• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Pioneer SX-780 VU Meter Issue

You can test them in place, you can tell if they are shorted or open. If you pull them you just as well replace them. If you have replacements on hand, pulling and testing may tell us where to go next. If you have replacements don't install them yet. Once the old ones are out it makes testing other components in circuit easier. The meter test can tell you if they are bad, it cannot tell if they are good.
 
Q11: B-C=0.675vdc/OL, B-E=0.687vdc/OL, C-E=0.663vdc/OL
Q13: B-C=0.662vdc/OL, B-E=0.681vdc/OL, C-E=OL/OL
Q15: B-C=0.648vdc/OL, B-E=0.663vdc/OL, C-E=1.6vdc (and climbing)/1.8vdc (and climbing)

R259=151.5 ohm
R261=100.5 ohm
 
I don't see anything definitive there.
Test D1, Measure resistance of R253.
Power up and measure dcv at the emitter of Q7, and each base of Q7, and the base and emitter of Q5, and the base of Q11 and base of Q13.
What color are the stripes on R259, Red Black Brown Gold or Brown Green Brown Gold? Or something else?
 
D1: 0.6vdc/OL
R253= 295ohms
Q7 Emitter=0.660vdc; Top Base= 0.63vdc; Bottom Base= 0.036vdc
Q5 Emitter=-0.571vdc; Base=0.036vdc
Q11 Base=-39.5vdc
Q13 Base=-40.2vdc
R259 is Brown Green Brown Gold = 160ohm
 
If you could fill me in on why you think Q11 & 13 are the culprits it would be appreciated.
Thanks
 
The collector of Q11 & Q13 should be much more positive, the base of Q11 is being pushed harder negative (-39.5vdc) than when it is balanced properly (-37.7vdc, from schematic), which makes me think the diff amp (Q7) is trying to turn off the darlington pair Q11/Q13 to drive the collector (STK pin 1) more positive. Since it acts like Q13 is saturated (full on) when it should not be, it points me at Q13. Meters are notoriously poor at detecting high voltage failures in transistors, so I am not surprised it did not show a fail on Q13 E-C. Since Q13 and Q11 are closely dc coupled it is better to replace both at the same time. I could be wrong, and the failure is Q7, that will be my next choice to change. Sometime you can't tell when you are inside a feedback loop, but the input at the connected bases of Q5 and Q7 are at 0vdc like they should be, and the output (pcb pin 9) is way too negative, and STK pin 1 is also way negative so . . .
 
Q11 & Q13 have been replaced.
No change in base voltages of Q11 & Q13
voltage readings
Q11 Base = -39.5vdc
Q13 Base = -40.2vdc
 
Well shucks! What are the voltage of pins 0 and 1 to the STK, did they change?
Do you have replacements for Q7 (512-KSA992FBU pair, match VBE,hFE if possible)?
Q15, 512-KSA1013YBU
Q9, (512-KSC1845FTA pair, match VBE,hFE if possible)?
 
Do I need to reinstall the STK's before measuring pins 0 and 1?
I don't have replacements for Q7, Q9 or Q15
 
jdr76 said:
Do I need to reinstall the STK's before measuring pins 0 and 1?
I don't have replacements for Q7, Q9 or Q15
Click to expand...
No, you should NOT install the modules before fixing the SX-780's voltages on pins 1 and 0, that is the whole point of using the resistors. I have never seen a good STK fail to work in a unit that has the dcv at the pins 1 and 0 inputs properly balanced. What I have seen is bad transistors and fried resistors causing those voltages to be unbalanced.
Acquire transistor replacements, while the old ones are out measure the resistors. If there is any question of correct resistance in circuit, lift one end of the resistor from the board and measure it that way.
 
The voltages on pins 1 and 0 are unchanged.
I've been doing some additional voltage checks in an attempt to better understand the circuit. In the process of making these checks I realized that the schematic and the circuit board layouts I have don't match the unit I have. The schematic and board layout show the collector of Q15 going directly to Pin 0 via a jumper. On my circuit board instead of a jumper there is a 430ohm resistor with a diode going to pin 9 of the STK as well.
The collector of Q15 is 20vdc where as pin 0 of the STK is 17.2vdc.

Also, should I be making these measurement with speakers hooked up or not?

Thanks
 
No speakers needed. If the relay is not pulling in they cannot affect the circuit anyway, until it does click in.
Is there a similar modification to the other channel, collector of Q16? Do they look like factory original or a later customization (if you can tell)?
This modification (I think) kills the constant current control of Q15. I would take out the modification and return the pcb to original configuration. The new/upgrade modules are intended for the original circuit configuration.
 
Last edited:
All of those are definitely original from the factory as they are part of the pcb traces and not scabbed on. Using post '11' as a marker, the position of the resistor and diode are shifted to the left versus where the jumper would have been to the lower right of that. This is not an aftermarket modification but part of the original pcb for sure. There looks to be other components in the general area that don't show on the service manual pcb layout. Looks like more feedback components to me. But I'd have to trace them out to be sure.
 
Curiouser and Curiouser, said Alice. What version of PCB does your unit have? It should be something like GWK-118, then another letter/number combo printed in a box near it? What year of manufacture does you serial number show? (https://www.audiokarma.org/forums/index.php?threads/pioneer-dates-of-manufacture.56786/)
OK, Pioneer made changes that never had documentation reach us in this latter day real world. Now on my schematic there is a diode and resistor from the base of Q15 going to the +39.5vdc rail that is connected to emitter of voltage regulator Q19. That regulated voltage would be a much better source for a constant current circuit (R353 300Ω, D1, and Q15) than the pin 9 unregulated "+40vdc (+34vdc)" supply that varies under load. Did you follow the trace to pin 9 of the STK connection?
Can you post pictures than clearly show what you are referring to?
 
You must log in or register to reply here.
Back
Top Bottom