Riddled TV Repair Forum

Jesse Mack · Joined: 22 May 2008 Posts: 600 Location: The Last Frontier

I do not think you or anyone on this forum is stupid, however, we all do stupid things without thinking and kuhurdler's advice could have saved your meter from being blown up if you, like I, got a momentary brain cramp of which I think I get too many already so no encourgement please.

Now, next thing in a row. I need to know if the fuse blows as soon as you plug it in or do you have to turn the set on to get it to blow.

If it blows the instant you plug it in you need to check any and all diodes using the meters diode position on the power supply board. Also check Q801 which is a mosfet that is mounted on a heat sink on the power supply board.

If you have to turn the set on to get the fuse to blow you need to check the Horizontal Output transistor which is large transistor mounted on a heat sink on the main circuit board. The nomenclature on all components will be printed on the component side of the board.

Caution: There are some VERY high voltage potentials so make sure to be unplugged before doing anything. Believe me when I say it is VERY EASY to forget to unplug before proceding from one thing to another. The potentials usually are not life threatening but they can sure destroy ones equipment and make you hurt physically for the rest of the day. I know this from EXPERIENCE.

Jesse

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shaolin29 · Joined: 09 Jul 2008 Posts: 9 Location: Orange County, CA

The fuse does not blow upon plugging it in. After plugging the TV set in and pressing power, the set does nothing. No lights, no noise.

Jesse Mack · Joined: 22 May 2008 Posts: 600 Location: The Last Frontier

OK, you now need to positively identify if it is a power supply problem or chassis problem. Please be careful on the next step as the power supply, if it is working, has 600 Volts present even when the set is in the off state. Have the set plugged in, Fuse verified good, using the tuner shield on the main chassis as ground and measure Pin 6 of connector P1 which is on the power supply with a harness going to the main chassis. It should have 12 Volts DC with respect to ground. If it is present the power supply is working. Stop at this point and let me know.

If it is not present measure across capacitor C805, also on the power supply. It is a very large capacitor and will have to be measured from the bottom of the power supply. there are only two legs on the capacitor. The capacitor will have a stripe on one side and that is tne negative side. There should be 160 VDC on this capacitor. If it is not there, measure R801 which is also on the power supply. It is a 7 Watt cement resistor. Make sure you have unplugged the tv set before measuring. It should measure 1.5 Ohms.

I'll stop for now, let me know what you find.

Jesse

shaolin29 · Joined: 09 Jul 2008 Posts: 9 Location: Orange County, CA

I followed your helpful instructions and here's what I found:

Measured Pin 6 of connector P1 using main chassis as ground - No volts present.

Continued onto capacitor C805. Located C805 no voltage found.

Continued then to R801. Found R801, measured only 0.5 Ohms, not 1.5 Ohms.

Thats what I have so far. Again, thanks for the kind assistance.

Jesse Mack · Joined: 22 May 2008 Posts: 600 Location: The Last Frontier

OK, Here's something important. When you measure across C805 you must be in DC mode on you meter not AC. Also, This section of the TV is hot woth respect to ground. Meaning using the tuner for ground will not work and give you incorrect readings... If you are using the correct settings on your meter, your failure involves very few components. Put you meter in the AC mode. Able to read 120 Volts AC or greater. Put one lead on the anode of D804 and the other on the anode of D802 with the set plugged in. (the anode is the end of the diode that does not have a stripe.) You should have 117~128 Volts AC. If you do not, check with one lead of the meter on post M2 and the other on post M1. These are the posts of the power cord at the set.

Here's a novel thought, Check to see if you have a house breaker tripped.

Jesse

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shaolin29 · Joined: 09 Jul 2008 Posts: 9 Location: Orange County, CA

I rechecked C805 and made sure my meter was in DC mode, but still no voltage was given.

I then put my meter on AC mode to read D804 with D802 with the set plugged in. This gave me a stationary 112 Volts AC.

I also checked M1 and M2, which gave me 117 Volts AC.

No breakers tripped during the electrical black out.

Jesse Mack · Joined: 22 May 2008 Posts: 600 Location: The Last Frontier

OK, look on the bottom of the board at the junction of the D802 & D804 diodes cathodes (the end with the stripe's). Either the diodes are bad or you have a blown foil between the cathodes of the diodes and capacitor C805. This is because if there was a short around C805 and the circuit was intact you would instantly blow a fuse. Also check for a blown foil between the diodes and between D801 and D803 where the common anodes come together to hot ground. Sometimes the foils can be very narrow and an open foil is not always obvious. you may have to use a magnifying glass or use your meter set on Rx1 to check continuity. When you find the open foil, remove the protective covering and solder a jumper across the open portion of the foil. or use a small piece of wire to jumper between the solder points. If you find the open foil check the resistance across C805 before you plug the unit in. You should start out with a fairly low resistance and the meter should start to climb to infinity fairly quickly. If you have a very low resistance, say between .1 and 20 ohms do not plug it in. You will have a short that must be addressed.

Jesse

shaolin29 · Joined: 09 Jul 2008 Posts: 9 Location: Orange County, CA

I wanted to take some time and look very carefully at the board for blown foil where you suggested. I even went ahead and double checked with a magnifying glass. After 2 evenings of looking I couldn't quite locate any blown foil at the cathodes you recommended me looking at.

I went ahead and took a few pictures of the diodes, cathodes, and capacitor you recommended me taking a look at for blown foil. I apologize in advance for the bad lighting.

Losing hope here...but again, as always, I appreciate your assistance in trying to solve this problem.

Jesse Mack · Joined: 22 May 2008 Posts: 600 Location: The Last Frontier

OK, I do not see any blown foils either but your problem is still the same. If you have 120 VAC across the bridge and no output voltage the bridge must be bad. Did you check the Front-to-Back ratio using the diode position on your meter? That means you measure each diode with the probes one way and then reversing them and measuring again. You should get an infinity reading one way and .640 ~ .800 the other. make sure the set is unplugged. If you get an infinity reading in both directions the diode is open. The diodes are very inexpensive and many times they are cracked in a longitudinal direction making the crack hard to see. Many times they will fall apart when removed. If you replace them make sure you replace all four.
You might check the continuity between the anodes (non striped end) of D801 and D803 to the negative side of the Capacitor. This will confirm the integrity of the ground circuit.
The part number for the diodes is GP15KL-042 and can be substituted with ECG125 or NTE125 diodes which should be available at most repair shops or radio shack.

Jesse