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Pioneer SX990 ?

Hi folks ! I'm new here ,and I have a question. I have a Pioneer SX990 that the right channel is distorting. Volume seems good it's just distorted. I found the OM/repair manual ,but there's really nothing in it about testing. I can see that someone has been in this before because they added RCA jacks to the A speakers. They also changed one of the 2SC 793 transistors on the power board to a TR 26.
Only thing I've done so far is poke around to see if I hit anything that changes the sound. Nothing yet. I also checked some voltages on these four transistors Q9 ,Q10 ,Q11, and Q12. Base ,and Emitter to ground. 11 ,and 12 showed very little to no voltage. Q9 showed 54volts on one leg ,and 46 on the other. Q10 voltage was fairly even on both legs at around 36-38.
The manual shows the board layout ,and some test points, but doesn't show what to look for exactly. Does anyone know where I can find the values of the test points ?

Thank you !



Re: Pioneer SX990 ?

That age of equipment needs fresh electrolytic capacitors first ! Just then and only you can hope it can works like new one. And secondly, those fellas who change something inside as you´d told never can service electronics right way. Most of the problems with 30 years or older electronic equipment came from bad electrolytic capacitors ! Thats a fact ! It does not cost fortune for change all those for fresh ones. Just a few $ or £ or €. You can also use NOS caps if you can reform them first - but thats a different story. Use high quality fresh ones. You dont need those supercaps which cost fortune. Take a time !

Re: Pioneer SX990 ?

Since you have one good channel you should be comparing to that, with no signal and volume at zero.
If someone has been in there modifying the amp they could have done just about anything; I suggest getting it back to factory first. Check for any soldering 'errors'. The original type transistors are always desirable.

Re: Pioneer SX990 ?

Thanks Johnnysan. I have new Toshiba transistors on the way. That's the first step. Then I will proceed from there.

Re: Pioneer SX990 ?

I agree with Johnnysan. Always get the unit back to original condition if possible. Once you get it to original, you can then start to troubleshoot. Distortion can come from bad (open or short) transistors, open resistors, bad diodes (open , short) on the driver/output board. Distortion can at times come from dirty controls and switches. Distortion usually doesn't come from bad caps. I would hold off replacing caps, until you fix the distortion problem.