CurtPalme.com Home Theater Forum

v_erich · Joined: 12 Sep 2006 Posts: 76 Location: Austria/Europe

Hi,
I have a modded Barco BG808s with P16 tubes (R762746 Retro amps 120MHz), 7.61 Firmware.

I have the problem, that the brightness rises over temperature.
I have no idea where to serch, when ABL is on, its not so much, but rises also significant.
Without ABL when I start with a black like a good CRT, after 2 hours I end up in a "black" like a 15 years old LCD Sad

Any ideas?

Regards,
Erich

v_erich · Joined: 12 Sep 2006 Posts: 76 Location: Austria/Europe

Did some tests today.
Measured the 200V supply on the RGB amps, they are all 3 ok (200,5V equal).
Measured the filament voltage for the Tubes (P16LNQ), was 6,5V - reduced to 6,3V.

No change on rising the brightness over time.

Then I muted the picture with the remote and let the CRT run for 1h.
After that the blacklevel was low (but CRT warm), after un-mute the tubes I can see with a black test picture (brightness 50, contrast 0) how the blacklevel rises from minute to minute when looking into the tubes.

???

I have no idea.

Regards,
Erich

v_erich · Joined: 12 Sep 2006 Posts: 76 Location: Austria/Europe

Problem still exists Sad

But to be more detailed, its only on the red and green tube.

So could it be something with the tube itself?

Please help me, it's not nice to have after 2 hours watching movies I have a blacklevel like a old LCD... Thumbs Down

Regards,
Erich

Ile · Posted: Thu Dec 10, 2009 12:53 pm Post subject:

Have you measured that +17V line is stable during this?

Have you tried to turn red/green G2 pots back and fort at G2 board?

v_erich · Joined: 12 Sep 2006 Posts: 76 Location: Austria/Europe

Yes, the +17V line starts at 17,58V and 1h later its 17,6V Wink

So rock solid i think.

With the pots I don't understand what you mean.
Do you think the pots are old/dirty/...?
What is interesting is, that the blue G2 pot is on a complete different position then the R+G ones.

The G2 I can change without problems, but no change of the problem.
Birhtness rises over time.

Thanks and regards,
Erich

Ile · Posted: Fri Dec 11, 2009 8:55 am Post subject:

**Curt Palme** · Posted: Fri Dec 11, 2009 3:28 pm Post subject:

I just had this with a local customer. Good thing too, as he made about 4 trips back and forth to my place for more boards. Smile

It ended up being the 762719/762720 video boards. I thought it was a G2 issue or video amp issue, so we swapped all those before we got to the video section...

Of course, he hasn't returned the old boards back yet, so I don't know exactly which board it was, but when he swapped that pair, the problem went away.

v_erich · Joined: 12 Sep 2006 Posts: 76 Location: Austria/Europe

Ok, these 2 boards I have heavy modded Sad

Maybe to much, but I cannot remember that this happened after some modification.

The swap with the neckboards I will made these days.

Thanks and regards,
Erich

v_erich · Joined: 12 Sep 2006 Posts: 76 Location: Austria/Europe

Hi again,
I checked the video chain of the 2 boards today, also changed some things (which I think are maybe risky back to original) - but no change in the brightness rising.

I checked the rising on all 3 tubes again, its on all tubes - but on blue less then on red+green.

What I don't understand is, that if I press Pause on the remote (and the tubes are dimm) for about 10 minutes, the black level is back to normal, but rises again.

?

Regards,
Erich

Ile · Posted: Fri Dec 18, 2009 8:32 am Post subject:

Now when you say it's in all tubes, makes it different.

It could be G2 or EHT board, that's where 1020V for G2 voltage is coming from. There is also quad between those two.

I think G2 is shut down when you press pause...

Tom.W · Joined: 09 Mar 2006 Posts: 6635

v_erich · Joined: 12 Sep 2006 Posts: 76 Location: Austria/Europe

Hi,
thanks for the hints.
So I will have a closer look on the G2 board.
But how can I take it out of the CRT?
Newer done this, the red cable is mounted on it.

Regards,
Erich

**Curt Palme** · Posted: Sat Dec 19, 2009 3:32 pm Post subject:

I can't see it being the EHT board. If it were, and the HV was shifting, the focus and picture size would change as well.

I would try and find some stock 762719 and 720 board. It seems lately that about 1/2 the repairs I've done have been to modded boards. The latest one was a modified Marquee HDM, which would die every 20 minutes or so at random.

It turned out to be a bad solder joint on one capacitor. The solder joint looked fine on the bottom of the board, but wasn't making a good connection at the top of the board where the connection was. That was 2 hours finding that one.. Smile

v_erich · Joined: 12 Sep 2006 Posts: 76 Location: Austria/Europe

I think I found the problem.

My timings form my Lumagen Radiance are to fast, so the backporches were to short, so that the barco clamping circuit could not do that well and the brightness rised over the time/temp.

Now with more time for the barco to clamp right this problem is gone.
Found it, because a upscaled PAL resolution (1440x864) looked well also after 2 hours.
So I started investigations with timing and found this hint at the service manual (circuit description of barco for minimum timing).
Changed the settings in the lumagen to longer timings and now it looks ok.

Hope this was the problem.

Regards,
Erich

Ile · Posted: Thu Feb 11, 2010 7:57 am Post subject:

Check this bulleting out if you still have that problem.

Decibel just sended this to me.

v_erich · Joined: 12 Sep 2006 Posts: 76 Location: Austria/Europe

Hi,

thanks for the information, but I knew that and its originally in my boards. I looked for that when I made the better filtering of the supply voltages for these boards.

In the meantwhile I found a guy which has a spare BG808s, so I borrowed some cards and now I test one card after the other.
The idea with the timing of the video signals was good, it changed a little bit but still the same problem.

Can it be something with the P16 tubes? They are not standard in this crt, maybe there is a problem?

Thanks,
Erich

Ile · Posted: Thu Feb 11, 2010 9:17 am Post subject:

Have you already swapped driver board?

My brightness/G2 drifted few hours when I was chanced el-caps to driver board, but then it settled down.

Maybe aged C28 and C12 from neck board socket board could also have effect to G2 adjustment circuit...

v_erich · Joined: 12 Sep 2006 Posts: 76 Location: Austria/Europe

Hi, C28 was changed during the better filtering of the power supplies.
I changed C12 on one board for testing, will check with the next movie if it helps.

Thanks,
Erich