element eldft551 not turning on

[budm]

Re: element eldft551 not turning on

If you monitor the 24V or the 12V output, do you see the output voltages jump for seconds when the TV is turn on, that will give us the clue if the power supply did try to start up and then goes into shutdown for some reason.

[cashkennedy]

Re: element eldft551 not turning on

Yes the 24v / 12v and 5v (not the 5vsb) all come on for 1-2 seconds when its first turned "on". After that 1-2 seconds they all go away the STB stays on (4v) and the blue light remains lit, and I have to unplug to set to turn "off".

Is the protection mode supposed to trip if there is not enough load on the 12 and 24? or only if it doesn't feel its accurately making those voltages?

All 3 voltages that come on temporarily appear to reach a correct voltage (and not go over) , but there could be a limit to my multimeters ability to see quick changes. (all voltages were within 2% of correct value)

[cashkennedy]

Re: element eldft551 not turning on

Which optocoupler is responsible for the protection mode?
From what I can make out PC1B is to sense when to turn on the power supply. So I would assume PC3B and PC4B are involved in monitoring the voltages on the secondary?

[tom66]

Re: element eldft551 not turning on

4V seems strange for standby?

PC4B seems to be the OVP (over-voltage protect) optocoupler for 24V, 12V (and possibly 5VSB) lines, it triggers if PC3B or associated circuitry fails.

PC3B looks to be 12V (and by relation, 24V) regulation.

PC2B looks to be 5VSB regulation.

PC1B looks to be on/off control.

[cashkennedy]

Re: element eldft551 not turning on

STB is the "PS_ON" for this power supply. the 5VSB is 5.0v and after pressing the power button STB goes from 0 to 4v and stays that way till I unplug the set.

I tested the voltage on the SHUTDOWN pin of UA4 (I assume that's how PC4B tells the power supply to go into protect mode?) and I get 4.1 V there after the set has been on for 1-2 seconds.

The way the datasheet describes shutdown , makes it sound like id need a oscilloscope to properly monitor it though, as it may be pulsed to control the output / or steady to turn off the output.

[tom66]

Re: element eldft551 not turning on

https://cdn.badcaps-static.com/pdfs/...c49d47f310.pdf

Looks like SHUT controls CSS (soft start capacitor.)

A high signal on SHUT will kill the oscillator and discharge CSS, disabling the IC. Once SHUT is released, CSS will have to charge to a set point voltage, at a constant current, then the oscillator begins. (The voltage on the CSS pin may also act to limit the current when first starting up.) CSS limits inrush current and gives everything a little while to start up (PFC bus etc) when the PSU is first turned on.

Things that could cause SHUT to be high (more than about 1V in this case):

VCC2 undervoltage monitor (ZA4, R93, R94, R96, D29) - The voltage on VCC2 must exceed the zener voltage plus base voltage of D29 (probably should be Q29?) Check ZA4 is not open circuit (should measure as diode one way.) Check the base voltage of D29, see if it is sufficient (>0.6V.) This might fire during operation if the startup cap is bad and it will keep SHUT high briefly on first startup, I think.

Some kind of strange current-limit detect circuit(?) comprised of CA16, RA42, RA8, DA8, R74 and DA13. Looks like more than ~2.4V (two diode drops 0.7V each plus ~1V required for SHUT) across CA17+R08+R07 will fire SHUT.

The OVP circuit on PC4A: Check zeners and diodes for shorts in that portion of the PSU.

[cashkennedy]

Re: element eldft551 not turning on

Im pretty sure DA5 and DA13 don't exist, let me go get the board back in my hands and confirm. (as in they weren't soldered to the board / those protection schemes were not implemented)

[budm]

Re: element eldft551 not turning on

Pin 10 is the shutdown control pin.
From spec sheet:
"An alternate approach is the use of the shutdown circuitry
of Pin 10 which has been improved to enhance the available
shutdown options. Activating this circuit by applying a
positive signal on Pin 10 performs two functions: the PWM
latch is immediately set providing the fastest turn–off signal to
the outputs; and a 150 mA current sink begins to discharge
the external soft–start capacitor. If the shutdown command is
short, the PWM signal is terminated without significant
discharge of the soft–start capacitor, thus, allowing, for
example, a convenient implementation of pulse–by–pulse
current limiting. Holding Pin 10 high for a longer duration,
however, will ultimately discharge this external capacitor,
recycling slow turn–on upon release.
Pin 10 should not be left floating as noise pickup could
conceivably interrupt normal operation."
As you can see on page two of the SCH, pin 10 (shutdown) is connected to OR GATE Diodes DA13, DA5, D31: anyone of those Diode when the Anode is HIGH will cause the IC to go into shutdown.

[tom66]

Re: element eldft551 not turning on

Interestingly, it appears the error detect for OVP is a latch (NPN, PNP bipolar latch circuit.) Only a power cycle can reset the latch, which may explain the symptoms observed.

[budm]

Re: element eldft551 not turning on

Yes, that is the latching circuit when it detects OV of the 12V and the 24V outputs.

[cashkennedy]

Re: element eldft551 not turning on

Yall are over my head now.... I have checked most of the diodes involved with the overvoltage protection areas, there are a few parts that seem to be mislabeled or changed name /

Parts DA5 and DA13 were not soldered to the board ever from what I can tell (the rest of the parts in circuit around those are still there though)

There is no Q6 , the part on my real board that seems equivalent is D29.

The part in the schematic labeled D29 is labeled Q16 on the actual board.

So far the everythings tested as working the way it should though.

Is there a way to test PC4B on the secondary side? Im not sure what voltage I should be expecting to be crossing there to indicate an error. And is that error signal only supposed to exist for a second or two while the 24v is on?

[cashkennedy]

Re: element eldft551 not turning on

I also don't get the difference between brown ground (the heatsinks) and negative of the filter cap. They are about 10 ohms apart, not sure what they should be apart.

Are there 4 different "grounds" on this board ? lol

[budm]

Re: element eldft551 not turning on

So are you getting DC voltage at pin10 the shutdown pin after the TV is turn on?

[cashkennedy]

Re: element eldft551 not turning on

Yes I get 4.1 vDC at the shutdown pin 1-2 seconds after turning it "on". It stays at 4.1 vDC till I unplug the set.

[tom66]

Re: element eldft551 not turning on

See if you can measure voltage at anode of D31, if this is high, then we know for sure it is OVP circuit detecting fault.

[cashkennedy]

Re: element eldft551 not turning on

There is 5.01 vDC at the anode of D31.

This circuit only detects over-voltage? No possibility the error is from under voltage? And is over voltage possibly from a lack of load?

[tom66]

Re: element eldft551 not turning on

Looks to detect over-voltage and something on 5VSB, not sure what, need to analyse circuit a bit more.

[budm]

Re: element eldft551 not turning on

The OVP OPTO is fed by Diodes OR gate:
DA7 + ZA1 for 24V OVP. ZA1 may be leaky.
DA7 +ZA2 for 12V OVP. ZA2 may be leaky.
DA4 (Dual Diode) is fed by the window comparator UF4A/B. The input to the comparator resistor-string (RA61, RA32, RA34, RA35) is connected to the GND of the 24V power supply but it looks like the GND of the 24V is not tied to secondary side circuit ground like the 12V power supply section up above. The only thing I can think of is that it is looking to see if the GND of the 24V is grounded to the gorund of the Inverter board which is grounded at the inverter board chassis mounting screw. You can verify that by measuring the Ground pin of the 24V connector to see if it shows oepn circuit, then check againg when the cables are attached to the inverter board. The 5VSTB for the comparator circuit is B+ for running the two op-amps.

[cashkennedy]

Re: element eldft551 not turning on

I tested ZA1 and ZA2 earlier and they showed diode readings only 1 direction ( I assume a leaky one would have a large diode reading (large reading = little voltage going through, right?) in the reverse direction?, or would i not notice a small leak with my multimeter?)

All the grounds comming from the inverters are part of a continuous ground plane that also includes the screw that holds the power supply in , so I dont think there is a special ground coming from the inverter that is being monitored.

[cashkennedy]

Re: element eldft551 not turning on

Although they didnt mark the GND with a red triangle, it does have a GND yellow flag at the end of the path for 24V. I think it was just an ommission, as you'll see in the picture all ground pins for all connectors are all tied to the same ground, which screws the power supply to the frame.

What voltages should i be hoping to see leaving the comparators? (im pretty sure the comparators are both inside a 8 pin IC? the one that has no markings / posisbly cause they burned off ...