Power amplifier repair question.

The mono block powers on with no issues. The light bulb is off when connected to the dim light bulb tester. All fuses good, power supply voltage present +68 V and -68 V. However, the amp outputs no sound. All output MOSFETS have been replaced. I have found earlier three were bad. With speaker connected (amp powered off), I can hear the click in the speaker when power supply terminal blocks being attached to the main board.
I do not own an audio generator or oscilloscope, just DMM. I guess, the first question if would be possible the troubleshoot the unit without the measuring equipment.
What transistors needs to be checked in first place in this case?

Thank you in advance.

 

Is the Mute function working, check Q10, Q11, Q13 for function and voltages.

What is the voltage at the junctions of RP1 to RP10 it should be zero volts compared to ground, any higher and Q2 will be turned on which is an automatic safety mute control for speaker protection.

 

The sound got restored after replacing the Q10 and Q11. Both failed. I can hear the output to the speaker, it's not clean rather sound choppy.
However, now the rail voltage drops to -48/+48 V with main board connected. Also, -38 V detected on each point from RP1 to RP10. The dim bulb now is now glowing when connected in series with the amplifier. I guess, the short is present on the main PCB. Very strange with all new outputs MOSFETS installed earlier.

 

Qp1 to Qp6 Mosfets must be hard on or one has blown.

 

Were the output MOSFETs replaced with the exact same parts? Biasing of these things is quite a bit different than the biasing of BJTs - the cut-off voltages vary all over the place.

If you did replace with the same type, then try this:
First note the position (or measure the value) of P2 so you can return it to its original position if this doesn't help. Now set it to its minimum resistance setting. Power up and see if your rails came back up.

If they did, you can monitor the voltage across one of the source resistors and slowly increase the resistance of P2 until you see a few mV across the source resistors. Now measure the voltage on the speaker output to ground and confirm it is very near zero. Connect speakers and give it a try. USE CAUTION - I DO NOT SEE ANY SPEAKER PROTECTION IN THIS UNIT. A bad output device can put the entire rail voltage across a speaker.

If you replaced the MOSFETs with something other than original part numbers, all bets are off.

 

The MOSFETs were replaced with matched pairs of ECX10N20-S and ECX10P20-S. It is not really possible to find these days the 2SK1058 and 2SJ162 of the same series. I will try to go with the suggested steps.

 

Looks like a good choice of substitutes. But bias will still need to be readjusted for the replacement parts.

 

Continuing to post updates on the started repair of B&K Components Mono Block 200.1. Unfortunately, I did not have a chance really to focus on this over the past few months.

-Rail voltage was restored back to specs (-70V/+70V) after examining the both sides of amp PCB under a very good light. I've located a small piece of wire (access cut off) that just created a short of PCB tracks in once place.
-The Q10 and Q11 were fried again, which have been replaced now for the second time.
-After replacement, checked the other transistors again. All were in working order.
-The amplifier's sound got restored.

Now with the sound restored, I wanted to focus of the two final adjustments "DC offset" and "Setting the bias".

I was able to use the my digital multi-meter and P1 pot to set the amplifier's DC offset to 0.5 mV. The initial value was around 286 mV. Look's that was an easy part. Not sure how to proceed with bias adjustment, as I have not done it yet.

I will try measure the voltage on RP1-RP5 resistors and use the P2 pot to make an adjustment.

For example: Desired current value ~ 50mA
I x R = V
I x RP1 = V
0.05A x 0.47Ohm = 0.0235V

Please let know if these are correct steps. Do I need to take into consideration number of output MOSFETs and RP resistors for this calculation?
Thanks.