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Old 29th January 2011, 10:46 AM   #1
jazz is offline jazz  Netherlands
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Default Help wanted,aleph (cl) ono problem (long)

Hi,

I've build a clone of the aleph ono using boards from R. stens. Build up a proto on a wood board and tested it. it worked flawlessly from the get go and i didn't even have to adjust any settings. It remained stable for three months until i decided to finish it and gave it a case.

Once i had made the case i made one mistake, i reversed + and - on the power supply. I was quick at it and no damnage seemed to be done. correct the mistake and it works perfectly again (4 hour test on the bench). Finished it up and listened to it for a couple of hours and then one channel suddenly ran away. Massive current in the positive phase output channel, burnt out the source resistors and a severely scorched pcb (somehow the fets survived).

No problemo i thought. I replaced all the transistors following the input diff, checked the caps and resistors and fired it back up again. Hey seems fixed, runs fine on the bench with the lid open. Close the lid and there it goes again.
Ok replace transistors again, replace the psu reg, some of the caps and a few of the resistors. resoldered all connections and components on the faulty pcb. back to the bench, works perfectly fine. close the lid (not fully this time because of the measurement leads but close enough), there it goes again. The other channel keeps working perfectly. Don't know what to do next.

These are the symptons:
- It biases up correctly until about .31 volts across the 33R output resistors. then it starts to run away.
-the voltage across the 150R resistor (r53) is a bit high but the same as the other channel:1.4V. stable until it starts to run away.
-the voltage across the bias led is stable until it starts to run away (1,88V), then skyrockets (led goes out, measured 10V before pulling the plug)
-Voltage across the load resistor of the input diff is 3.69V (R4, same as other channel)
-with the lid open it biases up much slower (of course) and after about 20 minutes it starts to run away again but at a higher voltage across the output resistors (3.5V), the input stage remains stable untill the run away accelerates from about .5V to high in a matter of seconds. I've attached the scope to the output but can't detect any signal up to 10MHz.

Right now i'm lost for what to do next. I even build up the output stage bug style to bypass the scorched pcb. shouldn't the negative phase behave the same if it is oscillation? why doesn't the other channel behave the same? Could it be the input stage diff, the led or what else? I don't understand how the voltage across the led can rise in such a way. If the sk389 would be damnaged how could it bias up ok until runway?

I've attached the schematic from R. Stens' site for reference. If this is not appropriate please delete and tell me so.

Please help, I really want to get this one to work again.

Kind regards,
Joris
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File Type: pdf AlephOnoMM.pdf (35.5 KB, 191 views)
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Old 30th January 2011, 02:32 AM   #2
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did you try to replace the zener?

Last edited by AudioSan; 30th January 2011 at 02:36 AM.
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Old 30th January 2011, 10:59 AM   #3
jazz is offline jazz  Netherlands
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Hi Thanks,

Yes i did, it's the other phase but i did anyway.

grtz
Joris
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Old 30th January 2011, 05:43 PM   #4
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Quote:
Originally Posted by jazz View Post
Please help, I really want to get this one to work again.
Kind regards,
Joris
Did you try to replace/check output fets, Q3, Q6 led and Q17 BC550C ?

Does the cascode on the 1st stage work ? how is the 2SK389 Q5 ?


Regards,

nAr
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Old 31st January 2011, 06:53 AM   #5
jazz is offline jazz  Netherlands
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Hi,

I've replaced Q16,17,6,3 and 18 to no effect. Up til now I've assumed the led and Q5, 1 and 2 to be ok. The input diff biases up ok until the runaway starts. Could the led be defective when it is emitting light and running at about 8,9mA until the runaway starts?

I'm going to try and desoder the sk389 and measure idss.

thanks,
Joris
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Old 31st January 2011, 07:06 PM   #6
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I'm not sure I can help more. It seems your runaway could be caused by blown fets showing erratic behaviour instead of stable biasing and thermals. I think about the TO220 ones.

Moreover, I really don't know if the 6k8 resistor for the led is correctly wired in the schematic you posted ... It seems it has only 6k8 between V+ and led biasing resistor, and led would also be referenced to V- ??? it seems a very high potential, I'm unsure where GND (at the place nothing is shown) in the attached schematic instead of V+ would work ... The led is only supposed to be a 1,65 or 1,90V voltage reference so.

Pearl II biasing

Click the image to open in full size.

Note R25 top is referenced to ground.

AlephOno original drawing detail

Click the image to open in full size.

Nothing is written in the same place on top of R5, so what ?

So, I would go first to replace the 3 x TO220 fets by fresh ones, and having no zener plugged anywhere in at first. There are none in the original schematic. Try swapping GND to 6k8_top_connect instead of V+, it might help !!!

I could also tell a lot of stupidities. If it's the case, lol gurus flame me you're welcome lol

Best,

nAr
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Old 1st February 2011, 08:46 AM   #7
nar is offline nar
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Maybe I was not so clear with my previous posts. Here is a comparative view in this particular circuit bias taken from 3 revisions: 1st, AOno original drawing; 2nd, R. Stens drawing; 3rd, Pearl II biasing, just for comparison purposes. My guess is the unidentified point in AOno original should be tied to GND ( top connection of R5), as shown in red; I think on Mr Stens schematic, +30V on top of R5 should also be replaced by GND; in Pearl II schematic it's very clear, top of R25 is referenced to GND no mistake possible.

Click the image to open in full size.

Hope this helps,


nAr
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Old 1st February 2011, 09:40 AM   #8
jazz is offline jazz  Netherlands
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Hi Nar,

Thanks, good one about the 6k8 resistor. I had noticed the difference but didn't give it much thought as i believed that electronically, connecting it to either ground or the plus rail doesn't make much difference. That is until I actually calculated the dissipation requirements for both.

Maybe overheating has caused the resistor to become temperature sensitive. I'll see what i can do.

Thanks for thinking with me!

regards,
Joris
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Old 2nd February 2011, 06:51 AM   #9
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Quote:
Originally Posted by jazz View Post
-the voltage across the bias led is stable until it starts to run away (1,88V), then skyrockets (led goes out, measured 10V before pulling the plug)
There's your problem, replace the LED.
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Old 2nd February 2011, 07:08 AM   #10
nar is offline nar
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Quote:
Originally Posted by nar View Post
Did you try to replace/check output fets, Q3, Q6 led and Q17 BC550C ?
Quote:
Originally Posted by Itsmee View Post
There's your problem, replace the LED.
I also think the led might have a problem ...

Regards,

nAr
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