Unusual Clipping on Pioneer SX-750

[lucapatt]

Hey Folks,

I have a peculiar condition that I am looking to get some perspective on regarding a Pioneer SX-750 that I just finished restoring with new caps and transistors. After completing the job, I proceeded to do a power test with an 8-ohm dummy load and scope to look for channel symmetry and max watts per channel. During the test, I noticed that both channels were not tracking as closely as I had hoped and that the left/right channels were experiencing some asymmetrical clipping.

Here is the maximum voltage I can squeak out of both channels before clipping. About 55 watts per channel (although sometimes I am unable to reach this wattage). Notice the deviant left and right channel tracking.


Upon further digging, I decided to isolate the pre-amp from the power amp by unhooking the lead wires and scoping the pre-amp only. Again, I found more interesting behavior. The results are below.

Notice the asymmetrical clipping on the negative cycle of both channels.



I am not sure what to think about these results and if they are contributing to the poor tracking in the power amp.

The volume pot and balance knob have been cleaned thoroughly and track perfectly. I have validated this by scoping the voltages directly before the pre-amp section.

Good overlay of both channels before pre-amp:


I initially thought the ICs on the tone board may be a suspect. I checked all passive components surrounding the ICs as well and they all checked out. Any thoughts here? Am I chasing my tail to solve this issue? or is this kind of performance to be expected and I should just move on and enjoy the amp.

Thanks all!

 

[ecluser]

Hey! What are you thinking by putting 500mV at the input? The sensitivity of the SX-750 is 150mV at the auxiliary input. Consult the level diagram on page 23 of the service manual. You will see that the full output power is reached with only 900mV at the input of the power amplifier stage. Your second picture shows a voltage of 2.35V at the output of the preamplifier. No wonder that you have severe clipping.

You got 55W per channel, both channels driven. This is a 50W per channel receiver. No problem here.

For the gain mismatch, it could be a few things. The first one is bad tracking of the volume control. To rule out this possibility, you must reduce the input and compare the output at different positions of the volume control. It is not rare that old volume controls don't have exactly the same output level, particularly on the lower side of the volume (between 0 an 50%) where there is more wear.

 

[lucapatt]

Hey @ecluser, thanks for the message - you are right. I did not pay close enough attention to my input voltage at the aux line. I am generating this function with my iPhone and I’m not 100% sure what the max output voltage of the headphone jack is. I will turn my volume down and measure until I get 150mv then repeat my testing.

As far as the volume pot, I will also check lower levels for channel symmetry.

 

[lucapatt]

@ecluser After decreasing aux input voltage to 145mv, both channels track quite nicely during a power test. 50 watts per channel picture below.

I will note that this is with the volume pot turned nearly all of the way up.

 

[ecluser]

I will note that this is with the volume pot turned nearly all of the way up.

That's normal. This is the meaning of the sensitivity specification: the input signal needed to give rated output power with the volume control at maximum (highest gain) and tone controls in their middle position (flat response).