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SX-6000 Bias issue

RyeGuy859

RyeGuy859

Well-Known Member
Hello! Ive been working on an sx-6000 that had a channel out. It was the outputs and 1 driver on the Left channel. I replaced the sanken 1080s with some 2SD555s I had pulled from a unit a while ago. That channel came up nicely and the midpoint and bias was set good. Now the other channel which was working from the start will not bias nicely. I can only get it up to around 6-7mv with the midpoint set around 34v. If I raise the midpoint closer to half my B+(73.2v) The bias in that channel drops even lower.

What ive changed in the Right channel, all 4 transistors excluding outputs. Ive checked the STV-3 and it measured 1.3v in circuit so I dont think its that either. Im not sure where to look next if anyone as any suggestions.
 

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Check the resistors associated with the bias circuit - R14, R20, R22, R24, R26, R28, R30 and VR4. You should probably replace VR3 and VR4 - they are known to degrade over time and go open causing the outputs to blow. . . . YMMV. What did you use for Q@, Q3, Q4 & Q8? Make sure your pinouts are correct for the replacements.
 
Richard P said:
Check the resistors associated with the bias circuit - R14, R20, R22, R24, R26, R28, R30 and VR4. You should probably replace VR3 and VR4 - they are known to degrade over time and go open causing the outputs to blow. . . . YMMV. What did you use for Q@, Q3, Q4 & Q8? Make sure your pinouts are correct for the replacements.
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I’ll check the resistors tomorrow. I used 1845, 2x TTC004, TTA004. pin out are good, i verified before swapping and compared to the other rebuilt channel. Originals were EBC i believe, I made sure the board markings were accurate before pulling as well. Replacements are ECB. It plays good, but something’s wrong due to the bias not increasing. Those VRs measure 100 ohms i believe, i’ll give it a check too
 
The channel never biased in the first place to be clear, I hadn’t checked it until I rebuilt the bad channel.
 
Checked all resistors and they all were good. For VR3/4 VR3 (Good bias channel) tested from 0-57ohms. VR4 (Bad bias channel) Tested from 0-43ohms. I guessing those last 10 ohms are whats making my bias not go high enough. Dont think I have any on hand to replace sadly.
 
I doubt the 10 ohms would have that much effect, but it's possible. I vaguely recall that when I worked on an SX-6000 3-4 years ago, I found one of the emitter resistors was open . . . can't hurt to check those as well while you wait.

1711501127020.png

There are also 4 separate ground points on the board - make sure they all have continuity to the chassis.

1711501437418.png
 
I believe I checked them before I got something like .8/.9 which is typical on my meter. I do have extras laying around i could swap in but I will measure again. As well as the grounds here shortly
 
To get the 13mv drop across one resistor I had the pot at around 50ohms so with the other channel maxing at 7mv around 43ohms it might make sense
 
Emitter resistors are good, this problem may be a bit different than Im thinking. My B+ should be around +76v. I only measured around 73v at idle. I didnt think much of 3v but maybe those last 3 volts are enough to lower increase the drop across my emitters. Id have to look at the circuit to be sure, but I haven't touched the Power supply yet, ill give it a refresh and see how that gets me.
 
After taking a look at this similar issue thread, Even though the STV-3 measured fine in circuit drop replacing it did the trick after not increasing above 6mv (just like this unit) I have 3 1n4148s ill throw together and see where that gets me too.

Need help with Pioneer SX-6000

no input for sure .. load doesn't matter as far as i am aware . i normally have speakers attached but i dont think it matters anyway .
audiokarma.org
 
Makes sense. The STV-3 was the next logical place to look. I recall that they typically measure 1.4V ~ 1.5V on a diode test (just measured some on a Sansui 4000 this past weekend and that's what they measured) so maybe your 1.3V measurement should have indicated that it's getting weak. I think the 1N4148s work OK but UF-1004s are the preferred solution . . . let us know how you make out.
 
Richard P said:
Makes sense. The STV-3 was the next logical place to look. I recall that they typically measure 1.4V ~ 1.5V on a diode test (just measured some on a Sansui 4000 this past weekend and that's what they measured) so maybe your 1.3V measurement should have indicated that it's getting weak. I think the 1N4148s work OK but UF-1004s are the preferred solution . . . let us know how you make out.
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I don’t have any UF-1004s but I do have some 1N4004 why appear to be the same.
 
So I replaced the STV-3 with 3x 1n4004 and it measures a 1.6v drop. Pulling the existing STV-3 measured 1.3v drop. After putting in the replacement I get goo bias tracking but It is VERY high. With the pot fully CCW I read around 100mv drop, It tracks nicely if I turn it more but that is still way to high. Im going to put the original back in and see what it reads again to double check.
 
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Original stv-3 back in same story, can only get around 2-3mv bias

Tried 2x 1n4148 and the bias doesnt come up at all.
Tried 3x 1n4148 and bias starts to high around 100mv again. Measured a 1.7v drop.

Not sure when im only getting the two extremes here with my bias diode changes
 
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The sweet spot for voltage drop seems to be 1.4 ~ 1.5V from what was found in the thread linked above. That's why the UF1004 seems to work OK where others don't (as a drop-in replacement). I recall seeing an Xraytonyb video where he adjusted a resistor value or two to compensate for the higher or lower voltage drop of a replacement 'stabistor' as he called it. I think it was this one:
 
I've been messing with it all day now, Swapped the STV-3s and had to change, then I swapped the pots and I am able to get the bad channel bias to 11mv now while the other previously good channel dropped down to only 5mv max. Im thinking these pots are really a problem now.

I also rebuilt by Power supply and raised the B+ from 71v-73 so a bit closer to what im seeing in posts.
 
I have nothing to add. I am just commenting so I can follow along. I recently worked on my SX-6000. I followed Merlynski's instructions from the thread below for setting the bias. I could get one channel to 13mV as he suggested, but the other would only go as low as about 17mV so I set them both to that.

SX-6000 Main Amp repair

I'm working on an SX-6000 refurbishment and have done the Power Supply and Main Amp capacitors and transistors, including the filter caps (except the main output transistors). That work solved a crackly intermittent channel and, if I'd just done a listen test, I'd move on. Since I've read a...
audiokarma.org
 
Byron Boldt said:
I have nothing to add. I am just commenting so I can follow along. I recently worked on my SX-6000. I followed Merlynski's instructions from the thread below for setting the bias. I could get one channel to 13mV as he suggested, but the other would only go as low as about 17mV so I set them both to that.

SX-6000 Main Amp repair

I'm working on an SX-6000 refurbishment and have done the Power Supply and Main Amp capacitors and transistors, including the filter caps (except the main output transistors). That work solved a crackly intermittent channel and, if I'd just done a listen test, I'd move on. Since I've read a...
audiokarma.org
Click to expand...
Seems strange similar to my issue but in a little different way. Does yours have original pots?
 
RyeGuy859 said:
Seems strange similar to my issue but in a little different way. Does yours have original pots?
Click to expand...
yes it does. The next time I open it up I should try and measure their values. I recapped everything but the tuner boards and those two black 4 pin 1000uF caps. I tried unsoldering one of them, but it just didn't want to come off. Maybe glue was holding it on?
 
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