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Old 08-09-2019, 08:43 PM   #1
Duranitron
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Default AOC 1620sw powersupply fault

like to share this design problem concerning the AOC e1620 - e1660sw

Symptom: the monitor is turning on and off randomly. the 14.5 volts supply the power to the LCD driver board.. measuring fluctuating 15 to 17 volts the reason the lcd is turning on and off due to over voltage the protection circuit is kicking in.
i notice the e1620 service manual page 29 as you can see 14.5 volts output but in my circuit its measuring 15-17 volts. all parts checkout good but the output voltage is fluctuating. i check the zener diode 18B in aoc e1620sw and 20b in aoc 1660 sw same circuit. 18b is 17.8 volts zener and 20B is 19.8 zener there is no way you can come up with 14.5 volts out..
so i decided to replace it with 14volts zener and the output now its 14 volts regulated and its working perfectly. hope this helps.
Attached Images
File Type: jpg design fault aoc 1620sw.jpg (147.8 KB, 16 views)
Attached Files
File Type: pdf AOC e1620swb service manual.pdf (3.72 MB, 8 views)
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Old 08-10-2019, 07:18 AM   #2
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Default Re: AOC 1620sw powersupply fault

Have you checked 5V? If also 5V is higher than expected could be a problem of the feedback circuit.
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Old 08-10-2019, 02:27 PM   #3
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Default Re: AOC 1620sw powersupply fault

If you look at the schematic, the feedback resistor that monitor the output of the 14.5V power supply is not used, it only monitor the 5V and regulate the 5V power supply.
That shunt circuit is not on when the output Voltage is less than the 18v, the transistor is not biased on at that point to put the 250 Ohms resistor as the load.
Since the monitor has been working fine before with 18V zener, then something else is causing the problem. AOC uses this circuits in many of the power supply boards and I have yet have the need to use lower Zener Voltage for the circuit to function.
"the reason the lcd is turning on and off" What do you mean by that? The whole monitor turning on and off?
Do check the 5V to see if it is maintained at 5V or not.
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Old 08-10-2019, 06:47 PM   #4
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Default Re: AOC 1620sw powersupply fault

maybe the old 18V zener is just bad. Op never tested the zener that came out. Doesn't make sense that the monitor is working fine with 14V but not with 15 to 17V. Unless that original zener was bad, I bet the actual problem is something different.
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Old 08-10-2019, 07:01 PM   #5
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Default Re: AOC 1620sw powersupply fault

I believe the problem is somewhere else since that circuit is made so the 250 Ohms load will be present if the output Voltage has gone up more than 18V (actual will be 18+ Vbe of the Transistor) for the Zener to conduct and turned on the Transistor. That 14.5 V power supply is used for the backlights inverter circuit also, so it is not clear as to what the OP means about the LCD goin on and off, does he mean the backlights, the video, or the whole monitor is turning on and off.
250 Ohms is actually pretty light load.
I wonder what the actual marking of the Zener is.

Last edited by budm; 08-10-2019 at 07:07 PM..
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Old 08-12-2019, 07:35 PM   #6
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Default Re: AOC 1620sw powersupply fault

i have notice that also the protection circuit is kicking in the lcd monitor not the power supply. solid 5 volts is monitored in the main power supply through out the testing even when the lcd is turning on and off. i already fixed it by replacing 18b zener with 14.5 volts zener and it has a solid 14.5 volts output voltage. with the 18b zener in circuit the output is unregulated 15-17 volts that is why the lcd monitor overvoltage protection is kicking in..
the AOC 1620 its lcd monitor is separated to the main circuit it is like a power supply with laptop lcd monitor inside .
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Old 08-12-2019, 07:54 PM   #7
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Default Re: AOC 1620sw powersupply fault

i test the monitor for almost 3 days now. no issue what so ever. budm is right i also notice this circuit in my years of servicing monitors. yes i agree with budm the circuit will not turn on the zener 18b unless the voltage is higher that 18volts, so how come the label in its output is 14.5volts maybe the protection circuit of the lcd monitor has gone bad that is why it turns on and off through the years of supplying it with unregulated 17 volts. thanks for the reply guys..
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Old 08-13-2019, 09:18 PM   #8
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Default Re: AOC 1620sw powersupply fault

"maybe the protection circuit of the lcd monitor" are we talking about the LCD panel that goes into shutdown? Or the backlights go into shutdown?
As you notice, the schematic does not match the black diagram of the power supply, the block diagram shows the Voltage regulation circuit sample the output Voltage of the 5V and the 14.5V, but the schematic of the power supply shows the feedback resistor for the 14.5V output is not used. So may be they never correct some of the errors. What is the LCD panel model number? May be you can get the spec sheet of the LCD panel, it will tell you what the running Voltage for the T-CON is,and if it has any protection circuit, but so far I do not see any feedback pin of the LVDS connector for it to be used to tell the logic board to shutdown the monitor.
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Old 08-14-2019, 07:18 PM   #9
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Default Re: AOC 1620sw powersupply fault

Quote:
Originally Posted by budm View Post
I believe the problem is somewhere else since that circuit is made so the 250 Ohms load will be present if the output Voltage has gone up more than 18V (actual will be 18+ Vbe of the Transistor) for the Zener to conduct and turned on the Transistor.
My thinking too.
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Old 08-14-2019, 10:26 PM   #10
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Default Re: AOC 1620sw powersupply fault

sorry for that budm i already returned it to the owner. the problem is in the lcd panel board because before i suspect the powersupply for fault i substitute my old laptop lcd panel to it and i connect the lvds and it works no issues what so ever. but the supply that is supposed to be 14.5volts is unregulated 15 to 17 volts which is my point that i brought this concern to you guys. the zener diode 18b is checked good. my first move is to correct the voltage output to 14.5volts so i decided to change the 18b zener which is 18volts to 14volts zener to provide a solid regulated 14volts output after that i connect the original lcd panel and it works without issue. thanks again
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Old 08-14-2019, 10:58 PM   #11
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Default Re: AOC 1620sw powersupply fault

there is really no connection to the error correction circuit because as you can see the circuit is a dc regulator circuit employing an 18b zener which is supposed to be an 18volts regulator circuit not a 14.5volts regulator circuit. thanks again.
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Old 08-14-2019, 11:25 PM   #12
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Default Re: AOC 1620sw powersupply fault

That 250 Ohms shunt resistor is not really doing that much of the current draw to bring down 17V to 14.5V, something is strange going on, at 14.5V and if the Transistor is fully on the current flow through that shunt resistor will be about 58mA @14.5V, it is pretty weak shunt regulator circuit, or that 14.5V power supply is really weak. The Inverter circuit can easily draw more than 2A so when it is running it should be able to bring down the output Voltage.
I wonder now if that shunt resistor is really 250 Ohms, it may be less.
If you get another one you can try experimenting by bypassing the Transistor so the 250 Ohms is always active and see what the output Voltage will be.

Last edited by budm; 08-14-2019 at 11:33 PM..
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Old 08-15-2019, 02:20 AM   #13
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Default Re: AOC 1620sw powersupply fault

actually i substitute a transistor with much bigger value with the same effect it brings down the voltage to 14volts with the 14v zener and i don't see much of a change if the 18 volts zener is placed because the 18volt cant be reach by the supply voltage of 17volts.. its really strange why aoc put that circuit there. in substituting a supply voltage to the lcd panel my supply reads 14.5volts with current draw of less than 500mA. ill try to experiment using the circuit if how much current its capacity. do i need to put up again another post? nice to have an experienced person around badcap forum like you sir. thanks again.
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Old 08-15-2019, 02:37 AM   #14
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Default Re: AOC 1620sw powersupply fault

im also amazed with that circuit. if you bypass the transistor and placing the resistor in parallel of the supply its really very weak. that resistor is rated 250 ohms 2 watts wonder why its 2watts hehehe if the current draw is just 58mA.

Last edited by Duranitron; 08-15-2019 at 02:39 AM..
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Old 08-15-2019, 06:19 AM   #15
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Default Re: AOC 1620sw powersupply fault

Probably a 1W resistor is too borderline, would not live for long.
Is R914 really not present?

Last edited by Dumah Brazorf; 08-15-2019 at 06:24 AM..
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Old 08-15-2019, 06:41 AM   #16
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Default Re: AOC 1620sw powersupply fault

yap R914 is empty in the board. maybe next time i will post pictures so that it will be clear. maybe the circuit magnifies the current rating of the zener to about 2A perhaps i'm just guessing hehehehe because the circuit doesn't make sense.
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Old 08-15-2019, 09:51 AM   #17
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Default Re: AOC 1620sw powersupply fault

Quote:
Originally Posted by Duranitron View Post
im also amazed with that circuit. if you bypass the transistor and placing the resistor in parallel of the supply its really very weak. that resistor is rated 250 ohms 2 watts wonder why its 2watts hehehe if the current draw is just 58mA.
14.5V @58 mA is about 0.80w, so good design practice will double that, so 2W resistor is being used.
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Old 08-15-2019, 09:53 AM   #18
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Default Re: AOC 1620sw powersupply fault

Quote:
Originally Posted by Duranitron View Post
actually i substitute a transistor with much bigger value with the same effect it brings down the voltage to 14volts with the 14v zener and i don't see much of a change if the 18 volts zener is placed because the 18volt cant be reach by the supply voltage of 17volts.. its really strange why aoc put that circuit there. in substituting a supply voltage to the lcd panel my supply reads 14.5volts with current draw of less than 500mA. ill try to experiment using the circuit if how much current its capacity. do i need to put up again another post? nice to have an experienced person around badcap forum like you sir. thanks again.
500 mA is for the LCD T-CON, the 14.5V is also used by the inverter circuit that uses lots more power to light up those CCFL backlights.
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