Sharp LC32D43U TV, R7001 open

[boss2250]

No power at all from PSU. While checking for the obvious, fuse, burns, open caps, I found R7001 to be open. On this board, R7001 is the first resistor coming off the neutral side of the pwr plug. The PSU is - RDENCA198WJQZ. The resistor is marked 6.8 megohm, M1WJ. Near as I can determine this is an anti-surge power resistor. The Sharp parts list ONLY show it as a resistor, no part # or spec. Anyone have any idea as to the correct replacement part?
Thanks in advance!!

[Andrew F. Ali]

Re: Sharp LC32D43U TV, R7001 open

This resistor will not cause a 'no-power' symptom.

[boss2250]

Re: Sharp LC32D43U TV, R7001 open

Thanks Andrew for the reply, but I have to disagree. This resistor supplies the ground return for the whole board. Just for test purposes, I put a temp jumper in place of the resistor ( no power). All grounds check OK, pull out the jumper, No grounds on the board. Still need to ID this r7001.

[budm]

Re: Sharp LC32D43U TV, R7001 open

This resistor will not cause a 'no-power' symptom.

+1, that resistor has nothing to do with operation of the circuit, the TV will function without the resistor in place.
The primary side of the of the circuit does not use safety/chassis GND as the primary circuit ground reference.
When you make the Voltage measurement in the primary circuit, the GND ref will be the negative leg of the main filter cap, not the chassis.
The Ground of secondary side (cold side) circuit is tied to the chassis ground, Do not jump anything in the primary side of the circuit the the safety/chassis ground!
There are also safety X/Y caps in the primary side that are connected between L and Chassis, N and chassis, L and N, they have to do with EMI and AC Noise filter to pass EMI compliance test.
How about pictures of your boards and the whole back side of the TV? Do you have about 160~170vdc MEASURED between the two legs of the main filter cap, that will be the starting point, do not waste your time on that resistor.
BTW, 'NO POWER' do you mean you do not have and DCV on any one of the connectors on the secondary side of the circuit? Do you have the service manual?

[boss2250]

Re: Sharp LC32D43U TV, R7001 open

PSU is stone cold dead. I am attaching front close up, rear view and rear close up. I've only been in electronics for 40+ years, so I guess I'm still new, BUT everything needs a ground to work. According to the schematic, this is that ground or neutral return.
Thanks for your post
ps> On the parts list, this resistor is marked as a "critical" component with the triangle. All I need is to find what this r7001 is. If it isn't needed, I'm pretty sure the manuf would not have included it. BTW, if I ever DO find it's replacement AND it doesn't fix the problem, I will just replace the psu.

[budm]

Re: Sharp LC32D43U TV, R7001 open

"BUT everything needs a ground to work. According to the schematic, this is that ground or neutral return." of course but we are talking the primary circuit ground and secondary circuit ground here, you will never want to have primary circuit ground tied to the safety ground or the cold side ground unless you want to be electrocuted.
"ps> On the parts list, this resistor is marked as a "critical" component with the triangle. All I need is to find what this r7001 is. If it isn't needed, I'm pretty sure the manuf would not have included it. BTW, if I ever DO find it's replacement AND it doesn't fix the problem, I will just replace the psu." Like I said, it has nothing to do with functing of the circuit but has to do with EMI/FCC radiation requirement of the AC noise generated by the switching power supply or the noise from AC line from getting into the TV. It is safety resistor with high breakdown Voltage, it is for discharging X/Y ac filter caps. If you look at the diagram you will understand why the resistor or the X/Y caps, Gas discharge, the MOV are not need for the circuit operation.
You can waste your time finding and replacing that resistor. Why don'y you start checking the DCV at output the bridge rectifier instead. Voltage testing is the first thing to do in any circuit troubleshooting after checking the fuse and the visual inspection, no DCV between the two legs of the main filter cap = no functioning power supply.
If you look the schematic you will see two kind of ground ref points, one for the primary side and one for the secondary side (cold side) which is tied to chassis.
Upload the service manual will help.
BTW, most electronics teaching never emphasize about the ground of difference circuit especially in the hot side of the circuit, it is one reason people blew up the scope and the circuit when they try to use the scope to make measurement in the primary side of the circuits, or by cutting the Safety Ground prong of the Scope AC plug which makes the scope chassis to be at LINE Voltage.

[budm]

Re: Sharp LC32D43U TV, R7001 open

Double posted.

[Andrew F. Ali]

Re: Sharp LC32D43U TV, R7001 open

OK. 40+ years of electronics is impressive,............ so check if you have AC going to the Bridge rectifier and 160 Vdc across the main filter caps as budm suggests. Let us know the results.

[budm]

Re: Sharp LC32D43U TV, R7001 open

"BUT everything needs a ground to work." Actually that is not true, GND is just the common ref point with 0V potential (but not in the real world), it can be positive or negative. Battery operated electronics do not need the safety ground or how about circuit that powered by the isolation transformer, and do not confused Safety Gnd and circuit Gnd ref.point. The current leaves the power source then flows back to the power source through the load. Safety ground is for fault current to provide the fault current path to earth for protection.
And when you deal with return path the so call GND, that is another complex problem that you really need to pay lots of attention to.
http://www.edn.com/design/test-and-m...alled--ground-
http://www.edn.com/design/consumer/4...off-the-ground
http://www.edn.com/design/systems-de...mplex-circuits
BTW, I also have 40+ year of real world experience, not just from the text books.