BK Precision 8500 Repair Attempt - Doesn't draw current

[Shay]

Hello,

I am trying to repair a BK Precision 8500 DC electronic load.

The load powers up just fine and passes a self-test; it measures the correct input voltage. I am able to access the menu and set any mode I'd like without any issues.
However, as I set up CC mode or any mode, choose any current, and click on the ON/OFF button, the unit just does not draw any current at all.
I tried resetting the config in the menu to default, and I just couldn't get the unit to draw any current.

Upon further investigation, here is what I have come up with so far:

The control loop for the mosfets is working well.
The supply voltages are good.

I am able to make the unit draw current and show the drawn current only when I move the encoder constantly. Otherwise, it just sits at 0A.
Upon testing the DAC (https://www.ti.com/lit/ds/symlink/dac7631.pdf), I've noticed it does produce output voltage when playing the encoder fast, which means the DAC is probably working OK and the control loop for the mosfets is fine.

Probing the SDI pin (serial data input) on the DAC shows that there is some communcation and there is a CLK signal on the CLK pin. However, on the CC/CV Mode I only get those pulses of communcation when rotating the encoder fast. on the CW/CR I constantly get bursts of data, but no current at all (no output on the DAC)

Could it be a faulty DAC?

I would appreciate any help in investigating the problem.
Shay.

[Swake]

Is there only one DAC? If so, is it used for the input voltage too, because then it is working.

You might have a look a the remote sense inputs. Potentially there is some fault here.

Would there be some external input/trigger required? This machine can likely be commanded with signals on the back.

[Swake]

There is a short FAQ in the manual with a question about your situation. They suggest to verify some specific settings.


https://bkpmedia.s3.amazonaws.com/downloads/manuals/en-us/85xx_manual.pdf
Page 73

[Shay]

And as always.. when repairing something, do the easiest thing first. No, its not measuring the PSU rails.. it's reading the manual. 
Thanks @Swake, that was the issue. Although I tried resetting the config settings, its not related to the "system" settings.