CurtPalme.com Home Theater Forum

gravymaker · Joined: 01 Dec 2006 Posts: 58 Location: Winnipeg

Hey gents,

Recently, my green tube stopped turning on when powering on the unit. I traced the problem to the neck board by swapping the green and blue VNBs and the problem followed the neck board. These are MP-modded VNBs, quite a few years old now.

One of the caps on the neck board looked suspect, so I replaced it. This worked the first time I fired up the projector, thought I'd solved the problem.

But the next day, the tube did not turn on when I powered on the unit. After 10-15 minutes while searching the forums, the tube suddenly fired up, good picture.

So at this point, I've replaced both caps on the neck board, and re-flowed the solder traces on the two large resistors. The problem persists.

Any other suggestions as to where to look? I'll likely re-flow the RCA connector next. (*replaced stock VNB picture with my actual VNB)

Thanks!
Scott

cmjohnson · Joined: 03 Apr 2006 Posts: 5180 Location: Buried under G90s

Does the projector ALWAYS power up OK in red and blue?

Because if it didn't, then I would suspect the LVPS.

I once had an issue that was just like yours. Green would not reliably power up, red and blue were OK,
and it turned out to be a weak LVPS. I did an LVPS swap and no further problems.

Now, why would this problem only happpen on the green tube? Because the green tube always draws more current than red or blue.

Like I've said, I've SEEN this happen. Red and blue fired, up, green didn't. Problem went away with an LVPS swap.

However, I'm more inclined to think it's not the LVPS in your case, assuming red and blue always power up OK and you
have no fail indicators on the CLM status light cluster.

In this instance, you could have:

LVPS weakness (not impossible)
HVPS is probably NOT at fault.
Anode lead connection to green tube, or HV splitter: POSSIBLE. No HV to tube means no picture.
WARNING: Do not mess with anode lead connections unless the projector has been turned off AND UNPLUGGED for
not less than two minutes. Do not touch the anode lead tip after removing it from the HV block. Touch the anode lead
tip to the aluminum LC housing to discharge any residual CRT potential. You may get a SMALL spark and snap. Which
doesn't look like much but it'd light YOU up pretty good if you touched it instead.

WARNING AGAIN: ANODE VOLTAGES ARE LETHAL. You must not touch the tip of the anode lead connector with the
unit plugged in (even if it's off...murphy's law and all that) unless you have ensured it is fully discharged to chassis ground.

Then again, it MIGHT be the neck card. The correct way to test this is to swap the neck boards between the green tube and one other.

If the problem continues to be with the green tube, then it's either a power supply issue or, very slightly possibly, it's actually a green tube problem.

gravymaker · Joined: 01 Dec 2006 Posts: 58 Location: Winnipeg

Thanks CM. Yeah, originally it was the green tube that didn't fire up, but when I swapped neck boards between green and blue, the problem moved to the blue. Right now, Red and Green reliably fire up 100% of the time. Blue takes about 15 minutes to finally come on.

I did swap the HVPS anodes previously, to no benefit.

The problem definitely seems to follow the neck board. I'll take a very close look at it to see if I can spot any suspect components or bad solder joints - maybe my solder job on the new capacitors on that neck board wasn't up to snuff.

cmjohnson · Joined: 03 Apr 2006 Posts: 5180 Location: Buried under G90s

On the suspect neck card, watch carefully and see if the tube filament glows at the same brightness as on the other two tubes when (a) the projector is plugged in and on standby, and (b) the projector is plugged in and turned on. This will probably have to be done in a dark room and may require you to remove the top half of the tube cover to see the filament area of the tube.

If the filament is not operating at the correct temperature (voltage) then the picture will be dim or may not display at all. If there's an intermittent connection to the filament on the neck card, you'll see evidence of that by the glow, or lack thereof, of thee filament.

Another test...and I caution you, you have to do this right...is to pull off the flying G2 lead and see if you get a bright picture out of that tube. If you DO, then it implies an issue with the neck card's G2 circuit. Or an issue with the G2 drive signal coming from the power supplies.

I want to divert to that for a moment. I HAVE seen multiple cases where the black G2 lead had become pinched, or in one case, cut clean in half, by getting into the hinged rear heat sink and getting pinched off when swinging the heat sink into its normal position.

So do check for possible damage to the G2 leads. Just as a routine check.

Now, as for that G2 test: Locate the P14 filament connector on the side of the upper motherboard. Tie a string to it so you can grab that string, yank it, and pull the P14 plug right off its connector without having to fiddle around with it.

When you pull the flying G2 lead off the pin on the CRT neck, the CRT will go bright. (Maybe not if there are other problems on the neck card.)

When it's bright like that, you can't control the brightness or contrast.

Attempting to plug the flying lead back in is risky. Miss the pin and touch something else with it, and you can wreck the neck card.

So, you'd want to power down the projector before reattaching the flying G2 lead.

But if you power it down in this condition you WILL spot burn the tube. Permanent tube damage.

That's where the string around the P14 plug comes into play.

After pulling the G2 flying lead off and getting a bright picture, NOW you pull the P14 plug. Wait until the picture COMPLETELY dies and THEN turn off the projector via the remote. Now that it's safe, reconnect P14 and reconnect the G2 lead. Be sure you put it back on the proper pin.

ALWAYS remember that if you get into a situation where you have a bright picture that shows retrace lines (diagonal lines in addition to a bright raster) and you can't control it with brightness or contrast (or even the G2 and drive settings in the service menu) then you need to pull the P14 plug and let the image die out before powering down the unit, in order to avoid spot burns.

So, what does pulling the G2 flying lead tell you? Easy...it tells you that G2 voltage is present (lead attached) and controlling the tube's overall brightness as it should G2 power supply and cables OK. And also, that the circuitry associated with G2 on the neck card is working. That's a simple circuit, consisting of just one capacitor and three resistors. Odds are that the G2 circuit on the neck card is not at fault.

barclay66 · Posted: Wed Sep 07, 2016 9:11 pm Post subject:

Hi,

This is another option:

Get some freezing spray (the one for electronics which has an additional plastic tube for spraying onto small places). Switch on the PJ and wait until everything is working (warmed up). Now apply the freezing spray onto components on the faulty neck board until the picture disappears. Voilà: The last component that You freezed should be the culprit...

Regards,
barclay66

mp20748

Nashou66 · Joined: 12 Jan 2007 Posts: 16171 Location: West Seneca NY

Look at the 33 ohm Resistors and check the solder joints on those. Also the pic you're showing is the newest style neck board. What year vintage is your PJ? Or Give the Part number of the VNB.

Athanasios

cmjohnson · Joined: 03 Apr 2006 Posts: 5180 Location: Buried under G90s

The photo indicates a board number of 65518-01 Rev A, which would make it a 50-270340-01A board.

**Curt Palme** · Posted: Sat Sep 10, 2016 2:41 pm Post subject:

I'd suspect a cracked SMT resistor or solder joint. This is something that needs to be attacked with a scope or meter, randomly swapping parts will probably make things worse.

gravymaker · Joined: 01 Dec 2006 Posts: 58 Location: Winnipeg

Appreciate all the help guys, I've replaced the pic with my actual VNB in the first post.

gravymaker · Joined: 01 Dec 2006 Posts: 58 Location: Winnipeg

When I zoom in, the end of R33 (the end of the resistor closest to the socket opening) looks a little dark/toasty, doesn't it...It might just be the bad lighting in the pic, but I'll start testing the resistors with the meter tonight.

cmjohnson · Joined: 03 Apr 2006 Posts: 5180 Location: Buried under G90s

So you have actual MP modded 338 cards instead. That is a WORLD of difference from the newer 340 cards.
Putting up the wrong photo was really very misleading. "Hey, I've got a problem with my Chevy!" and you post pics of a Ford.

Resolder those big resistors, all four connections. That's routine.

I see that the 549 transistor in the lower left has been torqued. That's a little bit scary. Those leads are fragile. Without messing
with that, do take a moment to carefully inspect the leads for cracks right where they enter the body of the transistor.

How long ago was this card modded? There may be a date code on the big black capacitors which will give a clue. What is that date code, if there is one?

I'm asking that because capacitors have a limited service life. If they're more than 10 years old they should be replaced.

gravymaker · Joined: 01 Dec 2006 Posts: 58 Location: Winnipeg

I'll check the transistor leads and test all the resistors. The mods are over 5 years old. I've already replaced the caps a week ago - one of them was a little ballooned and tested out of spec, so I replaced the caps on each neck board.

gravymaker · Joined: 01 Dec 2006 Posts: 58 Location: Winnipeg

Well guys, this is interesting. I've tested all the thru-hole resisters, re-flowed the solder on as many components as I could and tested the capacitors. Everything specs out ok. The filament light looks just like the other tubes.

The problem follows the neck card, as follows:

When I fire up the projector, the tube is either very very low output, or completely off. If I gently tap the back of the neck card with the handle of a screwdriver, the tube comes to life and the pic looks great. It doesn't seem related to the RCA cable connection, as I have tried jiggling the RCA separately but nothing changes - it seems to need the "tap" to move the card slightly. It almost seems more like the socket connection itself is flaky - doesn't seem to matter where I tap, as long as the tap moves the neck card a little bit on the tube.

I tried spraying some De-Oxit into the socket holes on the card to no avail. I also tried crimping the G2 Bias connector a little to make a stronger connection, but the symptom persists!

Any other ideas, or do I just leave a screwdriver near the PJ?

cmjohnson · Joined: 03 Apr 2006 Posts: 5180 Location: Buried under G90s

Reflow the solder on the pin sockets. You may have a flaky one.

Got a good small magnifier? Look in the pin sockets for anything unusual about them, "One of these things is not like the other",
maybe there's a dead bug in one or maybe the spring contact inside one has slipped out of position.

barclay66 · Posted: Wed Sep 21, 2016 8:11 pm Post subject:

Hi,

If You still should have the effect of the board working after warm up, the procedure with the freezing spray (mentioned above) could help identifying the faulty part/area...

Regards,
barclay66

gravymaker · Joined: 01 Dec 2006 Posts: 58 Location: Winnipeg

Thanks for the tips guys. I tried re-flowing the socket pins, and took a look into the pins with a glass but couldn't really see anything out of sorts. Unfortunately it didn't solve things.

It may still be a component - I tried to hold the board still while I tapped the components and it seems like the tube kicked on and off more when I tapped near one of the capacitors. I've checked and rechecked the solder joints, as I recently replaced those caps, but I'll keep trying.

I even took a close look at all the SMT resistors like Curt suggested, but I'm not sure if I have the "eye" to catch anything less than an obvious problem on those tiny things.

Barclay, thanks for the suggestion of the cold air shot - I'll see if I can figure out how to try that - unfortunately its a tight fit and the components on the neckboard face inward - I'm not sure if I could shoot cold air with any precision, but I'll definitely have a look - thanks.

cmjohnson · Joined: 03 Apr 2006 Posts: 5180 Location: Buried under G90s

No, the component side of the board faces OUT. Otherwise you wouldn't be able to plug the connectors in!

Remove shield, components visible.

You can go to your local office staples depot store and buy a can of compressed air used for dusting things. Turn the can upside
down and it becomes cheap freeze spray.

gravymaker · Joined: 01 Dec 2006 Posts: 58 Location: Winnipeg

too funny - of course you're right! I'll try this tonite - tap the board to get it working, and then chill the key components one by one - i'll try not to let the board move during this process to eliminate any potential socket connection issue.

gravymaker · Joined: 01 Dec 2006 Posts: 58 Location: Winnipeg

I tried the cold shot, but it didn't cause any immediate change. I tried chilling the caps and the resistors, but it didn't seem to have any effect.

If I gently (and carefully!) "wobble" the neck card back and forth on the tube neck (using an index finger to gently push the large resistors alternately), it seems like I can make the tube cut in and out.

So strange - really points to an intermittent socket connection, doesn't it? Maybe I'll try moving it over to the red tonite to see if its a "better" fit on the red, but I don't have high hopes.