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Pioneer SX 1010 - Issue with Phono (Equalizer) board

Nuel

Nuel

New Member
Good day. I've been working on a Pioneer SX-1010 that has issues with its Phono board. It gives out constant cracking noise, sometimes very loud, getting the amplifier into Protection Mode. This issue is only in the Phono stage. All other inputs work fine.
With some investigation I found the issue was only with the Right channel of the Equalizer Board AWF-013-0 . So I started off with replacing the electrolytic capacitors and transistors on only the Right side. From other posts, I understood the difference in pin configuration with the modern transistors, so I checked multiple times to install them correctly.
The issue persists and I have no idea what's going on.
I took the voltage measurements and parts of the Right channel definitely has incorrect voltages. Haven't been able to get to the bottom of this.
The below image has the voltages at different points, green for correct measurements and red for incorrect ones.
Would appreciate any help.
Thank you.

1702074454012.png
 
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I’m rebuilding a SX-950 that had a similar problem. One channel on EQ board was crackling badly and was very obvious when scope the output sides. One of the C1313’s was getting hot and hitting it with freeze spray would quiet it briefly. Since both SA725 and 2SC1313 are on the Top 10 worst transistor list, I changed them all and the board was immediatelyu silent. Then did the caps.
 
What transistors did you replace, and with what? What caps were replaced? If you have a noiay phono board, check the ground components on the board, there's some small ceramics and resistors that could cause this.
 
jd3rdcoast said:
I’m rebuilding a SX-950 that had a similar problem. One channel on EQ board was crackling badly and was very obvious when scope the output sides. One of the C1313’s was getting hot and hitting it with freeze spray would quiet it briefly. Since both SA725 and 2SC1313 are on the Top 10 worst transistor list, I changed them all and the board was immediatelyu silent. Then did the caps.
Click to expand...
@jd3rdcoast , Thank you for the note. I replaced the transistors first. It didn't fix the problem then I changed all the capacitors. No change! None of the original transistors were getting hot.
 
SamIAmNot said:
What transistors did you replace, and with what? What caps were replaced? If you have a noiay phono board, check the ground components on the board, there's some small ceramics and resistors that could cause this.
Click to expand...
Transistor changed:
Q2, Q4 - KSA992FBU
Q6, Q8 - KSC1845FTA

Capacitors changed (All electrolytes for one side) - C2, C6, C10, C12, C20.
I also changed the C8(15p) and R16(22k).
The problem is just on one side and so I changed the components only on that side.
R16 was my prime suspect due to the base voltage for Q8. But that changing that didn't help either.
 
Nuel said:
Transistor changed:
Q2, Q4 - KSA992FBU
Q6, Q8 - KSC1845FTA

Capacitors changed (All electrolytes for one side) - C2, C6, C10, C12, C20.
I also changed the C8(15p) and R16(22k).
The problem is just on one side and so I changed the components only on that side.
R16 was my prime suspect due to the base voltage for Q8. But that changing that didn't help either.
Click to expand...
Positive you recapped the correct side? I’ve mistakenly lost correct R/L orientation while working on boards before.
 
If using this board template, just a reminder that it shows the FOIL (Solder) SIDE. Orientation will be backwards from the top. This backs up Mikiemiki's comment. ScreenHunter 24.jpg
 
You only have bad voltages right around Q8. I'd check the components right around there to see what I'd find. Re-check the pin orientation of Q8. I've put some in, checked them over a couple times only to find I put them in wrong.
There aren't many parts in that area to mess up the voltages.
 
@kwayde, for Q2 and Q4 I happened to pick the only two that had very different hFEs from a set of 10. The others are within a range of +-3. What are the odds?
So anyway, during this time the other side also started to crackle. So I replaced Q2,Q4 and Q1,Q3 with matched pairs on each channel. Also replaced all the electrolytic caps and remaining transistors.
Now the there is no crackle on either side, runs as it should, Yay! However the messed up voltages remain around Q8 as in the image I had posted earlier and now also around Q7.
Both Q8 and Q7 run a little hotter.
I replaced R15, C7 and R16, C8 around those transistors.
Not sure if it is safe to leave it as it is.
All the voltages around the Power Supply board match the service manual. But may be a recap could help as the caps in that board are quite worn out?
 
Recapped the Power Supply board. Voltages are still messed up around Q7 and Q8 in the Equalizer board and they both run a little warm. No crackling sound in either Phono channels, run very quiet, even played a few records.
So I'll leave it as it is for now and see what happens over time.
Thank you everyone for all the help.
 
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