NGMO1 - unregulated > 10V

[orion242]

Tearing into my R&S NGMO1.  Works fine up to 10V output even under load.  Set the output to 10.1 and it falls to ~7v.  The fan bearing is gone as well.  No error on the display, reads correct voltage / current.  Not in OVP or OCP.  All the power rail LEDs on the main & analog boards are lit.  Seems to have all the power rails to the analog board. 

Initial pic/notes on the analog board here.

https://www.eevblog.com/forum/testgear/ngmo1-0-15v-0-2-5a-power-supply-teardown-(rohde-schwarz)/msg2154727/#msg2154727

Thanks to those that posted the service manual here.  Block diagram is there at least.

https://www.eevblog.com/forum/testgear/calibration-procedure-service-manual-for-rs-ngmo2/msg988839/#msg988839

I'll post more photos with comments and readings as I work through it.

[orion242]

Seems to be an issue to set the voltage.  The LT1166 is getting a ~7v to pin 2 when the supply is set for 10.1v.  Below that pin 2 & 3 match. Light blue is where pin 2 of the LT1166 seems to go.  Note the test pad.  Voltage there = output voltage, lest in this unit.  Green is the main gate drive. Note without the heatsink in place, the main mosfets get hot quick and require attention.

[orion242]

Poked around some more.

The LTC1650 seems to be the DAC in charge of setting the voltage.  Test pad right next to it (labeled V Set) measures ~0.146v per volt set on the screen.  Tested this @ 15, 10, 5, 2.5 & 1 volt, lined up nicely. Assuming its fine up to that point the fault has to be in the sea of op amps between there and the AD825s that lead up to the LT1166.  Spent some time looking into the other test pads and trying to figure out what they correlate with while I'm tracing out the whole mess.  Took a bunch of measurements at various settings around the DG409, test pad near the top (orange line) seems to be a fixed 2.440v.  Pin 11 on the DG409 I suspect has to do current measurement in to ADC.

Guess its either follow from the LT1166 up or the V set pad down.

[orion242]

DAC08

Trying to wrap my head around this part.  Am I correct to assume this is a current based DAC where the digital end is setting a multiplier that's applied analog reference current?

What's the most likely purpose?  Setting the output current limit I assume.