I recently picked up an SX-980 and was going through the service manual to set the center voltage and idle current. The right channel is unable to get below 680mv even with the trimmer all the way around. I turned it back and forth a bit thinking it might be dirty but that doesn’t seem to be the case. The jumper wire has already been cut. The left channel adjusted just fine. I’d like to take a shot at fixing this myself. I’ve been wanting to get my feet wet with electronics repair and do more than use DeOxit. I have all the basic tools and an oscilloscope. Can someone point me in the right direction as to what to check first? I realize I’m a newbie at this and appreciate any assistance as I try to learn.
-
Feeling lucky? Check out the AK 24th Anniversary Giveaway thread and join in the fun!
Pioneer SX-980 - Unable to Set Center Voltage
- Thread starter Dlathem
- Start date
Register to hide this ad
You can also pull the jumpers and recheck the DC Balance. The Flat amp has some troublesome transistors that can influence the Balance.
Also, Amp board Resistor (Edit) R21 is part of the feedback, which could be checked for value (5.6K).
Also, Amp board Resistor (Edit) R21 is part of the feedback, which could be checked for value (5.6K).
Last edited:
Freeze spray: "Cold in a Can" Many brands and prices are all over the place: Amazon.com: Max Pro 1234 Max Pro® 1234ze Freeze Spray 10 oz. : Industrial & Scientific
A usually less costly and easier to find substitute is "Air in a Can" spray duster, keyboard cleaner etc. Hold the can upside down and it gets plenty cold.
You can find it in office stores, home stores, lots of places. An example: https://a.co/d/09nnoott
A usually less costly and easier to find substitute is "Air in a Can" spray duster, keyboard cleaner etc. Hold the can upside down and it gets plenty cold.
You can find it in office stores, home stores, lots of places. An example: https://a.co/d/09nnoott
Looking at the schematic (attached) it shows that the input stage is compensated for DC offset ("center"). Because the bases of the left and right transistors of Q1 look into a different impedance Q2 compensates this by stealing away base current from the left Q1. If Q1 is bad (freezer spray will confirm) this won't work properly. If Q2 is bad VR2 won't work at all to make any difference in your offset voltage.
R26 is the base resistor for Q102 and has nothing to do with the feedback. You mean R21? If that one is bad the amp won't work at all.
R26 is the base resistor for Q102 and has nothing to do with the feedback. You mean R21? If that one is bad the amp won't work at all.
How interesting. Thanks! I’ll try that.
For either item, a little goes a long way. If the can has a straw use it to direct the flow right on the suspect part. Usually a very short squirt, 1-2 seconds max is all it takes.
So if I'm monitoring the DC voltage, hit the Q1 transistor with the freeze can and my voltage changes drastically then Q1 is bad? I can't check that in circuit with a meter correct?
Yes R21. Edit made to post #3.
Disconnecting the feedback can help in determining offset issues. Checking for value and connection issues is a quick and easy process of illumination.
I have to disagree. Disconnecting the feedback will make the amp rail to one of the power rails. I would start with disconnecting Q2 to see if the offset changes. Is VR2 at minimum? That would suggest Q2 being bad.
Or let TS try the freezer spray on Q1. But usually devices in these positions fail by exhibiting intermittent behavior, this offset fault seems persistent. Either Q2 steals too little base current from Q1 or too much. My bet's on Q2.
Last edited:
The AI generated answer from Google is patently stupid. "By removing this feedback, the amplifier operates in "open-loop" mode, allowing you to measure the raw offset voltage multiplied by the very high open-loop gain." Which will cause this amp to rail. The other 10 links were not relevant.
Pioneer's troubleshooting guide certainly is an interesting read but does not cover this particular case, namely the function of Q2. On which my bet still is.
Pioneer's troubleshooting guide certainly is an interesting read but does not cover this particular case, namely the function of Q2. On which my bet still is.
Correct. Also check the stuff with Q2. Be aware that results with freezer spray can be inconclusive.
Not gonna help. Will show transistor OK.
Yes, I do.
You can always remove Q1 the dual transistor, and re-install it 180 degrees turned (swapping the transistor locations), and see if the DC offset issue changes, possibly to the opposite polarity offset. If the two transistors are reasonably matched, there shouldn't be much change.
Last edited: