My sister came up with "raster disaster". I give her full credit for that one. :)

I brought my 8500 down to Orlando to give to my brother-in-law. It had been sitting idle for a couple of years. We fired it up and the blue raster was collapsed vertically. All rasters were "jumpy". I suspected the VDM but had no tools, schematics, or scope to debug it so I brought all of his boards back up to Huntsville with me.

I replaced my VDM with his and the same symptoms appeared, so it is a bad VDM.

I called it a night (too tired to do the schematics/scope thing) but need to get back at it tonight.

Question is, for anyone that's actually fixed a VDM, what's the most common failure component? Might save me some time. Can I still get parts? I'll post back here with the fix.

Thanks,

Cary

Hello

It could also be the vertical windings in the yoke; try swapping blue and green yoke connectors and see if the problem moves to green. Look close on the vertical board for anything burnt. Check the yoke lines for cuts or pinch damage.

Thanks Tim!

I tried that when I was with the original 8500 that has the problem. I swapped blue and green cables on the VDM and the problem went to the green tube and blue was fine so I think the yoke is ok. My 9500 is solid, but when I put this VDM into it, it failed the same way, so it's got to be the VDM.

I didn't see anything (or smell anything) on the board to make me suspicious of any one component. I'd ordinarily go for the high-current parts (those deflection drivers) first, but thought that maybe there's a part, or parts, that fail often on these. Doesn't hurt to check.

Cary

Nope, the few that I've repaired have all had different problems. Don't even get me started on the convergence board when you insert the yoke connectors one pin off. bad components were littered all over my bench.

7 hours to repair 3 boards...

You have two problems with that VDM, and the worst one is being able to see the blue collapsed. That's not supposed to happen. The protect circuit is supposed to blank the video before that happens.

When you're able to see the collapse, the problem is more difficult than a normal repair, though that would be fine, but you'll need a test unit to use so as to not damage good tubes during your troubleshooting.

Replace U900 and at the same time, replace check R920 and R921.

Thanks Mike!

The blue is collapsed down to about 1/4" on the tube face. It's not totally just a horizontal line (which would scare me that I'm burning a line in my tube). I can see that there's really a crosshatch pattern in there, it's just bunched up and looped over itself (there's even a twist near one end). I guess that's enough raster height there to fool the scan-fail logic. I've turned the brightness down to where it's just visible.

Thanks for the info on U900, R920, and R921. I don't have schematics with me here but I assume that's one of the deflection drivers (on my way home so I can check).

Cary

it's just bunched up and looped over itself (there's even a twist near one end). I guess that's enough raster height there to fool the scan-fail logic. I've turned the brightness down to where it's just visible.

Cary

That twist also indicates that there's either an unusual off-set (+/-) on U900, or U900 is defective or not getting equal 14v on both Vcc's (+/-). The two resistors supply the Vcc's.

A safe way to run this problem down is the pull P14 (CRT heater connection) and use a scope on the output (where yoke connects) of the board (comparative amplitude measurement to either of the other two working channels), or use a digital meter to do the same. From there, remove both R920 and R921 and put in their place a 4.7ohm ohm 1/4 watt resistor.

Using these resistors in the place of R920 and R921 puts some protection on that chip while you're running the problem down...

Wow, great idea pulling the heater. I just checked R920 and R921; they are ok (not opened up). I'll have to wait until after work tonight to get the scope on it. Thanks Mike.

Cary

Finally fixed this one. Here's the skinny for the next guy.

Pulled the heater P14 for testing (thanks Mike). I started looking at the blue deflection driver, U900. The output, which drives the coil directly, was oscillating badly. The inverting input was also oscillating (non-inverting input is tied to GND). It would oscillate for 3 or 4 seconds, then settle down to what looked like the ramp waveform, then oscillate again. This pattern repeats. I believe the part must have been going in and out of thermal shutdown.

The inverting input to this driver is a summing junction from several sources (size, ramp, S, C) and they all looked good. It had to be the deflection driver, U900.

I replaced it with a part from a scrap board and it still didn't fix it.

If the thing oscillates then the feedback path is suspect. Sure enough, the three 10ohm resistors, R912, R913, and R914, all in parallel from the deflection coil return to ground, and also in the driver's feedback path, were *not* 10ohms. I removed them and they were not open, but around 290ohms (not all the same). They all had either a tiny burn spot or bubbles on the side towards the PCB (that's why I couldn't see it). I replaced these and the board works fine now. I should have checked these before wasting time replacing U900.

The question is then, why did they fail? This board works fine in my 9500 but I'll be putting it back into my brother-in-law's 8500 this weekend and I'm wondering if something with the deflection yoke could have caused the failure. It's a longshot I think, but I'd hate to fry the board again.

In other words, when I pack for Orlando this weekend, should I pack the whites AND the yokes? :) :) :)

Cary

yep, you're gonna have to replace that Yoke.

When they short, the pulse from the HDM wipes out those three guys (10 ohm resistors).

Thanks Mike! I'll bring a yoke with me and replace the one on the tube now. Is there any way to test the one that's on there now? Does the resistance go down slightly if it's shorted? Test to see if it's shorted to the horizontal coil?

Cary

Thanks Mike! I'll bring a yoke with me and replace the one on the tube now. Is there any way to test the one that's on there now? Does the resistance go down slightly if it's shorted? Test to see if it's shorted to the horizontal coil?

Cary

When that happens, it's just best to replace it. It could have been caused from a flash-over of horizontal pulse from the horizontal section, but that may be rare. So you may get by with just leaving it in there.

Me, I would replace it. And I'm saying that from experience.