Viewsonic VT2430 fluctuating 5V rail

[rddube]

Re: Viewsonic VT2430 fluctuating 5V rail

It did disable the audio, I couldn't hear anything but had my meter on the 5V rail and saw it was pulsing. R_J do you think I should test a load on the 12v rail without the main board attached to see if it has any effect on the 5V rail?

When I did the load test on the 5V rail it wasn't fluctuating, but maybe it still comes from the power supply when there is a load on the 12V rail. What do you think?

Or could it be one of the caps on the main board? I am tempted in trying to change them (there are quite a few) but there are some 16V caps, 35V caps and 50V caps on the main board. Some are 1uf 50V, wonder if that could be a source? I checked all of them with the transistor tester ESR function, but a lot of them are in parallell so I don't really get good readings.

[R_J]

Re: Viewsonic VT2430 fluctuating 5V rail

You could try loading the 12 volts, but like the 5 volts, how much load? 1 amp, 2 amps? 5 amps?
I guess you could try adding another 470µf - 1000µf across c823 and see if the 12 volts and 5 volts are more steady.
as far as those 1µf caps go I don't think they are the problem.
What you can do is solder another cap across any you suspect and see if has any effect, sometimes all that is required is half the capacity to make a diference.

[rddube]

Re: Viewsonic VT2430 fluctuating 5V rail

A question: I've been checking those small 10uf 16v caps on the main board, there are 21 of them. Brand is Lelon. They all measure out of circuit ok on the uf side, but average 4.2 ohms (some 4.5, some 3.9). Doesn't that mean that there is about 80 ohms resistance in total?

I have some Panasonic 10uf 16v caps, that measure .9 to 1.2 ohms. Could that change something if I changed them all?

The other thing is, what if I measure amps being pulled by the main board on the 5v and the 12v rail, and then do load tests without the main board at the level of amperage I will have measured? Not sure how to do it yet besides disconnecting indidual pins or cutting on wire on the cable, but could that tell us anything?

I will try what you suggest R_J about adding a cap to C823 and see what that gives.

[budm]

Re: Viewsonic VT2430 fluctuating 5V rail

If you check the resistance between 5V pin and the GND pin of the main board without having the board connecting to anything else, what resistance do you get in both directions of the meter?

[R_J]

Re: Viewsonic VT2430 fluctuating 5V rail

If you can measure the current on each of the 5 & 12 volt lines that might help, If you know the current being drawn by the 12 volt line, you could then load it without the 12 volts connected to the main and see if the 5 still pulses. adding the extra capacity should change the pulsing (make it slower) if it is on that line

I'm still thinking about when you replaced those low esr caps and the tv did not work at all. that seems to point at the power supply, you should havechanged them one at a time, maybe that would have given a clue as to which line had a problem.

[rddube]

Re: Viewsonic VT2430 fluctuating 5V rail

Gentlemen, sorry just got back from a business trip, which is why I didn't reply earlier.

BUDM: In one direction, I get 250 ohms that climbs to 3.08K and stops at around that, in the other direction I get 3.9K ohms.

R_J: Before doing the amp test (which will require me to cut the cables), I'll wait to see what Budm may suggest with the resistance test. As for when I changed the caps and the TV didn't power at all, I think it was because of my soldering. Because when I removed them again and resoldered them it came back to normal.

[budm]

Re: Viewsonic VT2430 fluctuating 5V rail

The resistance reading on the 5V line is fine. Is 12V also fed to the main board also? if it is the check the resistance on the 12V also.
There is possibility that the heavy load may be at the Voltage regulator output so you should also check the resistance on the output of the Voltage regulator IC.
Since the 12V may be used for the T-CON board, you may also want to disconnect the T-CON cable from the main board to see if the power supply will become steady or not.
Can I see the pictures of the whole main board?

[rddube]

Re: Viewsonic VT2430 fluctuating 5V rail

The resistance reading on the 5V line is fine. Is 12V also fed to the main board also? if it is the check the resistance on the 12V also.
There is possibility that the heavy load may be at the Voltage regulator output so you should also check the resistance on the output of the Voltage regulator IC.
Since the 12V may be used for the T-CON board, you may also want to disconnect the T-CON cable from the main board to see if the power supply will become steady or not.
Can I see the pictures of the whole main board?

Hi Budm,

Yes the 12V feeds the main board (I actually get 13.1V steady when connected) and powered on. The resistance on the 12V rail is in megaohms, so I suspect it is ok.

Which voltage regulator should I measure I103 or I104?

I tried it with the Tcon disconnected, same result i.e. pulsing of the 5V rail.

Here is a photo of the board, not mine actually but identical. On my board, I have installed a small video card fan in the center of it so it is hiding most of the components - it hasn't been connected however to do these tests as I didn't want to add any load to what is already there. If you need me to, I can remove it (it is only held by some hot glue on the silkscreen) and take an actual photo of my board. Note that I have changed caps on the top left of the board from C110 to C131 down to C291 with panasonic or nichicon low esr caps, but it hasn't changed anything. The brand of the caps the bigger ones are Hermei and the smaller ones 10uf are Lelon.

The brands of caps on the power supply are Elite and I'm thinking although I did check the output caps for the 5V rail and they were fine, but maybe I should recap the whole power supply board, as I'm not sure I can trust Elite caps? Also, when I measure voltage on the big cap, I get fluctuation. Anyhow, I'll listen to your advice (or R_J'S). Thanks again to both of you for walking me through this, it's an incredible feeling to have experts looking over your shoulder!

What a wonderful forum, the best on the net!

[rddube]

Re: Viewsonic VT2430 fluctuating 5V rail

Ok, so I just ordered a set of caps for the power supply, including the 150uf 400v which is an Elite brand. Chose Panasonic and Nichicon caps. Should have them on Monday.

[budm]

Re: Viewsonic VT2430 fluctuating 5V rail

What Voltage does the fan run on?
The POWER monitors outputs of all 3 power supply via R824, 825, 862. So if one is fluctuating then the other will be affect as well.
Regulator IC I103, 104, 204, 205 Output pin resistance should be checked against GND.
I also see at least 4 switching power supplies on that main board.

[rddube]

Re: Viewsonic VT2430 fluctuating 5V rail

Wow Budm, you're a genius!

The fan runs on 12V, but I don't have it connected to run the tests.

Ok, I took the resistance readings with everything disconnected from the main board.

I103 output one way measures 454 ohm climbing, the other way 326 ohm steady
I104 output one way measures 175 ohm steady, the other way 174.4 steady
I204 output one way measures 2 ohms and the other way 2ohms, looks shorted
I205 one way measures 90 ohms steady the other way 85 ohms steady

[budm]

Re: Viewsonic VT2430 fluctuating 5V rail

Wow Budm, you're a genius!

The fan runs on 12V, but I don't have it connected to run the tests.

Ok, I took the resistance readings with everything disconnected from the main board.

I103 output one way measures 454 ohm climbing, the other way 326 ohm steady
I104 output one way measures 175 ohm steady, the other way 174.4 steady
I204 output one way measures 2 ohms and the other way 2ohms, looks shorted
I205 one way measures 90 ohms steady the other way 85 ohms steady

OK, lets see good close up of that I204 to able to see what components are connected to the output pin, I hope it is just bad MLCC.
I see LYTICS cap C250, MLCC C251.
I also see D207 and L210 are connected to the output pin.
BTW, you did verify the pinout of the IC to make sure we are looking at the correct Output pin, correct?
BTW, what is the P/N as printed on I204?

[rddube]

Re: Viewsonic VT2430 fluctuating 5V rail

Here is the photo. Part no is 1084-33. I103 is also a 1084-33 while the other 2 are just written 1084.

As for your question about the output pin, I got the datasheet to make sure I was measuring the right thing. Tks Budm.

[budm]

Re: Viewsonic VT2430 fluctuating 5V rail

OK, that is a fixed 3.3V output regulator IC.
So something on the 3.3V line is shorted out, I would try lifting the Caps connected to the output pin first to see if the low resistance will go away.

[rddube]

Re: Viewsonic VT2430 fluctuating 5V rail

Ok, took out C250, still there. Took off C251 still there. What should I do next?

Should I try and lift the ground leg of the regulator?

[budm]

Re: Viewsonic VT2430 fluctuating 5V rail

See if inductor L210 is connected to the output, it is then lift it also.

"Should I try and lift the ground leg of the regulator?" Yes you can.

[rddube]

Re: Viewsonic VT2430 fluctuating 5V rail

I found L210, but it doesn't seem to be connected to it. Looking at the schematic, I see it elsewhere connected to the tuner section? I'll wait for your reply before lifting anything. Tks Budm.

[rddube]

Re: Viewsonic VT2430 fluctuating 5V rail

Oops, I guess I took my readings wrong, it is connected to the output. Still don't see it in the schematics, but then I'm probably cross-eyed!

So Budm, should I lift L210 first and see and then the regulator leg, or both at the same time? L210 is the easier part

[rddube]

Re: Viewsonic VT2430 fluctuating 5V rail

Budm, while waiting for your reply I measured C322, and I get .5 ohm across it's legs. I didn't remove it, but it is near L210. I'm getting confused on what to do!

[budm]

Re: Viewsonic VT2430 fluctuating 5V rail

It may be easier to lift L210 first to see what happen. Right now it is a matter of finding which component that is connected to the output is shorted out. At least we are getting close to solve the problem.