DS160 H selection problem

I have a DS160 H that has the scan twice then stop problem but with a twist
It runs fine for the first few minutes then it stops seeing the selection.
The fact that when I turn it on it has several selections queued leads me to believe its the read out circuit rather than the write in.
other things that lead me to believe this is that selecting all the torroids by applying a battery to the phono plug has no effect nor does pulling out and reinserting the phono plug.
However when I pull the 12aX7 out and reinsert it I get a selection while tells me the thyratron is working
Looking for any advice on where to go next and what the model number for the controller is ( this one is missing the nameplate) and finally where I might find a schematic online
Thanks in Advance
BOb

Bob;

Sounds like it's time to replace the 0.05 uF capacitor and 5.6 M resistor in the pulse amp.

Cheers,

Tony Miller

Clean the tube sockets with oxide remover.

They corrode over time.