Please help! Pioneer QX 949 Offset set to zero but unable to set bias appropriately

[backmd]

Please help! Offset set to zero but unable to set bias appropriately

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when dc offset adjusts, yet idle current doesn't - differential voltage reading are your friend.

measure across the bases of the driver's (Q9, Q11 are they new? a1220/c2690) for 1.8v and that it IS adjustable over several tens of millivolts (at least) - use another channel for comparison
Then switch to the emitters and look for 1.2 volts less overall voltage BUT the SAME amount of millivolts adjustment as the bases had.
the way the voltages are determined is to start at zero volts and add in each stage's Vbe drop.
thus data is below
+/- 0.6v or 1.2v base to base at the outputs 1.30 is measured
+/- 1.2v or 2.4v differentially base to base at the drivers 2.1 is measured how much adjustment?
2.2 to 2.7 so it can vary about 0.5 volts
Then switch to the emitters and look for 1.2 volts less overall voltage BUT the SAME amount of millivolts adjustment as the bases had.1.31 to 1.52 so it varies only 0.22 volts.
What does this mean?

 

[backmd]

More data, trying to get the bases of the drivers at the 1.2 volt spec

We have a real clear schematic now.
Spec Measured
Base of 2SB528 -1.2 -1.1
Base of 2SD358 1.2 1.1
R38 -28.0 -27.5

 

[backmd]

Here are the links to schematics and service manuals

http://akdatabase.com/AKview/albums/userpics/10004/Pioneer QX949 Schematic.pdf

http://akdatabase.com/AKview/albums/userpics/10004/Pioneer QX949 Service Manual.pdf

 

[backmd]

Mystererious Extraneous .005 or so ceramic cap connected to ground and?

There may be different reading on My Fluke multimeter on mv and volts, perhaps the amp is oscillating and has different readings on mv and volts. There is a ceramic capacitor that is not on the schematic. One end is connected to ground and the other was connected to pin 19, the -42 volt negative supply

 

[backmd]

After further analysis and thought, the .01uf ceramic capacitor is in parallel with the 10uf electrolytic. Electrolytic capacitors according to Walt Jung's article on picking capacitors, cease to be an ideal capacitor at high frequencies. The ceramic bypass capacitor in parallel makes up for the deficiencies and performs well in the high frequency range. Therefore oscillations will be less likely. I probably need to measure the power supply capacitor also and make sure it is 10uf and add the .01uf ceramic bypass capacitor. Boy I did not realize that we must think of both AC and DC circuit analysis, otherwise risk an unstable amplifier. Wow, I removed the 10uf capacitor that was not on the power supply schematic by mistake. Let's reestablish the 10uf and .01uf caps on the 42 volt negative supply

 

[JURB]

Set the bias first with no load. Then set the offset. Also with no load.

 

[backmd]

Back to the drawing board!

Set the bias first with no load. Then set the offset. Also with no load.

Retried the procedure and got the offset to zero, but the bias is 78mv, at extreme counterclockwise rotation of the potentiometer.?

 

[petehall347]

check values of emitter resistors ..

 

[backmd]

Direct short across the diode/bias pot

Retried the procedure and got the offset to zero, but the bias is 78mv, at extreme counterclockwise rotation of the potentiometer.?

Well maybe this starts to make sense. When measuring the bias pot. There is a direct short. Will have to pull a leg of the diode to determine whether the short is in the diode versus the pot. But pots usually do not fail that way.

 

[petehall347]

is this one or both channels ?

 

[backmd]

Diode short?

Just one of the four channels

 

[petehall347]

pot will show short extremely one way as centre tap (wiper) is joined to one outside ... pull diode to test properly

 

[backmd]

Pot is shorted.

Very poor access to board, had to crush diode since I could not cut it. After crushing the diode, the pot still had a short ruling in a shorted potentiometer.
Probably have to find a 15 turn 100 ohm potentiometer now, and if I do recall Radioshack used to sell such. Addendum, they do have the 1k ohm 15 turn cermet potentiometer, so we may be able to bias tonight!

 

[petehall347]

that's a new one on me ..never seen a shorted resistor of any kind ... they normally go from ok to high then open ,,,

 

[backmd]

Remeasured before sacrificing the 100 ohm potentiometer

Before replacing the potentiometer, I remeasured it to reconfirm that it was shorted, and it WAS NOT shorted. Will solder in the D4 that was sacrificed, and then try to bias. Will start with minimal ohms on the bias pot as measured with the Fluke.

 

[backmd]

Next will check the emitter resistors.

Retried the procedure and got the offset to zero, but the bias is 78mv, at extreme counterclockwise rotation of the potentiometer.?

. The emitter resistors are connected in series and they total 1.0 ohm and that is what I measure on the Fluke multimeter.

 

[petehall347]

voltages q8 q10 q12 ..
voltage across d6
voltages q14 q16 .

 

[backmd]

Pioneer published specs versus measured; AWH-027

voltages q8 q10 q12 ..
voltage across d6
voltages q14 q16 .

We have a real clear schematic now.
Spec Measured
Base of 2SB528 -1.2 -1.1
Base of 2SD358 1.2 1.1
R38 -28.0 -27.5
Base of Q6 -41.4 -42.1
Base of Q8 1.8 1.56
Junction between
R34 and R36 19 19.6
R18 .07 0.0
R14 -.12 .56
Later I will measure the voltages at Q6, Q10, Q12, Q14, Q16 and the voltage across D6. Hopefully D6 is still functional as that would be difficult to replace.

 

[petehall347]

righto according to the drivers base voltages +- 1.1v bias should be low.as emitters should be at 500mv +-
check r42 and r40 . 150 ohm 1/2 w
and voltages of drivers q10 q12 ... collectors and emitters ..

 

[backmd]

Befuddled and confused

righto according to the drivers base voltages +- 1.1v bias should be low.as emitters should be at 500mv +-
check r42 and r40 . 150 ohm 1/2 w
and voltages of drivers q10 q12 ... collectors and emitters ..

righto according to the drivers base voltages +- 1.1v bias should be low.as emitters should be at 500mv +-
check r42 and r40 . 150 ohm 1/2 w Checked the driver emitter resistors in series and they measure 295 ohms respectively. Spec is 300 ohms
and voltages of drivers q10 q12 ... collectors and emitters ..
We have a real clear schematic now.
Spec Measured
Base of 2SB528 Q12 -1.2 -1.1
Base of 2SD358 Q10 1.2 1.1
R38 -28.0 -27.5
Base of Q6 -41.4 -42.1
Base of Q8 1.8 1.56
Junction between
R34 and R36 19 19.6
R18 .07 0.0
R14 -.12 .56
Q12 emitter -0.67
Q10 emitter 0.64
Q12 collector -34.3
Q10 collector 34.3
So the drivers have around 0.4 volts forward bias?