[rescue] sun2 monitor repair

[John Hudak]

The neck of the CRT glowing indicates that the heater voltage of about 6
volts is present.  The heater heats the cathodes.  Each cathode has a ~ 50
v p-p signal is present that controls the emission of electrons that end up
striking the face of the CRT.  if this is a color CRT then there are 3
cathodes (aka 'guns'), if B&W, only one cathode.

Looking at the *Front *of the CRT - if you can see some glow or retrace
lines (turn the brightness all the way up), that is an indication that the
annode voltage is present.  The time varing voltage on the deflection
plates control the scanning (raster) on the face of the screen.  The annode
voltage is generated by the flyback transformer or in some monitor
implementations, a voltage multiplier  (usually a 'trippler').

As I indicated earlier, you may want to check the annode voltage with a
high voltage probe, at some point.  the HV generation ckt *usually* is an
all or nothing deal.
A trouble shooting tip is not to run the CRT without the CRT neck connector
(sometimes a board) attached to the CRT.
Also, the CRT will hold a significant charge.  Do not attempt to dislodge
the annode connection without first discharging the CRT by taking a jumper
wire that contains a 1K resistor, attach one end to the metal chassis and
the other end to a flat blade screwdriver.  Insert the flat blade of the
screwdriver under the annode cap and bleed off the charge. Some sources say
the resistor is optional...I've done it both ways.
If you are going to be poking around the circuitry with a scope, you should
use an isolation transformer on the monitor.  Sam's article describes this.
A significant shock hazard exists if you don't.  Scope probes have been
known to vaporize (j/k but you get the point)

Once you get something to appear on the screen you can worry about focus,
picture orientation, etc.
This overview may be of help
http://www.ti.com/lit/an/snla017/snla017.pdf
good luck
BTW, do heed the cautions in Sam's article I pointed you too..you can get a
HV shock/burn if your hands are in the wrong place. They have been known to
be fatal.  Always work with one hand in your pocket (an old TV tech
guidance...)

Also, all the computer mfg had their monitors sourced by a CRT mfg.  If you
can find out who made the monitor, you might have better luck finding
schematic.  The mfg should be printed somewhere on the unit. During the 90
s, Ball and Sony were big suppliers.



On Sat, May 4, 2019 at 8:19 AM Walter Belgers <walter+rescue at belgers.com>
wrote:

> Hi,
>
> Thanks for the tips.
>
> I donb t believe it is a TTL/ECL thing. In fact, the monitor I tried that
> *did* give me something, is of the wrong kind.
>
> > On 1 May 2019, at 23:27, John Hudak <jjhudak at gmail.com> wrote:
> > Easiest thing to check is the CRT - does  the cathode in the neck glow?
> or
> > more directly, does the front of the CRT light?
>
> No glowing on both sides. Guess that is the first thing to look. If the
> flyback transformer does not produce the required voltage, it would still
> glow, right?
>
> > This may be of help
> > http://www.repairfaq.org/sam/monfaq.htm
> <http://www.repairfaq.org/sam/monfaq.htm>
>
> Ah nice, I will have a look. My first attempt finding anything like this
> only
> led me to commercial parties doing monitor repairs.
>
> Walter.
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