Mitsubishi VS50501A another thrown away TV with bad convergence.

[MRX37]

Yup. Found another big screen TV with messed up convergence. It's a Mitsubishi VS50501A, made in December 1997.

The green raster has a kind of hourglass shape to it. I think this is what's screwing up the convergence, as I can access the convergence menu, and move the crosshairs.

I just know someone here is going to tell me to replace the IC's... I looked up this model number, and the problem may not be the IC's. I would like to rule out other causes before saving up to order IC's...

[MRX37]

I set up a crosshatch pattern on the screen, and, well. A picture is worth a thousand words...



The whole raster seems messed up. Also, the green doesn't seem to be as stable as the red and blue...

[mbates14]

Ive worked on that set.

2 things: first you need to look at the STKs as the solder is terrible on those. (and replace if necessary). also check yoke returns on convergence yokes for opens (resistors).

And one more thing: On the physical yokes themselves, youll have to melt/razor off the cap to each yoke where the cable plugs in, and resolder all those joints on the winding interconnects. (known to cause random shutdown/startup probs).

[MRX37]

Originally Posted by mbates14

Ive worked on that set.

2 things: first you need to look at the STKs as the solder is terrible on those. (and replace if necessary). also check yoke returns on convergence yokes for opens (resistors).

And one more thing: On the physical yokes themselves, youll have to melt/razor off the cap to each yoke where the cable plugs in, and resolder all those joints on the winding interconnects. (known to cause random shutdown/startup probs).

You've worked on this model? Great! You can tell me where everything is!

PS: I mentioned that the green color wasn't stable. Any idea why?

[mbates14]

Like i said, you need to go through all the solder joints. mitsubishi was bad for this.

[MRX37]

Originally Posted by mbates14

Like i said, you need to go through all the solder joints. mitsubishi was bad for this.

Okay, someone's been on that convergence board before. the STK IC's were either all replaced, or all resoldered. (I'm gonna bet on replaced as there were also some resistors that were replaced.)

I touched up all the solder joints, but that accomplished nothing.

[mbates14]

well Did you check to verify your positive and negative voltages were present on the STKs? because if someone has been into that convergence board, it makes me worry. hehe.

need to verify the proper voltages are present on the STKs, and the resistances check out. if everything checks out here, and the STK heatsink dont get super hot, then i would look at the digital convergence module.

[MRX37]

um... Where should I check those, and what are the voltages supposed to be?

[mbates14]

becuase by looking at that screenshot, it is DEFINATELY not getting proper drive, or missing voltages on the STKs. (if the STKs arnt bad already). might want to check the outputs for extreme DC offset also.

[mbates14]

Are they STK392-110s? or what STKs are they. (theyve used different ones from set to set). Also im not sure if this is the source of the problem, but the reason i ask for voltages, becuase that set has a tendency to loose the 24v supply to the STKs. via Z905 pico fuse openining.

if Z905 is open, youll need to test/replace D909, D908, D913 before attempting to replace fuse and power back up

[bgadow]

Wins the award for worst convergence I've ever seen...other than that, I've got nothing

[mbates14]

And its also possible that Z905 opened when the old STKs shorted, and when they replaced the STKs, they didnt replace this fuse. (didnt have any clue it existed).

Best case, is to look up the datasheets to the installed STKs, and figure out which pins are positive and negative rail supplys in respect to ground. i forget which is which off the top of my head, but it appears to me your missing the -22v supply. whcih is supplied by Z905. Thats why you still have SOME correction, as the positive rail is still present. the negative rail causes the "bowtie"

[MRX37]

The IC's are all STK391-020 IC's

The middle one has a date of August 2006 on it, so it's definitely been replaced.

Where is this Z905 fuse?

[mbates14]

youll have to look around. i cant remember exactly. (its been 2 years since ive seen this set).

its a pico-fuse. so its going to look like a resistor, but its labled Z905. just trace around the board where the power comes in, youll find it.

[MRX37]

So it's on the convergence board itself, and it looks like a resistor?

Is it under that metal covering, or can I rule that out?