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Pioneer SX 990 Bias Issue Too High Won't Adjust

T

tonebrain

Active Member
Pioneer SX 990.

Went at it cuz the left channel was posting bias readings of 200 plus mvolts. Variable pot makes little difference in bias reading but does adjust
voltage into base of Q6 similar to good channel.

I have tested just about everything in the circuit and replaced a few resistors and C11/C12. All the left channel transistors, the diode, the VRs
seem to test fine. pulled C6 and C8, powered it up with a 60 watt bulb and results were the same. Even voltage drops on transistors look good.

Pulled the outputs {which are fine}. The collector of Q6 reads 800 mv. So with the output transistor voltage drop that gets the bias to 200 mv. Collector of Q7 is
closer to 300 mv and with everything in place, that channel ran proper bias. Right channel read normal bias when everything was hooked up. VRs both sides seem to drop comparable voltage when adjusted and read out 50 ohm resistance range. One thing I noticed was the voltages on the bad channel read a bit lower than the good channel. Chalked this up to higher bias current, but not sure?

At a loss here and really don't want to start replacing everything just to get it up and running. Actually, it does run as it is, but am afraid 200 plus mvs will
fry the one channel and cause a lot of damage. Any suggestions?
 
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R13 and R19 should be checked for value one leg removed for confirmation.
50 ohm trimmer?
A high bias reading means a high resistance between the drivers. As you dial the trimmer up you are increasing the resistance and increasing the current to the outputs.
Aged resistors, poor solder connections and wrong value replacement components can cause this.
Also on rare occasions the STV can short one of the several series diodes and this will cause a high bias. A fluke will check and confirm the forward voltage. You could read the right side and compare the two? For a -3 it should be around 1.5v.
 
That helps, as I was struggling to figure out exactly how the bias resistance affects the bias itself.
The bad side VR pot does have a slight effect {enough so that if the bias was within acceptable range it would dial the bias in}.
The STV seems good, as both sides measure forward voltage around 1.3 volts and the right side is working fine.

I will go over the bias circuit resistance again and post later today what I find. Thanks!
 
Okay, with 60 watt limiter bulb and no outputs installed.
B+ 56.8 v
R19 52.8v R20 52.7
R19/D1 29.5v R20/D2 29.7v
D1/VR3 27.9v D1/VR4 28.1v
VR3/Q4C 27.9 {adjusted to 0 ohms} VR4/Q4C 27.9 {midpoint adjustment}

R25 29.7v R26 29.2v
Center rail 28.7 28.6
Q7 E 28.4 Q8E 28.5
Q7 C @850mv Q8C @300mv

I don't get it? Q7 tests good, is it possible its starting to
short out, even though the Vdrop test shows good?
 
I would change out the bias pot. Set the new pot at 30 ohms and use a dim bulb tester when adjusting. I have rebuilt many of these Pioneers of this era. (1000, 1500, 2500, 990, 770, etc.) When I get one where one channel's outputs are blown, more often than not, there is a bad bias pot on the amp board. I now replace them as a standard practice.

I have never come across a bad dc balance pot, though.
 
ohm check the emitter resistors. Don't have a schematic in front of me to tell you what numbers, but they are the large low value ones that you are measuring across. If they have gone high in value for some reason, the voltage drop across them will be higher for the same current level.

and yeah those bias pots are crap. My Allied 495 is an SX-1500TD under the hood, and it lost a channel because the bias pot failed. I replaced it with a used original from an SX-990 parts unit, which was perhaps not a moment of brilliance on my part but I had the part on hand and combined with good used output transistors from something else it got it working again for no money.
 
The emitter resistors seem okay, they are suppose to be .5 and all read 1 ohm on my meter, which is probably as accurate as my
meter can get at that low register {although I will check my meter with some new 1 ohm resistors just to be sure}.

As crappy as the bias pots are, the bad channel pot seems to be okay, testing both resistance without power and voltage adjustment with
power, it seems to be as good as the bias pot in the good channel. I have a new one that I may stick in anyway, just to be sure.

What I did find is that the cold resistance on the STV 3 diodes is off by a meg. The STV in the good channel measures 11.21 M while the
high bias channel STV measures 10.38 M. They both drop about the same voltage {cold}, somewhere @ 1.3v, which seems to be okay for
a triple diode?

I don't see how a 50 ohm VR pot can make up a meg of difference between the STVs? although I get that when they heat up they
both should drop their resistance. Are they somehow going to reach the same resistance warm, even though they are a Meg apart
when measured cold? Is the meg difference the cause high bias on the one side or can't we trust a cold measurement of a triple diode?
 
The bad side bias trimmer also seems to adjust the voltage properly when powered on.
at least as good as the good side VR, it just can't compensate for the 800 mv being sent to the
output base {bad side}.
 
Time to pull that amp board and start testing transistors, caps, and resistors. Both sides. It'll be faster in the long run.
 
Q7 did diode test good, with the proper voltage drops, could it test good and yet send too much current base to collector when powered?
Did a lot of testing {board is out} and not finding any obviously bad components.
 
Since it's out, replace all of the caps and the two 2SC458 transistors.

It would also be better to test the transistors off of the board. Take lots of pictures. You'll find the issue.
 
usually when I've had a transistor that passed the diode check but didn't actually work properly in circuit, the gain went low, not high. Without looking at the schematic I couldn't guess if low gain would cause high output stage current but I tend to doubt it.


what do you mean the ohm check on the triple diode, for reverse leakage? What that does i wouldn't think its super critical, especially with both being north of 10M. Forward voltage drop is generally the more important value. I guess if there was a doubt you could switch it channel to channel to see if it moves. I have done that in the past when I just couldn't figure things out.
 
gadget73 said:
usually when I've had a transistor that passed the diode check but didn't actually work properly in circuit, the gain went low, not high. Without looking at the schematic I couldn't guess if low gain would cause high output stage current but I tend to doubt it.


what do you mean the ohm check on the triple diode, for reverse leakage? What that does i wouldn't think its super critical, especially with both being north of 10M. Forward voltage drop is generally the more important value. I guess if there was a doubt you could switch it channel to channel to see if it moves. I have done that in the past when I just couldn't figure things out.
Click to expand...
Okay, I wasn't sure if the resistance of the triple diode was a critical factor. I thought of switching them and may do that, just to be
sure. I think I may go at Q7 first. It appears to be getting all the correct voltages, but not acting correctly. Been holding out on that, as
I don't have a sub laying around.
 
tonebrain said:
Okay, I wasn't sure if the resistance of the triple diode was a critical factor. I thought of switching them and may do that, just to be
sure. I think I may go at Q7 first. It appears to be getting all the correct voltages, but not acting correctly. Been holding out on that, as
I don't have a sub laying around.
Click to expand...
tonebrain said:
Okay, I wasn't sure if the resistance of the triple diode was a critical factor. I thought of switching them and may do that, just to be
sure. I think I may go at Q7 first. It appears to be getting all the correct voltages, but not acting correctly. Been holding out on that, as
I don't have a sub laying around.
Click to expand...
you mentioned when you had transistors that tested good but didn't function properly, they usually went low, not high. In the SX 990 bias circuit that is running too high and won't adjust, I was previously thinking that the driver transistor that was sending .8v to the base of its output transistor was at fault {output tran drops .6v and we end up with 200 plus mv of bias that won't adjust out}. So, I pulled that driver and replaced it with a new replacement and nothing changed, still 800 mv at base of output.

I then noticed that the other driver in the faulty bias circuit which had also tested good, was when under power was only dropping .2v {as you mentioned, tested good but performs weak}. I don't have a proper replacement to see if that would fix the issue and have to order some. My question would be in the interim, if the complimentary driver in the bias circuit is weak and not consuming proper current, could that be sending excess potential to the other driver in the bias circuit, which responds by sending too much current to the base of its power transistor, which in turn pushes the bias too high and out of adjustment range of the VR?

I think {but am not sure}, wouldn't all that align with what was mentioned above, as a driver in the bias circuit that is weak would be adding resistance to the bias circuit thereby raising the bias beyond adjustment?

All other resistances in the circuit and supplying the circuit are spot on. Replaced quite a few resistors and the VR.
 
0.2 volt drop on a silicon device would be unusual. About 0.6v is typical for an Si junction.

as for the rest of the question, honestly I don't know without some study time and probably some liquid thought enhancement to puzzle it out.
 
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