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**budm** · 07-20-2018, 12:01 PM

Re: Viewsonic VT2430 fluctuating 5V rail

Are the 12V, 19V steady? 5V, 12V, and 19V are generated by the same transformer.
The feedback for the output Voltage monitor all 3 power supplies outputs so I expect the other two outputs to fluctuate also.

**R_J** · 07-20-2018, 12:09 PM

Re: Viewsonic VT2430 fluctuating 5V rail

The first cap. I would replace would be C810 10µf/50v

**rddube** · 07-20-2018, 12:25 PM

Re: Viewsonic VT2430 fluctuating 5V rail

Thank you Budm and R_J for stepping in!

Budm: the 12V fluctuates for 11.75V to 12.5V (when load is on) but not in standby. I can't find a 19V??

R_J: C810 on board is a 22uf 50V which I took out and measured with my ESR meter and it looks fine. But you are right, the schematics calls for a 10uf, so I don't know if that could be the cause or not?

I did measure the voltage on the big cap, and I do get a fluctuation there: 160V steady on standby, when power up, fluctuates from 148V to 152V ???.is that normal?

**R_J** · 07-20-2018, 02:52 PM

Re: Viewsonic VT2430 fluctuating 5V rail

If C810 is a 22µf then I would leave it, schematic could be different. any small change of voltage on the large cap won't have any effect on the output,
The led is supplied by the VCC5V-ST, there is a 1500µf/16v (C110) on the main board that should take care of any voltage fluctuation, unless it's bad?

**rddube** · 07-20-2018, 05:18 PM

Re: Viewsonic VT2430 fluctuating 5V rail

Ok R_J will check C110 and report back...thanks again!

**rddube** · 07-20-2018, 09:24 PM

Re: Viewsonic VT2430 fluctuating 5V rail

Ok, so I took out C110 and it gives me 1285uf with ESR of 1.2. So I decided to take out C101 and C108 which are same value, and got C101 1356uf ESR .03 and C108 1555uf and ESR 0.

So I guess you're right on R_J, I should change C110 and C101, right? Will try to find a replacement tomorrow and report back with the new capacitors.

**rddube** · 07-21-2018, 05:01 AM

Re: Viewsonic VT2430 fluctuating 5V rail

Replaced C110, C101 and C108 with 1500uf low ESR Panasonic caps. And now, I have no power??

Measured my voltages at p101 and 13V is down to 6-7V, no 5V. So I disconnected the main board from the power board and got back 13V steady, 5V fluctuating.

Rechecked my soldering on the new caps, redid them, but same result. Now I am really lost

Help!

**R_J** · 07-21-2018, 11:55 AM

Re: Viewsonic VT2430 fluctuating 5V rail

It could be that you power supply can't supply the current required, do you have around a 10ohm 2watt resistor, place it across the 5 volt line (plug disconnected from main) that should draw around 500ma, if it can't supply that then fault is with power supply.
Simply replacing the caps should not have caused that problem. but being new caps the extra load might be enough to effect the 5 volts, and the 5 volts controls the feedback so if goes low so do the other voltages.
You could have a poor D822 diode, going open under load

**RepairMonkey** · 07-21-2018, 01:55 PM

Re: Viewsonic VT2430 fluctuating 5V rail

Originally posted by rddube View Post

Replaced C110, C101 and C108 with 1500uf low ESR Panasonic caps. And now, I have no power??

Measured my voltages at p101 and 13V is down to 6-7V, no 5V. So I disconnected the main board from the power board and got back 13V steady, 5V fluctuating.

Rechecked my soldering on the new caps, redid them, but same result. Now I am really lost

Help!

I'm assuming Electrolytic caps - is the polarity correct?

**rddube** · 07-21-2018, 01:56 PM

Re: Viewsonic VT2430 fluctuating 5V rail

Thanks R_J for your patience! I removed the caps (the low ESR Panasonice 1500uf 16v) and reinstalled them, and now everything is back to normal, so monitor/tv fires up and 5V still strobing.

I will try the test you suggest with a 2 watt 10ohm resistor - so I plug this resistor on the 5V line and ground (without being connected to main) and what should I do after that, measure the 5v line?

I'll also check the D822 diode out of circuit and report back. Thanks again!!

**R_J** · 07-21-2018, 04:43 PM

Re: Viewsonic VT2430 fluctuating 5V rail

Yes to checking the 5 volt line, I choose a 10 ohm as that would draw 500ma, the supply should at least do that, if the 5 volt line drops with that resistor then the power supply has an issue.
I would think it should do 1 amp so you could use a lower value higher wattage resistor, 5 volts, 5 ohm = 1 amp of current,
Checking the diode will likely not tell you much, it will likely check ok.
Now that I look closer I see it is a dual diode so it should be ok

**rddube** · 07-21-2018, 07:28 PM

Re: Viewsonic VT2430 fluctuating 5V rail

Ok, so what should I do?

**R_J** · 07-21-2018, 09:12 PM

Re: Viewsonic VT2430 fluctuating 5V rail

I was giving a suggestion on how to check the 5volt standby of the power supply, by providing a load other than the main board. If you don't have a resistor to load the 5 volts I can only assume the main board is ok and the fault is with the power supply not providing enough current.
Did you check C827? I see from your picture it is a smd cap.
This could be hard to troubleshoot since it is working just not supplying enough current.
I see you checked the caps like c833,c824 but have you tried replacing them to see if that stabalizes the +5

**rddube** · 07-22-2018, 05:28 AM

Re: Viewsonic VT2430 fluctuating 5V rail

Hello R_J,

I tried with a 10ohm resistor on all 3 lines of 5V and it is rock steady at 5.03V. So I tried what you suggested a 5 ohm resistor to get a bit more load, and all 3 lines of 5V are rock steady at 5.01V. So I guess the problem is not with the power supply, right?

What else?

**rddube** · 07-22-2018, 06:21 AM

Re: Viewsonic VT2430 fluctuating 5V rail

I should mention that when it is powered on and the blue Led (it is blue when powered on and amber when powered off and in standby), I can hear a faint throb through the speakers of the monitor/tv. It doesn't change if I increase volume, it remains faint. Not sure if that can give a clue?

**R_J** · 07-22-2018, 11:17 AM

Re: Viewsonic VT2430 fluctuating 5V rail

Have you checked if the sound is ok? it could be a fault with the audio amp ic. If you play music and turn up the volume how does that effect the blue led.

**rddube** · 07-22-2018, 05:23 PM

Re: Viewsonic VT2430 fluctuating 5V rail

Ok, I checked the sound and it is fine, in fact very good. Do you think it could be some of the capacitors on the main board, there are quite a few, especially the 10uf 16v, there are tons of them but none of the caps are bulged or anything.

Really don't know what to look at next. Tks R_J

**rddube** · 07-22-2018, 07:03 PM

Re: Viewsonic VT2430 fluctuating 5V rail

So I decided to do a temp test with an infrared thermometer on the main board, wanting to see if some caps might heat up....didn't find any that looked abnormal.

However in the EEprom section, I did find the resistors around the EEProm to be heating quite a bit (some of them), one of them R338 100R went up to 77C, still climbing slowly. So I decided to check their values and found 3 that don't correspond to the schematic. R336, should be 4.7K is at 2.4K, R287 should be 4.7K is at 3.9K and R310 should be 4.7K is at 4.2K. I know this section runs on 3.3V but maybe it might affect the 5V rail, what do you think?

Finally, on all capacitors the only one that seems to heat up a bit more than the others is C259 220uf 16V.

**R_J** · 07-22-2018, 07:18 PM

Re: Viewsonic VT2430 fluctuating 5V rail

Those resistors were likely changed during a board revision, they are pullup resistors on clock and data lines, not likely to effect the problem.
C259 is panel-vcc filter, turn off tv, then unplug the lvds cable from the panel, turn on tv, do you still here the throb in the speakers?

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Viewsonic VT2430 fluctuating 5V rail

Viewsonic VT2430 fluctuating 5V rail

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