Krell KSA-250 low frequency oscillation - Page 2 - diyAudio
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Old 6th November 2012, 12:33 AM   #11
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Quote:
Originally Posted by willcycle View Post
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
Will,

Before you swap driver boards, give some thought to a couple O'scope tests. I often use small Test Hook Clips that can safely spring attach to the lead-in on a resistor or capacitor.

UNPLUG AMP
GROUND THE INPUTs
PUT A DUMMY LOAD RESISTOR ON THE OUTPUTs

Put a test hook clip on the lead wire on one of the 3uF DC servo caps at the output drive side(away from NJET input). This is the output of the DC servo amp.

POWER ON!

xxxxxxx
The second opto isolator monitors the current in an output predriver emitter follower. Too much predriver current implies unsafe output current. This is part of the safe operation circuitry.

xxxxxxx
A change in output bias current should be handled by the amplifier's negative feedback. That's why a fault in the servo circuit feedback, or the complex input circuitry seems more logical. Changing driver PCBs might confirm this without any information for a solution.

What's the iine... Measure twice, cut once.
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Old 6th November 2012, 10:10 AM   #12
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Too bad I can't post videos here. It is interesting that the DC oscillation goes positive and negative but is slightly biased in positive voltage territory. Just to rule out an issue with the bias circuit on the output board - and avoid having to desolder and solder 28 power transistors again - I plan to replace the bias transistor and IL74 optocoupler first. Do you think any of the input transistors need to be matched if I have to replace them?

By the way, I just noticed your Apogee Full Range avatar. I have biamped Divas powered by two KSA-250s; a version 2 and 3. The amp I am working on now is the version 2. The two DC servo caps are 2uF on mine.

Will
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Old 6th November 2012, 10:14 AM   #13
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you can't post video's here but you can post them on youtube and then link them here! for example here is an excellent video showing lots of details about the insides of the big 250's

Krell KSA-250 recapping - YouTube
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Old 6th November 2012, 10:17 AM   #14
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LineSource, can I contact you off list? I don't see any contact info for you. can you send me a PM??
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Old 6th November 2012, 04:59 PM   #15
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Here is a video of a scope display of the low frequency problem with my amp.

Video Link
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Old 7th November 2012, 03:10 AM   #16
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Quote:
Originally Posted by willcycle View Post
Here is a video of a scope display of the low frequency problem with my amp.

Video Link

"Right channel output of a Krell KSA-250 amplifier with no input signal displaying problem with DC oscillation."

Will,

Is the input grounded or just floating open in your video?

The front-end circuitry is complicated. There are both a single ended input and a differential input, plus a mechanical relay for input selection.

The KSA-250 is shipped with shorting pins in the XLR inputs. These pins must remain in the XLR inputs if the units are to operate in the single ended mode. These pins must be removed to connect either unit for balanced operation. The shorting pin connects pins 1 and 3 and grounds the inverted input.

I'm a blind man in a mind field.


""F. PROTECTION CIRCUITRY
The KSA-250 has a sophisticated series of circuits that protect them from dangerous input signals or loads on the output. These circuits also analyze various amplifier functions to prevent speaker damage that may be caused if the amplifier fails to function properly.

There are two protection modes: Input and Output. When the Input Protection is engaged the amplifier will remain on, with the front panel LED illuminated, but it will not pass a signal. This indicates a problem with the input signal caused by the preamplifier or source component. This is most often a DC condition. The amplifier will resume passing signal when the problem is resolved.

When the Output Protection is engaged the amplifier will turn off, with the front panel LED turning off as well. This usually indicates a problem with the speaker wires, their connection, or the speakers themselves. Once the problem is located and resolved the amplifier will turn on normally with activation of the front panel switch. The Output Protection will also engage if the amplifier fails. If the amplifier will not turn on with all inputs and outputs disconnected, please contact your dealer or Krell Service""

Last edited by LineSource; 7th November 2012 at 03:23 AM.
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Old 7th November 2012, 10:15 AM   #17
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The input is open. It would not make a difference to the oscillation as I looked at the left channel in the same condition. You are right, I should short the input prior to measurement.

Last edited by willcycle; 7th November 2012 at 10:19 AM.
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Old 28th November 2012, 12:36 PM   #18
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Linesource,

I am going to replace the matched input JFETs as I understand this maybe the cause of the problem. The parts, J203 N-channel JFETs are rated at 40V Vds. As these parts are no longer available the Motorola recommended substitute is rated at 25V. Do you think this would be a problem in the circuit?

Thanks,
Will
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Old 28th November 2012, 02:31 PM   #19
Arick is offline Arick  Canada
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Quote:
Originally Posted by willcycle View Post
Linesource,

I am going to replace the matched input JFETs as I understand this maybe the cause of the problem. The parts, J203 N-channel JFETs are rated at 40V Vds. As these parts are no longer available the Motorola recommended substitute is rated at 25V. Do you think this would be a problem in the circuit?

Thanks,
Will
Linear systems recommends this part http://www.linearsystems.com/datasheets/J201.pdf
rated at 40V
Cheers,
Eric
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Old 28th November 2012, 05:56 PM   #20
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Will,

You might give some thought to simple tests that can confirm that the DC servo is the source of the oscillation. The DC servo circuit that was pictured earlier is connected to a ???K resistor that then drives the feedback inputs of the main dual diff-pairs. If you can locate this resistor on your PCB you could unsolder the resistor lead coming from the Servo. NOTE: the servo output will still go to the safe operation circuitry(op amps), but this should function just as before. We are only removing the servo from the input feedback point. If this removes the oscillation, then the servo circuit is likely at fault and needs additional testing... JFETS? ServoCaps? Local power supply Caps? Connection link to safe operation circuitry?

At the same time you could solder a test wire to the pad where the resistor lead was unsoldered and lifted, and use an O'scope to also monitor the now isolated DC servo output.

You can probably come up with other servo test ideas.

Remember from the PCB pictures that the input to the DC servo comes through a metal spacer that is attached by screws to the output transistor PCB. A funky mechanical connection.

GROUND THE INPUTs
PUT A DUMMY LOAD RESISTOR ON THE OUTPUTs
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