E3631A output/limit issues

[dorkshoei]

I just picked up a E3631a.  I read this repair (https://www.eevblog.com/forum/repair/agilent-e3631a-problems-i-just-keep-getting-lucky-not/msg3610050) but my situation seems different.

At power on:  +6V reads 0.201V 0.036A CV

I can set a limit voltage,  say 4V.   I can set a limit current.

If I connect a voltmeter without adjusting the output voltage, the voltmeter displays the limit voltage (4V).

If I reduce the output level (not in limit mode) the display does not change,  still shows 0.201V  but the connected voltmeter shows the lowering value.

If I increase the output level (not in limit mode) the display does not change,  still shows 0.201V  but the connected voltmeter shows the increasing value.     

In addition,  I can adjust the output so it exceeds the previously set limit value.  The display still shows 0.201V but the connected voltmeter displays the increased voltage (above limit) AND if I switch back to limit mode,  the limit value is now HIGHER than what I previously set and it matches the voltmeter.

Anyone have any suggestions on where to start debugging?

[HighVoltage]

Start debugging always with ALL power supply rails.
Look for correct voltage and low ripple.

 

[dorkshoei]

Start debugging always with ALL power supply rails.
Look for correct voltage and low ripple.

All the power supply rails are in-spec.   Schematic is on page 127 of the service guide.

As I said.  The issue occurs on all 3 outputs.

The value on the display is not the value output,  rather the limit value is what is being output (to an attached voltmeter).

Nothing changes on the display (in normal mode) when the output knob is being turned until (what should be the displayed) the output exceeds the set voltage limit value, at which point the display suddenly shows this value (>limit),  the attached voltmeter shows this value (> limit) and if you switch to the limit display, the limit has now increased past it's previous set value.

I suspect whatever part of the circuit is responsible for this output vs limit comparison is faulty.