Pioneer SX-780 STK-0050 voltages

[lebrun53]

Hello all,
I have a SX-780 with voltages on the STK-0050 pins that are slightly high.
(This based on other posts here)

Electrolytics on the amp/power board have been changed
Transistors on the amp section all changed per recommendations on other posts here.
Transistors in power supply circuit have been removed, tested and reinstalled.
Voltages from power supply circuit all good.

Here are the voltages on the STK-0050 pins with the power packs removed and the 1k resistors installed.
Left:
Pin 1 -7.81v
Pin 2 -41.16v
Pin 3 -.02v
Pin 8 -.01v
Pin 9 +41.12v
Pin 0 7.78v

Right:
Pin 1 -7.85v
Pin 2 -41.27v
Pin 3 -.01v
Pin 8 -.01v
Pin 9 +41.20v
Pin 0 7.83v

My question is:
Pins 1 & 0 are mentioned to be +- 6v
Is my difference of 1.8v within tolerance?

I have new (clone) power packs ready to install, but wanted to check here for opinions before installing.

What do you Gurus have to say?

(Thank you for being here.... a tremendous resource)

Thanks in advance,
Larry

 

[Watthour]

Are the 1KΩ resistors installed between pin 0/8 and 1/3? They should be pulling voltage down to a lower level.

With the resistor installed, can you adjust the DC Balance (offset) trimmers VR5/VR6 to get the voltages at pins 3/8 down closer to 0.00V Your indications are that they are presently around 10-20 mV, and getting to ±5mV would be good.

With zero input signal, output modules installed, and everything functioning properly Pins 0 and 1 should be 1½V.

 

[lebrun53]

Hi Watthour,
1K resistors are on the proper pins as you noted.
With lots of tweaking I now have:
Left:
Pin 3 -1.5mv
Pin 8 +2.5mv

Right:
Pin 3 -3.8mv
Pin 8 -2.1mv

slight fluctuations in the +- 1.5mv range

1. Could my Pin 1 & 0 voltages be slightly high due to my high Pin 2 & 9 rail voltages?
2. Could my Pin 1 & 0 voltages be slightly high due to the replacement transistors in that circuit?
3. Is there a way to bring the voltage on Pins 3 & 8 down?

Sorry for lots of questions....
I really do not want to blow my new (albeit fake) power packs.....

again, thanks....
Larry

 

[Watthour]

Yes, the voltages you are measuring could be high due to several causes, including the higher than typical DC bus voltages, as well as tolerances of the components installed.

I would suggest that you construct a dim bulb tester before trying your output modules in the unit.

 

[merlynski]

I would suggest that you construct a dim bulb tester before trying your output modules in the unit.

+1 for the DBT. I have talked a bunch of people through trouble-shooting the VA-Driver-STK stage and have yet to see one fail (with a good STK or custom module) once the pins 1 and 0 are nicely balanced +/- around 0v and the DC offset will adjust and be stable with the 1kΩ resistors in place.
The constant current through the STK pins 1 and 0 is established by the transistor (Q15 in the left channel), diode and 2 resistor circuit connected to pin 0, which sets the separation of the pins voltages across the 1kΩ resistors. The DC balance feedback path (and audio negative feedback) comes from the junction of the STK output emitter resistors (set dc offset to 0vdc here), through R255 (left ch.) to one base of Q7. The other base of Q7 is the 0vdc (offset) reference and the audio signal input of the Q7 differential amp and Q9 current mirror. The dc offset correction and audio signal is then fed to to Q11-Q13 pair to drive the STK. Once the STK is in place it is an active component in the feedback loop, which the 1kΩ resistors are not. From what I have seen the pin 1 and 0 voltage differences are always greater than the voltage with the STK in place. But use a DBT for the first power up . Sorry if I rambled on too much there .

 

[lebrun53]

Thank you both for the replies....
I always use a DBT when any changes are made.
I'll install the STKs tomorrow and report back....
lb

 

[lebrun53]

Update....
Before installing the new STK power packs, I replaced all the power diodes, the D1, D2 diodes and the HOT 2SD712 in the power supply.
Temperature went from 70 degree C to 52 degree C. (MJE15032)
Re-adjusted the pin 3 and 8 voltages back to the lowest I could achieve.
Installed the STK-0050s
ALL IS HAPPY.
Re-adjusted DC offset to .5mv +-

Thanks to the Gurus for the help.

Larry

 

[merlynski]

Great, enjoy your SX-780!
What voltage did your STK pins 1 and 0 end up with once they were installed?

 

[lebrun53]

All at 1.18v
DC offset .5mv
again, thanks

 

[merlynski]

All at 1.18v
DC offset .5mv
again, thanks

Excellent, great results, I appreciate the feedback, helps me help others down the line, and you are welcome!