Re: Dell ST2421LB Dead monitor
Unfortunately I do not know enough about how the circuit works.
pin3 on the optocoupler measures 5.84V (against hot ground) which is the same as pin2 on the TOP258PN IC which it is connected to. pin4 on the optocoupler measures 7.96V against hot ground, so about 2.12V when measured across pin3-pin4 of the optocoupler. Across pin1-2 of the optocoupler measures about 1.126V.
On the TOP258PN IC, pin4 measures ~337V, pin2 (=pin3 on the optocoupler) is 5.84V, pin1 is about 1.26V.
Thanks for your patience.
llphat
Member
Last Activity: 03-10-2024, 10:41 AM
Joined: 12-26-2010
Location: United Kingdom
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Re: Dell ST2421LB Dead monitor
R606 measures 6.6ohm in circuit. C611 checked OK approx 10uF out of PCB.
I could see 340V (against negative leg of C605 large filter cap) on the Pin 1,2 and 0V on Pin 4,5 on primary side of the transformer T601, but no output voltage on the secondary side (pin 6/7, pin8/9 and pin10), does this mean transformer is broken?Leave a comment:
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Re: Dell ST2421LB Dead monitor
Could anyone help with the value for C607 please. I have the same board 4H.17B02, zener ZD601 appear shorted in circuit but once taken off PCB appear OK. Took off C607 but found pads for ZD601 still shorted eventually found that it was C608 47uF 25V that was shorted. However in the process of taking off C607 end terminals was destroyed.
Anyone would could help with value of C607 would be much appreciated.
Tried 80nF which gave approximately 5.8V at pin2, ~1.26V at pin1 and 330V at pin4 of TOP258PN.
Thanks.Last edited by llphat; 02-25-2022, 10:01 AM.Leave a comment:
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General Power board question - Fuse, Rectifier, TVS
I had some issues recently with a few power boards with blown fuse, on checking it seems one of the bridge diode has shorted. After replacing the fuse and rectifier bridge, on power up the TVS (transient voltage suppressor) always explodes. Is this expected?
Or should the TVS also be changed once the bridge has a shorted diode?
The most recent case I had is the Dell 1800FP power board. Fuse F901 blown, on checking rectifier BD901 has a shorted diode after changing both TH902 explodes on power up. -
Toshiba NB510 Netbook - no 12V?
Dear All,
I have a Toshiba NB510 netbook which doesn't power up, and show very odd symptoms. Unfortunately I could not find the schematics despite trying very hard to find it online.
-On connecting the AC adaptor the battery charge LED or adaptor connected LED lights up.
-with either battery or adaptor then pressing the power button, the power on LED comes on, but nothing else happens, hard disk doesn't spin, screen remains off, etc
-3.3V and 5V appear to be present, since the hard drive is not spinning up the 12V (or V+ Batt??) appears missing.
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Re: Dell E178fp repair
The model is E178FPv the boards are similar to the images posted in the first post.
Fuses checked OK and there is a ~330V voltage across the big cap at the bridge exit.Leave a comment:
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Re: Dell E178fp repair
Apologies for digging out such an old thread, I have recently come across an E178FP which does not power up. Opening up and checking the PCB, there were no obvious bad caps or burnt areas. There was no voltage across pin 1 which should be be 5.1V when on.
The only obvious thing is when I measure across ZD2 which gives 0.001V (in circuit) which surely does not sound right - is this the culprit?
Oh if ZD2 does turn out to be blown, is there a way to find out the values from the marking on the component?Leave a comment:
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Re: Foxconn WinFast NF4K8MC power up problem
Thanks for your response.
I had tried at least two different PSU, with them having either 24pin or 20pin power connectors but the effects are the same.Leave a comment:
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Foxconn WinFast NF4K8MC power up problem
I have a Foxconn WinFast NF4K8MC motherboard which does not seem to power up if you short the PWR_SW leads. If you however short the PS-ON (green wire on ATX power connector) to ground it will power up and boot without problems. Once powered up, you can switch the board off by shorting the PWR_SW lead for a few seconds, after switching off though it will again not power up if you short PWR_SW, the only way to power up is to short the PS-ON to ground on the ATX power connector.
Yes I can probably make two switch, one to hot-wire PS-ON to ground to start and the other is the normal... -
Re: Iiyama ProLite481S - Bad Caps Found - Advice Please
I have a similar monitor which I have fixed. Mine did not have the problem with the big 150uF 400V cap. I had to replace the two 1000uF 16V. I simply went down to the local Maplin and asked for two 1000uF 25V 105C, the stock code is DT70M costing 44p each. The caps from Maplin are Jamicon which I find are quite reliable.Leave a comment:
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Re: Technika 20" TV LC-TM2018
Voltage across C904 reads about 330V which is what I would expect for a 240V input (U.K.). Fuse F901 is intact.
With regard to V909, I think perhaps you mean C909. I will check with a suitable replacement. I will probably also check C910 while I'm at it.
Thank you.Leave a comment:
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Technika 20" TV LC-TM2018
I have recently found a Technika 20" LCD TV which appear to be non-working. After opening up I found that the three bigger capacitors at the output side on the power board is bulged. After changing the caps the 12V output is still not present, what else could I check?
Please see attached schematic for the power board LM-TM2018. I have replaced the three bulged caps C914, C915 and C916. What damage could these bulged caps have caused to the rest of the circuit?
Thanks.... -
Re: LG L1718S no power
Thanks for the replies.
When I measure the AC input on my cheap analog meter, I only get around 200V AC. Switch it over to 500V DC range to measure the voltage across the primary filter capacitor I get around 300V.
For the two capacitors near the transformer, one reads around 4V across it and another about 4.5V across it.Leave a comment:
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Re: Iiyama Prolite E430S click sound
Image of the back of the power supply board attached.
Initially I had suspected ZD904 at the top left of the image as it looks rather burnt.
I had just tried to measure the voltage across C907 the 22uF 50V cap. When I put a voltmeter across it the clicking sound slows down, reads around 15V steady but when it clicks (now slower) the voltage drops to about 10V....Leave a comment:
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Re: LG L1718S no power
Image of the back of the power supply board attached.
IC802 is TOP245YN.
Thanks for your kind assistance....Leave a comment:
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Re: Iiyama Prolite E430S click sound
Image of photo board now attached.
I have replaced two 1000uF 16V caps on the top right corner of the power supply board image.
Indeed there is a 22uF 50V cap next to the IC901 at the bottom right of the photo but I do not have equipment to test. I will find a suitable replacement to see if it solves the problem....Last edited by llphat; 12-26-2010, 11:24 AM.Leave a comment:
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Re: LG L1718S no power
Many thanks for your prompt reply.
I have found the service manuals at below, both look similar:
As for myself, I'm residing in the U.K.
I have previously measured the voltage across the main filter capacitor to ensure the primary rectifier is working, the voltage is around 300V. I only have a very cheap multimeter so can't get precise values.
I have enclosed an image of the power supply board....Leave a comment:
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LG L1718S no power
Hello All,
I have a LG L1718S LCD which does not appear to power up. After taking the monitor apart and inspecting the power supply board, I could not see any bad caps or other bad components.
I have found the service manual and followed some of the troubleshooting routes for no power on page 15.
1. Primary rectifier voltage is present, approx 300V at the bridge
2. Pin3 of IC802 approx 1V present, pin2 abt 2V and pin1 abt 6V all taken against ground of C805 capacitor.
Service manual doesn't go on from there.
There... -
Iiyama Prolite E430S click sound
Hello All,
I'm new to this forum and am looking for some advice on fixing a few broken LCD which I have. One of the LCD is a Iiyama Prolite E430S.
I have taken the monitor apart, on inspection the power supply board is emitting a constant click sound on power up. I have replaced two bulged caps C922 and C925 (1000uF 16V). The click sound still appears but I can't seem to locate where it is coming from. My suspicion is the ZD904 zener diode which might be broken.
Any help would be appreciated. Thanks in advance
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