Need help DCLCD 19" LCD Monitor - I already replaced caps

[CapBlown]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

What is the good way to test the voltages? Put the power/inverter board on the desk, plug in the power cord in and test

I would not dare to try that way. I'd make sure that whole circuitry is correctly connected to LCD before trying to measure any voltages. That ensures that you have ground where it's supposed to be.

I always try to make sure that I can't short any 2 connections accidentally so I usually connect my multimeters black wire in some safe ground position (away from measurement points) before getting into measuring voltages. It's easier and safer to measure when you operate only one measurement point at a time.

[wenter]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Are you getting any stable secondary voltages like 5V or 12V DC on the connectors to the logic board?

If not, are you getting mains voltage * 1.4 across the big filter capacitor?

Did you test the fuses to see if they are open?

What is the good way to test the voltages? Put the power/inverter board on the desk, plug in the power cord in and test, or I need to have LCD panel connected to inverter and the rest of connection?

Is there a good FAQ on how to perform basic tests on the power/inverter board?

[retiredcaps]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I also have same monitor, with little bit different symptom. Monitor will not turn on. The power LED will flash on and off.

Are you getting any stable secondary voltages like 5V or 12V DC on the connectors to the logic board?

If not, are you getting mains voltage * 1.4 across the big filter capacitor?

Did you test the fuses to see if they are open?

[wenter]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Hi, all. My first post on this forum. I already read some threads here and I have found tons of useful information. Great forum.

I also have same monitor, with little bit different symptom. Monitor will not turn on. The power LED will flash on and off. There also clicking sound coming from the area colored green. I suspect that the K2645 MOSFET is the problem. But can not tell for sure.

No leaked caps on the board. Four transistors on the bottom of the board seems to be working.

Any ideas?

[archt30]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Great, thanks for the help.

[PlainBill]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Guys, Im sorry for butting in but as a newbie I have a quick question. I have that same board shown above in a Sceptre Komodo monitor. The problem is the same as I keep reading about: display stays on for about 3 seconds then shuts down. If left on for a while it will finally hold the display when I turn it back on. OK finally here is my question. I have the caps to make a change out. Do I need to be concerned about draining any of those on the board before I start?

Nope. The only one that might pose a hazard is the big one (100uF, 450V, and if the monitor was working when you shut it down it will be pretty well drained.

PlainBill

[archt30]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Guys, Im sorry for butting in but as a newbie I have a quick question. I have that same board shown above in a Sceptre Komodo monitor. The problem is the same as I keep reading about: display stays on for about 3 seconds then shuts down. If left on for a while it will finally hold the display when I turn it back on. OK finally here is my question. I have the caps to make a change out. Do I need to be concerned about draining any of those on the board before I start?

[PlainBill]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

the only 16pin chip on the bottom of the board says:

4A FXFHK TL1451A


I honestly don't think I have the tools to handle desoldering these surface mount chips without damaging the board. If you think it is going to be one of those chips, I may just have to throw in the towel.

Let me know

Working with surface mount chips is no big deal. 60 years ago radio repairmen were up in arms because the manufacturers were building stuff on PC boards rather than using point to point wiring. They would lift the traces using 100 watt soldering irons and didn't know what they would do if they had to replace a vacuum tube socket. Ever since then every new innovation is met with wails of despair, and the old timers complain the new stuff is impossible to repair.

In some ways surface mount boards are easier to work on. With the proper tools, it's pretty hard to lift a trace or tear out a feed-through. The best tools are a 40 watt soldering iron with a fairly wide tip, a 20-25 watt soldering iron with a tiny tip, some solder braid, and a product called Chip-quik. Chip-quik is a low melt alloy (Woods metal) that dilutes the solder so the chip can be easily removed (It's melting point is well below the boiling point of water). Clean up the left over solder and Chip-quik with the solder braid, drop the new chip in place and solder it with the smaller iron. Yes, it's a skill that improves with practice, but so is just about everything.

If you don't want to go that way, I suggest offering the monitor to one of the other people here, or advertising it locally on Craigslist.

In the meantime I'll investigate the inverter controller and see if we can fake it out long enough to do some meaningful troubleshooting.

PlainBill

[martini]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Something is not copacetic; the resistances should match closer than that. I don't think just replacing the transistors is going to fix it.

Now we may be venturing into dangerous territory. The first step is for you to give me the part number of the inverter controller - the 16 pin IC on the bottom of the board.

PlainBill

the only 16pin chip on the bottom of the board says:

4A FXFHK TL1451A


I honestly don't think I have the tools to handle desoldering these surface mount chips without damaging the board. If you think it is going to be one of those chips, I may just have to throw in the towel.

Let me know

[PlainBill]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I just measured the resistance across the transistors. They looked like the following:

Q12

126 Ohm
170 Ohm
73.6 Ohm

Q11
126 Ohm
170 Ohm
73.4 Ohm



Q6
1.4 kOhm
600 Ohm
600 Ohm

Q7
1.4 kOhm
600 Ohm
700 Ohm



so the two pairs do not match, but the transistors within a pair match. And nothing is anywhere 00.0 ohms (which I didn't think it would be)


So whats next?

Something is not copacetic; the resistances should match closer than that. I don't think just replacing the transistors is going to fix it.

Now we may be venturing into dangerous territory. The first step is for you to give me the part number of the inverter controller - the 16 pin IC on the bottom of the board.

PlainBill

[martini]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Did you reflow the solder on the transformers yet? It will take thirty seconds and that's one more thing we can cross off the list.

I just measured the resistance across the transistors. They looked like the following:

Q12

126 Ohm
170 Ohm
73.6 Ohm

Q11
126 Ohm
170 Ohm
73.4 Ohm



Q6
1.4 kOhm
600 Ohm
600 Ohm

Q7
1.4 kOhm
600 Ohm
700 Ohm



so the two pairs do not match, but the transistors within a pair match. And nothing is anywhere 00.0 ohms (which I didn't think it would be)


So whats next?

[martini]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

sorry guys, haven't gotten back to this in a bit... i redid the transformers. I can check the transistors.

im looking for close to zero ohms across each combination of the transistor leads?


youd never believe it, but my projector died and it had a blown 470uf 25V elite cap in it... my life is filled with bad caps and transistors!

[weirdlookinguy]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Did you reflow the solder on the transformers yet? It will take thirty seconds and that's one more thing we can cross off the list.

[PlainBill]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I think Plainbill meant you to check them with the low 200 ohms setting, looking for a 00.0 reading or close to that across the solder points, three readings each transistor, he should confirm this later when he comes online
Also Google chip quik

Absolutely correct on both points.

PlainBill

[Bobdee]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I think Plainbill meant you to check them with the low 200 ohms setting, looking for a 00.0 reading or close to that across the solder points, three readings each transistor, he should confirm this later when he comes online
Also Google chip quik

[martini]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Did you check for shorts as Plainbill asked you to do first
No point removing them if not shorted

is there an easy way to test that with them on the board?

[Bobdee]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Did you check for shorts as Plainbill asked you to do first
No point removing them if not shorted

[martini]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

i just tried the transistors... i believe they are above my ability/tools. I have just been using my metcal soldering station and some braided copper solder remover. I don't think I can get the transistors off without damaging the board.

any tips?

[martini]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

i just swapped the square capacitors.... one on the broken board seemed to have a crappy solder joint so i as hopeful that it would fix it.. however it didnt. i guess ill look at the transistors now

[martini]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I've circled the transistors in red. Even if only one of a pair is shorted, replace both. The transistors are Rohm 2SC4672; they are available from Digikey, their stock number 2SC4672T100QCT-ND. Also check and resolder all components in the area, especially the transformers and the gray rectangular caps.

PlainBill

i will look into this - I swapped both transformers with no luck. When you say rectangular caps, do you mean the things next to the transformers on the topside of the board?

the transistors are small... hopefully i have a steady enough hand to solder them... chances are ill just swap them with my other monitor ... if i mess that up ill have two broken monitors!

haha