H.P. vs19e good170vdc. no 5v or 12vdc

[alexanna]

Hello everyone.
I have a H.P. vs19e.That will not power on. I do not see a power led any ware on the monitor.
I have verified 170vdc across the large capicatior.There is no 5v or 12vdc that I can find on this board. I have only been able to locate one fuse and it is good. I also am not noticing any poor solder joints. The 25v 1000uf caps below the 2 schottky diodes have been replaced.
I need help understanding how this power supply works.Thanks.

[PlainBill]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

Hello everyone.
I have a H.P. vs19e.That will not power on. I do not see a power led any ware on the monitor.
I have verified 170vdc across the large capicatior.There is no 5v or 12vdc that I can find on this board. I have only been able to locate one fuse and it is good. I also am not noticing any poor solder joints. The 25v 1000uf caps below the 2 schottky diodes have been replaced.
I need help understanding how this power supply works.Thanks.

This is a quick walk-through on the power supply. Refer to the picture I've labeled.

AC comes in through the AC input connector, the fuse, NTC, and an assortment of inductors and caps designed to filter out noise. It goes to the bridge rectifier. The output from the bridge rectifier is filtered by the 'mains filter cap. You have verified that there is 170 volts there, so the entire input circuitry is good.

The SMPS is controlled by the SMPS controller, which is on the bottom side of the board. That picture is so bad I didn't even try to decipher it. The SMPS controller gets it's power through the start-up resistor; the start-up cap holds enough energy to keep the SMPS controller operating until the power supply 'comes up'. Once it is running the operating energy for the controller comes from the a tertiary winding on the SMPS transformer and is stored in the run cap.

The outputs of the secondaries for the SMPS transformer are rectified by the schottky diodes, and filtered by the caps and inductors. The voltage regulator monitors the ouptut voltage of one of the secondaries (usually the 5 volt output) and uses that to vary the current into the optoisolator. The output of the optoisolator is fed into the SMPS controller.

Possible causes of the problem: 1. Bad start-up or run capacitor. I ALWAYS replace both unless all caps are of a known good brand and look perfect. 2. Bad start-up resistor. Check voltage at both ends. One end should be at 170 volts, the other at 10 - 25 volts. 3. Shorted schottky diode. 4. Bad transformer or 5. Bad SMPS controller. Both of those are very rare.

PlainBill

[alexanna]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

I am going to try to get better pitcures of the back of the board.
I have a question on where to pickup ground on the board.When i measure voltage across the big cap I read the 170vdc,If I use a screw terminal on the board I start reading 78vdc. on the pc123 octocoupler I am reading -78vdc
on pins 3 and 4. an nothing on pins 1 and 2.

[PlainBill]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

I am going to try to get better pitcures of the back of the board.
I have a question on where to pickup ground on the board.When i measure voltage across the big cap I read the 170vdc,If I use a screw terminal on the board I start reading 78vdc. on the pc123 octocoupler I am reading -78vdc
on pins 3 and 4. an nothing on pins 1 and 2.

I've marked up the bottom of the board. The area marked 'Live Side!!' has two grounds, the AC ground as marked, and the DC ground, the negative terminal of the large cap. Be careful!!! When measuring voltages on the SMPS controller, use the negative terminal on the large cap. It should not be necessary to measure anything on the AC side.

For the area marked 'cold side', use any of the ground screws.

PlainBill

[alexanna]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

checking the start up resistor with neg lead of dvm to neg leg of large cap I am reading no voltage on either side. if i move neg lead to ground screw i read -78vdc
I am guessing the start up resistor is on the live side of the board?

[PlainBill]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

checking the start up resistor with neg lead of dvm to neg leg of large cap I am reading no voltage on either side. if i move neg lead to ground screw i read -78vdc
I am guessing the start up resistor is on the live side of the board?

Assuming I identified it correctly, yes.

What is the part number for the SMPS controller?

PlainBill

[alexanna]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

sg6841sz

[alexanna]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

let me try to add photo

[japlytic]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

This IC is a System General SG6841 switching power supply PWM controller.
The datasheet can be found at: http://www.alldatasheet.co.kr/datash...TC/SG6841.html

[alexanna]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

Ok I have also checked the start cap and there is no voltage.So if I see the 170vdc on one leg of the sps pwm controller, can assume it is defective? Also I am not seeing any fuse protecting the 5 or 12vdc circut, If there was a short to ground could that have caused this to fail or is this something that can fail over time w/use?

[PlainBill]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

Ok I have also checked the start cap and there is no voltage.So if I see the 170vdc on one leg of the sps pwm controller, can assume it is defective? Also I am not seeing any fuse protecting the 5 or 12vdc circut, If there was a short to ground could that have caused this to fail or is this something that can fail over time w/use?

I'm not sure I have identified those components correctly. This SMPS controller is a newer design than I anticipated.

With the power on CAREFULLY check the voltage from pin 1 to pin 3 of the SMPS controller. In the picture pin 1 is the pin in the lower left corner, the pins are numbered counter-clockwise.

PlainBill

[alexanna]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

I am reading 2.7vdc

[PlainBill]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

I am reading 2.7vdc

Something was tickling my mind about this part so I did a search on Badcaps for SG6841SZ and found others who had the same problem and either decided not to purchase the part, or did purchase them and repaired the monitors.

At this point there a couple of options. You can order the IC from several different sellers on eBay; one company has them for $6.00 each with free shipping from California.

Or we can continue troubleshooting. That will require a good quality picture of the 'hot' area of the power supply so we can identify the components that connect the 170 volt supply to pin 3 of the SG6841SZ.

PlainBill

[alexanna]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

I did a littel further testing last night,And ihave some questions.And got a better pitcure.On the purple resistor just infront of the yellow coil [smps coil]
with the dvm neg lead to neg of big cap,I am reading 170vdc on both sides,Also on the mosfet? attatched to heat sink I read 170vdc on center leg and o v on 2 outter legs.Also there is no voltage to start up or run caps.
http://www.mouser.com/ProductDetail/...87v%2fdg%3d%3d. is mousers part number and specs. Is there a possibility the 800v mosfet could be defective and how can i test it. Thank you.

[retiredcaps]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

It could be the lighting or angle of the picture, but the cap at C917 looks slightly bulged? Of course, you have the best view.

[alexanna]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

C917 dose not look to be bluged.At this time i do not have a cap of that value to try.
I might have misunderstood Plain Bills post but i thought there might be a question on the location of the start up resistor.At this time the 800v mosfet is removed from board to see if there might be away to test it. I would like to continue to trouble shoot
and I will provide any pitcures nessary

[PlainBill]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

C917 dose not look to be bluged.At this time i do not have a cap of that value to try.
I might have misunderstood Plain Bills post but i thought there might be a question on the location of the start up resistor.At this time the 800v mosfet is removed from board to see if there might be away to test it. I would like to continue to trouble shoot
and I will provide any pitcures nessary

In post 4 I attached a marked up a picture of the bottom of the board. The 'hot' side is delimited by a red line. I need a large, clear picture of that area. With that I can possibly identify where the start-up resistor(s) are.

Frankly, other than verifying the resistor(s) supplying the startup voltage is within spec, there is little additional troubleshooting.

PlainBill

[alexanna]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

I have tried to get a crisp clear photo if I use 10m on my camera it is very clear but to big to upload. here is one on 8m.
one question dose pin 1 of smps controller go to one terminal of start resistor?
pin one shows that is connected to resistor r928,That i have marked in first photo. Thanks again for taking the time with me.

[PlainBill]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

I have tried to get a crisp clear photo if I use 10m on my camera it is very clear but to big to upload. here is one on 8m.
one question dose pin 1 of smps controller go to one terminal of start resistor?
pin one shows that is connected to resistor r928,That i have marked in first photo. Thanks again for taking the time with me.

I've defaced the picture. That large resistor appears to be a current sense resistor. Pin 1 is definitely Gnd, while there are a series of 3 SMD resistors providing startup power. I've also numbered the pins on the SMPS controller.

PlainBill

[alexanna]

Re: H.P. vs19e good170vdc. no 5v or 12vdc

I have verfied voltage on the three sm resistiors. starts out at 167 and I see 2.7vdc at the last one.Also 2.7 vdc at pin 3 of smps controller.This way above my head,What do we know at this point?