Just a note on service on the 25" chassis, I had one that worked on a 25" tube, and was testing on a 27" tube. After bad results on the 27" I transfered it back to it's 25" and the chassis would not come up.
Even though the 27" was powering up, between shutting it down and reseating it on the 25" somehow the HOT got roasted.
The symptoms are upon power up you can hear a high pitched whine repeated every 5 to 10 seconds as the power supply tried to ramp up.
Replaced the hot and all is good.
Later,
dabone