven't measured the large capacitor though. There is no thump or anything when powered on.
I'm at a loss as to what to actually do about it, but as you say... one step at a time. I'll try again tomorrow.
Thanks for everything.
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Old 27th August 2013, 01:22 PM   #7
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If pin 4 has 0volts on it pin 2 should be similar as that is the inverting input. If there is 0v on pin 2, there should be 24v on pin 4 likewise if pin 1 is 12v and pin 2 is 13v then pin 4 will be 0v and the other way around, pin 1 is 12v pin 2 is 11v then pin 4 will be 24v.
That is how the inverting and non inverting input works the amplifier.
Have a look at the data sheet for the IC. It will explain in more detail how it works.
Sounds like if you have not made a mistake with your measurements, the IC has died.
Hope that helps
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Old 29th August 2013, 03:59 AM   #8
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Originally Posted by JonSnell Electronic View Post
If pin 4 has 0volts on it pin 2 should be similar as that is the inverting input. If there is 0v on pin 2, there should be 24v on pin 4 likewise if pin 1 is 12v and pin 2 is 13v then pin 4 will be 0v and the other way around, pin 1 is 12v pin 2 is 11v then pin 4 will be 24v.
That is how the inverting and non inverting input works the amplifier.
Have a look at the data sheet for the IC. It will explain in more detail how it works.
Sounds like if you have not made a mistake with your measurements, the IC has died.
Hope that helps
I really can't thank you enough for your help.
I measured the pins again on the LM1875 and this is what I got...
Pin 1 1.1V
Pin 2 0.8V
Pin 3 0V
Pin 4 0V... no surprise in this instance
Pin 5 24V

So I was clearly wrong about what the chip was getting on pins 1 & 2. Any Idea why this would be?
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Old 29th August 2013, 04:03 AM   #9
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Oh, and zero volts across the large capacitor.
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Old 30th August 2013, 09:23 AM   #10
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Hi.
If the junction of R6 and R7 is not at 1/2 rail. (=12v) pin 1 will be low causing pin 4 to go low and as pin 4 controls the inverting input pin 2 and sets a low voltage on there, pin 2 will try to return pin 4 to a higher level. The overall control is pin 1, which is at the wrong potential.
Check the voltage across C6, it should be 1/2 rail.
Hope that helps.
Jon Snell Electronic
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