NO NEVER test a Diode under load. Any Diode that has THREE legs, is either a Dual Diode, OR a Power Diode. ALL, Diodes, usually measure 0.6 - 0.7 on the Diode test PN on a Multimeter.
(The Multimeter supplies the current to test the junctions)
Are you sure they are Diodes, and Not transistors? NPN... PNP ? If they are transistors the you should measure Junctions, between the Base and Collector and Emitter. again, between 0.5 - 0.8 depending on type, a Go/No GO tester is NOT suitable to test semiconductors in a TV set, you need a Multimeter.
Before we can attempt to repair anything at all, is to get a service manual, and then there is one thing that one MUST do before anything else is done. Now that is, the making sure that the Power Supply,of the unit, no matter what it is, has the correct Voltages, coming into it, and more importantly, coming OUT of it. This is absolutely imperative, as, IF these Voltages are NOT correct then NOTHING will work, properly at all. So we must measure these voltages with a Volt Meter. All Voltage measurements are made with reference to ground. Meaning we clip, our Black, Minus, Lead onto the Earth, or Chassis, or Negative (-) "Rail" of the Power Supply. Now with the Red lead, we can now, measure these Voltages. Now sometimes, the Voltage of each Output is annotated,or written, onto the PCB, that will indicate the Voltage of that output. However most usually they are NOT, so in this case we MUST have a manual to tell us. Now this manual is called a "Service Manual" or sometimes a "Repair manual" This manual is absolutely vital so much so, that it can be near impossible to do without it. As apart from having "Troubleshooting" step through's, it also has our Voltages. When we measure these Voltages they are 100% EXACT, so IF the Voltage you measure, except for a few rare exceptions, the "Rail" that is NOT at the correct voltage is the one, that the "Fault/Cause" is "ON". Therefore we can now work outwards from the Power Supply, looking for the cause of that problem. Now if the Voltage is Lower than specified, we are invariably looking for a "Short", something has "Faulted" and is "Dragging" the Power Rail down to earth. Now if the "Rail" is "Higher" than the specified Voltage then the fault we are looking for is usually some sort of "Open Circuit" possibly a Diode, or resistor that has changed it's "Value". Now knowing this and knowing that the Power Supply is the FIRST place to start you troubleshooting and repair from. You now have a MUCH better chance of a "Fix". One thing more, DO NOT forget to get your SERVICE MANUAL
. for you exact make & model. even if you pay for it, you will be glad you did.
Links below, are your user manual, a pay for service manual, link for a free service manual.