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Need help troubleshooting Oatley K281 kit

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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!!
 
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!!
 
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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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.
 
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
 
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?
 
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
 
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

Hello again, well I've been dicking around with the resistors trying to get these voltages right. I've got it all reading pretty much ok but still no luck with any form of output. Here is the schematic with the voltages I'm now reading.
An externally hosted image should be here but it was not working when we last tested it.

I'd like to get it going but I don't know how long I'll persist with this. I'm feeling pretty defeated by the whole thing.
Cheers
 
I replaced the LM1875 out of desperation, and as soon as I turned it on I heard the thump from the capacitor charging up. Hooray... this is progress. I am yet to get any actual real sound out of it but I am feeling more encouraged. I haven't taken any voltage readings yet but I think I'll need to undo all the resistor mods I did trying to get the voltages right. This may have been a bad chip all along... Fingers crossed.
I won't have a chance to do any more for a day or so but I now have some hope.
 
Hooray!!! It works!!
but I'm not out of the woods yet. There is way too much hum at all times there is plenty of signal getting through.
Time to consult the forums on how to reduce this annoying buzz / Hum.
Thank Jon for all your help. I'd have been totally lost without you.
 
Hi all. Still having issues.
I have had some email correspondence from Jon Snell and I'll paste it here for anyone whom it may benefit.
From Jon...

Pleased to be of assistance.

Is the sound a buzz or a hum?
If a hum check your power supply for ripple. Place a large value capacitor across the supply and disconnect the mains. Does the hum/buzz go away? If it does check your earthing/ground point.
Check the input C6 capacitor is decouplig the supply. Try another one from the ground point to pin 1 if the chip. If the hum is still there it is nothing to do with the valve stage.

All the best!
Jon
 
I put a large capacitor across the mains and when powered off, the signal was clean while the capacitor drained.
I am a bit confused about checking the earthing/ground point. As far as I can tell the only earth is the - DC supply and that all seems fine.
I think there is some very basic electronics knowledge that I'm missing. I also have a 12v TDA 2003 guitar amp that I built inside an old radio and that has a similar hum issue. My solution with that was to power it off a car jump starter pack and that works fine. If I can fix this hum with the K281, maybe I can implement the same fix for my radio guitar amp.
 
THE END!!!
Overall volume is up and power supply hum levels are acceptable. It would be nice to get it quieter but it's really not bad.
I really just went overboard with shielding and making cables as short as possible.
So this brings my K281 adventures to a welcome end.
Yay!!!

Thanks Jon
 
I really wanted to build a balanced headphone amp with two of these K281 kits but the shipping charge to the USA is over $50! Oh well.

I know your pain. I am in New Zealand and as such, am plagued with the curse of shipping expense. In the case of the Oatley kits I am fortunately close to Australia which makes the cost of these kits appealing.
I often have to contend with shipping costs from the US and it hurts my bank balance. Unfortunately we have no good supplier of electronic components here and everything must be sourced overseas. I can't tell you the number of times I have read "you can get it at radio shack" on these forums and felt extremely hard done by.
 
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