Before you try to clear a short, you should be very sure that it is
indeed the tube that is causing the control failure.
With the CRT out of the socket, or atleast with the filament only
disconnected, measure the difference potential
between the cathode & the control grid. The brightness control should be
able to swing the grid to -50 or maybe
-100 volts with respect to the cathode.
If not, the control circuitry might be defective.
Next, remove the tube and do the test again. If it still fails, the
control circuitry is defective, not the tube.
Having decided which is the culprit, if it's the CRT, inspect the socket
& the base & all the wiring to see if there
might be some external leakage path.
If you decide it HAS to be the CRT, then try the following with due care
for life & limb. You have little to lose
with the CRT at this point.
Never having tried this with a CRT, I can only suggest a method that has
worked for me with standard vacuum
tubes. (Transmitting sizes)
With a variable high voltage power supply, take a suitably rated
electrolytic cap of say, 10 or 20 Mfd & charge it
to say, 100 volts. Disconnect the cap from the supply & discharge it
through the short path while the filament is
energized. Use polarity on the cap so the negative is applied to the
grid. You DONT want to drive the grid 100
volts positive - it'll melt.
Do this once & measure the resistance. See if it changes. If so, repeat
until the short is cleared.
If no change, increase the charging voltage in 25V increments and repeat
step 1.
Let me know how this works.
Arthur VE3ZV
leon545b wrote:
Hello all -
A friend gave me a replacement CR tube for my 545B that has an odd
problem. When I plug it in, the brightness goes to full, and can't
be controlled. Suspecting leakage from the cathode to the 1st grid,
