Krell KAV500i problems

[Marionks]

Hello,

I have an unfortunately defective Krell KAV500i. All transistors in the output of one channel were broken. I have now replaced it, but it plays totally distorted at low volume.

Strangely enough, I have this distortion on both channels. When I turn up the volume the distortion disappears.
I can't even imagine that now.

Attached are the two circuit diagrams that I found.

 

[BSST]

Have you set the bias current? Did it behave normally?

 

[Marionks]

Where exactly should I measure the quiescent current? If I measure directly at the end transistors (resistor) I don't get anything displayed, which surprised me.
Normally I should at least measure something as soon as I turn the quiescent current pot.

 

[BSST]

You should be able to develop a few mV across each of the 0.33 ohm resistors. What voltage are you able to develop across C3 (i.e. collector to emitter at Q17) ? Be very careful to avoid massive over-current. R22 set for minimum corresponds corresponds minimum spreader voltage.

 

[Marionks]

on the 0.33 ohm i measure 0mV over the C3 i get an Voltage about 2,7V i can change it with R22 in the mV area

 

[BSST]

I assume that Q17 is mounted on the same heatsink as the output power transistors so that Q17 can sense the temperature of the transistors and adjust the spreader voltage.

R21, R22, and R58 (?) controll the spreader gain. The spreader voltage is about Vbe*[1+(R22+R58)/R21]. R22 of course can vary from 0 to 200 ohm. So gain from 1+562/90.9 minimum, and 1+762/90.9 maximum; so spreader voltage adjustable from Vbe*7.18 to Vbe*9.38. Taking Vbe=0.6V, voltage would be adjustable from 4.31V to 5.63V.

The output stage is a cascade of 4 NPN emitter followers for positive swing and 4 PNP emitter followers for negative, or a total of 8*Vbe, roughly 4.8V. So the spreader design looks reasonable.

But you observe only 2.7V. I believe the spreader is misbehaving or there is a shorted output transistor(s). You should probably use a Dim Bulb Tester to minimize risk to the output stages.

Good luck!

 

[Marionks]

The crazy thing is that I have this problem in both power amplifiers. That can't be right, can it?

 

[BSST]

What is history of this amp? Did you acquire in broken condition? Has it been worked on before?

Try to find why bias spreader isn’t woking. Why doesn’t spreader voltage rise with changing pot rotation? Look at base emitter drops across all 8 emitter-follower chain.

Good luck.

 

[Marionks]

I don't know the history of the amp. I aquire it in this condition.

The red marked parts are replaced. The blue ones are OK.

Spreader Voltage rise between ~2,3V to ~2,8V when I turn on the R22.

 

[BSST]

I suggest disconnecting the output stages from the front end. This will allow you to troubleshoot the bias spreader and front end independently from any problems in the output stages.

Isolate the output section by disconnecting the wires at “Drive(+) P1 PAD”, “Drive(-) P2 PAD”, and “Out P3 PAD”. Let the output section float. From the “Out P3 PAD” on the PCB, run a jumper to the junction of R9 and R10. This connects feedback and when you apply power, the “Out P3 PAD” should bias to about 0V. You should be able to exercise the bias pot and generate the range of spreader voltages estimated earlier. Or troubleshoot if necessary.

If this proceeds as hoped, we can then do some checks of the output section. When all seems correct, we’ll reconnect everything.

 

[Marionks]

Now i found it. Someone hase changed the R21 to 180ohms and the R58 to 480ohm. i replaced the parts an now i have the voltage about 4.31V to 5.63V
In this case i can adjust the idle current.
On the second channel the parts also replaced..... Very strange

 

[Marionks]

What is the official value for the quiescent current?

10mV or lower?

 

[BSST]

I wasn't able to find anything specific to the KAV500i. I saw some posts that seem to call for 12mV across each emitter resistor, but couldn't find a definitive link to your amp.

I found this link for a different model:

https://www.diyaudio.com/community/threads/krell-kav-300i-information.313933/

Perhaps other members will offer opinions.

 

[levinson mark]

Hello,

I have an unfortunately defective Krell KAV500i. All transistors in the output of one channel were broken. I have now replaced it, but it plays totally distorted at low volume.

Strangely enough, I have this distortion on both channels. When I turn up the volume the distortion disappears.
I can't even imagine that now.

Attached are the two circuit diagrams that I found.

Do you have the circuit diagrams of Krell KAV-250p ?

 

[Marionks]

Sorry, I don't have this circuit diagram for this preamp.

What is the problem about it?