troubleshooting TV SMPS sequence

[budwich]

Re: troubleshooting TV SMPS sequence

:-( again not sure what to say.

first, your investigation is much like a "search". You look at things at a particular point, in this case, the output of the supply going towards the panel / scaler. You look at what's there. It appears "funny" as in not regular. It is unlikely that the output is caused by the scaler since it ain't connected. That means that something further "upstream".... so you need to "paddle upstream"... :-) Look at the waveform coming into the diodes... what does it look like... is it similar or different than that of the output.... that perhaps tells you there is an issue with the diodes... maybe depending on what things look like. Does this make sense?

Anyways, before you go there, you need to determine any kind of "cycle" / "period" that you might be seeing. You are less concern with the value of "ripple" as oppose to period. That might tell you if something is wrong with any diodes or capacitors not doing their function maybe. NOTE: yes ripple is important if it is really bad... not sure if you are seeing that level of "ripple".

The usual purpose of a scope is to provide details about the waveform in terms of frequency, peak to peak values, and timing. These are the aspects that you need to compare and look at each point along the circuit. People use multiple trace / channels to do this more easily but a single channel can be used as well if "general waveforms" are being looked at as opposed to when closely timed digit type signal circuits are involved.

PS... as cap... just stated you need determine more about the output's "cycles"... if any repeatable trace patterns show up.

[CapLeaker]

Re: troubleshooting TV SMPS sequence

I am a little bit late today, but there is something wrong with the 5v supply. It should 'the do that. Change the 5v output capacitor for now, retest.

Now for the other 2 rails: they really seem to go up and down in voltage. Question is: do they do that in a repetitive timely manner, or erratically?

[ilikesteel]

Re: troubleshooting TV SMPS sequence

Budwich,

remember, I spread those two signals out so I could capture on camera the fall and rise of the signal. It does not look like ripple. it's almost like a square wave or a degrading square wave signal that has a definite cycle of low to high. we're talking about a 2V swing from low to high. when I switched the time back the other way (to the left), you see two solid lines jumping up and down.



where could that kind of signal be generated from?

on the 5V vsb, that's a jump of about 4.75V to 5.36V. is that signal a being generated on the 1W small 5V board? both boards have separate signals that are jumping around. I'm really confused. what other measurements can make with the scope??

can we determine if the problem is on the HOT side or the COLD side of the PS, any guess? how can we determine if the problem is on the HOT or Cold Side? I'll be glad to make more types of measurements, just give me some guidance.

I can reconnect all cables and put TV back to normal. recheck the output voltages with the scope and see if they remain at +10-11v on the +12 and 20-22V on the +24 with the TV in normal operation.


what do think is the cause of the 5V VSB signal, cap 616 on the 1W PS board, 470uf at the output? I'd be glad to change that out and also the Main cap 180uf 400v if you think that may help?



Thanks very much,

ilikesteel

[budwich]

Re: troubleshooting TV SMPS sequence

hmmm... not sure what to say. The "5v thing" looks like the scope wasn't triggering and hence the waveforms were all over the place. Might want to "line trigger".

On the others, not sure that you should be measuring with a load on the outputs but that just a comment... I would normally do both to see if there is a difference. Although you are thorough with your observations, I do believe you need to choose a time scale that allows one to see multiple "cycles" of the "DC" ... why... because you previous indicated that the bulbs flashed... but the wave form that you show doesn't really indicate that would be the case.... but maybe a "1.5" volt "swing" is enough to cause a bulb to "flash".

[ilikesteel]

Re: troubleshooting TV SMPS sequence

just a BUMP.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

Guess photos didn't attach. here theya re.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker and Co,

here we go again.

powered TV on, NO LOAD on the PS, +12 and +24 are only on the PS.
attached are 3 photos of the results I received.

I used a X10 probe. and the GND line is the very bottom line on the scope screen.

The first photo is the +24V.
.5V per voltage/division
.1 of a volt Time/Division
looked like 18V-22V change, note the photo.

The second photo is the +12V
.2V per Voltage/Division
.1V Time/Division
looked like 10V-11V change, note the photo.

There really isn't any ac ripple that I can tell. it looks like a square wave with the lower reading going up to the higher reading and back down.

Both voltages exhibited the same as did the DVM readings.

here's the KICKER. I decided to check the 5V-VSB.
.1V per voltage/divison
.2 MS Time/Division
That's the 3rd photo. wow, what's going on here?


thanks,
ilikesteel

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

OK, will do and report back.

thanks
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

There is no problem on the secondary to put the o'scope GND to the Secondary GND.
However DO NOT hook up the scope GND to the primary. You run the chance to blow up the scope, unless you have the power supply on a isolation transformer. You can also check with your DMM between scope GND and the point you want to hook up your GND to and see if you get voltage.
TO check for ripple just shorten up the time and voltage on the o'scope. Every cap has some ripple. Maybe you should test that on a different PSU that is working first.
also check youtube for "ripple on capacitor".
W2AEW has something on youtube over the AC and DC coupling... also something to watch.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

how can I check for ripple at the same time? also, can I gnd the scope to the DC gnd of the PS?

thanks,
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

Check both of the secondary rails. Also post the Voltage / Time per division you used.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker and Company,

I did not try the hair dryer trick ....... yet. I could not find a CAP 180uf, 400V that I could replace the original cap with. I did manage to resurrect a tektronix 314 oscilloscope. attached are some pics of the front of the scope.


now what can I do with the scope to further isolate the problem? I do NOT have an isolation transformer.

Thanks,
ilikesteel

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker,

alright, let me poke around and see what I can find. I'll update when I've done something.

thanks again,
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

CapLeaker.

is this it, the end of the road?

remember when it was pulsing, I think it was when we had lower voltage on both supplies. right now, they are almost at operating voltages. perhaps I should run the lamp test again?

If it is pulsing on the primary side, what do you think would cause it? main cap?

thanks,
ilikesteel

We are kind of limited to do tests with a DMM and a load, makes it harder. Yes on the lamp test, because this way you can see the flickering, while measuring something else. Actually did you try to heat up the PSU with a hair dryer yet then turn it on? Desperate times = desperate measures. Everything goes.
I could rule out the main cap for sure if I had a picture of the ripple from a scope, or measure the ESR. Not having these, you can go and replace it with something close from the junk pile. At least 400V and preferably the same or more capacitance.

[budwich]

Re: troubleshooting TV SMPS sequence

no problem... you have to make do with what you got.... which means you might have to do a lot more than you would otherwise. But again, you have done lots of testing / changes and "stuff", I think you are probably close based on the help you got so far. If you had a scope ( and knowledge / help from the forum), you would likely have the answer fairly quickly.

I admit that I don't know enough about smps power supplies to be "dangerous"... but just to be curious especially around the "test techniques".

I suspect that if the input side had issues, there would be other issues / voltages apparent which doesn't appear to be the case... maybe.

Anyways, keep going... what have you got to lose.... time and frustation... :-)

So I will ask again, what is the flashing rate of the bulbs? and are they both flashing at the same rate? (I hope)

one other question, have you "swapped" the output "diode packages" between the two outputs to see if things change in some manner...... forget this, I see that the diode sets are not the same between the outputs.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

BudWich,

what about the steady 360V at the main cap?

This is my first time going this deep into a power supply. It's virgin territory for me. I don't have a scope or an isolation transformer. the reason I thought I'd give it a shot at repairing, the set is an older one. I don't want to spend $50 or more on a PS. if I can fix it for my time and everyone else's contributions on this forum, I'd take a chance buying some parts to replace.

???

thanks
ilikesteel

[budwich]

Re: troubleshooting TV SMPS sequence

I didn't go thru things "again", but iirc, your "bulb" test showed "reasonable" voltages on both legs (ie. within a couple of volts)... but there was some sort of "flashing" which just doesn't work for me... and probably for your meter as it is trying to "find" a "steady state" measurement of something that should be reasonably stable which the lights are telling isn't true, so your dc measurement is somewhat "fictitious".

So related to this, with bulbs attached, do you know what the input of the transformer is /was reading?. Maybe you lost a leg of the bridge rectifier such that the resulting incoming wave form is "gross" as opposed to "slightly rippled / frequency chopped"... :-)

Related to the above, you do need to count the flashes to get an idea of what you are "seeing" with your "bulb scope"... :-)

[ilikesteel]

Re: troubleshooting TV SMPS sequence

CapLeaker.

is this it, the end of the road?

remember when it was pulsing, I think it was when we had lower voltage on both supplies. right now, they are almost at operating voltages. perhaps I should run the lamp test again?

If it is pulsing on the primary side, what do you think would cause it? main cap?

thanks,
ilikesteel

[CapLeaker]

Re: troubleshooting TV SMPS sequence

I almost think there has to be something bad with the transformer itself. Remember the blinking light bulb on the secondary? That shouldn't be. Now if we could proof that the primary input is not doing that then we'd be a step ahead again.
See, as more juice you draw on the secondary, the PWM gives wider pulses to the FET's. Unfortunately that requires an O'scope. Problem is you going to need a isolation transformer for the PSU, you don't want to blow the O'scope up while fiddling around in the primary.

[ilikesteel]

Re: troubleshooting TV SMPS sequence

BudWich, CapLeaker, Budm,


I checked the 6 capacitors to make sure I installed them correctly on the board. all caps have their NEG lead inserted into the board going through the hole marked as GND with the slanted white lines near the hole.

with the 360V on the main and with NO load on the power supply for the +12 and +24; what would be keeping the voltage from being up to their required value? I would expect the stated voltage with NO load to be a few volts over the stated value of the supply, not under it.

Now What, what do I do from here?

Thanks guys,

ilikesteel