CurtPalme.com Home Theater Forum

front_range · Posted: Tue Apr 01, 2008 9:49 pm Post subject: Barco 708 Scan Fail problem

I have a Runco 943 (aka Barco 708) that arced somewhere around the blue tube, and now has a scan fail LED. I tried unplugging the convergence boards, but still have the SF LED. The only spare deflection and SMPS boards I have are the ones from a 701s, which look nearly identical, and have the same cables (different part numbers). Anyone know if I can temporarily swap them safely?
TIA
Steve

**Curt Palme** · Posted: Tue Apr 01, 2008 9:54 pm Post subject:

You can, but they might not work exactly right. You might get double side by side images with the deflection board, but at least it should point you in the right direction as to what board is bad. Change the blue and green HV leads at the same time...

I just got my new lot of 20 HV leads in if you need one or two.

front_range · Posted: Tue Apr 01, 2008 10:11 pm Post subject:

Thanks Curt, will do. Can you please PM me a price for a couple of those leads shipped to 80921?

**Curt Palme** · Posted: Tue Apr 01, 2008 10:55 pm Post subject:

$25 each incl shipping.

Email me at curtpalme@shaw.ca

front_range · Posted: Wed Apr 02, 2008 12:46 pm Post subject:

Thanks again Curt, right now I just have blue unplugged from the splitter. I'll get with you when I know everything I need.

front_range · Posted: Wed Apr 02, 2008 2:25 pm Post subject:

I swapped the deflection board with no change, still have the SF LED. The power board doesn't seem to play here as long as the individual voltage indicators are green, right?

**Curt Palme** · Posted: Wed Apr 02, 2008 2:36 pm Post subject:

There are two red LEDs on the deflection board. WHich one is on?

I agree, the SMPS should be OK at this point.

front_range · Posted: Wed Apr 02, 2008 2:49 pm Post subject:

Neither. The 3 G2 LEDs on the RBG board are lit, and SF & COINC on the diagnostic board. I haven't seen any others that are lit except for the greens on all the low tension supplies & the appropriate RGB input LEDs when a video source is applied.

**Curt Palme** · Posted: Wed Apr 02, 2008 3:07 pm Post subject:

That doesn't make sense. If the two red LEDs on the deflection board aren't lit, then the SF on the SMPS should not be on. Are you sure it's the SF light and not one of the other red LEDs that are on? (EHT holddown, etc?)

front_range · Posted: Wed Apr 02, 2008 3:12 pm Post subject:

Yeah, it's the third one over from the right. According to the sticker, from the right to left it's COINC, Hold down HD, then SF (the lit one)

**Curt Palme** · Posted: Wed Apr 02, 2008 3:14 pm Post subject:

OK, I'll hvae to look at the manual. From what I remember, the SF light on the front is driven directly from the SF lights on the deflection board. I could be wrong though. I'll post later today, I'm in the office all day working in the shop.

front_range · Posted: Wed Apr 02, 2008 3:20 pm Post subject:

Thanks Curt, in the mean time I'll put the original deflection board back and see if it's doing the same thing.

front_range · Posted: Wed Apr 02, 2008 3:50 pm Post subject:

OK, the original deflection board does the same thing. I think I have a clue what is going on now. The SF light is a pass through from the deflection board, but the HT holddown will also set the SF. The HT hold down isn't lit on the diagnostic board, but I pulled the SMPS out of the chassis and fired it up again and the STATUS_HTHD is lit. Does that mean I have a high tension problem and a faulty indicator on the diagnostic board?

front_range · Posted: Wed Apr 02, 2008 4:09 pm Post subject:

Oops, I misread the schematic. It looks like I reversed my reading on cause and effect, and SF causes HT shutdown. Back to the drawing board.

front_range · Posted: Wed Apr 02, 2008 8:55 pm Post subject:

OK, I'm now stuck. I pulled the F200 HT fuse and tried to isolate the source of the SF signal, since it doesn't seem to be coming from the horz or vert driver. I pulled J762 off the SMPS, which is the SF signal going back to the HT hold down circuit. I still had the SF LED. Then I tried pulling J7 off the deflection module to isolate the harness going back to the SMPS for a short, no change. Then I pulled J14 off the deflection board to make sure the source of the SF indicator was not a short in the harness to the diagnostic board. The SF LED went out, so the SF sig is definitly coming from both the original and the swapped deflection board, but niether lites the horz or vert fail LED.

front_range · Posted: Thu Apr 03, 2008 1:45 pm Post subject:

I got the scan fail problem resolved. I have seen two of these chassis now that had P16 tubes instead of the older T180s and both the P16 sets had a "mod" that consisted of a small daughterboard on top of the RGB input that had 3 long leads going to thermistors on each CRT's heatsink. I found this odd because I've seen other references on this forum that I read as indicating you could put P16 tubes in an older T180 chassis, but not the other way around because the T180 tubes would overheat. Seems like it should be the other way around if the P16 chassis have thermal protection... But FWIW, my problem was a disconnected thermistor.

**Curt Palme** · Posted: Thu Apr 03, 2008 1:50 pm Post subject:

Ahhhhhhhhhhh, I forgot about those. YEs, one bad thermistor or one disconnected wire and the set will go into shutdown mode.

front_range · Posted: Thu Apr 03, 2008 2:20 pm Post subject:

Now for the next oddity, I have STATUS_HTHD lit on the SMPS, but not on the front panel. I'm trying to figure out what set of circumstances would lead to that. I noticed a blister on the back of the quad, so I know I have an issue there, but that would lite the front LED wouldn't it?

Edit - Hmm I think I misread the STATUS_HTHD and it is supposed to be on? Looking at the divider on the quad, it looks like ~19VDC, so I'm guessing the next thing is to get a new quad in the set and see if that will light it up.