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Monitor Compaq V500 (chassis Samsung CHB5705); black has red hue.

Hi,

I've been given a monitor. It works pretty fine, but for one thing:

There's a red teint across the screen, especially in the black areas. When I turn contrast fully down, the screen doesn't become black but red. With contrast fully turned up, it's perfectly normal picture, except in big black areas, where it shows a red hue again.

When I turn brightness down, it also becomes more red-dish, but not so extreme as with the contrast fully turned down.

No retrace-lines, or other problems.

Could it be a heater-cathode short? Or something else? I suspect the red channel, of course. However, I'm reluctant to operate this monitor without being pretty certain where to start looking. At the moment, it's the only monitor I have...

Also: I use this monitor in 1024x768 mode, but when starting up, it goes to 640x480 (I think). In that resolution, it shows severe >< distortion. (barrel distortion?). For my standard resolution, 1024x768, I can perfectly correct this, but in other modes it shows up again, severely. Is this expected behaviour of this monitor?

Model: Compaq V500 Chassis: Samsung CHB5707

A schematic I've downloaded from here:

formatting link
(click on download).

Hope anyone can help.

Peter.

Reply to
peter_dingemans
Loading thread data ...

This is common in these when the CRT starts to go weak. The bias circuits can no longer properly bias the CRT.

If you call Samsung, maybe they will sell a copy of the service manual, to you. Normaly, they only supply their authorised dealers. With the schematic, you should be able to check the bias drive circuits. But, failure in these is rare.

Jerry G. ======

peter snipped-for-privacy@hotmail.com wrote:

Reply to
Jerry G.

Presuming this is a CRT display - monitors invariably have the heater supply derived from the main chopper transformer with the usual rectifier/reservoir cap, the cap can go high ESR so the power to the heater is reduced which under-runs the cathodes and causes a rapid deterioration in emission!!!!! Also some years ago some 'genius' at Philips issued a memo to the trade that due to the better regulation of modern PSUs, CRT heaters should be run at

6.15V instead of 6.3V and quite a few monitor manufacturers actually fell for it!!!

If you find the heater voltage is too low (I've seen as low as 5.75V!) there is sometimes an alternative to risking safety by cranking up the B+ preset in the hope of increasing the Vhh. Check if the heater rectifier is a fast soft recovery type or Schottky-barrier, if it is not an SB type then replacing it with one will increase efficiency and increase the rectified heater voltage. However since the chopper PSU is usually a flyback type converter you have to start by making a peak detector with opposite polarity to the heater voltage to measure the peak reverse voltage the SB diode will be subjected to! Use a diode similar to the existing heater rectifier - but pointing the other way round, and give it it's own reservoir electrolytic (voltage rating 5x that of the heater reservoir cap!) the SB diode must withstand the sum of the reverse voltage and the 6.3V on the reservoir cap! Try to get hold of a 60V 3A SB diode, this may be sufficient but if not leave the peak detector diode/cap combination and load it with a power resistor - trial & error to determine what value loads the reverse voltage low enough. Note that SB diodes have higher junction capacitance so it helps reduce heating in the reservoir cap if you pad it with a decent size polyester cap - say 1 or 2.2uF.

Another thing I find causes loss of emission is long periods of low beam current (like many screen savers!) scan a blank sheet of white paper and save the file as (BMP I think) to the wallpaper folder then use this as the background, turn up the Bri & Cont controls and leave it running awhile, this often wakes up sleepy cathodes.

Reply to
ian field

Thanks for the replies!

I'll check for bias and heater voltage. Thought that 6.15 V is still within the margin (6.0-6.6V) for heaters, but still. At least the lower heater voltage should assure long life of the filament :)

To be honest, the monitor isn't really worth all the trouble. It's just that I'd like to know what is going on.

I'm assuming the barrel-shaped distortion is completely unrelated to this red-hue problem, right?

Peter.

Reply to
peter_dingemans

Short of hitting it with a hammer under-running the heater is the quickest way to ruin a thermionic device!!!

The life graph was posted on a guitar amp forum at a time when some guitarists started discussing reducing the voltage supply to a tube amp as a means of producing a specific type of distortion. The graph as posted shows a steady downward slope for cathode life against excessive heater voltage, and a nearly vertical drop for underrun heaters! In all my years servicing CRT based displays, I've seen maybe less than a dozen O/C CRT heaters - it just doesn't happen that often, if the heaters are overrun it will boil off the emissive coating long before heater life becomes an issue!

Reply to
ian field

There are three little trimpots labeled R G and B Screen. Most of the time you find them on the little PC board on the end of the CRT, but sometimes they're on the bigger PC board. Take great care and try turning the "Red screen" control a smidgen counterclockwise. There's a good chance this will fix the problem. Don't kill yourself.

Reply to
Ancient_Hacker

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