Gun needs adjusting

How can i do that?

i2lgames wrote on 2023-10-28, 14:13:

How can i do that?

Hello Electrosoldier,

CRTs often have some adjustable resistors on the internal board. There you can adjust the horizontal and vertical line. But be careful - there may be high voltage on some contacts inside! Maybe look on the net for maintenance documents for the CRT before you open it. Such documents often describe how to adjust it.

Best regards,
JF

Found the resistors, adjusted for h/v size but the verical is only a bit larger. Nowhere near the full screen. The brightness and clarity of the monitor is top notch! I dont think that it is the flyback. Can a capacitor cause this?

It is unlikely to be a capacitor issue, there's no raster distortion or anything (or is there ? any jitters or other instability), just that it is not tall enough. I can suspect some cold solder in the frame amp section, perhaps a component in the feedback section isn't connecting well and ends up causing it.

Getting access to the entire board for visual checkup can be a challenge, a number of clear and focused photos can be most helpful.

It could be a cap issue, I haven't worked on many color CRT monitors but usually there is an electrolytic cap in series with the deflection coil. It's there to prevent destruction of the driving transistor/IC in the unlikely event the frequency goes so low that the pulse saturates the coil. If that cap was to loose a lot of capacitance (dries out or leaks) then you could get reduced deflection angle. Picture might not seem distorted (or not much) because modern SVGA deflection circuits often have feedback for linearity purpose and that might be correcting most of the distortion.

Figuring out which cap it is without schematic and without some experience in fixing CRT monitors/TVs is going to be somewhat difficult though. Dried out caps can look pretty normal. I would first suspect any cap about 220-1000uF that's close to the wires going into the coil. Note horizontal deflection will have it's own cap but usually way smaller value and higher voltage (100V or so). Anything close to a heatsink should be checked and possibly replaced if in doubt.

Cracked solder sounds very plausible and doesnt require much knowledge. Get access to solder side and resolder all solder joints shotgun style 😀 start with those belonging to heavy elements.

Does it remember its OSD settings after unplugging from power/computer for couple of minutes? Maybe it lost calibration and requires going into service menu?

Deunan wrote on 2023-10-28, 18:09:

It could be a cap issue, I haven't worked on many color CRT monitors but usually there is an electrolytic cap in series with the deflection coil. It's there to prevent destruction of the driving transistor/IC in the unlikely event the frequency goes so low that the pulse saturates the coil. If that cap was to loose a lot of capacitance (dries out or leaks) then you could get reduced deflection angle. Picture might not seem distorted (or not much) because modern SVGA deflection circuits often have feedback for linearity purpose and that might be correcting most of the distortion.

Figuring out which cap it is without schematic and without some experience in fixing CRT monitors/TVs is going to be somewhat difficult though. Dried out caps can look pretty normal. I would first suspect any cap about 220-1000uF that's close to the wires going into the coil. Note horizontal deflection will have it's own cap but usually way smaller value and higher voltage (100V or so). Anything close to a heatsink should be checked and possibly replaced if in doubt.

Series cap like that you mostly find on low end stuff like (smaller) TVs, monitors (and large TVs) tend to have direct coupling for best linearity and framerate performance. But if there actually is one, it can be responsible for this problem, so definitely worth investigating.

Be very careful inside a CRT when its opened up. The power in there is more than enough to kill you and the contacts hold the charge even after its unplugged from the power.

Be sure to look up how to discharge a CRT, it’s not hard.

Also be aware that the tube itself when disconnected from the fly back will slowly regain charge over time (sounds crazy but it’s true), after working for a few hours I’ve had CRTs regain enough to give a nasty zap when discharged again before reinstalling the fly back, although that was a fairly large CRT. Technically you’re unlikely to actually die (although it is possible) from getting zapped by an unplugged CRT, but it is a seriously nasty zap that you’d want to avoid.

Be aware that even with a bleed resistor most CRTs will still hold a decent charge for as much as 24 hours sometimes, depending on the CRT.

A plugged in CRT on the other hand will absolutely kill you if you touch the wrong thing.

The big capacitor on primary side is the one that will be able to kill and one should discharge it before handling the board. Deflection supply one on secondary can bite pretty hard but that one usually discharges pretty quickly on its own, cathode amp supply likewise, but it is always a good idea to use a resistor to discharge anything with 50V or more written on it. I use this :

Dielectric absorption is definitely at play on a kinescope itself. When you disconnect the anode cup, you always want to keep it connected to aquadag to prevent the charge from building up. Also it'll pick up all the static electricity from just any handling, the kinescope itself is a pretty good leyden jar (read : capacitor) and when you go back to connecting the anode, it will be able to surprise you in the most unwanted way 🤣

Tiido wrote on 2023-10-28, 19:09:

Series cap like that you mostly find on low end stuff like (smaller) TVs, monitors (and large TVs) tend to have direct coupling for best linearity and framerate performance.

I see. My experience with "modern" color CRT SVGAs is limited. The only things I've had to deal with so far was CRT gun rebalance and generic issues like PSU caps or cracked solder around the flyback.