TV Trouble

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qdgjcl
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TV Trouble

Post by qdgjcl » Mon Apr 28, 2003 1:20 pm

I have a ten year old Zenith TV and the picture started shrinking, and would then pop back up. Now the picture is reduced to a small thin, very bright line. There is still sound and it looks like you can see some movement in the line. Does anyone know what the problem is and if it can be fixed?<p>Thanks for any help,<p>Brad

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Externet
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Re: TV Trouble

Post by Externet » Mon Apr 28, 2003 2:05 pm

Hi.
The vertical deflection output stage is kaput.
Miguel
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Re: TV Trouble

Post by russlk » Mon Apr 28, 2003 2:17 pm

It should be easy to trouble-shoot and repair, if you can find someone knowledgeable.

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Edd
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Re: TV Trouble

Post by Edd » Mon Apr 28, 2003 7:50 pm

Sir Bradley:
First, just confirmation that your “bright line” is from left to right across the screen, specifically just like you took the picture and collapsed it down from the top and bottom..
into that thin line.
If this is the case, provide us with the model and year of manufacture off the back. Also on the giant module there should be a 9-xxxx identifier #, usually on a heat sink member
or larger physical component.
In the interim you might pull the back and keep one hand in your pocket and steer clear of the large red ultor wire coming from the IHVT (Flyback transformer)……specifically, loosely translated, if you let it touch you, you’ll…FLYBACK !….really though, its quite well insulated. You should see at least two heatsinks ,either dedicated Al extrusions or possibly mere aluminum sheets. One will be the larger one, for the hoz out xstr.The next largest for the vert out IC which will probably be in a 1”L x1/2” Hi x~1/8” thick, electrically ~10 to13 pin SIP packaging. That’s the one of interest, might be a LA 7837. Look at it from the bottom of the chassis and inspect those pins for microscopically fine,circular, cold solder ring joints. Time ,temperature ,and differential expansion may have produced an open circuit(s) for you on those IC pins and that may be your only problem…..clean, rosin flux and resolder. If that should be it fire up the TV and observe the pic and then give the heat sink a final vibrational analysis with a plastic screwdriver with the old tap-a-tap-a-tap-a test.. If this shud not get it, give destructive feedback.<p>73's de Edd
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qdgjcl
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Re: TV Trouble

Post by qdgjcl » Tue May 06, 2003 2:12 pm

TO Edd Whatley, SORRY ABOUT GETTING BACK THIS LATE. I HAVE BEEN GONE. THE MODEL NO IS SR2573DT AND THE YEAR IS 1995. WHAT IS THE GIANT MODULE?

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jollyrgr
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Re: TV Trouble

Post by jollyrgr » Tue May 06, 2003 3:35 pm

The "GIANT MODULE" is the main circuit board that contains all the components. In other words not one of the CRT boards, patch panels, or switch modules. <p>Zenith TVs CAN be fixed (in some cases) to the component level. But usually you do a module swap. Or you can buy the module outright. Depending on where you live you might be able to do this local, if you find the right shop, or you will have to do it by mail. Here are some examples of web based sites that do exchanges. (NOTE: I have not tried any of these as I have a somewhat local shop that exchanges these over the counter.)<p>http://www.tristatemodule.com/Products/ ... enith.html<p>
http://users2.ev1.net/~snoopee/mx/<p>
There was a company that would repair your module and send it back. But I was unable to locate this site. <p>If you still want to fix this yourself, here are some hints and warnings. Keep one hand in your back pocket at all times. (No joke; this will prevent you from getting zapped and creating a path for current flow across your heart.) Fixing a computer, radio, or similar electronic product is fairly safe. The inside of a typical color monitor or television has voltages in the 25,000 (25 thousand) volt range. This voltage can be present for a long time. (I have heard stories of people disposing of old CRTs getting zapped by tubes that had been disconnected for YEARS. Use pleny of light while working on the circuit board so you can see what you are touching and you don't accidentally short anything or touch live contacts. Setup a small mirror in front of the set while you are in back. This will aid you greatly in seeing what is going on and you won't have to lean over the set to see what is happening.<p>Where to start: Check out the vertical output circuit. Hopefully this will be labeled as a section on the board. Look for a somewhat large transistor or amplifier semiconductor in this section and the surrounding capacitors. SOMETIMES (but not always) you will see a fried resistor or leaky capacitor. This would be a good sign as you can fix this rather easily. Otherwise you must troubleshoot the section. Since you state that there is a thin line and you see movement and there is sound sections of the set are still working just fine. Thus you can stay out of the audio, tuner, "display", and possibly the power supply section. Considering you have a line I would think the high voltage section is operating correctly. But it could still be related to this section. Check the capacitors and resistors in the vertical section. It may be as simple as a dried out capacitor or a fractured resistor. It could be a bad (cold) solder joint.<p>Some other things to consider. If the picture was "blooming" (getting bigger during bright screen appears) I would then start checking the power supply section. Look there for a fault B+ voltage. It might be labeled on a sticker inside the cabinet or at the test point. <p>Good Luck<p>[ May 06, 2003: Message edited by: Jolly Roger ]</p>
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Edd
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Re: TV Trouble

Post by Edd » Wed May 07, 2003 6:55 pm

Sir Bradley:
Looks like your set does use the popular and infamous LA7837 vert output power I.C. Also the main module on that particular set is a 9-1551…located where I told you. If you will look at the rear of the chassis, you will see the Flyback transformer located at the right rear of the chassis’ corner, just in front is the square U channeled alum heat sink for the Hoz out xstr and just towards the front of the chassis is the sink for the vert out I.C…. which is mounted to the left side of that sink.It is Zen part #221-637-01
,but this will be obscured by its mounting clip. It has 13 pins so if you had not found that IC yet, check its pins with a mag glass as I mentioned for floating solder ring joints on its pins.Then, not likely, but also check yoke connector 2Y2 that is about ½ an inch left and forward of the long leg of that vert heat sink for joints/ connectivity problems.If U haven’t lucked out yet , time to check for 2 supply voltages to the vert out stage.
With no AC power plugged in, back at the extreme right rear corner of the chassis locate RX3242, a 1 ohm 1/2 w Carbon film fusible/resistor…is it burned or open ? If not then locate service link (bare jumper wire) L3273 about 1 in towards the front of the chassis from the afore tested resistor. This is the test point for the ~24 VDC that feeds your vertical I.C. For ground metering reference the gnd of the tuner is adequte and easy for you to locate. Now perform DC metering of that source as you power up the TV for an instant. If that is ok ,shut down and move over to inspect inside the vert heat sink area ,where there are 2 diodes that are side by side,the left one is Cr2150..a 9 V Zener diode that is the derivatant of Ur 2nd supply to the vert I.C..Put your probe, or a Pomona style of micro zip clip on the cathode (closest to the heat sink proper) of that zener and then power up and see if ~ 9 VDC is there. If either of the voltages are non existent or grossly low I would expect the vert IC of crowbarring its output stage. That shud be ‘nuff data initially . Feedback pls.<p>
73's de Edd
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:( :( :(<p>[ May 08, 2003: Message edited by: Edd Whatley ]</p>

qdgjcl
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Re: TV Trouble

Post by qdgjcl » Wed May 28, 2003 9:30 am

Thanks for the responses. I started to troubleshoot the TV everything checked out, but I got stuck at the point where you said to use the ground of the tuner for reference. I don't know where the tuner is. Please send help.<p>Thanks,<p>Brad

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Edd
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Re: TV Trouble

Post by Edd » Wed May 28, 2003 2:44 pm

Sir Bradley:
On your chassis there are two ground references, specifically, over on the right rear quadrant where the AC power cord enters, that portion is a Switch Mode Power Supply section and using its " hot ground" reference could put your meters neg (gnd) wire ~ 62-124 volts above ground if you were to touch a real external ground , like another gnd terminal of an AC outlet, a grounded ant/coax/cable lead, a cold water or plumbing fixture, or even damp concrete floring, etc. The reference I specifically mentioned to U was that one of the main chassis whose ground is the "cold chassis ground" by virtue of being connected to the secondary side of the output winding(s) of that SW mode PS transformer, thus being electrically isolated away from the AC line.
The tuner is easy for you to find as it is the small quasi square/rectangular metal shielded/encased unit on the top rear of the chassis that you had connected to its input F connector with your F59/62 connnector that was going to your ant/cable line or VCR ( if you had one in the circuit)
I assume that if you are to this stage that you havs closely vis inspected the Vert IC's pins and found no microscopically floating pin connections to same. Also that the 1 ohm carbon film fuse/curr lim/ resistor was ok....not burnt nor open. So you are now ready to check for the presence of proper supply voltages to the vert ckt. Feedback time.<p>73's de Edd
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