tektronix TDS500 TDS700 power supply models PSU repair

[sambor]

Thanks to all of you for many useful hints in this thread and others that concern Tek TDS LV PSU repair.

I am a troubled owner of Tek 784D scope that simply stopped switching on after several months of storage. It has later style power supply with schematics available here: https://download.tek.com/manual/070971003.pdf

I was able to pinpoint component U3 on the vertical board which happens to be dual flip flop latch dealing with 'on/off' signal as well as 'one-shot trigger'. After replacing U3 all back to normal but after few days it got damaged again after plugging PSU to the grid and flipping switch on the backplane.
After failure U3 has a resistance around 3 ohm between VDD and VSS which causes standby circuit to go into hiccup. Replaced U3 again and after a day or two it's the same story. I went through several U3 components after which I soldered DIP socket which obviously didn't sort my problem.

I am able to bypass U3 but don't want to give away front power button and 'one-shot trigger' functionality. Any ideas what could be causing surge that is damaging U3 upon connecting to the grid ?

I checked and replaced VR26 and CR17 on the main board (although found no damage to the components) as well as replaced most electrolytics as a matter of precaution. This change nothing regarding repetitive U3 failure. 
I was thinking of replacing VR24 and CR48 but they measure fine in the circuit using diode test. BTW, my 15V standby voltage is around 12.4V and not 15V which I find odd. Could it be related ?

Many thanks
Sam

[sambor]

Update: U3 component changed to TI supplier instead of Fairchild. Unit is operational again although requires NVRAM battery replacement.

[TERRA Operative]

Ebay user kellyjo_13 has replacement NVRAM modules that work really well. I have a them in a couple scopes and they work perfectly.

https://www.ebay.com/str/kellyjo13

https://www.ebay.com/itm/302913384290