1. Home
  2. Electronics Repair
  3. bizarre self-healing CRT focus?

bizarre self-healing CRT focus?

About two months ago, my Samsung Tantus Digital 32" CRT (non-HD version) developed a focus problem. It appears to have been a horizontal focus problem that was minor in the middle of the display, and pretty bad on the left and right. Vertical focus was not appreciably affected. Simultaneously, the display became noticably dimmer with lower contrast. (The latter I adjusted.) How bad was the focus? Not *that* bad: we couldn't read credits and other small text, but it was tolerable enough that we didn't immediately rush to fix it.

At one point, we observed that physically unplugging the TV for ~30 minutes temporarily and partially restored the focus. The blur would settle back in after another ~10 minutes.

This morning, the problem was gone. Focus was sharp, and it looks like the brightness and contrast were bumped back up too (I haven't gotten around to adjusting them back to sane levels.)

Any ideas? What could cause this problem to come, and then months later, go away? Do you think the problem will return?

-Ed

Reply to
edwinolson
Loading thread data ...

Intermittents in the focus circuit aren't unusual. It could also be a short in the CRT. It might be fine now, but chances are it will come back eventually. If it's still under warranty, you should have it looked at because the CRT is an expensive part. Andy Cuffe

snipped-for-privacy@psu.edu

Reply to
Andy Cuffe

Sadly, it's out of warranty.

That being the case, would it be worth it for me to have someone look at it? Is there a common cause that an ordinary electrical engineer can repair?

-Ed

Reply to
edwinolson

Thermal/Mechanical Intermittant to the "Aquadag coating" on the CRT . (could be internal to the CRT) The Odd Focusing problem is due to a fluctuating Focus Voltage occuring at the Horizontal Rate. The Focus Voltage is a rectified Voltage derived from the Horizontal Sweep/Flyback Transformer. The Focus Voltage is taken off a Voltage divider across the HV supply that is filtered by a capacitor formed by the Internal Coating on the CRT and the Black "Aguadag" coating on the Outside. If this Outside Coating is poorly Grounded (usually a length of coiled spring wire) the HV will fluctuate slightly at the Horizontal Rate but will not be readily apparent because the Picture Tube Phosphours glow at the Frame Rate ~ 16 milli-second. The Focus Voltage fluctuation are Synced to the Scan Rate and would appear somewhere near the same distance from the edge of the screen on every horizontal scan. This Voltage is Not supposed to fluctuate !! The only Filter Capacitor in this part of the circuit is the Coatings On and In the CRT. A small sheet of Aluminum Foil under Spring should improve the contact with the Aquadag Coating.

Is it practical to pay someone to TRY to repair an INTERMITTANT !!! A lesson in frustration for ALL parties involved. There is no Joy in repairing an intermittant, only Relief. (could still be internal to the CRT)

Yukio YANO

Reply to
Yukio YANO

Along the sme line - while adjusting an older (1988) RPTV, I noticed that the focus would change (go blurry) when vhite screen appeared; immediately return in focus on a dark scren; put a meter on the B+ and noted it stayed steady. Any ideas?

Emilio

Reply to
Emilio

Focus and ultor voltage are intimately related. The meter acted as a load across the ultor supply, making it more stable between low and high current (or brightness).

See if there's some way to improve the HV regulation.

Isaac

Reply to
Isaac Wingfield

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.