Pioneer SX-1250 Repair Manual

[t j]

Just had my SX-1250 looked at and the tech mentioned a typo in the manual. In regards to VR2 on power amp assembly on page 35, he said that 100 mV was written in place of 10 mV, and that 100 would cause damage of some sort. Can anyone confirm or deny the existence of this error in the service manual? What if he set mine to 10 instead of 100? This person seems legit and I'm pretty concerned about this.
-Thanks All

 

[Tom B]

I have never heard of this typo. I do know that the pioneer SX-950 has idle set for 20mv but the adjustment takes into account that you are reading through two 100 resistors and the 0.5 ohm emitter resistors. the Pioneer SX-1050 is set for 50mv and reads through two 22 ohm resistors and the 0.5 ohm emitter resistors. It to me would make more sense for the SX-1250 to be at least 50mv if not 100mv as the adjustment reads through the same design as the SX-1050 (two 22 ohm resistors feeding the 0.5 ohm emitter resistors)

Being set for 10mv shouldn't cause any damage, but it may have an effect on the power produced. With it being set at 10mv, I expect the heatsinks will stay real cool if not cold, especially at idle. Maybe others could chime in as far as the effects of running bias too law, as it relates to output performance.

As a point of reference, these adjustments are on page 38 not 35. There is a Fig. 35 on page 38 that is confusing.

 

[t j]

I have never heard of this typo. I do know that the pioneer SX-950 has idle set for 20mv but the adjustment takes into account that you are reading through two 100 resistors and the 0.5 ohm emitter resistors. the Pioneer SX-1050 is set for 50mv and reads through two 22 ohm resistors and the 0.5 ohm emitter resistors. It to me would make more sense for the SX-1250 to be at least 50mv if not 100mv as the adjustment reads through the same design as the SX-1050 (two 22 ohm resistors feeding the 0.5 ohm emitter resistors)

Being set for 10mv shouldn't cause any damage, but it may have an effect on the power produced. With it being set at 10mv, I expect the heatsinks will stay real cool if not cold, especially at idle. Maybe others could chime in as far as the effects of running bias too law, as it relates to output performance.

As a point of reference, these adjustments are on page 38 not 35. There is a Fig. 35 on page 38 that is confusing.

Thanks so much for the info. So what's my move if I have an established tech telling me that it's 10 mV, not 100 mV, and making me feel like I'm a turd for bringing it up?

 

[Tom B]

I would wait for further feedback. I am not dumping on your tech, its just that it doesn't seem to make sense to me, and I have restored a lot of pioneers.

As a test, The heatsinks should develop some warmth (very little) with the unit idling at no volume, and speakers connected. Try running the 1250 for a while like this, (30 minutes) and see if the heatsinks stay cold cold. Then turn up the volume a little to get the unit going (say 9 oclock position). Let it run for at least 20 minutes. If the heatsinks are still noticably cold, then the bias is too low. If they have more warmth than room temperature the bias may be fine.

Another way to verify is to actually measure the voltage between the B and E connections of the output transistors. They should all be running at 0.5V to 0.6v between the base and emitter connection of these transistors at idle. But this is a test that only qualified techs should do, A simple slip of the test probes can cause a lot of damage. If the bias is indeed low, you will see the voltage reading B-E on the transistors very low (below 0.4v) on at least some

 

[steveUK]

FWIW, in the version (?) of the service manual that I have it is on page 38 and it says adjust for 100mV between terminals 7 and 19. Two 5.1K resistors are used across the speaker jacks. 10mV is the kind of figure that is used with many amps and receivers but not when load resistors are deployed across the speaker jacks. Maybe that is what your tech is getting confused about - if he is getting confused. I am not saying he is right or wrong, it may be a typo. Someone will clarify soon I expect.

 

[t j]

This is what the reaction from the person I paid to do this work. Name and Company will be posted shortly if he fails to do the right thing.

"By looking at the schematic and understanding the transistor parameters, you should be able to calculate the correct biasing current. I'm not going any further regarding this issue since I'm totally swamped right now."

What a guy!

 

[Tom B]

FWIW, in the version (?) of the service manual that I have it is on page 38 and it says adjust for 100mV between terminals 7 and 19. Two 5.1K resistors are used across the speaker jacks. 10mV is the kind of figure that is used with many amps and receivers but not when load resistors are deployed across the speaker jacks. Maybe that is what your tech is getting confused about - if he is getting confused. I am not saying he is right or wrong, it may be a typo. Someone will clarify soon I expect.

Actually the two 5.1k ohm resistors are put across the "Main in, or power in " jacks, not the speaker outputs. For the idle setting and usually balance setting there shouldn't be anything connected to the spoeaker jacks.

 

[t j]

So as this progresses, this guy claims that the manual is wrong based on his calculations. He writes:

"You paid me to diagnose any issues with your unit and to set the bias. I set the bias to the correct operating point I explaiined to you that the bias point in the manual is incorrect and set it based on calculations based on the circuitry and transistor paramters. What more do you want? I can't spend hours on in explaining the circuitry of the amplifier. I'll be more than happy to spend more time with you if you want a more detailed explanation on biasing techniques as a tutor if you'd like but I would have to charge you by the hour if that's what you want."

Am I tripping? Tell me that my expectations are unrealistic so I can go about my life, because I tend to stick on matters of principal.

 

[steveUK]

Probably the ever-stylish butt-joint construction, recessed a strategic inch or so from the front baffle and propagated on through the framing whenever possible up top that drew you in. The nice lacquer job sells it.

Sure, veneer would be the obvious choice, but would not beef up the original like this approach.

Actually the two 5.1k ohm resistors are put across the "Main in, or power in " jacks, not the speaker outputs. For the idle setting and usually balance setting there shouldn't be anything connected to the spoeaker jacks.

Yes you're right, I skimp read the section in the service manual.

 

[Tom B]

So as this progresses, this guy claims that the manual is wrong based on his calculations. He writes:

"You paid me to diagnose any issues with your unit and to set the bias. I set the bias to the correct operating point I explaiined to you that the bias point in the manual is incorrect and set it based on calculations based on the circuitry and transistor paramters. What more do you want? I can't spend hours on in explaining the circuitry of the amplifier. I'll be more than happy to spend more time with you if you want a more detailed explanation on biasing techniques as a tutor if you'd like but I would have to charge you by the hour if that's what you want."

Am I tripping? Tell me that my expectations are unrealistic so I can go about my life, because I tend to stick on matters of principal.

I think your guy is having a rough day!!!

 

[t j]

I think your guy is having a rough day!!!

Allright Tom B, thanks for the help. Can you make any sense of his responses? If it's necessary to calc the "bias point" why does the manual have a hard value? I'm really trying, and want to understand what he's saying, but evidence to the contrary is piling up.

 

[gslikker]

Usually 10 milliamps is enough for good operation of a class AB amp, providing temp feedback is such it never gets under.
Now, I crosschecked against a spec 4 schematic it is stated each power transistor gets 50 milliamps (in the schematic it is noted).
Looking at SX1250 schematic it would be 100 milliamps (100 mV over 2 x 0.5 ohm in series) for ONE transistor pair, so double of that in the 1250 compared to the Spec-4, and some 5 times more than a lot of other amps?

The schematic is okay but I CAN understand the techs reasoning, bias is " generous" at the least...Or I am making some calculation error.


Fortunately, the Pioneer experts will probably chime in as soon as they read the topic!

 

[t j]

I don't completely follow what you said, but you've done WAY more to help me understand, than the dude who I payed cash money to. I'd really rather find out that he's 100% correct, but the dots are not connecting.

 

[gslikker]

I don't completely follow what you said, but you've done WAY more to help me understand, than the dude who I payed cash money to. I'd really rather find out that he's 100% correct, but the dots are not connecting.

I just found in the BIG schematic indeed the current to each main amp channel is specified at 200 milliamps so the schematic is correct.
The dissipation because of bias , if voltage is plus and minus 70 Volts , 400 milliamps, be some 56 watts for bias reasons alone for this receiver.

Maybe they (Pioneer) want to run the amp in class A for very low currents, maybe they do it because getting less distortion.

As a comparison, the SX1250 apparently runs at TWICE the bias current each output transistor compared to the alike SPEC-4 !!

So, your tech is 99% sure "wrong" officially, but my opinion is you have one having more common sense than a lot of others. He is keen at things "fishy". So do NOT turn him down in favor of the average one....


Now, lets wait for MTF and Restorer-John and other Pioneer gurus I forgot, who have the real knowledge about this stuff, as myself I am only "mind-jogging" ...

 

[Tom B]

Usually 10 milliamps is enough for good operation of a class AB amp, providing temp feedback is such it never gets under.
Now, I crosschecked against a spec 4 schematic it is stated each power transistor gets 50 milliamps (in the schematic it is noted).
Looking at SX1250 schematic it would be 100 milliamps (100 mV over 2 x 0.5 ohm in series) for ONE transistor pair, so double of that in the 1250 compared to the Spec-4, and some 5 times more than a lot of other amps?

Probably the schematic is okay but I CAN understand the techs reasoning, bias is " generous" at the least...Or I am making some calculation error.


Fortunately, the Pioneer experts will probably chime in as soon as they read the topic!

Look at the schematic and service manual closely. The 100mv is being measured across two sides (the PNP side and NPN side). Each side has 2 transistors in parallel. So in effect 50mv per side. And the way it measures the voltage is that each .5 ohm emitter resistor is in series with a 22 ohm resistor. If the service manual, instead of asking for measurement from pin 7 to pin 19 told you to measure from pin 10 to 13, or pin 8 to pin 13 this would measue the voltage directly across the Emitter resistor and I could understand a lot lower reading. The Spec 4 actually has the reading taken directly across the emitter resistor pairs in series, not through 22 ohm resistors. (and has you verify that the other pairs are OK, in that this is a similar 4 output trANSISTOR per channel (configured as a PNP and NPN Dalington).

As a cross check on the SX-1250, take voltage measurements between pin 8 to 10, 10 to 11 then pin 16 to 17, and 16 to 18. All of these measurements should read at least 0.5v foward bias on these transistors identifying them as conducting. If they are not reading at least 0.5v then the bias is too low.
But great to see other feedback!!!

 

[gslikker]

I think the 22 resistor should not have impact on the reading, it is in series. Now the voltage is over TWO 0.5 ohm resistors parallelled from the NPN transistors, which are in series with TWO others to the PNP transistors. So total is still 0.5 ohms which should read 200 milliamps being 100 milliamps each transistor if matched close.. Now, in the big schematic you see EACH of the two power supply bridges specify 200 milliamps for each power amp channel.

So, the 100 millivolts wanted is voltage corresponds with the 200 milliamps (each channel) you find in the drawing. The big one on Page 53. One of the diode bridges is almost on top right, the other one is a bit hidden with the other power circuitry. You see the 200 mA marks and they go for each channel.
It is a bit fuzzy on mine although the manual is a 60Mb pdf.
A single error could imply something wrong, but as voltage and indicated current correspond the conclusion can only be the schematic writeups are right.

 

[t j]

Look at the schematic and service manual closely. The 100mv is being measured across two sides (the PNP side and NPN side). Each side has 2 transistors in parallel. So in effect 50mv per side. And the way it measures the voltage is that each .5 ohm emitter resistor is in series with a 22 ohm resistor. If the service manual, instead of asking for measurement from pin 7 to pin 19 told you to measure from pin 10 to 13, or pin 8 to pin 13 this would measue the voltage directly across the Emitter resistor and I could understand a lot lower reading. The Spec 4 actually has the reading taken directly across the emitter resistor pairs in series, not through 22 ohm resistors. (and has you verify that the other pairs are OK, in that this is a similar 4 output trANSISTOR per channel (configured as a PNP and NPN Dalington).

As a cross check on the SX-1250, take voltage measurements between pin 8 to 10, 10 to 11 then pin 16 to 17, and 16 to 18. All of these measurements should read at least 0.5v foward bias on these transistors identifying them as conducting. If they are not reading at least 0.5v then the bias is too low.
But great to see other feedback!!!

I've been told that the service manual is flat out wrong. Are you able to confirm or deny this? I've seen guys on Youtube set

Look at the schematic and service manual closely. The 100mv is being measured across two sides (the PNP side and NPN side). Each side has 2 transistors in parallel. So in effect 50mv per side. And the way it measures the voltage is that each .5 ohm emitter resistor is in series with a 22 ohm resistor. If the service manual, instead of asking for measurement from pin 7 to pin 19 told you to measure from pin 10 to 13, or pin 8 to pin 13 this would measue the voltage directly across the Emitter resistor and I could understand a lot lower reading. The Spec 4 actually has the reading taken directly across the emitter resistor pairs in series, not through 22 ohm resistors. (and has you verify that the other pairs are OK, in that this is a similar 4 output trANSISTOR per channel (configured as a PNP and NPN Dalington).

As a cross check on the SX-1250, take voltage measurements between pin 8 to 10, 10 to 11 then pin 16 to 17, and 16 to 18. All of these measurements should read at least 0.5v foward bias on these transistors identifying them as conducting. If they are not reading at least 0.5v then the bias is too low.
But great to see other feedback!!!

Is 100mV on vr2 (between 7 and 19) of the power amp a typo? Anyone ready to say that it should read 10mV as I've been told by a guy who comes off as a very capable tech? I feel like it should be set at 100mV as the manual states.

 

[zebulon1]

The 100mv is high/unbelievable but it's correct!
We in the Pioneer forum usually set our 1250 Idles to 65mV's (This is with the original outputs installed). No adverse effects and cooler running output temps.
Notes:
When using the On-Semi outputs MJ21195/96's the Idles are 60mV's

 

[Tom B]

I believe our friend from the Netherlands, now Zebulon1, as well as I have confirmed that the manual is correct and not a typo. That being said I usually look to set bias a bit lower at around 70 to 80 to keep temperature down on the output heatsinks and the outputs as well.

But the guys from the pioneer forum setting SX-1250 idle to 65mv sounds OK as well.

 

[t j]

Amazing how an experienced tech can be so wrong and so sure about it at the same time. It's got me completely spun. Thanks for the help fellas!