Hello e-music,
I know that they utilize some potentially lethal parts/sections that I do not come across on a daily basis
When ever servicing any electronic devices employing a Switch-Mode Power Supply design, precautions MUST always be
observed.
The Hot side of any SMPS is in effect connected directly at mains potential. Due to no isolation between the 240V or 120V
mains power and the Hot side of the power supply, it has the potential to kill. The current that could possibly pass through
your body would only be dictated by the current rating of the PSU's main fuse.
The chopper transformer, opto-couplers and high voltage caps and/or high value resistors are virtually the only components
that are providing any isolation between the Hot and Cold side of any appliance.
You work on the Hot side as little as possible when having the appliance turned on.
There are instances where you need to take Hot side voltage measurements, but you need to be extremely careful.
When ever servicing, say a CRT Television, you need to switch it on prior to any disassembly to observe any symptoms
while powered. Any ticking noises, hiccuping (PSU trying to start, senses a short and shuts down, then repeats), sparks etc.
Next, remove the rear cover, then take a look at virtually anything on the main board, the CRT and the ultor cap (suction cap),
and any other items connected to the main board. eg. speakers, deflection yoke and CRT board.
If nothing amiss, then blow the insides out with compressed air. A vacuum cleaner isn't the best to use, as it doesn't get the
dust up sufficiently, plus you risk damaging components.
Best way is just to remove the main board and dust it down with a clean paint brush. But before doing that, read the next step.
Turn the TV on again and observe to see if there is any distress such as arching, components giving off smoke etc.
If all good, turn off the set. Turn it off at the mains and remove the plug.
Next thing you need to do is to discharge the ultor cap connection to the bell of the tube.
You just need a jumper cable, preferably with crocodile clips on each end, then connect one end to the shaft of a flat bladed
screwdriver with a plastic or rubber handle. The other end of your cable should go to any good earth point, preferably to the
braided wire that partially surrounds the bell of the picture tube.
Next, gently push the blade of the screwdriver under the ultor cap until you hit one of the solid wires or metal plug underneath
or you actually hear a discharge. Gently jiggle the screwdriver to make sure its completely discharged.
You can now remove the ultor cap.
Next, you remove the main board, which ever way it needs to come out, but only hold the board by the edges. Try not touching
the solder side of the board with your fingers, especially near the mains connector to the PCB.
Once removed, take note of where the main filter capacitor/s are located.
Now, flip it over and place it on a bench with the solder side up.
Use the crocodile clip cable, and place it across the main filter cap/s and discharge it/them. But expect a mild c.r.a.c.k.i.n.g sound and
possible sparks when doing it.
Its preferably better to make up a cable system using crocodile clips and inserting a 100 ohm 5 Watt resistor in the middle of the
cable. This will prevent the sparks and/or c.r.a.c.k.s you'll hear, as it will discharge it at a slower pace. Though this is a great cable if
you do a fair bit of SMPS repairs, but otherwise wasted.
The main board will now be safe to handle without copping any nasty shocks.
Take note: Every time you need to energize the TV, the procedure above will have to be repeated.
Next you should give the whole main board a close inspection on both sides of it.
Solder side: Check for any dry or cracked joints, especially around heavy current areas, as well as the flyback and chopper
transformers, as well as heavily heat-sinked components.
Component side: Check for any parts cracked, burned or deformed, including coils and connectors. Check for hot spots on
the PCB itself for any overheating.
Now to actually answer your question, and I'm sorry it took so long, but the biggest risk is, as I've said already, the Hot side
of the PSU is the most dangerous section of any CRT TV, as well as any other appliance that uses SMPS technology.
You can be zapped by a multitude of other sections on the TV, even the ultor cap, but unless you have a heart condition, the
worst you'll ever get is get thrown across the room.
Large screen TV's usually pack 30kv at the ultor, but due to the very small currents involved, it would rarely kill anyone.
Though being thrown across the room isn't pleasant, and even the blase attitude I seem to have with the above, caution
is still the key. No one likes to be zapped, weather its a large one or a very mild one.
Once in my servicing days, I got a fair jolt from something, can't remember exactly, and my reaction was to jerk my hand up.
I ended up hitting the CRT board and cracking it. Thankfully I didn't break the neck of the tube.
If ever you need to take voltage measurements on the Hot side of the PSU, you need to plan it out before you do so.
Make sure yo know what component leg you need to target, weather you have the board with component or solder side
up. Keep your hands on the probes and try not touching any other components or heat-sinks in the process.
When doing Hot side measurement, you can only use a grounding point on the Hot side. NEVER use one on the Cold side.
Same goes with cold side measurements: only Cold side ground points.
This is what I mean in regards to planning what you need to do on a live set.
Most important is wearing good shoes, preferably with a rubber sole. NEVER barefoot or in socks only and definitely no
water on the floor.
I hope the above helps relieve any doubts you have with CRT tellys, even though it was all a bit long winded.
If you still have any reservations, please let me know.
Regards,
Relayer