Pioneer SX-850 Protection Relay and DC Offset

Brivan

Well-Known Member
I have a recently-acquired Pioneer SX-850 with a known protection relay issue. I purchased the receiver with the protection issue as a known problem. I removed the bonnet and base plate and checked power supply board voltages - all within 0.5v - 1v of spec. O-scope probing of all supplies gave me a nice, clean line. I started the power amp calibration per the service manual, but am having issues with the DC offset. My meter reading fluctuated all over the place - especially on pin 25 of the power amp board. The scope showed the same thing, with the average voltage reading being around -6v with VR1 fully counter-clockwise:
20181012_130800(0) (Small).jpg

Pin 10 is a little better, but I can't get close to 0v - about 30mV is as close as I can get:
20181012_130940 (Small).jpg

Output transistors checked good, as well as the 8 transistors on the power amp board (all were checked while soldered in place). With the power off, I ran VR1 - VR4 back and forth several times, then set them back to calibration position before powering back up. I'm guessing a power amp capacitor replacement is in my future - what do you think?
 
put 512-KSA992FBU transistors into q1, q2, q3, q4

Facts: the power amp runs off of +51.5v, +36.5v and -51.5v regulated.
The DC offset setting circuits are run off the +36.5v and -51.5v regulated AND ARE CHANNEL INDEPENDENT .
Pin 25 of the power amp board is the RIGHT channel.
the power amp uses the
input:
2sa726 or 2sa763

Vas stage:
2sa818 or 2sa898
2sc1628 or 2sc1903

driver stage:
2sd381 or 2sc1903
2sb536 no alternate listed

outout stage:
2sb531 no alternate listed
2sd371 no alternate listed

so:
2sa726 in the input circuit
and the collector of one of them on each channel is directly connected to the -51.5v power supply.
A failing 2sa726 might just jerk around the -51.5v regulated supply.
That COULD have a lesser effect upon the LEFT channel, pin 10 DC offset.

low noise caps could have film caps subbed in...

power amp awh-059
awh-059 c1 2.2 25 cssa 850 2.2 50 647-UKL1H2R2KDDANA **** low noise ****
awh-059 c2 2.2 25 cssa 850 2.2 50 647-UKL1H2R2KDDANA **** low noise ****
awh-059 c3 1 25 cssa 850 1 50 647-UKL1H010KDDANA **** low noise ****
awh-059 c4 1 25 cssa 850 1 50 647-UKL1H010KDDANA **** low noise ****
awh-059 c5 33 16 cea 850 33 50 647-upw1v330mdd
awh-059 c6 33 16 cea 850 33 50 647-upw1v330mdd
awh-059 c11 330 10 cea 850 330 25 647-UPW1E331MPD6
awh-059 c12 330 10 cea 850 330 25 647-UPW1E331MPD6
awh-059 q1 2sa726 512-KSA992FBU
awh-059 q2 2sa726 512-KSA992FBU
awh-059 q3 2sa726 512-KSA992FBU
awh-059 q4 2sa726 512-KSA992FBU
awh-059 q5 2sa818 512-KSA1381es
awh-059 q6 2sa818 512-KSA1381es
awh-059 q7 2sc1628 512-KSC3503estu
awh-059 q8 2sc1628 512-KSC3503estu
awh-059 q9 2sd361 512-KSC2073TU
awh-059 q10 2sd361 512-KSC2073TU
awh-059 q11 2sb536 512-KSA940
awh-059 q12 2sb536 512-KSA940
awh-059 vr1 10k multi 652-3296P-1-103LF
awh-059 vr2 10k multi 652-3296P-1-103LF
awh-059 vr3 100 single 652-3386W-1-101LF
awh-059 vr4 100 single 652-3386W-1-101LF
awh-059 d7 1s1885 512-UF4004
awh-059 d8 1s1885 512-UF4004
awh-059 d9 1s1885 512-UF4004
awh-059 d10 1s1885 512-UF4004
output transistor q13 2sd371a 863-MJ21194G
output transistor q14 2sd371a 863-MJ21194G
output transistor q15 2sd371a 863-MJ21194G
output transistor q16 2sd371a 863-MJ21194G
output transistor q17 2sb531a 863-MJ21193G
output transistor q18 2sb531a 863-MJ21193G
output transistor q19 2sb531a 863-MJ21193G
output transistor q20 2sb531a 863-MJ21193G


upload_2018-10-12_15-14-12.png
 
put 512-KSA992FBU transistors into q1, q2, q3, q4

Facts: the power amp runs off of +51.5v, +36.5v and -51.5v regulated.
The DC offset setting circuits are run off the +36.5v and -51.5v regulated AND ARE CHANNEL INDEPENDENT .
Pin 25 of the power amp board is the RIGHT channel.
the power amp uses the
input:
2sa726 or 2sa763

Vas stage:
2sa818 or 2sa898
2sc1628 or 2sc1903

driver stage:
2sd381 or 2sc1903
2sb536 no alternate listed

outout stage:
2sb531 no alternate listed
2sd371 no alternate listed

so:
2sa726 in the input circuit
and the collector of one of them on each channel is directly connected to the -51.5v power supply.
A failing 2sa726 might just jerk around the -51.5v regulated supply.
That COULD have a lesser effect upon the LEFT channel, pin 10 DC offset.

low noise caps could have film caps subbed in...

power amp awh-059
awh-059 c1 2.2 25 cssa 850 2.2 50 647-UKL1H2R2KDDANA **** low noise ****
awh-059 c2 2.2 25 cssa 850 2.2 50 647-UKL1H2R2KDDANA **** low noise ****
awh-059 c3 1 25 cssa 850 1 50 647-UKL1H010KDDANA **** low noise ****
awh-059 c4 1 25 cssa 850 1 50 647-UKL1H010KDDANA **** low noise ****
awh-059 c5 33 16 cea 850 33 50 647-upw1v330mdd
awh-059 c6 33 16 cea 850 33 50 647-upw1v330mdd
awh-059 c11 330 10 cea 850 330 25 647-UPW1E331MPD6
awh-059 c12 330 10 cea 850 330 25 647-UPW1E331MPD6
awh-059 q1 2sa726 512-KSA992FBU
awh-059 q2 2sa726 512-KSA992FBU
awh-059 q3 2sa726 512-KSA992FBU
awh-059 q4 2sa726 512-KSA992FBU
awh-059 q5 2sa818 512-KSA1381es
awh-059 q6 2sa818 512-KSA1381es
awh-059 q7 2sc1628 512-KSC3503estu
awh-059 q8 2sc1628 512-KSC3503estu
awh-059 q9 2sd361 512-KSC2073TU
awh-059 q10 2sd361 512-KSC2073TU
awh-059 q11 2sb536 512-KSA940
awh-059 q12 2sb536 512-KSA940
awh-059 vr1 10k multi 652-3296P-1-103LF
awh-059 vr2 10k multi 652-3296P-1-103LF
awh-059 vr3 100 single 652-3386W-1-101LF
awh-059 vr4 100 single 652-3386W-1-101LF
awh-059 d7 1s1885 512-UF4004
awh-059 d8 1s1885 512-UF4004
awh-059 d9 1s1885 512-UF4004
awh-059 d10 1s1885 512-UF4004
output transistor q13 2sd371a 863-MJ21194G
output transistor q14 2sd371a 863-MJ21194G
output transistor q15 2sd371a 863-MJ21194G
output transistor q16 2sd371a 863-MJ21194G
output transistor q17 2sb531a 863-MJ21193G
output transistor q18 2sb531a 863-MJ21193G
output transistor q19 2sb531a 863-MJ21193G
output transistor q20 2sb531a 863-MJ21193G


View attachment 1303460
Wow. Thank you so much for all the good information! I've said it before - this kind of knowledge is indispensable! I'll get your recommended replacements ordered for Q1 through Q4. I'll report back. I've read through a few other threads on this - it looks like the best access to the main amp board is to separate the back plate from it, remove the plate, then unscrew the output transistor heat sink assembly and swing it up and over the main amp board. Does that sound about right? I'm a little concerned about the rigidity of the wires connecting the main output transistors to the amp board.
 
Wow. Thank you so much for all the good information! I've said it before - this kind of knowledge is indispensable! I'll get your recommended replacements ordered for Q1 through Q4. I'll report back. I've read through a few other threads on this - it looks like the best access to the main amp board is to separate the back plate from it, remove the plate, then unscrew the output transistor heat sink assembly and swing it up and over the main amp board. Does that sound about right? I'm a little concerned about the rigidity of the wires connecting the main output transistors to the amp board.
The replacement transistors should arrive tomorrow and I should be able to get them installed the next day. As has been warned on AK many times, I'll make sure the transistor orientation agrees with with the silkscreen print on the board.

Additionally, I removed the back plate from the amp board, then removed the screws from the output transistor heat sink and swung it up 90 degrees. It looked like it was still going to suck trying to solder with that heat sink in the way, so I photographed and labeled the output transistors, then removed them, which allowed me to remove the heat sink. I replaced all the old electrolytic caps on the amp board.
 
put 512-KSA992FBU transistors into q1, q2, q3, q4

Facts: the power amp runs off of +51.5v, +36.5v and -51.5v regulated.
The DC offset setting circuits are run off the +36.5v and -51.5v regulated AND ARE CHANNEL INDEPENDENT .
Pin 25 of the power amp board is the RIGHT channel.
the power amp uses the
input:
2sa726 or 2sa763

Vas stage:
2sa818 or 2sa898
2sc1628 or 2sc1903

driver stage:
2sd381 or 2sc1903
2sb536 no alternate listed

outout stage:
2sb531 no alternate listed
2sd371 no alternate listed

so:
2sa726 in the input circuit
and the collector of one of them on each channel is directly connected to the -51.5v power supply.
A failing 2sa726 might just jerk around the -51.5v regulated supply.
That COULD have a lesser effect upon the LEFT channel, pin 10 DC offset.

low noise caps could have film caps subbed in...

power amp awh-059
awh-059 c1 2.2 25 cssa 850 2.2 50 647-UKL1H2R2KDDANA **** low noise ****
awh-059 c2 2.2 25 cssa 850 2.2 50 647-UKL1H2R2KDDANA **** low noise ****
awh-059 c3 1 25 cssa 850 1 50 647-UKL1H010KDDANA **** low noise ****
awh-059 c4 1 25 cssa 850 1 50 647-UKL1H010KDDANA **** low noise ****
awh-059 c5 33 16 cea 850 33 50 647-upw1v330mdd
awh-059 c6 33 16 cea 850 33 50 647-upw1v330mdd
awh-059 c11 330 10 cea 850 330 25 647-UPW1E331MPD6
awh-059 c12 330 10 cea 850 330 25 647-UPW1E331MPD6
awh-059 q1 2sa726 512-KSA992FBU
awh-059 q2 2sa726 512-KSA992FBU
awh-059 q3 2sa726 512-KSA992FBU
awh-059 q4 2sa726 512-KSA992FBU
awh-059 q5 2sa818 512-KSA1381es
awh-059 q6 2sa818 512-KSA1381es
awh-059 q7 2sc1628 512-KSC3503estu
awh-059 q8 2sc1628 512-KSC3503estu
awh-059 q9 2sd361 512-KSC2073TU
awh-059 q10 2sd361 512-KSC2073TU
awh-059 q11 2sb536 512-KSA940
awh-059 q12 2sb536 512-KSA940
awh-059 vr1 10k multi 652-3296P-1-103LF
awh-059 vr2 10k multi 652-3296P-1-103LF
awh-059 vr3 100 single 652-3386W-1-101LF
awh-059 vr4 100 single 652-3386W-1-101LF
awh-059 d7 1s1885 512-UF4004
awh-059 d8 1s1885 512-UF4004
awh-059 d9 1s1885 512-UF4004
awh-059 d10 1s1885 512-UF4004
output transistor q13 2sd371a 863-MJ21194G
output transistor q14 2sd371a 863-MJ21194G
output transistor q15 2sd371a 863-MJ21194G
output transistor q16 2sd371a 863-MJ21194G
output transistor q17 2sb531a 863-MJ21193G
output transistor q18 2sb531a 863-MJ21193G
output transistor q19 2sb531a 863-MJ21193G
output transistor q20 2sb531a 863-MJ21193G


View attachment 1303460
Good news this morning - after replacing Q1 through Q4 (as well as Q9 through Q12, since I had it apart), I was able to go through the amplifier calibration and get everything set according to spec. I ran a 100Hz tone through it and the outputted sinewave looks just fine. Audio sounds good, as well. One problem remains that wasn't detectable previously - I have a loud rumbly, grating noise when selecting either phono 1 or 2 as an input. The sound is mostly coming from the right channel, but is also present in the left at lesser volume and the volume in both varies a little. I think the noise is telling me I have some crappy caps in the phono preamp section. I'll check on that shortly.
 
Good news this morning - after replacing Q1 through Q4 (as well as Q9 through Q12, since I had it apart), I was able to go through the amplifier calibration and get everything set according to spec. I ran a 100Hz tone through it and the outputted sinewave looks just fine. Audio sounds good, as well. One problem remains that wasn't detectable previously - I have a loud rumbly, grating noise when selecting either phono 1 or 2 as an input. The sound is mostly coming from the right channel, but is also present in the left at lesser volume and the volume in both varies a little. I think the noise is telling me I have some crappy caps in the phono preamp section. I'll check on that shortly.
I replaced all the caps on the phono preamp "equalizer" board, but still was getting the same noises. Some research on AK revealed that the transistors on that board - 2SA725/2SA726 and 2SC1313 are notorious for causing problems as they age. This was apparent when I replaced some of those same transistors on the amp board, per MTF's suggestion. So, I have replaced the two 2SA725s (2SA726 on the actual transistor) with KSA992 transistros and still have the same noises.

I'm now ready to replace the 2SC1313s, but have a question: In some of the AK forums, the 2SC1313 has a replacement of BC337. I have some BC33740BU transistors in stock, but not sure if I should use them. When comparing the 2SC1313, KSC1815 (more-common substitution), BC337 and BC33740BU transistors, it's like comparing apples to oranges to grapefruit to watermelons - there doesn't seem to be a lot of likeness between the four. Can someone shed some light on this for me?
 
From what I can determine from published data, the BC337 and BC337-40 types are not considered low-noise devices (not less than 4 dB noise figure) and may nit be suitable for the application. A BC550 might be a better choice if the KSC1815Y or KSC1845F is not available. The other operational characteristics would probably allow it, but it wouldn't be my first choice.
 
From what I can determine from published data, the BC337 and BC337-40 types are not considered low-noise devices (not less than 4 dB noise figure) and may nit be suitable for the application. A BC550 might be a better choice if the KSC1815Y or KSC1845F is not available. The other operational characteristics would probably allow it, but it wouldn't be my first choice.
Okay. That's the kind of info I was looking for. I'll order a few of KSC1845, as well as more KSA992 since these are to be replacements for common-failure transistors. Thanks for your input!
 
Last edited:
BC337 was not intended for low noise operation, meant as a high current low Vce saturated switch. i looked at Onsemi and NXP data, they do not have noise specs for this device. I know of others actually testing them for noise.
It inherently has low noise due to its construction, similar to a 2n4401. In the past designers, have used 2n4401 series in MC head amp applications due to their inherent LN.
I'd still use A992/C1845 as they have more applications and use BC337 in a pinch.
 
BC337 was not intended for low noise operation, meant as a high current low Vce saturated switch. i looked at Onsemi and NXP data, they do not have noise specs for this device. I know of others actually testing them for noise.
It inherently has low noise due to its construction, similar to a 2n4401. In the past designers, have used 2n4401 series in MC head amp applications due to their inherent LN.
I'd still use A992/C1845 as they have more applications and use BC337 in a pinch.
Thanks for the info! I'm not in a big hurry to get this receiver completed, so I'm fine with waiting for the correct parts to arrive - Wednesday of next week.

For the roughly two years I've been working on receivers, I've never managed to install a cap backwards - until yesterday on the SX-850 equalizer (phono preamp) board. Don't know how I missed it, but I actually felt it getting hot - before it exploded, thankfully. That bad part is, at the same time, I saw light smoke wafting up from the power supply board. Great. After quickly powering down, I gingerly started feeling components on the PSB for heat, but didn't feel anything. Turns out Q6 that supplies -19vdc to that capacitor I installed backwards is a smaller TO-92, which I skipped over, figuring the heat-sunk TO220s were one of the victims. Anyway, the capacitor has been replaced and Q6 on the PSB is still providing -19vdc, but I have a replacement coming for it, as well - just to be safe.
 
I'll be interested to hear how your phono amp comes out. I think I have the same issue with my 850. There's a thread here indicating that KSC1815 is possibly a better substitute than KSC1845:

http://audiokarma.org/forums/index....stor-substitutions.583009/page-3#post-9197541
I read a bunch of threads regarding the KSA1815 vs. KSA1845. At the end of all that, I was about ready to flip a coin, but ended up ordering the KSC1845 due to its presumed higher-voltage-handling capability. I just got them in today, so we'll see how it goes. Here's another thread:
http://audiokarma.org/forums/index.php?threads/transistor-substitutions.688515/
 
I'll be interested to hear how your phono amp comes out. I think I have the same issue with my 850. There's a thread here indicating that KSC1815 is possibly a better substitute than KSC1845:

http://audiokarma.org/forums/index....stor-substitutions.583009/page-3#post-9197541
I replaced all four 2SC1313 transistors on the phono preamp board with KSC1845. Seems to work just fine. Outputted sine wave looks the same as the input and the audio is good. Audio output has a small hum with the volume at about 50%, but I think this is normal for phono inputs. This project is finally complete with all the receiver functions working properly.
 
I replaced all four 2SC1313 transistors on the phono preamp board with KSC1845. Seems to work just fine. Outputted sine wave looks the same as the input and the audio is good. Audio output has a small hum with the volume at about 50%, but I think this is normal for phono inputs. This project is finally complete with all the receiver functions working properly.

Could that small hum be from the turntable, headshell, wires?
 
Could that small hum be from the turntable, headshell, wires?
I was getting the slight hum without any turntable connected, so it's down to the receiver. But at 50% volume, there's no way I'd hear that hum over the very loud audio that volume setting would produce. Even at the most tooth-rattling, ear-bleeding volume I can withstand, the volume knob doesn't come anywhere near 50%. At least on the mid-level audio equipment I have sitting around. I have several other Pioneer receivers around the house - I'll switch them over to phono input and see what kind of noise they make at 50% volume.

Additionally, I was getting the hum with the bottom plate removed from the receiver, which I'm guessing makes a difference. At 50% volume, even getting my hand close to the phono preamp board would cause a louder hum.
 
I was getting the slight hum without any turntable connected, so it's down to the receiver. But at 50% volume, there's no way I'd hear that hum over the very loud audio that volume setting would produce. Even at the most tooth-rattling, ear-bleeding volume I can withstand, the volume knob doesn't come anywhere near 50%. At least on the mid-level audio equipment I have sitting around. I have several other Pioneer receivers around the house - I'll switch them over to phono input and see what kind of noise they make at 50% volume.

Additionally, I was getting the hum with the bottom plate removed from the receiver, which I'm guessing makes a difference. At 50% volume, even getting my hand close to the phono preamp board would cause a louder hum.

So it was just the 1313 transistors that caused this problem?

Have you ever had any issues with the selector buttons on yours? Those seem to be real troublemakers.
 
Yes, the 2SC1313s were the last transistors replaced, so my problem was with at least two of them, since I had noise in both channels. But I replaced all four on the board. Prior to that, I replaced all the capacitors and the 2SA945 transistors.

Despite spraying the function selector push-button switches with DeOxit, I still have a very small amount of scratchiness in the phono inputs when I push the buttons in and move the switches around. It's not very audible and I get clear sound on both phono inputs once a phono function is selected. I've been spraying these types of buttons from the front, squeezing my DeOxit straw between the spring and the switch shaft in the bottom left and bottom right corners. That method seems to work fine, since it's taken care of noisy switches on some of the other Pioneer receivers I've worked on.
 
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