Krell KSA-250 low frequency oscillation - diyAudio
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Old 29th October 2012, 02:45 PM   #1
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Default Krell KSA-250 low frequency oscillation

I first realized this problem one day when I turned my system on to listen and saw the midrange undergoing extreme movement while playing due to reproducing some low frequencies (there is no capacitor in series with it to block low frequencies since the system is actively biamped). I performed a complete capacitor replacement on the amp, including the large power supply caps - the low frequency oscillation is still there. I have measured some peaks of 350 mV. This is only in the right channel. Observing the right channel output on a scope you can see the the trace bobbing up and down with the oscillation.
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Old 31st October 2012, 10:04 AM   #2
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I suspect the problem to be associated wit the DC servo circuitry. Does anyone know which board and section this circuitry is on?

Thanks,
Will
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Old 1st November 2012, 11:55 AM   #3
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Quote:
Originally Posted by willcycle View Post
I suspect the problem to be associated wit the DC servo circuitry. Does anyone know which board and section this circuitry is on?

Thanks,
Will
There is a discrete JFET input diff-amp circuit on the main input PCB that performs the DC-servo function. This circuit uses the standard capacitor feedback integrator topology.

There is also an opamp based circuit on the power-up PCB that performs a coarse check for DC shorts in the output stage transistors before soft-start power-up is allowed.
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Old 1st November 2012, 08:12 PM   #4
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If you carefully look over your main PCB the discrete servo circuit should be similar to the attached topology. A parallel pair of high quality 3uF capacitors are often used for the integrator function. Locating these caps might help you trace the servo circuit on your PCB.
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File Type: jpg ServoCkt.jpg (38.2 KB, 208 views)
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Old 2nd November 2012, 12:15 PM   #5
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Thanks for the info. I hope to be able to spend some time with the amp this weekend. I will let you know what I find.
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Old 4th November 2012, 06:12 PM   #6
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I can't seem to find a problem with the DC servo circuit; the related transistors seem to be OK. In studying my scope output watching the DC level fluctuate, it occurred to me if the bias level was fluctuating due to some fault with the auto bias circuit the DC servo would just be trying to compensate so there would be no DC offset on the output. Is there a way I can test the opto-coupler IC?

Thanks,
Will
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Old 4th November 2012, 07:53 PM   #7
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Quote:
Originally Posted by willcycle View Post
I can't seem to find a problem with the DC servo circuit; the related transistors seem to be OK. In studying my scope output watching the DC level fluctuate, it occurred to me if the bias level was fluctuating due to some fault with the auto bias circuit the DC servo would just be trying to compensate so there would be no DC offset on the output. Is there a way I can test the opto-coupler IC?

Thanks,
Will
Will,

The only safe suggestion I can give you is to CAREFULLY use your O'scope to compare key signals between the good channel and the bad channel. Study the main PCB to locate key input transistors, feedback points, and Vbe driver transistors. A faulty local power supply could also be the cause of your oscillation. Changes in output bias current causing your oscillation does not make sense to me as an outside guess'er.

I attached a photo of the KSA250 driver PCB outlining the DC servo circuit we have been discussing.

The identified screw hole connects a trace on the output PCB below to the DC servo input. Hence, the amplifier output signal comes from the PCB on the heatsink up through this screw hole to the servo input on the main PCB mounted above. 3D-SOUND. well.... 3D packaging anyway. :-)
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File Type: jpg KSA250_Servo.jpg (104.0 KB, 152 views)
File Type: jpg KSA250 Output PCB.jpg (98.7 KB, 149 views)
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Old 4th November 2012, 08:13 PM   #8
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I am looking for an and all info on these amps. I have a KSA-250 that i need to rebuild this winter that is not quite sounding up to par. so any detailed info anyone has, be it hand drawn schematics, photos whatever i would really like a copy of!

Zc
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Old 5th November 2012, 10:42 AM   #9
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Thanks for the helpful photos Linesource. Do you know what the other optoisolator on the input board is for?

I will check the input differential pair transistors. Working on a live amp seems too risky as some leads are very close together.

Do you think I can swap input boards between the two channels to narrow the problem to either the input or output boards?

Thanks
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Old 5th November 2012, 03:48 PM   #10
AVWERK is offline AVWERK  United States
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Did the 250 use the NEC j44/k163 combo For its servo?
These were used in earlier amps are getting a little hard to find these days

Regards
David
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