Hi all,
my name is Wolfgang and I live in germany. I found this good website and a few people have here Krell amps. We have a Krell KSA 150 power amp. This amp has now a fault. I can switch it on and the blue LED lights up. The right relais on the softstart board switch on and then nothing. I can not turn off the amplifier by the switch. Only by disconnecting the power cord. The right brown 700 Ohm inrush current resistor warms up, not the left 700 Ohm resistor. I take a look on the caps on the left side directly by the 700 Ohm resistors. It seems they are ok. I resolder all soldering points.The diode 1N5408 above this caps is mesaured and ok. The coil resistance of the relay left is ok and has the same resistance as on the relay on the right side. Directly at the 700 Ohm resistor on the left side are right from them are 4 resistors, white colour. On the resistors are following text: 10W 7.5 Ohm +- 5%. Now my question. They are realy 7Ohm5 or 7,5 Ohm? All 4 resistors are infinity ohms. So they must defect. But I wonder that all 4 defect without optical impact. Between 2 of this resistors is a NTC or PTC and the resistors are pressed with a metal clip at it. Is this a PTC or NTC and what value has it? The part references for the 7.5 ohm(?) resistors are R10, R11, R12, R13. I will replace the resistors but I must definitly know if they are 7Ohm5 (7,5 Ohm) or something else. When I place the wrong values the power amp will be killed. I dont have or find schematics. So is here somebody that can me give a information about my questions? Thanks in advance and sorry for the not perfect english. Have a nice evening.
Best regard and greets,
Wolfgang
my name is Wolfgang and I live in germany. I found this good website and a few people have here Krell amps. We have a Krell KSA 150 power amp. This amp has now a fault. I can switch it on and the blue LED lights up. The right relais on the softstart board switch on and then nothing. I can not turn off the amplifier by the switch. Only by disconnecting the power cord. The right brown 700 Ohm inrush current resistor warms up, not the left 700 Ohm resistor. I take a look on the caps on the left side directly by the 700 Ohm resistors. It seems they are ok. I resolder all soldering points.The diode 1N5408 above this caps is mesaured and ok. The coil resistance of the relay left is ok and has the same resistance as on the relay on the right side. Directly at the 700 Ohm resistor on the left side are right from them are 4 resistors, white colour. On the resistors are following text: 10W 7.5 Ohm +- 5%. Now my question. They are realy 7Ohm5 or 7,5 Ohm? All 4 resistors are infinity ohms. So they must defect. But I wonder that all 4 defect without optical impact. Between 2 of this resistors is a NTC or PTC and the resistors are pressed with a metal clip at it. Is this a PTC or NTC and what value has it? The part references for the 7.5 ohm(?) resistors are R10, R11, R12, R13. I will replace the resistors but I must definitly know if they are 7Ohm5 (7,5 Ohm) or something else. When I place the wrong values the power amp will be killed. I dont have or find schematics. So is here somebody that can me give a information about my questions? Thanks in advance and sorry for the not perfect english. Have a nice evening.
Best regard and greets,
Wolfgang
Those power resistors are 7.5 ohm. They are used in the soft start circuit to reduce inrush current charging the power capacitors when the amp is first turned on. A relay then shorts them out during use. They are all connected in parallel. When one blows they all go. I replaced them in my KSA 250 with higher rated (12W) wirewound types since I also replaced the filter caps with higher capacitance values.
the PTC/NTC with the metal clip could be a temperature sensor to trigger a turn off (safety) circuit if the resistor/s on either side get too hot.
If the 7r5 are in parallel they will give a very low resistance.
That won't do much good in limiting the start up current.
Could they be in series?
If the 7r5 are in parallel they will give a very low resistance.
That won't do much good in limiting the start up current.
Could they be in series?
Hello and good morning,
thank you all for the reponse of my question. This is my needed information for the nest steps. So I can now order the new parts at a electronic parts distributer/shop. Ok, when one resistor goes defect the other three will also go defect. Hopefully this NTC or PTC will work. To do the next steps will cost a few days of time. But then I will inform you about the result and hopefully the amp will live again. Thanks again and nice day.
Best regards,
Wolfgang
thank you all for the reponse of my question. This is my needed information for the nest steps. So I can now order the new parts at a electronic parts distributer/shop. Ok, when one resistor goes defect the other three will also go defect. Hopefully this NTC or PTC will work. To do the next steps will cost a few days of time. But then I will inform you about the result and hopefully the amp will live again. Thanks again and nice day.
Best regards,
Wolfgang
have you identified what these resistors do?
Are they current limiting resistors that limit the start up current during the short period start up duration?
Are they bypassed by a relay after a short delay?
A big transformer, like a 1kVA operating on 220/240Vac needs about 25r to 40r of added resistance to limit the start up current.
I can't see that some paralleled 7r5 can do this duty.
Are they doing something else?
Are they current limiting resistors that limit the start up current during the short period start up duration?
Are they bypassed by a relay after a short delay?
A big transformer, like a 1kVA operating on 220/240Vac needs about 25r to 40r of added resistance to limit the start up current.
I can't see that some paralleled 7r5 can do this duty.
Are they doing something else?
Hello,
I received the spare parts (7,5 Ohm/10 Watt resistors). After replacing and check thes caps, relais and soldering points again I tested the power amplifier. The power amplifier is working! I measured the offset votage of both channels and then connected two old speakers first. All perfect! Thank all peoples for support me!!
The Krell KSA150 works in combination with JBL speaker boxes and Marantz Preamp. I use Krell sources in combination with tube preamp and power amp. The power amp drives JBL and Electro Voice chassis in horn system.
Again many thanks!
Best regards and nice weekend,
Wolfgang
I received the spare parts (7,5 Ohm/10 Watt resistors). After replacing and check thes caps, relais and soldering points again I tested the power amplifier. The power amplifier is working! I measured the offset votage of both channels and then connected two old speakers first. All perfect! Thank all peoples for support me!!
The Krell KSA150 works in combination with JBL speaker boxes and Marantz Preamp. I use Krell sources in combination with tube preamp and power amp. The power amp drives JBL and Electro Voice chassis in horn system.
Again many thanks!
Best regards and nice weekend,
Wolfgang
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