Pioneer SX-1250 Low Bias on both channels after rebuild

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stack2duck

AK Subscriber
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Reposting because I think I posted in the wrong spot.

Happy Sunday everyone. I am working on rebuilding an SX-1250 and just rebuilt the power amp boards with all new transistors, caps, and diodes.

While the receiver works and plays music just fine out of both channels after rebuilding these boards, I am only able to achieve ~70-75 mV bias on both channels. I also can achieve a perfect 65 V for DC offset just fine. And yes, the original output transistors are still installed so I am thinking I should be able to achieve the factory set 100 mV bias with no problems.

Now I know most AKers set their bias for this receiver at 65 mV or so to have it run cooler, but am concerned of a deeper issue this might present that I want to address.

Should I just set it at 65 mV, call it a day, and continue rebuilding the rest of the receiver or is there a deeper issue I need to fix here?

Thanks in advance for the help.
 
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stack2duck said:
Reposting because I think I posted in the wrong spot.

Happy Sunday everyone. I am working on rebuilding an SX-1250 and just rebuilt the power amp boards with all new transistors, caps, and diodes.

While the receiver works and plays music just fine out of both channels after rebuilding these boards, I am only able to achieve ~70-75 mV bias on both channels. I also can achieve a perfect 65 V for DC offset just fine. And yes, the original output transistors are still installed so I am thinking I should be able to achieve the factory set 100 mV bias with no problems.

Now I know most AKers set their bias for this receiver at 65 mV or so to have it run cooler, but am concerned of a deeper issue this might present that I want to address.

Should I just set it at 65 mV, call it a day, and continue rebuilding the rest of the receiver or is there a deeper issue I need to fix here?

Thanks in advance for the help.
Click to expand...
First of all, I really hope by "dc offset" you are referring to the trimpots on the stabilizer board to set the rail voltages at +65V and -65V- otherwise that is truly alarming for an amplifier to be running at 65V offset! Second, it will depend on what drivers/pre-drivers/outputs you are using, but an idle of 75mV is fine. I set mine between 70-75 after a rebuild. With the modern transistors that is a good setting to balance heat load and frequency oscillation in the audio.
 
Something is wrong if you can’t get to 100mV.
What transistor subs did you use? I normally replace all transistors on the amp boards and leave the output transistors, just like you, and have never had any problem getting to 100mV.

Ditto on the 65V DC Offset, must be the stabilizer outputs….
 
Thank you for the replies. Yes, sorry, I meant the stabilizer board rail voltage adjustment. Easily set to 65V.

Here are the transistor subs I used:
Q1: 2SA750 --> sub KSA992
Q2: 2SA750 --> sub KSA992
Q3: 2SC1439 --> sub KSC3503
Q4: 2SC1439 --> sub KSC3503
Q5: 2SA858 --> sub KSA1381
Q6: 2SA858 --> sub KSA1381
Q7: 2SC1904 --> sub KSC3503
Q8: 2SA899 --> sub KSA1381
Q9: 2SD610 --> sub KSC2073
Q10: 2SB630 --> sub KSA940
Q11: 2SC869 --> sub KSC1845

This is consistent with the lists I have seen on AK for this board so I'm really not sure why I would be having issues getting to 100mV... Just double checked and I have all of them installed in the correct spots on the board and they all appear to be installed correctly.
 
If the outputs are the MJ21195/96 OnSemi then getting to the 100mv setting is almost impossible. We bring them to 65mv for the On Semi.
You also want full +- unregulated rail Voltage. Which is the voltage from the big filters. I can’t remember that voltage at the moment.
 
zebulon1 said:
If the outputs are the MJ21195/96 OnSemi then getting to the 100mv setting is almost impossible. We bring them to 65mv for the On Semi.
Click to expand...
The reason I made the post is because they are the original outputs, which from reading through posts means I should be able to reach 100 mV easily. I have the outputs pulled currently and they all just tested fine on the multimeter.
 
There’s a few things that need to be sorted out.
Bias/Idles is adjusted on the amp board.
DC Balance is adjusted on the amp boards.
The +-65v regulated is adjusted on the stabilizer board.
The +-65v is used for the prestage supplies in the amps.
The unregulated DC is used to power the outputs through the collectors.
Three test point pins on the amp boards.
Two for the idles, bias.
One for the DC Balance positive and the Chassis for the negative.
 
zebulon1 said:
There’s a few things that need to be sorted out.
Bias/Idles is adjusted on the amp board.
DC Balance is adjusted on the amp boards.
The +-65v regulated is adjusted on the stabilizer board.
The +-65v is used for the prestage supplies in the amps.
The unregulated DC is used to power the outputs through the collectors.
Three test point pins on the amp boards.
Two for the idles, bias.
One for the DC Balance positive and the Chassis for the negative.
Click to expand...
From what I’m understanding, you’re basically just describing pg 38 from the service manual, correct? The “Power Amplifier” and “Stabilizer” adjustment sections? If so, I am understanding what you’re saying and this is what I’ve been using to adjust these voltages.

Thank you for helping me understand the verbiage. I am pretty novice with that.
 
I’m worried your not following the procedures completely.
Both channels not achieving the 100mv bias setting (original outputs installed) is either a procedure, power supply issue or incorrect component placements.
Probability is a variable in my troubleshooting. I tend to blame myself on abnormal failures.
Before I completely put my foot in my mouth:
What are the voltages at the four large filter caps?
 
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zebulon1 said:
I’m worried your not following the procedures completely.
Both channels not achieving the 100mv bias setting (original outputs installed) is either a procedure, power supply issue or incorrect component placements.
Probability is a variable in my troubleshooting. I tend to blame myself on abnormal failures.
Before I completely put my foot in my mouth:
What are the voltages at the four large filter caps?
Click to expand...
Zebulon1, I really appreciate the help! I currently have the heat sinks off to touch up some chipped paint and am waiting for paint to dry. Once I have it back together this evening I’ll get the measurements at the filter caps.

Are you asking for the voltages across the + and - terminal across each of the main filter caps?
 
You can connect the black probe to the chassis and use the red to the terminal that’s not connected to the other cap. There’s four readings. One for each cap.
 
zebulon1 said:
You can connect the black probe to the chassis and use the red to the terminal that’s not connected to the other cap. There’s four readings. One for each cap.
Click to expand...
Well. As I was reattaching the STV-3H back to the heatsink, of course it broke… I was worried about this as I very carefully put it back on…

Just ordered a NOS one on eBay from Ben (Affinity For Artifacts/Peace Love Music). Guess I’ll have to wait to continue this project until that gets here and I install it. Thanks again for the help.
 
zebulon1 said:
You can connect the black probe to the chassis and use the red to the terminal that’s not connected to the other cap. There’s four readings. One for each cap.
Click to expand...
I apologize for the huge delay. I have been ill for the last couple weeks. I just got the new STV-3H installed and the outputs re-pasted.


Here are the voltages at each filter cap:

Right Channel Front: -70.2 V
Right Channel Rear: 70.3 V
Left Channel Front: -70.0 V
Left Channel Rear: 70.1 V
 
Would replacing the VR2 trimmers with 1k ohm trimmers to reach 100mV be an option here potentially? I read through some posts on here but generally it seems like a band-aid on a gunshot wound fix potentially. Any extra input would be appreciated!
 
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