ViewSonic VP171b takes a long time to turn on

[Player2]

I'm working on a ViewSonic VP171b monitor which takes a long time to turn on. From the time that the power button is pressed, it takes about 1-1.5 minute to come on. During that time there is no indication that it has power, no standby light etc. Once it turns on it works perfectly, and it can go into standby and come back on with no problem. As soon as it is turned off though, it will take a long time to come back on again.

I have narrowed it down to the power supply board, and in fact purchased a new board since it didn't cost much. Everything works normally with the new board. I would still like to find the cause of the problem though so I can repair the old board and add it to my parts bin. Since I have both boards, I have the luxury of swapping components until I find the issue. What I am looking for is advice on where to start, as I could easily spend a whole afternoon swapping components at random.

FYI all of the caps have already been replaced. Thanks!

[Bobdee]

Re: ViewSonic VP171b takes a long time to turn on

Replace the caps that will probably sort it

[Player2]

Re: ViewSonic VP171b takes a long time to turn on

Replace the caps that will probably sort it

That was my first thought too, but I replaced the caps and no change.

[Bobdee]

Re: ViewSonic VP171b takes a long time to turn on

That was my first thought too, but I replaced the caps and no change.

Strange that, the big AC filter cap as well

[Player2]

Re: ViewSonic VP171b takes a long time to turn on

Strange that, the big AC filter cap as well

Yep, changed that as well. Didn't have one on hand, so I pulled the good one off the new board and tried it in the malfunctioning board, still no change.

[ratdude747]

Re: ViewSonic VP171b takes a long time to turn on

maybe a slow-charging inverter?

[Bobdee]

Re: ViewSonic VP171b takes a long time to turn on

Have you tried measuring the VDC at that cap to see if its low when cold and rises as the monitor comes on, just a thought

[PlainBill]

Re: ViewSonic VP171b takes a long time to turn on

I'm a little unclear about the description of the problem. Do you mean when the power supply is turned off from the front panel power button (as opposed to the power switch on the back of some Viewsonic monitors) there is a 1 + minute delay before the power LED comes on? Or do you mean the delay only occurs when the power is turned on from either the back panel switch, a power strip, or by plugging it into an outlet?

Two places to test. 1. Check the voltage across the large cap as power is applied. It should rise rapidly (less than 2 seconds) to 165 volts DC. If it does not, check the thermistor on the AC input (either the green or the yellow disk near CON01).

2. Check the DC outputs of the power supply. They should also rise within 2 seconds of AC power being applied.

PlainBill

[Player2]

Re: ViewSonic VP171b takes a long time to turn on

Ratdude- Are you referring to U04 in the top right of the second picture?

Bobdee- I checked the voltage on the large filter cap and it checks out against the one on the good board- both in what voltage it rises too and how quickly.

PlainBill- The main filter cap charges immediately. However, your suggestion to check DC voltage on the output led to something helpful: When I flip the switch for the main power on the back of the monitor, the long delay is actually the 12v output to the signal board. Once that comes up, the monitor will turn on and off at will via the button on the front. On the good board, 12v comes up within seconds of flipping the main switch. FYI there are only 4 leads on the output of this PSU- 2x 12v and 2x ground, both 12v behave the same way.

Whatever is causing the slow charge doesn't seem to be affected by heat, as the monitor will behave the same way whether it has been running for hours, sitting overnight cold, powered off and then immediately back on (by the main switch), etc.

[PlainBill]

Re: ViewSonic VP171b takes a long time to turn on

Ratdude- Are you referring to U04 in the top right of the second picture?

Bobdee- I checked the voltage on the large filter cap and it checks out against the one on the good board- both in what voltage it rises too and how quickly.

PlainBill- The main filter cap charges immediately. However, your suggestion to check DC voltage on the output led to something helpful: When I flip the switch for the main power on the back of the monitor, the long delay is actually the 12v output to the signal board. Once that comes up, the monitor will turn on and off at will via the button on the front. On the good board, 12v comes up within seconds of flipping the main switch. FYI there are only 4 leads on the output of this PSU- 2x 12v and 2x ground, both 12v behave the same way.

Whatever is causing the slow charge doesn't seem to be affected by heat, as the monitor will behave the same way whether it has been running for hours, sitting overnight cold, powered off and then immediately back on (by the main switch), etc.

Excellent!!! There are a couple of possibilities. I note that you said the capacitors have been changed. On every SMPS power supply there is a small capacitor (10 - 100 uF) on the power supply pin of the SMPS controller (U04 on the bottom side of the board). Usually there is a high value resistor (or several resistors in series) from the + lead of the 100 uF, 450V cap which supplies current to this capacitor. When the capacitor charges to a certain voltage (typically 10 - 20 volts) the SMPS controller starts up.

Possible causes of a slow start-up are the dropping resistor increasing in value, or the capacitor becoming leaky. Often this cap is never replaced "'because they never fail". Of course, a resistor increasing in value is uncommon. If you want to take a methodical approach to troubleshooting this, identify the SMPS controller and get it's data sheet. From there you can identify the power supply pin, and the associated components.

PlainBill

[kc8adu]

Re: ViewSonic VP171b takes a long time to turn on

another gotcha is glue that gets conductive with age.had one that would act like that with the startup voltage too low for the chip to come out of uvlo.after a few minutes the voltage would creep up and it would start.there was some oily stuff under the glue.was small psu in a newall dro.that small cap is more likely though.

[Player2]

Re: ViewSonic VP171b takes a long time to turn on

There are no small (< 100 uF) electrolytic caps on this board. There is a ceramic disc not far off the SMPS, but I swapped that and no change. Also swapped all of the through-hole resistors.

I also peeled off all of the glue on the board but none of it left behind residue or had eaten into the traces. It all peeled off cleanly in one piece.

I'll dig out a magnifying glass tomorrow so I can read the number on U04 and find a datasheet.

[kc8adu]

Re: ViewSonic VP171b takes a long time to turn on

the cap of interest is the one at top center under the datecode.
check those 1w pink resistors too.one may have gone high and on its way to open.

[Player2]

Re: ViewSonic VP171b takes a long time to turn on

I will try swapping that cap. The one that's there is new, but maybe the board doesn't like it. I'll try the one from the working board.

I did check all of the through-hole resistors out of circuit and they are all good. One did appear wet on the bottom, I swapped it with the corresponding one from the other board, still no change though.

I haven't got a chance to look up that data sheet yet, but I'm going to try to get to it at some point today.

[momaka]

Re: ViewSonic VP171b takes a long time to turn on

I don't think this has been mentioned, but what brand and series of caps did you use as replacements?

[Player2]

Re: ViewSonic VP171b takes a long time to turn on

I don't think this has been mentioned, but what brand and series of caps did you use as replacements?

Nichicon HM

Sorry, forgot to include that in the original post.

[momaka]

Re: ViewSonic VP171b takes a long time to turn on

That's okay. I just wanted to make sure you're not using general purpose caps.
The Nichicon HM are new, though, right?

On a sidenote, the soldering on the underside doesn't look that great. Not sure if this could be causing your problem, but maybe there's a cold joint somewhere?

[Player2]

Re: ViewSonic VP171b takes a long time to turn on

Yep, they are new caps. I will check all of the joints. Now that you mention it there was a lot of flux residue on the board before I ever touched it, perhaps it's been repaired (or attempted) before. I'll check it out.

[thinkbot]

Re: ViewSonic VP171b takes a long time to turn on

I don't know where to post this message,

anyway this topic seems very good

I have VP211b (in Europe it's advertised as VP2130b, I guess it's the same)
and 2x VP 201b (VP 2030b)

VP2130b had the same problem as described here, it took some time to turn on, or to get up from sleep mode

The problem were Teapo caps, 6x 100uF 25V and 2x 100uF 6,3V
There were on the board were DVI and VGA are located

The same Teapo 100uF 25V are on the USB board, but I didn't use USB so those Caps seemed fine

Anyway I soldiered new ones, and now it works beatifully

Thank you very much for the forum, this topic, and all help

[Player2]

Re: ViewSonic VP171b takes a long time to turn on

Hehe, glad you got yours working! I did try replacing all the caps on my signal board too, but unfortunately it didn't work for me. I've shelved the project for now anyways, I only paid ~$25 for a new power board, I figure I've burned at least that much potential money since then screwing around with the old board!