onkyo a-8650

[spdf]

Hello guys! I am new here and after a little bit of reading I got impression what a great and experienced community you have here. So I decided to post here my question about repairing amplifiers as well.

Recently I've got faulty Onkyo Integra A-8650 as my learning repair project. I found that one channels output transistors were blown so after a lot of measuring I found all (as I expect) malfunctioning parts and replaced them.

Amplifier is working again, but I have concerns about output transistors getting really hot on one channel different from that I worked on (not sure how hot is bad but after 5 mins I can't hold finger on them). I measured dc offset
and I've got 1.5mV and 2.5mV than I measured dc voltages on output transistors:

on hot channel Q609 emitter 0.09V and Q611 on collector -51.0V
on channel that I worked on Q610 emitter 0.018V and Q612 -50.8V

Things I've changed on this hotter channel is R902 I have placed 12 ohm 2W metal oxide resistor instead of 13 ohm 1W wirewound. And D507 to 3.9V 0.5W Zener. All caps in amp where measured with esr meter and bad caps were replaced with the same new ones.

So the main question is where to look for a clue and how worried should I be?
Any help would be really appreciated!

Cheers.

P.S link for the schematics:
https://drive.google.com/open?id=0B8KUNmX2TYSUOU1VbkdGRUs2S1U

 

[petehall347]

any special reason you replaced a regular diode with a zener ?
change r901 to match r902

 

[spdf]

I couldn't find tlr112 locally so I was adviced that I could replace it by 3.9V zener

 

[roger2]

Have you checked or adjusted bias?

It would also be good to check temperatures on the heatsinks close to where the output transistors are mounted, to see how well heat is being transferred to the heatsink and away from the transistors.

 

[spdf]

Nor check nor ajusted.

Could you explain a bit more what do you mean by measuring bias in amplifier?

 

[roger2]

Bias is a constant DC current that flows through the output transistors. There should be information about adjusting bias in the Service Manual for your amp. In most amps there is a trim pot in each channel for adjusting bias, and a DMM (set to measure mV, not amps) is usually connected across an emitter resistor. Each amp will have its own recommended bias setting and sometimes special test points or procedures. If you don't have the Service Manual you need to get it. I tried HFE and they only have your amp's SM in German. But you can search for it via google, it may be available as a free download elsewhere. If not, get a hard copy SM http://www.stereomanuals.com/index.htm

When bias current is too low there is crossover distortion, when bias is too high excess heat is generated in the output transistors. Again, google can help you find a more complete explanation of what bias current is.

The second point in my previous post is worth looking into as well. If heat is not transferring properly from transistor to heatsink, the transistor can overheat. This heat transfer can be affected by amount and condition of thermal compound used in the junction between transistor-mica insulator-heatsink. As you have recently installed some new output transistors it would be a good idea to investigate (and compare) the efficiency of heat transfer taking place in both old and new outputs.

 

[spdf]

Thank you roger2 the bias was imbalanced ant that caused output transistors to heat. Adjusted that and everything is perfect now!

Still have one issue that I would like to solve. On phono input I hear hum and If I place a finger on Q101/Q102 the humming suddenly increases. And if try to move cartridge knob while on phono input it feels like explosion sound coming from speakers.

Is there any chance that new D507 3.9V zener instead of tlr112 led couse transistors Q581 and Q583 run hotter?

 

[roger2]

Most of that is beyond my knowledge level spdf. I can only suggest looking for bad solder joints, particularly anything to do with ground. Might be the cause of the hum in the phono input (when you are not touching Q101/Q102).

Hopefully someone else will respond. You could go back to your other thread to ask about D507