SX-980 for my dad

On the left channel power amp pcb (AWH-073) replace D6, D7, Q8 and C11 (see Post 19 above). If that does not resolve it lift one end and verify the resistances of R18, R19, R23, R24, R28, R29, R30 and R31. These are all parts of the overload detection bridge on the power amp pcb.

I replaced VR1, VR2, C1, C2, and C11. I also lifted a leg on all of the resistors that merlynski suggested that I check. All of the resistors measured spot on.

I did notice that R18 and R19 are 1ohm resistors on the left channel board, and .5ohm on the right channel board.

I measured C11 with my meter and the capacitance measures correctly. I replaced it with a WIMA.

D6, D7, and Q8 all test ok out of circuit. Any suggestions on what to use as replacements for these 3?
D6: 1S2471
D7: 1S2471
Q8: 2SC869

I also noticed that the bias pot seems like it’s gotten hot at some point. I can see a bubble on one side where it looks like it started to melt. Could this be what is causing the protection to kick in early in the left channel?
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I did notice that R18 and R19 are 1ohm resistors on the left channel board, and .5ohm on the right channel board.
There is your problem, wrong value resistors that trigger the Protection circuit too soon. Put 0.5 Ω resistors in place of the 1 Ω ones and your protection circuit should work correctly.
 
There is your problem, wrong value resistors that trigger the Protection circuit too soon. Put 0.5 Ω resistors in place of the 1 Ω ones and your protection circuit should work correctly.

So do you think that these were replaced by someone along the way? I can’t see any other evidence that anything else has been replaced inside of this unit.

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R18 and R19 should be 0.5 Ω 5 Watt wire-wound resistors. DigiKey has some: ALSR5F-.50-ND, about $1.77 each.
 
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So do you think that these were replaced by someone along the way? I can’t see any other evidence that anything else has been replaced inside of this unit.
They look original to me, but I have not looked inside an SX-980 before. But according to the schematic and the Service Manual they are the wrong value. Perhaps another Receiver used the same PCB but different value resistors and they got mixed up at the factory. Regardless, the improper value corresponds to the symptoms you report.
 
They look original to me, but I have not looked inside an SX-980 before. But according to the schematic and the Service Manual they are the wrong value. Perhaps another Receiver used the same PCB but different value resistors and they got mixed up at the factory. Regardless, the improper value corresponds to the symptoms you report.

I’ll get some new ones and change them out then. Can I reinstall D6, D7, and Q8 then, or should I still replace those also?
 
Since you found a definitive problem related to overload sensitivity the overload circuit was working correctly, albeit at the wrong level, I believe they can go back in if they tested good.
 
someone burnt the pot cover with their soldering iron, awful smell:smoke:
good one, the emitter resistors are the wrong size :) gee and they even went through the effort of putting the sleeves back on the leads,
put some glue on those STV4 leads where they enter the body to secure them :)
1N4148
ksc1845f
 
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What seems strange to me is why would someone replace the emitter resistors but nothing else (not disputing that they are the wrong size)? If the resistors failed wouldn’t that have taken out some transistors as well? Other than these two resistors the rest of the board looks untouched to me.

I wasn’t thinking of using these from Mouser.
https://www.mouser.com/ProductDetail/xicon/prw05wjw50kb00/?qs=lW4g0ZFxbL5gemc7pjuzZA==&countrycode=US&currencycode=USD

Those would probably work fine, correct resistance and wattage.
Usually what causes emitter resistors to fail is shorted output transistors. Are the output transistors original(Q101, Q102)?
 
Changing those resistors will change your bias setting, make sure to back it off and then readjust it afterwards before full power testing.
 
Here is what the 0.5 ohms in the other amp board look like. They are marked “NOBLE” as LesE mentioned.

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I ordered some 0.5 ohm replacements along with some parts for another project last night. I’ll report back once I’ve installed them.
 
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probably the o/p devices have been changed before.
We blew 2 sets of o/ps ( 1 set under warranty) in the 950 before I was wise enough to put in 250W Motorola devices. Still have those old 100W Toshiba devices in the drawer and the old pots :) Never could kill those Motorola devices.
hope that you are going to rebuild both channels
looks easier to service than the 950
 
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probably the o/p devices have been changed before.
We blew 2 sets of o/ps ( 1 set under warranty) in the 950 before I was wise enough to put in 250W Motorola devices. Still have those old 100W Toshiba devices in the drawer and the old pots :) Never could kill those Motorola devices.
hope that you are going to rebuild both channels
looks easier to service than the 950

Yes. Whatever I do to the left channel I will also do to the right.

So far I have replaced both trimmers, C1, C2, and C11 in both channels. I’ll swap in the 0.5 ohm emitter resistors once they arrive. Do you think that I need to do anything more than that on the amp boards?

It is similar to working on the amp board of the 950. First, remove the heat sink and then you can service the amp boards. This is a bit easier since the outputs just unplug from the amp boards, and the amp boards are separate in the 980. Way easier than the 1010 amp boards though!
 
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I think what you did on the amp pcb's is fine, could replace the 2sc869 if you want too, new paste on the drivers/HS, dusting/cleaning :)
 
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