Daytek F19AH

[UserName666]

Re: Daytek F19AH

OK so I just ordered a 10 pack of SG6841SZ chips for IC101 and a kickass soldering/rework station for getting the little bugger off the board. This monitor WILL live again.

edit: Pretty bad that I know the numbers "SG6841SZ" off by memory now

[UserName666]

Re: Daytek F19AH

So my replacement SG6841's finally came in. Just finished installing it. Plainbill, your suspicions about that chip were correct. I know have the clicking back in the transformer. Gonna hook everything up now and see if it works.

[UserName666]

Re: Daytek F19AH

Did not work Still no output voltage from the transformer though it remains clicking. IC101 now has it's 15~19v input voltage back and voltage has also returned to pins 1-2 of the opto-isolator but monitor still will not power up.

[PlainBill]

Re: Daytek F19AH

Did not work Still no output voltage from the transformer though it remains clicking. IC101 now has it's 15~19v input voltage back and voltage has also returned to pins 1-2 of the opto-isolator but monitor still will not power up.

Give me a few hours to get my day started and I will review the thread. We WILL get this working.

PlainBill

[UserName666]

Re: Daytek F19AH

Alright something I tried today was removing the 10uf cap next to the big filter cap since that is the only one that wasn't replaced brand new and still no go. According to the analog meter when this cap fills up the transformer clicks at the same time it discharges back to 0.

When around the board again and took some voltage readings again after installing the new SG6841.

IC101:
1-2 = 0~1.3v
1-3 = 11~19v
1-4 = .000~.298v
1-5 = 1.1~1.9v
1-6 = .000~.030
1-7 = 10~16v
1-8 = .000~.457v

Filter Capacitor = 158~163v (Usually around 162.5v)
Opto-Isolator pins 1-2 = .812~2.3v
Transformer - 161v in 0v out

Rectifier by filter cap and coil:
Pins 2-3 116vac
Pins 1-4 162vdc

IC1: all readings = .000-.040

IC4:
1-2 = 0v
1-3 = 0v
1-4 = 0v
1-5 = .825v
1-6 = .820v
1-7 = .850 ~ .9v
1-8 = 0v

Also for this monitor I have gotten some new tools.
I have a 0-30v variable power supply in for if I need to "APPLY" voltage somewhere to this board as well a a multi-meter with more options that my current. I included a pic of the multi-meter in case any of the function may be handy for figuring this out.

[PlainBill]

Re: Daytek F19AH

I'm trying to figure out meaningful tests. The problem appears to be that the tertiary winding is not providing run voltage to the SMPS controller.

What brand and model of 0-30V power supply did you get? I'm interested in the specs.

PlainBill

[UserName666]

Re: Daytek F19AH

It was this exact unit here. Just a cheap direct from Hong Kong thing but it does what its supposed to.

http://cgi.ebay.ca/ws/eBayISAPI.dll?...m=150501632382

[UserName666]

Re: Daytek F19AH

Could you possibly explain you last post a bit more? Does that mean the secondary of the transformer in not providing the voltage to pin4 of IC101? IC101 being the SMPS controller? Confused here.

[PlainBill]

Re: Daytek F19AH

Could you possibly explain you last post a bit more? Does that mean the secondary of the transformer in not providing the voltage to pin4 of IC101? IC101 being the SMPS controller? Confused here.

Here's the two bit explanation of how the SMPS is supposed to work.

The filtered mains voltage is fed through a resistor to pin 3 of the SG6841SZ. Internally it passes that through to pin 7, where it charges up the 10 uF cap. When the voltage across the 10 uF cap reaches about 17 volts the SG6841SZ starts sending pulses to the gate of the power FET. One end of the transformer primary is tied to the filtered mains voltage, the other end is tied to the drain of the power fet. The source of the power fet is tied to ground through a very low sense resistor. When the pulse hits the gate of the fet it turns on, current flows through the primary, building up a magnetic field in the transformer core. Then the FET turns off, the magnetic field collapses, and the energy is transferred to the secondary and tertiary windings of the transformer. The secondary output is rectified by the dual diode mounted on the heat sink, where it charges the output capacitors. The tertiary output is rectified by one of the diodes near the SG6841SZ, and charges the 10 uF cap.

Now if the pulse doesn't charge the 10uF cap, it discharges, and when the voltage across it drops down to about 10 volts the SG6841SZ stops until the cap charges up to 17 volts again. The problem is figuring out why the tertiary winding isn't charging the 10uF cap. I haven't reread the entire thread, but as I recall, most of the possibilities have already been checked.

Possibilities include excessive load on the secondary, a shorted secondary diode, a bad diode on the tertiary winding, the sense resistor too high in resistance.

One of the difficulties is the SG6841SZ is not at AC ground, so working on it is difficult with power on. Don't try any of these ideas YET, but some things I'm thinking of is using the variable power supply to provide DC power to the mains filter cap. Or using it to provide DC power to the output caps. OR (and this is very dangerous), using it to provide DC power to the SG6841SZ.

PlainBill

[UserName666]

Re: Daytek F19AH

OK I wont do anything yet but just for the record...

"OR (and this is very dangerous), using it to provide DC power to the SG6841SZ."

I don't know if your meaning blowing something else up on the board or frying the new SG6841 I put in but if it just a concern for the chip I bought a 10 pack so I still have 9 more to play around with.

[PlainBill]

Re: Daytek F19AH

Well, after a poor nights sleep, some things are much clearer. First of all, I've found a schematc for a power supply that is SIMILAR to the one we are dealing with. Page 9 has the relevant schematic.

Next, there are several possible causes for the problem we are seeing. I'm going to list a few.

1. Excessive load on output of supply

2. Defective diode on main supply.

3. Defective diode in run supply

4. Open winding on transformer

5. Shorted winding on transformer

6. Current sense resistor too high

7. SG6841 drawing too much current.

Let's start with item #1. Hook up your bench supply across the output of the monitor power supply. Looking at the picture of the back side of the board, C122 is across the output. Don't try hooking directly to that, instead try some other, more secure points. You could hook it to the output connector, or solder wires to the pads for ZD102. The top pad is negative, the bottom pad is positive.

Hook the monitor together - power supply / inverter, logic card, LCD panel and CCFLs. Don't plug in the AC cord to the monitor. Hook up the bench supply as I indicated, turn it on, and slowly increase the output voltage while watching the current. As a very rough guess, the monitor should function with less than 3 Amps at 12 volts. As you increase the voltage watch carefully for any signs something is overheating.

PlainBill

[UserName666]

Re: Daytek F19AH

OK I decided to go the way of hooking wires to the pads. As I was increasing the voltage the PS started to buzz louder and louder and by the time I got up to 4 amps I only had 3.1 volts output from the PS. Only thing I noticed was the parts circled in red got really hot. Attempted to push power button with no result. By PS I mean the external desktop supply.
(The again I have never used one of these kinds of PS before so maybe I'm doing/adjusting something wrong with it?)

Also not that this really matters but I think this monitor is a remake of the F19AH because the power and input button section says "F19AH keyboard KH172200B Rev:B"

[PlainBill]

Re: Daytek F19AH

OK I decided to go the way of hooking wires to the pads. As I was increasing the voltage the PS started to buzz louder and louder and by the time I got up to 4 amps I only had 3.1 volts output from the PS. Only thing I noticed was the parts circled in red got really hot. Attempted to push power button with no result. By PS I mean the external desktop supply.
(The again I have never used one of these kinds of PS before so maybe I'm doing/adjusting something wrong with it?)

Also not that this really matters but I think this monitor is a remake of the F19AH because the power and input button section says "F19AH keyboard KH172200B Rev:B"

The items circled in red are the drive transistors for the inverter. They would overload the supply if they are shorted. Darn, I thought we had eliminated that possibility.

J109 appears to provide power to the inverter. Unsolder one end of that and repeat the experiment. The external power supply should be able to power the logic card and panel; you obviously won't have backlights.

PlainBill

[UserName666]

Re: Daytek F19AH

The result of removing J109: Powersupply went up to 12 volts and when I got to .29amps the power LED came on green and I head the audio circuit fire up through the speakers and after about 3 seconds the power led went to orange indicated no input which is correct operation for this monitor!!!! The power button also functions in this state as well.

[PlainBill]

Re: Daytek F19AH

The result of removing J109: Powersupply went up to 12 volts and when I got to .29amps the power LED came on green and I head the audio circuit fire up through the speakers and after about 3 seconds the power led went to orange indicated no input which is correct operation for this monitor!!!! The power button also functions in this state as well.

OK, we have identified the cause of that problem. If you haven't already done so, check the resistance between the pins of the transistors. Set your ohmmeter to the 200 ohms range. On one transistor, measure from upper left pin to the tab on the bottom, then from upper left pin to upper right pin, then from upper right pin to tab on the bottom. Repeat for the other transistor.

PlainBill

[UserName666]

Re: Daytek F19AH

OK which transistors would you be referring to?
-Q101 by the filter cap
-IC103 between transformer and the heatsink
-All the little surface mount ones underneath labeled "Q" (16 of them)
-The 2 driver transistors for the inverter
-All of the above

[jetadm123]

Re: Daytek F19AH

I think PlainBill was referring to the inverter transistors marked 4060AL.

[PlainBill]

Re: Daytek F19AH

OK which transistors would you be referring to?
-Q101 by the filter cap
-IC103 between transformer and the heatsink
-All the little surface mount ones underneath labeled "Q" (16 of them)
-The 2 driver transistors for the inverter
-All of the above

jetadm123 is correct, the two that got hot. There are several explanations for the problem, but at this point the most likely scenario is that those transistors shorted, overloading the power supply. The next problem is figuring out WHY they shorted. The scorch marks in the area suggest one possibility.

While you are looking at the back of the board, what are the part numbers for IC1 and IC4?

PlainBill

[UserName666]

Re: Daytek F19AH

Q13:
1-top = 0 ohm
2-top = 23.8 ohm
1-2 = 0 ohm

Q14:
1-top = 27.6 ohm
2-top = 23.9 ohm
1-2 = 4.6 ohm

Q13 & 14 tested in board

IC1=
0Z9932GN
43014823T
D517A

IC4:393A
E26A

[jetadm123]

Re: Daytek F19AH

Looks like Q13 is shorted out. As for the scorch marks, maybe the marks are caused by the heat generated by the inverter transformer? Hopefully, it's not shorted out. What do you think PlainBill?