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Pioneer SX-980: popping noise on startup yesterday and now lights up but no sound

JoeDotCom

JoeDotCom

Super Member
I had some company over the other day and was hoping to fire up my Pioneer SX-980 for some tunes... I flip the power switch and i hear two pop/snap sounds, and then no sound at all. the unit lights up OK, but no sound comes out of the speakers, and when I flip the power off then back on again, I do not hear the protection circuit click. I am thinking it is related to the protection circuit because when i heard the pop/snaps I had flipped the switch... waited 2 seconds... first snap/pop 2 more seconds... 2nd pop/snap.

(EDIT: it was one single sound that happened twice. I am calling it a pop/snap because it did not really sound like just a pop or just a snap. it was a sharp popping sound... a 'snop'? haha.)

I checked all the fuses in the power supply area and they are fine. Looking at the manual, I think I found the protection circuit on the underside of the receiver. I set the DC Offest on this guy a week or two ago (Thanks AK! http://audiokarma.org/forums/showthread.php?t=383703)

Looking at the manual, it mentions some voltages that should be present on certain pins in the protection circuit. I just wanted to check in with the experts here before i start poking around in a live circuit. I may be in my mid 30's, but I am still a big fan of "adult' supervision. :)

I just realized that I can/should check the DC offset again. If i check the pins in the amp circuit, i should see voltage right? the protection circuit just isolates the speaker output? I will do that when i get home from work tonight.

Does anyone have any tips for troubleshooting the protection circuit? Am I headed in the right direction?

My baby...
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the power circuit with fuses. The 5 glass fuses checked out OK.
2011-08-18_20-29-53_334.jpg


The underside
2011-08-18_20-29-27_407.jpg


What I think is the protection circuit
2011-08-18_20-29-40_754.jpg


a slice of the Manual...
Picture%20162.png
 
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40V! yikes!

Holy Jumpin! I just checked the DC offset (between pin 10 and ground as the service manual instructs) and there is between 40 and 50 volts there! Not 40mV.... 40V! Of course turning the pot used to set the offset did not make much (if any) difference. I turned the unit off pretty quick after seeing those numbers from fear of damaging it more. Any ideas where to look from here?

I did notice that the largest of the caps on the power supply circuit board is swollen. I do not know if that is new or not. I am planning to recap the powersupply this winter... so my hope is that I can get this problem resolved so I am not trying to use a full recap as part of the troubleshooting process.
:no:

Help!!?? (Please!)

Picture%20163.png
 
The voltage you measured looks like the power supply rail voltage. You might want to pull and measure your output transistors.
 
Those two smaller caps beneath and to the left of the one you circled looks like they've seen better days too. :scratch2:
 
Regarding the large cap with the plastic top, they frequently look like that. The plastic gets warped, it's not necessarily a bad cap.
 
I had lots of fun learning in the past days learning about output transistors from other posts here on AK. (superthankful for all the info on this board!)

Luckily it is simple to take the output transistors out of circuit in the 980. There is a plastic plug for each transistor where it connects to the circuit board. I carefully disconnected each one, then measured from the bottom of the receiver using my meter in diode mode.

The service manual had a nice picture showing which pin is Collector, Emitter and Base.
Picture%20164.png


Here are my results... How do they look?

(my meter shows "1" for no connection)

NEC D746 S86 far from PS, close to front of unit
black lead to base, red lead to collector, result =1
black lead to base, red lead to emitter, result =1
red lead to base, black lead to collector, result =611
red lead to base, black lead to emitter, result =615
black lead to emitter, red lead to collector, result =1
red lead to emitter, black lead to collector, result =1



NEC B706 S79 far from PS, close to back of unit
black lead to base, red lead to collector, result = 580
black lead to base, red lead to emitter, result =590
red lead to base, black lead to collector, result = 1
red lead to base, black lead to emitter, result =1
black lead to emitter, red lead to collector, result =1
red lead to emitter, black lead to collector, result =1

Other side
NEC D746 S86 near PS, close to back of unit
black lead to base, red lead to collector, result =1
black lead to base, red lead to emitter, result =1
red lead to base, black lead to collector, result =607
red lead to base, black lead to emitter, result = 611
black lead to emitter, red lead to collector, result =1
red lead to emitter, black lead to collector, result =1


NEC B706 S79 near PS, close to front of unit
black lead to base, red lead to collector, result =586
black lead to base, red lead to emitter, result =594
red lead to base, black lead to collector, result =1
red lead to base, black lead to emitter, result =1
black lead to emitter, red lead to collector, result =1
red lead to emitter, black lead to collector, result =1



2011-08-21_21-09-20_648.jpg


2011-08-21_23-32-30_47.jpg
 
Well, reconnected the output transformers and still no dice. I have gone over all the circuit boards looking for any sign of charring, etc and everything seems fine. humm.
 
I sat and read the manual and looked over circuit boards for an hour or so today. I am not sure where to go from here. Can anyone give me a little nudge in the right direction.. please?
 
Did you use a correct chassis ground, like the frame of the receiver?
Your transistor test looks OK. Not sure where that 40v is coming from.
 
ryuuoh said:
Did you use a correct chassis ground, like the frame of the receiver?
Your transistor test looks OK. Not sure where that 40v is coming from.
Click to expand...

Thanks for the reply ryuuoh. :)

yes, when I check the dc offset I have my meter between pin 10 and chassis ground. (pics of my probe location s are in my DC offset thread that I posted a few weeks ago.)

This is frustrating...all the components look ok. But that 40-50 volts is still there.

Now that I know better, I feel a little dirty having to listen to music on my 5.1 receiver.

:nono:


Sent using a mobile device.
 
Need to pull and check driver transistors. Output transistors look OK.
 
1. remove output transistor connectors
2. remove any speaker loads from amp
3. power up amps, measure dc offset on both channels. amps are in dc balance (have correct feedback) without o/p transistors in place. TRY to zero using vr2, if no effect, return vr3 to original setting (measure it's ohms in circuit).

if not balanced (protection does NOT connect amps and speakers) then:

check driver transistors and their emitter resistors.

also check that the stv-4h diode leads are unbroken.

post q3,q4,q5,q6,q7 voltages... (ground referenced)

just a note, this is better served in the Pioneer Audio forum. Also mtf (me) is local to you...
I'm usually kept too busy to go round up strays in other forums.
 
Last edited:
avionic said:
Need to pull and check driver transistors. Output transistors look OK.
Click to expand...
Thank you for the push. I will be checking those (after i research how they work. :) ).

markthefixer said:
1. remove output transistor connectors
2. remove any speaker loads from amp
3. power up amps, measure dc offset on both channels. amps are in dc balance (have correct feedback) without o/p transistors in place. TRY to zero using vr2, if no effect, return vr3 to original setting (measure it's ohms in circuit).

if not balanced (protection does NOT connect amps and speakers) then:

check driver transistors and their emitter resistors.

also check that the stv-4h diode leads are unbroken.

post q3,q4,q5,q6,q7 voltages... (ground referenced)

just a note, this is better served in the Pioneer Audio forum. Also mtf (me) is local to you...
I'm usually kept too busy to go round up strays in other forums.
Click to expand...

Awesome thanks MTF! That is a great list for me to get started. There is so much to learn it is hard to know where to start, so i appreciate your list very much.

Should I abandon this thread and make a new one in the Pioneer forum?

re: Local... I saw you are in Bensenville. I was just in B-Vill this weekend for a christening (near york and grand) and I used to work at the Comcast TV studio that is there in Northern Elmhurst. It is great to know there are other AKers in the area. Any chance you are attending Central Fest this weekend in Barrington? link:Central Fest - Chicago area - Sat. August 27th

Thanks again everyone! I have lots of fun work to do now.
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