Very good advise from Bob. It astounds me how any of the components can survive literally mm’s from heat producing tubes. A recipe for failure. We also added a fan in our 21CT55 by cutting off the “chimney”. The hole is already cut in that location. Added a switch to shut it off if needed. As an added precaution, the power is regulated with a Furman power conditioner.
We had a failure after several months of operation. Suddenly lost convergence, focus and the picture shrunk. “Oh no, the flyback!” We traced the problem to an open resistor buried under the damper tube.
The High Voltage was reduced to 10 KV with this failure. It now adjusts nicely from 18 KV to 21 KV as it should. (Edit: Mike saw 25 KV, set it to 21.5 KV) The resistor was an original (made by International Rectifier) special film type, 3.4 Megohm 3 watt. It was replaced with QTY 2 (in series) OHMITE 2.2 Megohm 2 Watt film type which now makes the new resistor a 4 watt unit which is a 25% upgrade for power dissipation.
Did you check the serial number stamped on the wood pull out under the pencil box? Wishing you continued success in the restoration.
A few photos of the bad boy resistor, replacement and fan:
