28 Apr
2017
28 Apr
'17
9:41 a.m.
Hi Tony,
Thanks for the information, that is all very useful! I think I will open
up my VT420 which is working nicely to compare the glow in the base
followed by checking the voltages, as you describe.
Thanks again!
Aaron.
Tony Duell writes:
On Fri, Apr 28, 2017 at 4:38 PM, Aaron Jackson via
cctalk
<cctalk at classiccmp.org> wrote:
Yes we checked for the glow. It's very dim
but it is there. I will bring
What do you mean by 'very dim'? It should be a distinct orange glow,
best seen looking
up the base of the CRT.
I normally start by checking the CRT pin voltages. There's nothing on
the pins of
a monochrome CRT that can't be measured with a normal multimeter. The sort of
voltages I would expect are :
pin 2 -- cathode -- 70V-100V
pin 1 or pin 5 -- control grid (the 2 pins are linked inside the CRT) -- perhaps
10V-20V less than the cathode
pin3,pin4 -- heater. One will be grounded (0V). The other will be around 11V-12V
pin6, pin7 -- anodes. I forget which is the first anode and which is the second
(focus) anode, but expect a few hundred V -- say 400V-800V on each of them.
The higher voltages (cathode, anodes) will come from the flyback transformer
in the horizontal output stage, so if they're all missing, suspect problems
there. If the heater voltage is low, suspect PSU problems.
I assume you have the printset (I think I got it from bitsavers). The monitor
section circuit looks very standard to me.
-tony