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BG808 is very grumpy-pants for 30 minutes or so

post #1 of 12
Thread Starter 
I have a BG808 that is an excellent workhorse, but it has developed a problem where there is no image for a few minutes when it's been first powered up, and when it does show up, it is flickering and flashing. The problem is the same across all the tubes. It does this on the internal test patterns and all video inputs, and it gets better as time goes on, and it eventually goes away after about 30 minutes.

I did check the 17V supply and adjusted it, but it was only about 0.2 volts off, and that did not solve the problem. I also checked the video signals up to the driver boards (the ones with the G2 LED). The signals were also solid.

There are no funny sounds and no arcing. The green LEDs on the G2 diagnostic board are all good.

I'm thinking it is HV issue, but I have no interest in blowing myself and scope up. Does anyone have anything I should check?

I do have a perfectly functional BG808s I can swap parts with. I'm not sure what's common, but the part numbers should tell the story.
post #2 of 12
typically the EHT, quad or splitter goes on these

post #3 of 12
Thread Starter 
Thanks, I've spent some time reading through Curt's info and it has helped me immensely. I'm trying to figure out how to get a HV probe bought or built so that I can try to test each HV component. I've considered putting different parts either in the oven or freezer since it seems to be somewhat temperature related. (obviously the oven would be on the lowest setting). I may look around for some freeze spray to see if I can make it fail once it has warmed up.

I had a BG800 that the splitter failed on, but it was sizzling and the epoxy was burnt and cracked. I still get the normal crackle when it's turned on and off, so I'm thinking the starting to focus my attention to the focus/G2 board that generates the 1000V / master brightness. It does seem to be a bit blurry while it's going through its "Pink Floyd" warmup cycle.

I have the full service manuals and schematics for my 808s. I'll see what I can learn from those.
post #4 of 12
generally a HV issue causes the EHT board to go into shutdown mode, which the set won't get out of unless you power down and up, but internal arcing within the splitter can cause what you say.

No need for an HV probe, that won't tell you anything in this case, HV parts generally need to be swapped out to be properly tested. It could also be an issue on the G2 board, the video switching boards, and maybe the focus/ H shift board, and the power supply could still be a culprit as well.

The video boards and EHT board will not swap from the 808s to the 808.
post #5 of 12
Thread Starter 
I've swapped out the EHT, quad, and splitter modules and no change. The part number SMPS between the 808s and 808 are different, along with the G2 boards. Can anyone comment on the interchangeability of these parts?

7621706 vs 7621705

7625061 vs 762506

[edit] Curt, just saw your post. I should have hit F5 before replying.

One thing I can say is that the flashing does not always happen on all the tubes at exactly the same time. Is there a good place to probe? There was about a 2 minute delay in getting the HV crackle when I turned it on. I got this projector from a friend here in Elkhart. [edit]

[edit some more] the G2 board I was referring to was the one that has all the diagnostic LED's - not the three that are bolted to the chassis.
Edited by glankford - 1/1/14 at 9:17am
post #6 of 12
Thread Starter 
Ok, I've found the problem, but I don't know where it's coming from. On the chassis/backplane of the projector, there are 2 connectors that go to the tube socket. One goes to the driver board and then to the tube socket, the other goes directly to the socket. On the one that goes directly to the tube socket, there is a black wire and 4 yellow wires. The black wire has a high voltage (>700V). when the tubes go into "pink Floyd" mode, that voltage drops out/cuts out/etc.. When it's stable, it's a nice DC flat line. So I'm guessing there is something that causes the voltage to drop out, either because the supply is flaking out or there is a short somewhere that is pulling it down.

Ok, since my manual is for a 808s and not all parts are interchangeable,, is it relatively safe to assume that the backplane connections are the same and I can focus in on the G2 driver board? I can try to pull the connector for each tube socket to see if there is an intermittent short that pulls it down, otherwise I'm onto the G2 supply.

[update] I swapped out the 762120S that converts 15kv down (it's the potted brown block that's screwed down to the G2 board. that's not it either. I also pulled the connectors to each gun (one at a time), and that also didn't fix it.
Edited by glankford - 1/1/14 at 3:03pm
post #7 of 12
Thread Starter 
I have narrowed it down to a section of the G2 board that regulates the 1020V. I'm not sure how much time I'm willing to pour into this, but I think I know it's one of about 10 components on the board. I'll start with caps, then move onto active components.

Ok, I think I found the bad component. C12 is probably the problem. It has a temperature effect, so I heated everything up in that area of the board with a heat gun and fired it up. It worked perfectly. ok. I then took some freeze spray and hit the IC and big transistor. Nope. I then hit the capacitor. After about 30 seconds, the pink Floyd concert started. I'll swap it out and report back. I'm kind of talking to myself here, but maybe someone will find this info useful.

More info tomorrow after I swap that out.
Edited by glankford - 1/1/14 at 8:37pm
post #8 of 12
Originally Posted by glankford View Post

I'm kind of talking to myself here, but maybe someone will find this info useful[edit]
definitely, I've repaired cars, appliances, you name it based on some useful tidbit off the net that someone solved earlier. No point is climbing the same hill over and over again.
post #9 of 12
Thread Starter 
Ok, It wasn't that cap. I spent some time with a heat gun and some freeze spray. I've narrowed it down to three HV diodes or a transistor. The spray that I was originally hitting the cap with must have been dripping down and cooling off other things along the way. I started by hitting things that were low on the board, and then slowly moving up. when I hit that transistor, it went back into pink Floyd mode. More to come after I get that transistor replaced.

[edit] swapped the transistor, that wasn't it either. I'm now on the hunt for those diodes. [edit]
Edited by glankford - 1/4/14 at 8:47am
post #10 of 12
Just FYI, I've got good replacement boards, tested, for $100 incl shipping. Might be cheaper than trying to repair that one.
post #11 of 12
Thread Starter 
OK, IT'S FIXED! It wasn't the diodes either. it was a defective 22k resistor at the base of Q1. the freeze spray must have been dripping off of that transistor and causing the resistor to flake out. Either way, working projector again!
post #12 of 12
Well diagnosed and resolved.
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