Looking for couple tips please

[ohmmy]

Hi, I'm new and although I have a fair amount of electronics knowledge/experience I'm wondering couple things that could help me in my future troubleshooting.

1. I know usually bad filter caps or shorts would give low output voltage readings, but what about when voltages are wavering/not steady when measuring output?

2. How do you load a PSU without the mainboard to determine if the mainboard is loading down voltage readings or if it's a problem within the PSU?

Thanks in advance!

[tom66]

Re: Looking for couple tips please

1. I know usually bad filter caps or shorts would give low output voltage readings, but what about when voltages are wavering/not steady when measuring output?

In this order:
1. Startup capacitor on primary side
2. Shorted diode on one rail (but not on the wavering rail)
3. Bad output capacitor
4. Bad primary capacitor
5. Bad optocoupler or feedback circuit
6. Bad controller IC
7. Anything else (but pretty rare)

2. How do you load a PSU without the mainboard to determine if the mainboard is loading down voltage readings or if it's a problem within the PSU?

1157 car tail lights are a good option, as are some forms of car headlamp. Tie two in series for a basic 24V load.
For plasma TVs a 230V lightbulb or two 120V lightbulbs in series can form an effective Vsus load (nominal 200V.)

[ohmmy]

Re: Looking for couple tips please

Thanks a lot Tom!

Questions:
Do optocouplers or controller ICs tend to fail in this fashion? Hard to test?

Instead of a car lamp, anything else you can use? Resistors? Would a car lamp work for a 5v output?

[tom66]

Re: Looking for couple tips please

Do optocouplers or controller ICs tend to fail in this fashion? Hard to test?

It's very rare, but on this forum we have seen some controller chips with leaky Vcc input, this means they cannot start up properly and the output doesn't rise high enough.

Optocouplers do fail if overdriven (too much LED current.)

Instead of a car lamp, anything else you can use? Resistors? Would a car lamp work for a 5v output?

A car bulb would work but it would run dimmer.
Resistors are OK, but you need a large high power one.

[ohmmy]

Re: Looking for couple tips please

Awesome.

In regards to Plasma Tvs, I've repaired a couple with bad caps but at the time did not know to check Vs Va voltages to make sure they're outputting correct voltage and adjust if necessary.

Say the voltages were off a bit, what kind of symptoms would you get? And does the tv need to be on when adjusting or just plugged in?

[tom66]

Re: Looking for couple tips please

Missing Vs: No picture, no priming glow

Very Low (<20V from setting) Vs: Underdiffusion (picture not fully lit, missing patches, purple tint towards the middle)

Incorrect Vs: Sparklies/Dead pixels but otherwise OK picture

Very High (>20V from setting) Vs: Overdiffusion (picture too fully lit, very bright, no detail visible)

--

Missing Va: No picture, but panel is primed

Have never encountered low or high Va so could not comment. Most commonly derived off Vs supply so rare for just Va to fail.

Very Excessive Va: Fried plasma panel drivers, expensive paperweight, goes in trash.

[ohmmy]

Re: Looking for couple tips please

What are "sparklies"?

Do the Vs, Va go out of their range from tv use usually or? Like in the incidents where their were bad caps on psu, would after replacing them cause Va, Vs to change?

[tom66]

Re: Looking for couple tips please

Sparklies are misfiring random pixels, flashes of green or red usually, sometimes blue.

An example on one LG plasma I repaired:
https://www.badcaps.net/forum/attach...8&d=1326915294

Vs and/or Va can drop due to the potentiometer failing (contamination from dirt or moisture or simply age), or due to a power supply problem.

[ohmmy]

Re: Looking for couple tips please

Very helpful Tom, appreciate it.

When measuring/adjusting Va, Vs does the tv need to be turned on just plugged in, in standby?

[tom66]

Re: Looking for couple tips please

Needs to be powered on showing a picture.

[ohmmy]

Re: Looking for couple tips please

Thanks a lot. Could you help me understand the difference between the logic and main boards of a plasma tv?

[tom66]

Re: Looking for couple tips please

Logic board generates control signals for plasma panel drivers, similar to LCD panel T-con.

Main board generates video signal for control board, identical to LCD TV main board.

[ohmmy]

Re: Looking for couple tips please

Thanks so much Tom. Great help.

I'm curious, if you measured voltage at positive pin of a capacitor, if the voltage reading was steady and not wavering would that determine it was good? Or not a good indicator?

[tom66]

Re: Looking for couple tips please

That is only a test of a power board, not really of a capacitor. An ESR meter is the best device for verifying capacitor functionality.

[ohmmy]

Re: Looking for couple tips please

That is only a test of a power board, not really of a capacitor. An ESR meter is the best device for verifying capacitor functionality.

Of a power board? You mean the voltage outputs labeled? I'm talking about actually having positive lead on positive side of a cap to see voltage it's outputting, if that makes sense

[tom66]

Re: Looking for couple tips please

A capacitor isn't like a battery, it doesn't output voltage. The power supply outputs voltage. As part of the power supplies' operation, the capacitor is used as part of the rectification and filter stage to maintain a stable and regulated output.

[ohmmy]

Re: Looking for couple tips please

Can you use just wire to turn the power supply on from the 5v standby to the PS on or does it have to be a resistor to cut voltage?

[budm]

Re: Looking for couple tips please

The reason we use the resistor is because we do not know how the signal is to turn on the transistor or the opto has built-in current limiter in place or not, most do, but you do not want to find out the hard way. You can trace out the connection from the PS_ON pin to see where it is connected to.

[ohmmy]

Re: Looking for couple tips please

What value resistor should be used?

[budm]

Re: Looking for couple tips please

470~1K will be fine, which will limit the max current to about 10mA (470 Ohms).