TDS640A is a bit later modell: possible have Tek no problems with capacitor. I have one Tek TDS520, which their CPU Board has orginal capacitor, but no fault. Why? Because, this CPU Board is a later reversion (1993, maybe a replacement for repair)
A check: Soldering on capacitor, if it smells very bad (urine), this is a alarm signs.
If yes, someone trace is etched trought. Find of this fault, which Tek doesn't make complete startup. It is very bad !
One tipp: Doesn't power up it anymore and desoldering Dallas NVSRAM and save their data.
I have a bad experice with halfdead cpu board: It can make NVSRAM Data corrupt.
That you can read in dallas NVSRAM, what have Tek a problems. I'll writing into NVSRAM, even if it doesn't make a sign on CRT.
TDS640 ACQ Board Schematic exists. But not one for CPU Board.
http://w140.com/tekwiki/wiki/TDS640