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Old 13th April 2014, 05:53 PM   #1
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Default Help with ocillating amplifier !!

Hi!

Lets take this from the begining...
At first i have to say i'm not write or speak English so well...

A friend gave me a sub woofer (Audio pro SUB+) that was stone dead when plugged to the mains.
I thaught it would be an easy one to fix, and started right on to trouble shoot the electronics.
This is a special amplifier design (ACE bass) and can only drive the specific driver it ment for.
* Transformer was no longer able to put out any voltage.
* both output transistors shorted.

I replaced the transformer with two toroid transformers with almoast the same spec, and new Output transistors.
Now i try to adjust the negative resistance, following the servicemanuals instructions. The amplifier ocillate and i can't get a stable output voltage.
The instruction... Set the amplifier in Trim mode ( jumper) turn level and filter adjustment fully clockwise. Connect a tone generator on the input an adjust it to 50Hz. Adjust output level on the generator so you can read 1 volt on speaker output no load. Connect a 8ohm resitsor to speaker output and adjust trimpot VR202 so you can read 6,27volt on speaker output.
What i get is a voltage from 6-11 volt up and down ocillating...

I need help !!!
Attached Files
File Type: pdf Sub MkII S manual 230V.pdf (305.0 KB, 80 views)

Last edited by fragglan; 13th April 2014 at 06:05 PM.
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Old 13th April 2014, 08:53 PM   #2
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What about resistors R216, R217 and R241?
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Old 14th April 2014, 04:13 AM   #3
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Yes , R241 and the op-amps seem to be a speaker impedance feedback circuit.

Most of the current returns to 0V through R241 , as well as producing
a small signal NFB for the IC.

I could imagine what the FB/phase would be without R241
The output stage is "super over compensated" (220pF) and has basestoppers.
I doubt it would oscillate itself without a very serious feedback issue.

Very interesting !

OS
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Old 14th April 2014, 05:20 AM   #4
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Is R241 ok?

Oh - I see OS already asked that! Damn, I'm just too slow to the draw today

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Old 14th April 2014, 05:28 AM   #5
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Quote:
Originally Posted by Bonsai View Post
Is R241 ok?

Oh - I see OS already asked that! Damn, I'm just too slow to the draw today

Yeah , he could of damaged the IC with a shorted OP (and/or burnt R241).

He did not say what type of oscillation. What he describes is
"pumping" ,(a LF oscillation).
I commented as the circuit was new to me - I'll find a use for it .
PS - he should check the zeners + the IC.
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Old 14th April 2014, 09:15 AM   #6
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I think it is likely that the failure of the output transistors has destroyed the drivers or the op-amp. Its worth checking the phase of the replacement transformers, if the secondaries are in series. This is the first commercial application of the op-amp based push-pull Vas circuit I have seen.

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Old 14th April 2014, 09:22 AM   #7
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Quote:
Originally Posted by overall feedback View Post
What about resistors R216, R217 and R241?
R216, R217 and R241 Are all messured to spec... IC201 is changed for a new one. C203,C204 checked as well.
I allso have to tell, when testing with 16ohm load its 100% stable output but with less voltage.. (ca 3volt) When i messure the voltage on the main smoothing caps at 8ohm load i notice a change of some volt upp and down, the same behavior as shown on the output. Are the transformer simly to smal!! ??
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Old 14th April 2014, 09:48 AM   #8
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Quote:
Originally Posted by ostripper View Post
Yeah , he could of damaged the IC with a shorted OP (and/or burnt R241).

He did not say what type of oscillation. What he describes is
"pumping" ,(a LF oscillation).
I commented as the circuit was new to me - I'll find a use for it .
PS - he should check the zeners + the IC.
OS
About the Zener, ZD201.. As it looks to me this Circuit adjust the gain for the IC102A to prevent overdriving the amplifier. Correct me if i'm wrong. (still in the Learning stage)
And how do i messure a zener?
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Old 14th April 2014, 10:10 AM   #9
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Do you have 50Hz connected?
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Old 14th April 2014, 10:13 AM   #10
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Originally Posted by overall feedback View Post
Do you have 50Hz connected?
Yes, 50Hz sine wave
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