Krell KSA-250 failure

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#21
I also happen to be one of the owner of a bad KSA 250. My KSA 250 just start to have problem after I came back from a two weeks trip. When I try to turn it on, it wil only stay on for two seconds and automatic shut down. Any cue for me to start the repair? Would that be a problem from the soft start too?

Harry
 
#22
harrytam said:
I also happen to be one of the owner of a bad KSA 250. My KSA 250 just start to have problem after I came back from a two weeks trip. When I try to turn it on, it wil only stay on for two seconds and automatic shut down. Any cue for me to start the repair? Would that be a problem from the soft start too?

Harry
Click to expand...

Hi Harrytam,

I own two Krell KSA250 amps and I have replaced the soft start PCBs and major capacitors in both. I do not know if I can help you fix your amp, but I am willing to share what I have learned about my KSA250s. READ AT YOUR OWN RISK.

Here are my initial thoughts....

While you were gone for 2 weeks all the major power capacitors in you KSA250 probably fully discharged. When you came home and hit the power switch a large charge current was demanded by the main power capacitors that ran through the soft start relays. This current surge may have destroyed your soft start relays (BIG BLUE SPARKS), or one of your power capacitors might be leaky enough to trip the safe-operation test circuits.

0) GROUND THE INPUTS.
PUT A DUMMY RESiSTOR LOAD ON THE OUTPUT TERMINALS.
Physically locate the soft start PCB(front of chassis). Through the top vents watch for blue sparks on the main relays while you turn on the amp. SPARKS = (likely) bad power caps and/or bad relay

Some DIY repair ideas include:

1) UNPLUG THE AMP.
GROUND THE INPUTS.
PUT A DUMMY RESiSTOR LOAD ON THE OUTPUT TERMINALS.
2) remove the top cover from the UNPLUGGED amp
3) use a dummy power resistor (1Kohm is OK) to discharge any remaining voltage on the main power capacitors by connecting it between the ground center copper bus bar and to the (+) bus bar for a few seconds ... and then to the (-) bus bar for a few seconds.
4) use a soft brush and a vacuum cleaner to carefully remove the loose dust EVERYWHERE. Brush... do not physically scrub... vacuum does most lifting.
5) with a bright light and magnifying glass, carefully search the PCBs for burnt components or PCB traces. Closely check the soft start PCB and relays for evidence of arc burns. Use your nose to smell for burnt components.
6) take a drink of California Red wine. A cheap cabernet is fine.
7) with your new courage, you want to remove the screws from the copper bus bars on the main four power supply capacitors
8) TEST CAPS for LEAK: get a lab power supply and put a 1Kohm power resistor in series with the + terminal
9) TEST CAPS for LEAK: Slowly charge each power cap through the 1Kohm resistor for several minutes.
10) After several minutes of charging... measure the leakage voltage across the 1Kohm resistor. Any noticable voltage across the resistor = leaky power cap = replace all power caps
11) The soft start relays might have been destroyed by the bad caps. Say a $350 prayer and carefully use your light and magnifying glass to check the soft start relay and PCB components for damage. Black/brown grunge in the relay plastic case is not a good sign.


REGROUP and decide next options. take a drink of California Red wine. A cheap cabernet is still fine. Save a Pinot for when all is repaired.
 
#23
I check the main cap and appear to be o.k.

However, when I unplug the black and white wire from the main board to the soft start board, it will stay on. Does anyone know what kind of wire it is? I am hesitate to hook the speaker up with it now so I am not sure about whether the amp runs normally without those two wire. Any one know what those two wire is for?

Harry
 
#24
harrytam said:
However, when I unplug the black and white wire from the main board to the soft start board, it will stay on. Does anyone know what kind of wire it is?Harry
Click to expand...


Harry, Just a guess....

The two wires might be the output DC detection sense signal. There is also a clipping detection sense signal in the KSA250.

GROUND THE INPUTS.
PUT A DUMMY RESISTOR LOAD ON THE OUTPUT TERMINALS.
MEASURE FOR DC VOLTAGE ON THE DUMMY OUTPUT RESISTORS.
 
#32
also, since we are talking about KSA-250's. I noticed mine draws 7 amps at idle, not 12 as advertised?? and my amp just makes 250 watts at 8 ohms before clipping one channel driven. I thought it was supposed to do 320watts at 8ohms at clipping??

I think there was 3 versions of this amp and mine is a middle version.
 
#34
From what I understand, there is a auto bias circuit in the amp. I think there should be something wrong with that part of the circuit so the bias or DC offset will be off. This will provide signal from the black and white wire from the main board to the slow start board and make the over current/voltage protection circuit to shut down the power after two seconds. Does anyone have more info or experiance about this circuit?

Harry
 
#35
there is almost no info on these amps and i have yet to find anyone with any sort of real in depth knowledge of them! schematics just don't exist anywhere! i would love to one day find someone that used to work at the old krell service dept that would be willing to help!
 
#36
Chris, (anatech)

Form some previous post, I know that you have a lot of knowledge and experiance on this amp. Could you help me a little bit on this issue? I know some basic knowledge about the electronic circuit from school. Could you give me some idea about where to start?

Harry
 
#37
harrytam said:
From what I understand, there is a auto bias circuit in the amp. I think there should be something wrong with that part of the circuit so the bias or DC offset will be off. This will provide signal from the black and white wire from the main board to the slow start board and make the over current/voltage protection circuit to shut down the power after two seconds. Does anyone have more info or experiance about this circuit?

Harry
Click to expand...

I took the top off of one of my KSA250s. Many memories of my re-cap sweat years ago. When I take off the amp top plate I always look around and smell around for deformed capacitors, burnt resistors, and open traces.

The power up PCB includes a Bias Sense circuit, voltage reference Zener diode, and a comparator which are connected(wired) to an optical isolator on the auto-bias and bias transistor on the output board. These could be the wires you were looking at. This optical isolator is part of the Bias Relay and Time Delay circuit on that bias transistor stack on the output board. You should see a small relay as part of this circuit.

Do you think it is worth the effort to disconnect what you believe is the faulty channel and see if the amp powers up with one channel? I never tried a 1-channel power-up.
 
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