Jamie t wrote in
:

Steve wrote:

On Sat, 12 May 2007 18:16:45 GMT, Steve wrote:


The scope is a 465M, S/N is B056721 but it looks like that may not be
original, assemblies have been swapped etc..

The problem: the scope had been working fine until one day the crt was
dead. Fuse F558 was blown, (it goes from the 32V unreg to the HV
oscillator circuit). All other supplies are fine.

I first unsoldered pin 10 from T550 (feeds the 3x multiplier), but the
95V supply still wouldn't come up. I then checked Q556 & Q552, the
transistors driving T550, both are fine. I then checked the caps and
transistors in the 95V circuit, all fine. I then unhooked the jumper
leading from L582 to the rest of the scope, still no luck. I then
took out the crt (both the plug on the back and the anode lead), and
the supply came up with no problems. 94.5V, .07A, negligible ripple.

My readings are as follows.
Anode off & Rear Plug off: 94.5V @ .070A
Anode on & Rear Plug off: 64.2V @ .54A & climbing
Anode off & Rear Plug on: 26.7V @ .427A & climbing
Anode on & Rear Plug on: 21.6V @ .460A & climbing

Is this probably a bad tube? I don't have a whole lot of experience
with scopes, as this post would indicate, and don't want to get a new
crt if it's not going to fix it.

Also, just to make sure, there isn't any alignment procedure when
swapping scope crt's, is there? I don't figure there is, just want to
make sure.

I have another 465m, but the same fuse is blown, and I haven't gotten
into it yet.

All thoughts appreciated.

Thanks,
Steve


Well, I replaced the tube with the one from the other scope, and it
works just fine. So, my CRT must have gone bad, and the other scope
must have some deeper problems (I'm not even going to attempt to fix
it, it's best suited to a parts unit). What causes a CRT to just die?
It was working fine one day, the next day it's causing fuses to blow.
Now I'm curious. I always though they just wore out, lost brightness,
filament burns up, etc..

Again, all thoughts appreciated.

Thanks,
Steve
check the heater to cathode, it may have shorted..
that is a common problem in CRT's
Years ago, we use to isolate the heater circuit so that it wouldn't
ground any more and be able to still use the TUBE
it's also possible a grid wire could have broke in side.



you WANT the heater to be at the same potential as the cathode,so it will
not arc over.

Most likely,his geometry or astig elements shorted to another element.
IIRC,the +95 went to either one or the other.

The scope probably took a hard shock somewhere.

--
Jim Yanik
jyanik
at
kua.net