Qsc k10 won't turn on

[Bigdrums]

I followed the troubleshooting guide offered by QSC Service Manual starting pg 31 to test PSU/AMP board isolated. The line filter components test shown at the top of the page tested good.
Previously I found Q13 & Q14 suspect and replaced them.

I have checked the voltages shown on the bottom of pg 31 chart of the linked service manual and found only .6-.9 volts where there should be 41vdc between VIN and PRI_LO, measured at R147 to R150. I measured on both sides of the resistors in case they were bad.

The trace is good between the 2 resistors and the 41volts originates at U15 pin 16 I think. Schematic for that is on pg 36. Difficult to check an IC but found no shorts between it's pins.

Can anyone suggest why this might be, or what to check next?

Thanks!

Here's the link to the service manual:
https://elektrotanya.com/qsc_k-serie.../download.html

[Randall]

I have a pair of these. This has been on ongoing fear.

[Jazz P Bass]

While U15 'may' be loading down the voltage, it is not the 'source' of the voltage.

The source is the bridge rectifier +.
Verify that the bridge is working.

All Primary Side voltage checks of the SMPS must be referenced to Pri Lo.


Verify that the two strings are intact (R152 to R189 & R144 to R147)
These are the start up voltages for the controller ic U15.

qsc_k-series_power supply.pdf

Man, that is one heck of a decent service manual.
Hats off to QSC.

[Bigdrums]

I checked the strings which are good. Also verified Pri_lo to Pri_hi at R147 to R144 reads 163v so that means the bridge is working I believe.
But I still have no 41 v at VIN

[Jazz P Bass]

Well, that is odd.
What is the total resistance of string R147 to Pri Lo?
If they are accessible, you could measure that voltage at each node of the string.

It could very well be that U15 is toast.

[Bigdrums]

Difficult to get a solid resistance measurement I imagine due to charging caps but each of the R144-147 measure 11k.
Each end of those 1watt resistors are soldered to a pad shared with the previous one.
How can I test to tell if U15 is bad?

[Bigdrums]

Looks like D40 & D27 are bad. QSC is apparently using transistors for diodes as the board components are 3 leg.
Can you comment on weather these 2 diodes being shorted would cause the unit to not turn on?

[The Dude]

Did you check them out of circuit? Both of them are parallel with 10 ohm resistors, so you'll be reading across the resistors if you measured in circuit.

[Bigdrums]

Dude: so true. I have decided to send it off for repair. Thanks to all

[Jazz P Bass]

"QSC is apparently using transistors for diodes as the board components are 3 leg."

The three leg configuration is common in a surface mount diode, although not the only package type.
On a single diode package the third pin is not used.

SOT 323 diode.pdf