HP E3631A Power Supply bad display/output, potential data issue?

[Dereku3]

Hi all,

I picked up this PS recently that the seller said it "turned on and seemed to work". But unfortunately there was a bit more to it than that...

I have thoroughly searched the forum first, and although there are many similar symptoms I don't believe this exact problem has presented itself (or I'm just dumb)

The unit powers up and passes self tests without error - but gives 2 beeps.
The rotary switch and all buttons are responsive (can hear the clicks), all display segments work.
When setting a display limit, the rotaty switch is able to configure all decimal places without issue.

However when the outputs are enabled the +25v reads as -41.37v/2.339a, the -25v as +41.40v / 2.355a and the 6v as 1.000v / 8.839a.
Connecting a DMM to the outputs they all read 0v, no amount of adjusting the larger digits do anything to the displayed voltage or measured voltage.
However if I highlight the XX.X0v last decimal of +25 or -25 I get 2 large steps in the measured output when using the rotary switch. 0v to 10v and then 20v (-10v and -20v) - but still no change on the display.

Trying to perform a cal, it will accept the 6v cal, but fails on the +25/-25. However once exited from the calibration screen the the 6v remains at 0v;
unless I highlight the X.XX0a last decimal on the current side and rotate until the little CV changes to CC. And this point the measured output reads -0.270v.

Pulling the cover off I tested all bias and reference voltages. All are within spec. U3 in the ADC is outputting +10v -10v as expected.
R4 and R5 are good, along with R15 and R16.

Moving to the DAC circuit; measuring the CV/CC Ref of all outputs show a change between 0v and 2.7v, although in the same big 2 steps.

No output from +6V CV Ref, but the +6V CC Ref shows activity.

Looking at the input at Pin-3 of U13 on the scope I can see a stepped analog signal - and it does change in responce to the above findings that only the last decimal provides any impact (albeit incorrect) on the output voltage/current.
Attempting to adjust any other digit on any other voltage or current outputs shows no change on this pin.

This makes me think that U13 is functional, as it's responding correctly to what little output is coming from the DAC.
The data present at Pin-7 of U12 is the same as Pin-2 of U11 - so I believe U11 is passing the signal correctly.
But I cannot see any significant change of the data pulse when adjusting the settings - but I'm not sure I would with my 100mhz DSO?

There is a chance the DAC/U12 is faulty, but it seems weird that it would still output what looks like a coherient signal; just missing most of the actual data.
So I feel that U11/U12 and U13 are all functional. And instead the actual DAC Data from Pin-15 of U16 is actually bad/corrupt.

But I'm not sure of the relationship between U16 and U17 the microcontroller, and any other potential feedback circuits that might be causing an issue.

Some pointers would be appriciated. I don't really want to shotgun change all ICs at this point!