OSCILLOSCOPE LEADER 4081 = KENWOOD CS-5135 DOES NOT CONTROL THE BRIGHTNESS OF THE TRACE

Dear Foristas, I am grateful in advance for some collaboration with this LEADER 4081 analog oscilloscope equal to the KENWOOD CS-5135. With failures with the brightness control, I have the service manual.
Initially, when it was turned on, it took time to appear the stroke and when it appeared in 10 or 15 minutes, it did not let the brightness control.
Initially, the operating voltages were checked, being all normal, note that when starting the ignition did not show high voltage, the high voltage section was checked and an isolated high-speed diode was found at one of the ends due to oxidation, in the Inten adjust circuit consisting of VR401, coming into operation correctly.
The surprise came after armed and covered when a light was heard when a diode or resistance was broken, leaving the oscilloscope with excess brightness in the stroke and without intensity control this does not work.
The X74-1490-00 card was checked thoroughly to see if something burst or burned I want to emphasize that no burning smell was felt, everything being in normal condition.
Diode by diode, resistance by resistance, transistor by transistor, capacitors all manually with the ohmmeter were checked in detail, making different tests to verify the optimal state of each component, finding everything normal including the pot that controls the Focus intensity and astigmatism.
The approach works normal and astigmatism control. It just does not control the brightness or intensity. Attached scheme and photo of the failure presented.
I reiterate thanks for any suggestions.

I have a 1960s-era Tektronix oscilloscope that developed too-bright not-adjustable-brightness symptoms a few years ago. The fault turned out to be a cathode-to-grid short inside the cathode-ray tube (CRT). My repair was to install a new (well-insulated) transformer to run the CRT heater at half voltage, thus lowering cathode temperature and electron emission.

Thank you Mr. Anderson for your response, I think the same… .be that it is checked and voltages were taken on the electrodes of the tube… all negative and do not undergo changes when operating the brightness controls and VR401.

Thank you Mr. Anderson
for your response, I think the same… .be that it is checked and voltages were
taken on the electrodes of the tube… all negative and do not undergo changes
when operating the brightness controls and VR401. These were the
voltages taken with 10Meghomios probe, none presents changes when operating the
intensity control and neither VR401. All negative voltajes. Unable to conclude
anything.