Low and distorted output after amplifier heats up

[wabun]

my Krell KSA150 sings well when switch on, but after play for 3 songs, the left channel output become lower and lower with some significant distortion but when I switch it off, let it cool down, and turn on again, it sings well for 3 songs then left channel output become lower again, does anyone know why this can happen ? Sorry for my poor English, I am new to this Blog..

[Jan Dupont]

This can be caused by a bad soldering. When the component heats up the connection gets bad......
Do you have any skills for do such fault finding ?

[nigelwright7557]

Do an inspection of the soldered joints and look for a bad one.

Other than that could be a bad wire connection.

[smartx21]

Consider also searching for schematics and measure voltages along the circuit. An almost dead transistor, a blown resistor...
If cannot have schematics, try to search for lacks within the circuit, comparing to the functional channel. But be careful to manage the probes while power is on.
With power off, you can compare measures from selected parts, starting from power supply (if separate), and output stages.
Once my home amp had the contacts of electrolytics totally oxidated.
Good luck!
Regards,
Max.

[wabun]

thanks for the suggestion, I have some repaired simple amps like NAD3020 or Sansui, but not for Krell, 1st time handle complicated amp. I do inspect the solder joint with magnifying glass, could't foud any cold joint. Is there any way to identify weak transistor ?
Could it be the driver that provide thermal feedback faulty ? Krell amp uses very less jumper wires. schematics also cannot be found, headache..

[Nico Ras]

Krell is a relatively simple design and the reason for its good performance on a whole. I would be inclined to think that one channel is thermally running away and that the distortion is caused by break-down.

Have a look at the voltage developed across the output transistor emitter resistor and see if it continues to rise (no music) if it is then there could be a problem with the bias pot wiper or a bad thermal connection with the Vbe multiplier.

Nico

[Nico Ras]

A further problem might occur in that the pot in the offset adjust could be faulty, since Dan Agustino feeds DC into the feedback amp to adjust off-set and also places two small caps across this pot. If one of these caps are leaking then you would also see the output offset change with time and if allowed to rise to the one half cycle of the music clipping due to off-set drift could sound like your problem.

Nico