gtrplayer76
Active Member
Thank you . I will do that. I noticed the difference in voltage when I moved it from 110 to 117.
Pioneer SX-990 receiver problems/questions
- Thread starter djmtascam
- Start date
Thank you . I will do that. I noticed the difference in voltage when I moved it from 110 to 117.
djmtascam
Active Member
I didn't replace those 2 off board caps....only the two on the amp board which were 1000uf 35v. I believe they were (2) 3300uf caps mounted on the chassis?
Does your unit have those?
gtrplayer76
Active Member
I do have those ,and they are the same value as the ones on the board. 35 v 1000uf.
djmtascam
Active Member
Sorry you're right they were 1000uf/35V. I looked at my spreadsheet of parts I ordered and I only replaced the (2) 1000uf/35V that were on the amp board and my photos show the same black cans in the chassis mounts after I put it back together..so I'm pretty sure I didn't replace those.
djmtascam
Active Member
FWIW...this receiver was my friends Xmas gift to his brother and there was a budget as to how much I was spending on replacements to get it working.
The main problem was the amp board and that tone cap so that's what I ended up repairing...
If this had been mine to restore and keep, I would have replaced all the ecaps with new as well as rebuilding the amp board with new parts. It turned out replacing the output transistors and rebuilding the amp board and that tone cap was all that was needed.
So...YMMV as to how far you want to go with your's!
The main problem was the amp board and that tone cap so that's what I ended up repairing...
If this had been mine to restore and keep, I would have replaced all the ecaps with new as well as rebuilding the amp board with new parts. It turned out replacing the output transistors and rebuilding the amp board and that tone cap was all that was needed.
So...YMMV as to how far you want to go with your's!
gtrplayer76
Active Member
Which tone cap was bad ? This amp belongs to a friend who asked me to try to get the distortion cleaned up on the one channel. I don't usually work on these ss units I'm more into tube guitar amps 
djmtascam
Active Member
Which tone cap was bad ? This amp belongs to a friend who asked me to try to get the distortion cleaned up on the one channel. I don't usually work on these ss units I'm more into tube guitar amps
There is a small circuit board that the two tone controls are mounted on (bass & treble) at the front panel and it was one of the green mylar caps on there. Not sure now which one but I found it by running signal through that control board and removing caps and testing each one. All of those could easily be replaced with better polys. They're all in the signal path and cheap so why not...I also cleaned the controls with Deoxit....
gtrplayer76
Active Member
Ok I found the board I'm just not experienced enough on these to figure where/how to run a signal through it and where to test. Guess I'll wing it. Bound to figure something out eventually.
djmtascam
Active Member
I found it by injecting a function generator (or really any signal source) into the Aux input. I had those pre/amp jumpers removed. Switched the source switch to Aux, put my scope on the signal path following the schematic (starting at the Aux jack) to different points all along the way, when I got to that tone control board and turned the controls....found the one channel was messed up and didn't look right. So I pulled it out, tested the resistors and caps and found the bad cap. Stuck a cap I had laying around in there for a test and the signal cleared up fine. I had to buy a few of the right value and replaced it!
It's funny too because it's probably the simplest circuit in there only caps and resistors and was the most difficult to get to. I didn't even notice that board until I looked at closely and realized there were components down in there....LOL
But pretty easy to just "wing it" like you said and replace them all...super cheap, resistors never seem to go bad unless they get hit with something pretty bad and then you see them discolored or burnt...
That's one thing about components "in the signal path". If you do a signal path check like that you can locate crappy signal quality pretty quickly as you move along the circuit path using the schematic...then it's just a matter of finding the specific component....
gtrplayer76
Active Member
Thanks ! I'll give it a shot. What type signal did you inject ?I found it by injecting a function generator (or really any signal source) into the Aux input. I had those pre/amp jumpers removed. Switched the source switch to Aux, put my scope on the signal path following the schematic (starting at the Aux jack) to different points all along the way, when I got to that tone control board and turned the controls....found the one channel was messed up and didn't look right. So I pulled it out, tested the resistors and caps and found the bad cap. Stuck a cap I had laying around in there for a test and the signal cleared up fine. I had to buy a few of the right value and replaced it!
It's funny too because it's probably the simplest circuit in there only caps and resistors and was the most difficult to get to. I didn't even notice that board until I looked at closely and realized there were components down in there....LOL
But pretty easy to just "wing it" like you said and replace them all...super cheap, resistors never seem to go bad unless they get hit with something pretty bad and then you see them discolored or burnt...
That's one thing about components "in the signal path". If you do a signal path check like that you can locate crappy signal quality pretty quickly as you move along the circuit path using the schematic...then it's just a matter of finding the specific component....
djmtascam
Active Member
I just use a sinewave from my function generator at various frequencies into both L/R input channels and then check each one at various spots.
Do you have the service manual and schematics?
gtrplayer76
Active Member
I just use a sinewave from my function generator at various frequencies into both L/R input channels and then check each one at various spots.
Do you have the service manual and schematics?
Yes I do have both. Do you think it hurt anything that I used 2 watt .47 ohm resistors instead of .50 ohm ? It's less than 10% difference.
djmtascam
Active Member
Not real sure, maybe someone more knowledgeable will chirp in with a solid yea or nay. I would think that would be close enough but then I don't know how that may have changed the original design and how much wiggle room you have with those. I used 1% 0.5 ohm 2W.
I do know those emitter resistors set the crossover distortion of the output transistors and you'll need to check and readjust the bias/balance.
You could look at the output waveform with a scope to see if there's any crossover distortion....
Here's what I used:
http://www.mouser.com/ProductDetail/Ohmite/WHCR50FET/?qs=xbi1O6BJQnhnvWBV%2FoQO3A%3D%3D
Did you fire it up and have problems?
Did you replace both emitter resistors?
Not exactly sure what all you replaced.
gtrplayer76
Active Member
I replaced the four 2watt resistors on the new output board and the one damaged 150ohm resistor also be replaced the power transistors on the side that was distorted. I have no bias since I can replaced the resistors. I have ordered caps ,varistors ,and transistors to rebuild the board and the PS board.
djmtascam
Active Member
OK, well unless someone chirps in on those resistors, I would just proceed with your rebuild and see how it goes.
You will need to do the bias adjustment after everything is put back together and working. In that other thread is a bias adjustment info sheet that explains the how to...
gtrplayer76
Active Member
I've done the bias adjustment procedure before I changed the resistors. After the resistors everything changed for some reason. I checked them with a meter just to be sure they weren't packaged incorrectly. I changed the voltage selector and it got the voltage back down to a reasonable number.
djmtascam
Active Member
So now it's not working with everything replaced?
I don't understand?
You have to do the bias adjustments after replacing those components not before???
Then after it has run with speakers connected for awhile (like 10-15 mins)
After it starts to heat up, you need to check the voltages again to make sure the current throught he transistors hasn't drifted. If those thermal diode sensors mounted on the heatsink have failed, they won't compensate and the transistors and emitter resistors will go into "thermal runaway" and burn up.
That AC input voltage selector has nothing to do with the output transistor biasing adjustments. It's only affecting the DC power supply rail voltages in the receiver.
Your supply voltage may have changed slightly because of that but it should still be around +56V and -56V DC at the amp board B+ test points.
Did you follow the adjustment procedure correctly?
Or maybe you have failing diode sensors....???
gtrplayer76
Active Member
Ok let me step back a bit. I have not rebuilt the board yet. I only replaced the four .5 ohm 2 watt resistors. Before I replaced the resistors it was working except for the distortion in one channel. I went through the bias adjustment procedure then ,and all the voltages looked good. AFTER I replaced the resistors I had very little current through the fuses ,and it would not adjust ,and the voltage on the B+ was high at 76 volts. I changed the voltage on the back of the receiver to 130 ,and that brought it back down to 62 volts.
djmtascam
Active Member
Did you check the voltages on each channel. Sometimes you can find a bad component comparing readings from each channel. Good one to bad one.
You should test the driver transistors as well. Whatever caused that one resistor to burn originally probably took the drivers with it.
I assume you at least replaced the caps?
I attached a sheet someone here gave me that shows the different voltages and currents you should expect, maybe that will help find the problem....
You should test the driver transistors as well. Whatever caused that one resistor to burn originally probably took the drivers with it.
I assume you at least replaced the caps?
I attached a sheet someone here gave me that shows the different voltages and currents you should expect, maybe that will help find the problem....
gtrplayer76
Active Member
Nothing has been replaced except two output transistors and the resistors. I have the caps ordered. I'll get the voltages checked.