Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

[bamix]

Hey Guys,

I am new to this and have been struggling through. Will try and keep on rules as much as possible.

SUMMARY
======
I had 3 of these Monitors but disposed of one a couple of years ago as it failed from 2 seconds to black condition. One of the remaining two started going 2 seconds to black and then came good. After a few months both failed 2 seconds to black at almost the same time.

THINGS TESTED
==========

1) Unable to find a Mitsubishi Diamond Digital repair manual but located an AOC manual for the same Monitor.
2) Followed troubleshooting procedure in the manual as far as my level of understanding would allow. Researched on the Internet on similar problems and followed steps from this forum as a rule.
3) I have focussed on one Monitor board only and have replaced electrolytic capacitors, ceramic disc capacitors and some PP capacitors. I have found a problem at an Inverter Transformer on each Monitor board but it looks like this is symptomatic of a problem at another component.
4) The problem seems to be that one CCFL lights but the other doesn't so the protection circuit kicks in. I have rotated the boards in the display and the problem follows the board, ie: proper board position bottom lamp works, upside down board position top lamp works.

PROBLEM
======

1) The Monitor lights up when signal is received from the PC but shuts off after about 2 seconds. Turning power on/off at the Monitor repeats the symptom.

QUESTION
=======

I can't find much on these Monitors on the Internet. I have experienced a 3 out of 3 strike rate so I am sure it is not an extraordinary problem. However, even though I have tried investigating, measuring and replacing components the answer eludes me and whilst I admit electronics are not a strong suit of mine by any stretch I can't help but feel I am somehow close but just lacking the experience to make a connection. I am hoping that someone here has some experience with these Monitors that may be able to point me in the right direction.

DETAIL
=======

This is my story so far .

Hey Guys,

New here to BadCaps and seeking some kind advice that might assist me to resolve this issue of 2 seconds to black that I am experiencing with 2 x Diamond Digital (Mitsubishi) 19" Monitors. Long story but I had a 3rd one that had the same problem a couple of years ago and I ordered a new Inverter Board for it from Alibaba but when it turned up damaged I ended up binning both it and the failed Monitor. Wish I hadn't now...

Anyway, the 2 x remaining Monitors kept going until about 9 months ago one of them started going 2 seconds to black so I just put it aside. Then about 6 months ago, I upgraded that PC with new Mobo and Memory, etc and decided to plug in that same failed Monitor. Low and behold it started working so I was back to a 2 x Monitor set up, quite excited.

After about a month or so, its mate that had never failed started going 2 seconds to black and then it reverted to failure as well so I was left without any Monitors at all.

After doing some research, I was keen enough to have a crack at replacing some capacitors and see if I could resolve the problem. So far though, I have only replaced parts on one of the 2 x Inverter boards and have not yet had a result. Also, I flipped the boards upside down in their screens and checked to confirm the problem follows the board because one Monitor has a Top Lamp out but Bottom Lamp is OK and vice-a-versa. This way I think I proved that the CCFL lamps/connections are OK.

So, I couldn't find any repair manuals on the Diamond Digital but traced it back to an AOC LM928 and figured that Mitsubishi did a rebrand on this Chinese unit. I have been using a manual for this AOC model as my guide (I have attached a copy).

I changed all the cylindrical ESR capacitors with like or greater types as available from my nearest hobby/electronics retailer following what advice I could get from the web. Based on some of the capacitors being the allegedly poor quality purple Hermei type I was hoping this would resolve it but it didn't make any difference. Since then I have purchased a cheap Capacitor Tester and run all the displaced pieces through and they all measured as having been OK.

Since then I have been referring to the thread 'https://www.badcaps.net/forum/showthread.php?t=10419' and using the guide information compiled by retiredcaps as my bible. I have ordered in and replaced some ceramic disk and PP capacitors as well and still have the same issue.

So I have one Monitor identified as Monitor X and the other as Monitor O. I have been doing changes only on Monitor X so far and use the other Monitors board as a comparison when needed. I have attached a picture of the board from Monitor X (note that Q212 is removed for separate testing in this picture). Here is a brief description of some of my test results.

1. Fuse (one on board only): Checked not open circuit with a DMM, OK.

2. Bad Capacitors: Replaced all surface mount through hole type capacitors, C101, C102, C103, C201, C207, C213, C214, C215, C216, C217, C218 and C223.

3. Checked for shorted Mosfets/Transistors: On the board for Monitor X, I found faults at Q211 and Q212 as follows. Black to C and Red to E = 200 Ohms. However, after removing Q212 from the board this condition no longer exists. The board uses 2 x 8 pin Mosfets with a Gate, 3 x Source and 4 x Drain pins (Q203 and Q204). Connecting Black to S1 and Red to G shows no short, Black to S1 and Red to D1 shows no short and same for S2-3 to G and same S1 to D2-4. As mentioned by "retiredcaps" if there
was a fault there it would only be a brief flash or no light at all, not 2 seconds to black, so I think they are OK.

4. Transformers: I don't have a ring tester but I followed the advice in the guide using a DMM and figured I had found fault on both Monitors boards. I have attached a page showing the results as 'Initial Transformer Tests.pdf'. Happy that I had found a problem I have ordered 4 x Transformers from Hong Kong due in about 3 weeks time.
However, as an interim measure I removed PT201 from the board of Monitor O and used it to replace the faulty Transformer at PT202 on the board for Monitor X. Confident as could be I plugged the board back into the Monitor and switched it on but found the problem still there. I pulled the board back out again and did the same measurements of the board with the new Transformer and found it faulty again, in fact exactly the same readings (see 'Second Transformer Test.pdf'). So I don't know if there is another
part causing this faulty reading or if I fried the Transformer by powering it up again before checking.

Also, from the start I had 0 Ohms on what I thought was the secondary pins 8 and 9 on these Transformers and the guide suggested it should be around 950 Ohms.
So checking further, I found the thread 'https://www.badcaps.net/forum/showthread.php?t=12134' and saw at posts 9 through to 13 that this was an Inverter Transformer so the values were expected to be different. As "PlainBill" suggested, I checked Diodes at D203 and D204 and found that they were not shorted both showing flow in one direction only at about 1.34.

So what I thought was a bad Transistor (at Q212) turned out not to be a problem when removed from the board as mentioned above. I checked all eight Resistors (R224 - R231) and they all measure the same at .246 on the 2K scale. So still scratching my head.

Apart from the above, I followed the test flow chart in the Manual but only got so far until it mentioned square wave forms as I figured I would need an oscilloscope for that which I don't happen to have.

Maybe I have bitten off more than I can chew but this issue is really sticking in my side now. My family thinks I am crazy trying to save 2 x old Monitors but the thought that got me going in the first place was simply trying to save two more Monitors going onto the scrap heap. So what was initially an ideological view of saving these old beasts has now become an obsession and I will perhaps reluctantly give in and trash them only after I have exhausted any opportunity. Having the same problem on 3 of the same Monitor now and even experiencing it fail, come good and then fail again, I can't help but think that it is perhaps a common and minor issue plaguing them but try as I might at this stage I am seemingly going around in circles. Perhaps fresh eyes or the views of someone more experienced may deliver the break I need.

I have tried to include all the detail I can but if anyone needs any additional information then I will do what I can to provide same.

Thanks in advance. Please go easy on me, I learn eventually

[cpt.charlie]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

Are you sure that all lamps are OK? aged laps will draw more current and this will trip the inverter protection.

[bamix]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

Thanks cpt.charlie, I will remove the lamps and double check. I am trying to find a Cold Cathode Mod Kit that might help with this. If it is a lamp though, then would the values I found in the checks on the Inverter Transformers below be considered normal ?

PT201 (Pins numbered 1 from Right Edge of board)

Circuit and Result
1-2 0.246 2-3 0 3-4 0 4-5 0 5-6 0.246 6-7 1.369
1-3 0.246 2-4 0 3-5 0 4-6 0.246 5-7 1.076
1-4 0.246 2-5 0 3-6 0.246 4-7 1.075
1-5 0.246 2-6 0.246 3-7 1.075
1-6 0 2-7 1.075
1-7 1.369

Secondary Winding (8 & 9) 0

PT202 (Pins numbered 1 from Right Edge of board)

Circuit and Result
1-2 Open 2-3 0.246 3-4 0 4-5 0 5-6 0 6-7 0.246
1-3 Open 2-4 0.246 3-5 0 4-6 0 5-7 0.246
1-4 Open 2-5 0.246 3-6 0 4-7 0.246
1-5 Open 2-6 0.246 3-7 0.246
1-6 Open 2-7 0
1-7 Open

Secondary Winding (8 & 9) 0

After I carried out these checks, I learned from another thread that maybe Pins 7 and 8 are the secondaries and I actually got some readings there, about .244 on the 2K scale. Does that sound right ?

[cpt.charlie]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

If you can, remove the CCFL transformers and compare side by side out of the circuit, you should get the same values or prety close (they are the same model), btw, what are you planning to do with a CCFL kit?

[bamix]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

Thanks, I can do that. These are Inverter Transformers with 9 Pins. Is it best to still test as 1-2, 1-3, 1-4, 1-5, 1-6, 1-7 and then 2-3, 2-4 and etc as listed in prior post ?

Looking at the PCB it seems as though high voltage output is from both Pins 8 and 9 which is why I was getting a 0 Ohms reading when tested on the board. It looks like the ground is Pin 7 so I should check all 6 primaries to ground (7). Is that right ?

The CCFL mod kit was mentioned in another thread as a test device but they are harder to come by now as I think that PC case lighting has shifted more to LED's. I ordered two
of these from ebay anyway to make the job easier if I need in the future:
http://www.ebay.com.au/itm/NEW-CCFL-...-/221811364258

[budm]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

test lamp.
https://www.badcaps.net/forum/showthread.php?t=19987
With inverter transformer still on the board, check the resistance between either pin 8 (or pin 9) against all other pins.
It should show resistance against pin 7.
The inverter output is just a simple Royer Oscillator circuit, as long as it gets B+ form L201/L202 it should run.
Q203/204 are PWM controlled, it output Voltage is varied, the higher the B+ for the inverter output the brighter the backlights will be.

[bamix]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

Thanks budm, results below on 2K setting

PT201
8 to 1 = Open
8 to 2 = Open
8 to 3 = Open
8 to 4 = Open
8 to 5 = Open
8 to 6 = Open
8 to 7 = .248

PT202
8 to 1 = Open
8 to 2 = Open
8 to 3 = Open
8 to 4 = Open
8 to 5 = Open
8 to 6 = Open
8 to 7 = .249

Thanks for the tip on the test lamp, I was actually reading through that day before yesterday. I threw one of those compact lamps away about 2 weeks ago and now I wish I hadn't.

[budm]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

The HV winding realdings are within 3% of each other.

[bamix]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

Thanks budm, so that is all good also with no short circuits in the primary windings.

So back to the drawing board for me. I will double check the lamps and connectors as suggested by cpt.charlie. I was hoping that I had found the problem component but that's all part of the fun, hey. The odd thing is that one of these Monitors failed and then later came good and worked for weeks before finally failing again. Is that symptomatic of a lamp/connection or more indicative of a complex item failing ?

Thanks again.

[budm]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

To narrow down is to use test lamps (2) to verify.
Bad lamps, lamp wiring.
http://s807.photobucket.com/user/bud...?sort=3&page=1

[bamix]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

Understood. I will start at the beginning this time

Thanks for the link. That's a bucket full of sad tales to be sure.

[bamix]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

Hey Guys,

I dismantled both screens and removed all of the lamp assemblies. They look perfect, no discolouration, signs of overheated insulation or anything. In fact they appear as new. So, I set the boards back in the cases, hooked them up to a PC signal and powered them up. As I had found before, only the top lamp fired up on one of the Monitors and only the bottom on the other. I had everything colour coded with tape so I put the 2 x 'working' ones together on the same Monitor board and got the same result, still 2 seconds to black. Moving the lamps from position to position I did confirm that they all work but only at the known good Inverter Board outlets.

Doing some more research on here I remembered a thread I had checked before, https://www.badcaps.net/forum/showthread.php?t=7792. I went back to that again and picked up on a post by Bobdee about checking the transistors near to the transformers.

So I revisited this area carefully and found some had shorted, especially the two on the inoperative transformer side of the board from Monitor O. Monitor O is the one I was using for comparison so I hadn't done any work on this board yet and hadn't really looked at it closely otherwise. So, remembering to start at the beginning, I checked under magnifying glass and was excited to find what appeared to be bad solder joints at Pins 2 and 3 of the Inverter Transformer that was not working. I have attached a happy snap for reference with the problem pins circled.

So I removed 2 x Transistors from the other board (Monitor X) that had the most consistent readings and replaced the shorted transistors on Board O and then repaired the cracked solder at Pins 2 and 3 by removing the original solder, cleaning and then resoldering. I touched up the balance of the pins with a touch of the iron too.

So I set it up for test again and it works ! Two out of two lamps remain on whilst they are getting a video signal from the PC.

I am still waiting for some bits to come in so I will replace all the Electrolytic Caps and etc. before I finally assemble it but so far so good, one down and one to go

I will need some new transistors now to replace the failed C5706 units. I checked with RS Online but it seems they are only carrying surface mount types and I need the package for through hole mounting. I checked the data sheet (attached) and it says it is a TO-251 package.

What are the target values to look for in a substitute ? I gather that the Maximum Emitter Collector Voltage is key so I want 50v or greater I think. They have a MJE3055T which might work. Maximum Collector Emitter Voltage is 60v, Maximum DC Collector Current is 10A and Maximum Emitter Base Voltage is 5v all in a TO-220 package. This package seems much bigger than a TO-251 but Pin Centres are close, 2.3mm vs 2.4mm.

Does anyone have any experience using these in place of a C5706 ? I have attached the datasheets in case they are helpful.

[bamix]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

I am certain there is room for the TO-220 package and the Pin Centre variation is minimal so maybe the next best fit will be a D44H8. I can get sufficient of these quite readily.

What is the risk of changing the specification of this part from C5706 to D44H8 ?

[budm]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

So the inverter Transistors are bad after all.
Those original 2SC5706/5707 has very low Vsat, so if you use other Transistor that has higher Vsat, they will run hotter, that is one of the spec of the transistor that you need to match. I always use the original 2sc5707 for the repair.
Also the tuning cap next to those two transistors still have the same capacitance?

[bamix]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

Thanks budm.

Yes, I see that. In fact C5706 is even lower than C5707.

The D44H8 might be a better fit in that case with maximum Vce(sat) being 1v and Vbe(sat) being 1.5v whilst the MJE3055T has these at 1.1v and 8v respectively.

The data sheets for both the D44H8 and MJE3055T don't list a 'typical' value for these, just the maximum so am I correct to presume that keeping the maximum as low as possible in this case is the best course of action ?

The tuning capacitors you mention, are they the ones at C213 and C214 in the photo ?

If so, I am replacing these with Panasonic Radial polyprop cap,180nF 250V 10mm which I ordered from RS Components.

Thanks again.

[bamix]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

Update:

I found some C5706's on au.mouser.com so I will arrange some from there.

They come as Lead Free (E) and Lead + Halogen free (H). Is one type better or more durable than the other ?

[budm]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

Thanks budm.

Yes, I see that. In fact C5706 is even lower than C5707.

The D44H8 might be a better fit in that case with maximum Vce(sat) being 1v and Vbe(sat) being 1.5v whilst the MJE3055T has these at 1.1v and 8v respectively.

The data sheets for both the D44H8 and MJE3055T don't list a 'typical' value for these, just the maximum so am I correct to presume that keeping the maximum as low as possible in this case is the best course of action ?

The tuning capacitors you mention, are they the ones at C213 and C214 in the photo ?

If so, I am replacing these with Panasonic Radial polyprop cap,180nF 250V 10mm which I ordered from RS Components.

Thanks again.

Yes , caps c213, 214. Just check the capacitance, they do not look bulging out on the side so they may be OK. If the capacitance goes down, the oscillator will run at higher frequency so the transistors will run hot since it is no longer at the tuned frequency. Just make sure the pin out of the Transistor is the same as original.
Check the temperature of the transistors after you finish the repair.

[bamix]

Re: Diamond Digital (AOC LM928 ?) x 2, Two seconds to Black

I found out that the Lead Free type should be backward compatible with solder containing some lead anyway so I have ordered the (E) type. I gather that the Lead + Halogen free type must be in a higher demand for production as quantities are higher and cost is lower. I will keep things simple........