DIY Audio Team,
I was recently given a Krell FPB 300 amplifier that has a hum/distortion in the left channel. The right channel appears fine and has a DC offset of ~0.7mV which is very good for a solid state amp. The left channel DC offset is ~150mV and likely indicates the source of the problem. I would like to restore the amp but, need to first repair the problem in the left channel and get the amp functioning correctly.
Does anyone have the service manual for this amp and/or a complete schematic set?
Any help is appreciated, thank you - ALBQ
I was recently given a Krell FPB 300 amplifier that has a hum/distortion in the left channel. The right channel appears fine and has a DC offset of ~0.7mV which is very good for a solid state amp. The left channel DC offset is ~150mV and likely indicates the source of the problem. I would like to restore the amp but, need to first repair the problem in the left channel and get the amp functioning correctly.
Does anyone have the service manual for this amp and/or a complete schematic set?
Any help is appreciated, thank you - ALBQ
Ok,
Repair Time. Initial observations and measurements:
1) +/- 85V HV supply (F1/F2) -> 86.7VDC / - 86.7VDC
2) +/- 75V Reg Supply (TP4/TP5) -> 76.3VDC / - 76.5VDC
3) +/- 35V Reg Supply (TP7/TP) -> 36.5VDC / - 36.8VDC
4) +/- 18V Reg Supply (TP8/TP9) -> 17.8VDC/ - 17.8VDC
6) +5 V Reg Supply (TP10) -> 4.96VDC
7) Line Freq Monitor (TP11) -> 3.02VDC
Conclusion - supply is good
Next:
Noticed that when main breaker is off for a while and first power on the amp that the problematic left channel plays cleanly and sounds matched to the right channel for ~10sec before losing volume and becoming garbled.
Hanging my scope probes on the Non-Inv Bias/Inv Bias test points on the CPU card showed that when the amp is first powered on, I get +5VDC on each test point. After about 10sec, the value drops to ~0.3VDC and the channel drops out/becomes garbled.
Doing the same test on the working channel, voltage remains constant at 0.5VDC
Since I am not very familiar with repair of servo based amplifiers, I could use some advice on where to check next. How is bias sensed/adjusted in this amp?
Repair Time. Initial observations and measurements:
1) +/- 85V HV supply (F1/F2) -> 86.7VDC / - 86.7VDC
2) +/- 75V Reg Supply (TP4/TP5) -> 76.3VDC / - 76.5VDC
3) +/- 35V Reg Supply (TP7/TP) -> 36.5VDC / - 36.8VDC
4) +/- 18V Reg Supply (TP8/TP9) -> 17.8VDC/ - 17.8VDC
6) +5 V Reg Supply (TP10) -> 4.96VDC
7) Line Freq Monitor (TP11) -> 3.02VDC
Conclusion - supply is good
Next:
Noticed that when main breaker is off for a while and first power on the amp that the problematic left channel plays cleanly and sounds matched to the right channel for ~10sec before losing volume and becoming garbled.
Hanging my scope probes on the Non-Inv Bias/Inv Bias test points on the CPU card showed that when the amp is first powered on, I get +5VDC on each test point. After about 10sec, the value drops to ~0.3VDC and the channel drops out/becomes garbled.
Doing the same test on the working channel, voltage remains constant at 0.5VDC
Since I am not very familiar with repair of servo based amplifiers, I could use some advice on where to check next. How is bias sensed/adjusted in this amp?
Hello! Have you solved the problem with your amplifier? I have krell ksa-50s, and I also have problems with the system for regulating the quiescent current collected on the microcontroller. My amplifier always works at level 0, and does not go up to 1-2 bias level. at startup, these lamps light up, but within 20 seconds they go out alternately, and then, as stated in the instructions, depending on the signal level, they no longer light up and the bias does not rise. The amplifier plays well, but as I understand it, this is not its whole sounding character, because bias is nominal ..