Any CRT experts out there?

[tswan]

I have an old Dell P990 CRT monitor that seems to have finally given out. It's probably time to spend some $ and get an LCD, but I would like to take a shot at fixing the CRT for the fun of it.

For sometime now the monitor has appeared quite dim. I had to turn up the brightness to nearly 100%. I tried the "color return" option before the monitor finally stopped working and that did not improve the situation.

Right now when I power up the monitor the green power led cycles on and off about every second. I can hear a slight "ticking" noise keeping time with the led. I opened the CRT and observed a glow at the rear of the CRT that also came and went with the ticking.

If anybody has any ideas where to start looking I would really appreciate the advice.

Thanks.

[lucky13]

Re: Any CRT experts out there?

That glow is the filament.

It could be P/S failure (aka B+). You probably need a schematic to trace thru or troubleshoot.

Dim pix normally means the tube is getting weak and requires a boost of voltage but eventually there is not enough boost.

Just remember there is nasty voltages on those boards.

[Wizard]

Re: Any CRT experts out there?

Dried up capacitor in PSU section make PSU to cycle, more specifically a small filter cap for VCC for the PWM IC or for a all in one PSU IC. Results in slow cycle. Shorted loads made PSU to cut out once or quick cycling or screaming.

If the color of the glow from heaters is nice orange, nothing you can do with the CRT. CRT is totally gone. Provided that you do confirm the G1 circuit (if this monitor uses it has good cap somewhere for that).



Cheers, Wizard

[Toasty]

Re: Any CRT experts out there?

I think this is a "software adjustable" monitor. Looking at the back, lower right, do you have a 4 pin connector? It looks like a floppy drive power connector. If yes, then this is a software adjustable monitor. It uses a microcontroller and EEPROM to store the settings.

It's a Sony Trinitron and *may* be correctable. Longevity of the adjustment is the real question. If indeed it is the tube is failing, then it's time for a new one.

Get the PSU issue fixed first. Once you take the top shield off, the power board is on the left mounted vertically and all the voltages are measurable through the edge connector at the top. Voltages are marked on the board right at the connector.

Once the PSU issue is corrected, you'll need a RS232 to TTL converter card and cable (<$20 DIY) to make the adjustments.

See:
https://www.badcaps.net/forum/showthread.php?t=6770

http://www.geocities.com/gregua/windas/

http://www.geocities.com/gregua/windas/install.html

http://www.geocities.com/gregua/windas/cable.html

http://www.nkcelectronics.com/rs232-...ert232356.html

Good Hunting!
Toast

[Wizard]

Re: Any CRT experts out there?

Sony CRT uses this cap in the CRT board and this will make picture DIM. Find it with ESR meter. I fixed couple this way.

Cheers, Wizard

[tswan]

Re: Any CRT experts out there?

Just an update for anybody else who may experience this problem.

I found that the 6V regulator that supplies power to the CRT heating element was only producing about 4.5V. On the Oscope it looked like a square wave with the high at about 6 and the low around 4. I have not built an ESR tester yet so I used a function generator and a oscope to test ESR on LOTS of capacitors. I replaced many on both the main board and CRT board.

In addition to a much brighter display this exercise also corrected some other display issues.

I'm going to order the RS232 to TTL converter from Digikey today so I can put together the circuit to make changes to the EEPROM to see if I can improve the image even more.

Thanks to everyone for their advice.

[Toasty]

Re: Any CRT experts out there?

>>replaced many on both the main board and CRT board. <<
Please clarify - What are you referring to as the "main board" ?

Just out of curiosity... Why would you order it from DigiKey? The link I gave was a complete kit with PCB & all.

Toast

[tswan]

Re: Any CRT experts out there?

I already had the caps and a DB9 connector so I thought I'd just breadboard the circuit. I didn't think I would need to use it frequently enough to need the parts permanently mounted on a PCB and I was placing an order with Digikey for some other items anyway.

[Toasty]

Re: Any CRT experts out there?

Okay... and the other question....?

[tswan]

Re: Any CRT experts out there?

Sorry, did not see the other question.

There are a few different boards in the P990. The A board is the one that mounts the the CRT. There is a large horizontal board beneath the CRT that has the power supply and the FBT on it and it is called the D board. When I said "main board", I was referring to the D board.