Question about Mitsubishi WS-73907 TV
The problem is intermittent. The three colors at the screen of this Mitsubishi WS-73907 Rear Projection TV Set are “bowing” or arching from the sides to the center of the screen. It happens sometimes at turn on, or after a while of a nice and clear picture. All the images are there, I mean, we don’t loose the image at all. It seems that the tubes were suddenly out of geometry without a particular reason; always from sides to center making the image looking like a sand clock or hourglass. That’s it. Would you be as kind as help me to figure it out what is causing the problem?
Thanks a lot in advance.
Just checking to see if you have fixed your set? Looks like you have much help here. Hope you can resolve issue. I would like to help if I could.
Posted on Jan 02, 2008
As Terryfic correctly pointed out 9a04 and 9a05 feed the convergence amps in that circuit and one very important question he asked seems to have been overlooked in your zealous albeit elusive attempt to get answers, where did you purchase the STK392-570's from??
I've been a licensed technician more than twenty years and like Terryfic I can tell you from that level of experience that our industry is flooded with chineese knock-off transistors that wouldn't last ten minutes in that set. So if you didn't get these as Sanyo originals from some place like Acme Enterprises in Florida - or some place that you can without a doubt know that they are 100% Sanyo originals you haven't done step one yet!
These are difficult sets to work on - not something for the 20 minute technician to cut his teeth on thinking s/he will save a buck or two by not having to pay those screw-you-over technicians and I see 3-5 of these V17 chassis a month. That said, your problem is in the convergence circuit as indicated by 9a04 & 9a05 opening. I would suggest from those symptoms that your convergence amps (one or both) has failed and that they should be replaced with original Sanyo Ic's.
As Fwadley pointed out you could also pull that board entirely and send it to PTS Electronics. They charge $50.00 dollars to bench test it for you and can even go as far as entirely rebuilding that board if you elect to do so.. Of course by the time you do all this and pay for it you might have been a bit more penny wise to have taken it to reputable shop.
Posted on Dec 19, 2007
I have often repaired problems like this in my shop, what you experienced is called a pincushion effect. This is usually caused by the pincushion amp going bad. Let me explain. Your CRT's uses two magnetic fields to deflect an electron beam from left to right and from top to bottom. in the old days one yoke transformer was used to control three beams so the pincushion amp rarely went bad, and if it did you saw the effects right away (the hour glass pattern) decrease of horizontal sweep towards the center of the picture. This is due to the field being weaker at the center so the pincushion amp provides a counter wave to the deflection wave which makes the picture fit the geometry of the CRT. That is a lot to say in one breath. However this problem could have been taken care of by troubleshooting the pincushion circuit and determining what component was at fault. Now what has happened is that you (and others) have fallen prey to the "convergence amplifier swap out fiasco" The problem is too many people are using this shotgun form of troubleshooting without knowing the basics of TV repair, they may know electronics, but as with all electrical and electronic systems they each have their own functionality, and if you don't understand what is happening you may know what should happen, but you don't know what is happening, that is the case here. you treated this problem like a convergence issue when what you had was a pincushion issue. Now you have two problems because in swapping out the ICs you may have created a short circuit or a bridge between two unrelated circuits. Your best bet now is to take this set to a qualified TV repair shop to first repair what was done to it, and then to solve the original problem.
I have been working in the TV repair industry for over 25 years, and I still don't consider myself an expert because I am always learning something new about modern TVs and their functionality. That's why when a person brings a TV into my shop which had been subjected to "do it yourself" work, I sit the person down and show them what errors they made and a few times I even taught a few individuals electronics and TV repair. I like do it yourself, however I am one of those tech who believes that you should know what you are doing if you plan on doing it yourself, and talking people through procedures that require certain skills is never really a good idea no matter what your intent is. That is why solution #2 by fwadley is a very good solution. I just thought you needed an explanation about what happened. Terryfic gave you good advice, however (This is me being old fashioned) what was needed was additional troubleshooting before removing and replacing those ICs.
What is causing those resistors to burn is the fact that there is no offset signal to the amplifier the out put of the amplifier should almost be 0 volts, this is because the amplifier is actually a current amplifier and the offset voltage caused a 0v output. check the solder connections and make sure that all of the conductor traces are good. You may have created a broken trace during your working with the board, use a magnifying glass to look over the connections.
I hope this helps,
Posted on Dec 19, 2007
The two fuses blow when there is a short circuit in B+ feeding
convergence IC's that you changed.....so...........either
1) one or both of new IC's are defective ( what quality did you buy ? )
2) after soldering in new parts, did you clean pins with alcohol
and check for solder shorts between pins ? I use magnifier and ohm meter.......
3) Leaving connectors unplugged or plugging incorrectly
or possibly reversing plugs could result in blown fuses
Note: If you post comments as you proceed, I can comment back............................Terryfic.
Posted on Dec 19, 2007
I would take it to a TV repair place, they will charge around $50 and let you know what the problem is with it. this is your cheapest way out, it you want to get it. If you chose to take it please give me a fixya rateing. Thanks, Farren
Posted on Dec 19, 2007
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