One procedure is to actually flash the CRT with the focus voltage. Ground
the heater and quickly touch and release the cathode pin(s) with the focus
lead. Of course the set must be powered when doing this, or another source
of similar voltage may be used. Be advised: This is a probably destructive
procedure and may be marginal regarding safety of the technician. Absolute
interest regarding your safety must be considered.
Yes, I have done this with positive results and with a totally flacked CRTs
as results. Good luck.
"hr(bob) " wrote in message
...
On Feb 3, 4:57 pm, "
wrote:
This set came in with bright red screen and retrace lines. The CRT has
a red Cathode to filament short. I disconnected both sides of the
filament circuit from the flyback and ran the tube filament from a DC
supply at 6.0V, and although the picture was watchable it still had
some red smeariness through it. I've advised the customer that we can
try to remove the short but it may open the filament. But the set is
worthless either way. I don't have a rejuvenator but I would like to
give this a shot before pronouncing it DOA. Has anyone had any luck
doing this? Would I have a better chance with say a CR70? Or can I
duplicate what the CR70 would do in this instance? I would hate to
destroy the tube by using a procedure possibly not proper for this
particular problem. With a low ohms meter I can possibly isolate which
side of the filament the short is on if that would help. Thanks for
any advice. Lenny.
They used to, when CRTs were run from 6.3 V AC on a transformer
winding from the 120V power mains, have a special 6.3V transformer 1:1
turns ratio, but with very good high-frequency isolation, that you
used to isolate the heater-cathode from the low impedance of the
regular transformer windings. The smear you describe is due to poor
low-frequcny isolation. You could run the tube with an ordinary 6.3V
power transformer if you could put inductors in each of the secondary
windings and also maintain low capacitive coupling across the
windings.

Now that CRTs are run from a high-frequency source, either the pwer
supply or the horizontal output circuit, things are not so easy
beacause they are running on pulsesthat have the equivalent heating
power, but which are nothing close to sinusoidal and thus impossible
to use an isolation transformer.

H. R. Hofmann