Hp34401a resets itself in 2 seconds cycle.

[henas]

I hope someone can push me in the right direction.
Ebay purchased Hp34401a after switching on lit all the segments followed by displaying GPIB address and shortly after resets itself. This can go in 2 second cycle forever. There is no "Error" message segment lit on the display.

This unit has single chip display but as far as I can see it works fine and is communicating with main CPU. If I press any front panel button quickly enough after GPIB address appear and before the reset is invoked there is some response. It depends on the function button pressed I have either single short beep or some solenoid. In addition the display is acknowledging rear/front switch press displaying "Rear" segment.

Just to test if CPU can detect an error and report it to the display I disabled communication with earth referenced logic.
As previously all segments are lit followed by GPIB address but in addition "Error" segment is lit  too.
As previously the unit rests itself, now in longer 7 sec cycles.

I checked all floated voltages 5v/18v/-18v and earth referenced 5V. All well with in spec with no excessive noise. I checked -15v/+15v on CR402/CR401/CR304/CR305/CR306/CR307 all good. Reference voltages +10V, -10V,  5V and 7V are OK too. Tantalums seem to be fine but I replaced a few with slightly higher ESR anyway.

It appears  the unit resets itself in sync with power on reset signal XPONRST. I lifted pin 5 on LM2725 but no change. XPONRST still present.
What am I missing? Any help is appreciated.

[Kwakerman]

Do all the power supply rails stay up during the reset process (just wondering if it may be the PSU dropping out and causing a reboot rather than a reset)?

[henas]

I recheck all the voltages including reference and they are rock solid, no dipping.

[Samogon]

I would look at digital part, MCU RAM ROM.