Pc Engine LT/Screen Repair

wolfman · 01/13/2014, 03:30 AM

Hello,

I am new to this forum. I already had a good look around, there is quite some info here fortunately

Recently I acquired an LT, which has a non-working Screen. Backlight seems to work though. Since I did not find any info, I am asking here for help.

Is this connected perhaps to a damaged video processor?
Or is the caps? Which ones could that be?
Or is it related to the mainboard?
Are there any replacement screens that would fit? Where could I obtain them?
Does anybody here have some experience with fixing an LT?

I would be happy for any useful input.

Many thanks,
Wolf

BlueBMW · 01/13/2014, 08:09 AM

Replace all the mainboard caps first. Ill bet that once you do the screen will come back to life. Ive had a few LTs do that.

wolfman · 01/13/2014, 08:52 AM

Thanks for the hint, hope you´re right!

Do you know what screen would be a good alternative, if this won´t help it?

BlueBMW · 01/13/2014, 11:19 AM

Turbokon has done screen swaps on express systems. You might consult him on that.

GohanX · 01/13/2014, 09:24 PM

I turned mine on due the first time in over a year yesterday and got the same thing, backlight was on but nothing on the screen. After a few minutes the caps warmed up and the screen came to life. I ordered some caps, hopefully there's no damage from leakage.

wolfman · 01/14/2014, 06:08 AM

Hey, thats interesting. I also have a TE that takes a few seconds until you actually see the screen. Sound is also weak, so maybe thats really a cap-related issue. Will have a look at this.

What if its not cap-related? What else could it be? Videoprocessor?

wolfman · 01/15/2014, 07:32 AM

Is there some sort of documentation of the LT somwhere on the web available, like it is for the TG?

BlueBMW · 01/15/2014, 08:14 AM

I dont think we have a service manual or anything like that for it. There should be a cap chart somewhere though.

I really doubt its a video hardware failure. I would bet money its the capacitors.

wolfman · 01/15/2014, 01:47 PM

Quote from: BlueBMW on 01/15/2014, 08:14 AMI dont think we have a service manual or anything like that for it. There should be a cap chart somewhere though.

Oh, thats sad. I hope it will surface some day....

wolfman · 01/20/2014, 05:09 PM

I have been working on the unit today, and found 3 caps that had been soldered by the previous owner very very badly, looks like they didn´t hold longer than 10 minutes. Unfortunately a few lines already had come of the board, loosening the caps.

Can someone help me find their destination? Since there had been some brutal soldering I am not sure where the original leads ran to...at least I have to guess, which isnt too good. If I would find another spot I could solder a cable to one leg, and put the end where to contact was supposed to be.

The caps concerned are on the left side of the LT main board, around the 9V DC power jack/loudspeaker connector.

I have attached a few pics. You can see the missing lines, I am not sure about where they originally were attached to.

Is there someone who knows that or has good photos where you could see it, and could possibly trace the original route on the board (for maybe a 2nd solution)? That would be awesome.

http://www.flickr.com/photos/65709578@N06/12058745826/#
http://www.flickr.com/photos/65709578@N06/12057881735/#

Any help on this is greatly appreciated!

turbokon · 01/20/2014, 05:26 PM

Hold the board up against a light source like a lamp. You should be able to see the trace.

thesteve · 01/20/2014, 09:09 PM

Looks like you found the traces (correct arrows on pics)
Have seen a bad diode cause no pic, but caps most often 2

wolfman · 01/21/2014, 12:25 AM

Yes, but where do the traces END, thats what I want to knkow. Anyone?

Keith Courage · 01/21/2014, 01:06 AM

The easiest solution is to scrape off the green to expose some contact to solder to.

wolfman · 01/21/2014, 02:43 AM

Look, I want to know at which part the trace ENDS. I know that I can scrape the green off...

BlueBMW · 01/22/2014, 01:18 AM

Looking at those pictures I want to say first clean all that old solder off and wash off all that flux and crud. Once you have a clean board you should be able to see where the traces were. The first picture Ill bet just goes to that large green plane that is attached to the exposed copper your arrow points to.. The second picture is too dirty to tell anything useful from.

wolfman · 01/22/2014, 04:45 AM

Quote from: BlueBMW on 01/22/2014, 01:18 AMLooking at those pictures I want to say first clean all that old solder off and wash off all that flux and crud.

Well the solder that is left in the 1st picture I wanted to leave there since it still seems to have a solid and good contact to the board. I already cleaned everything and even scratched a little on the right contact to see if there would be any copper left (the brownish-blackisch rectangular spot left of the arrowhead), but there was nothing.

Is there a special cleaning fluid you recommend? I was using IPA so far, and q-tips.

wolfman · 01/22/2014, 05:15 AM

Quote from: BlueBMW on 01/22/2014, 01:18 AMThe second picture is too dirty to tell anything useful from.

Same here. That´s the best I could get with my camera. I thought someone here would know probably from experience or by memory where the trace would end or point to, so that I could solder directly to that point or part.

BlueBMW · 01/22/2014, 07:52 AM

Id have to take apart an LT to look and see. Ill try and do that tonight.

wolfman · 01/22/2014, 09:38 AM

Quote from: BlueBMW on 01/22/2014, 07:52 AMId have to take apart an LT to look and see. Ill try and do that tonight.

Awsome! Thanks a lot!

wolfman · 01/23/2014, 06:03 PM

Quote from: BlueBMW on 01/22/2014, 07:52 AMId have to take apart an LT to look and see. Ill try and do that tonight.

Did you have a chance to find out yet?

BlueBMW · 01/23/2014, 09:57 PM

Quote from: wolfman on 01/23/2014, 06:03 PMDid you have a chance to find out yet?

Alright... regarding CC505... the trace you pointed out does connect to that pad. It also seems to have continuity to all three pins on the left side of that T500 component. You should be able to solder to that trace that you pointed to in your picture.

Regarding CC506, the pad you pointed out does connect to that plane next to it.

/lt1_zps3e15ded5.jpg

wolfman · 01/24/2014, 02:46 AM

Thats great news, many thanks!

Do you know what that T500 labeled "800V" is? Transistor? Or a MOSFET? What kind of?

I am not sure but in case I would have to replace it, it would be good to know....

Great Forum btw, its really cool how much help there is and so much USEFUL info.

Fidde_se · 01/24/2014, 05:48 AM

It's a coil, might be 800mH, very unlikely to go bad.

(Mosfet is a type of transistor)

wolfman · 01/24/2014, 06:31 AM

Quote from: Fidde_se on 01/24/2014, 05:48 AMIt's a coil, might be 800mH, very unlikely to go bad.

(Mosfet is a type of transistor)

A coil with 7 pins?

Fidde_se · 01/24/2014, 07:08 AM

Yupp, usually not all legs are connected, the same package can be used for the ones that use the extras legs, the ones that are not connected are then used for stability and support, and the ones that use more then one lining are transformers and they use them, if the big top would break off it would still work, it's just unmagnetic ferrite.

The only way for it to stop working is if one of linings burn off or the varnish on the linings gets so hot it melts and burn.

wolfman · 01/24/2014, 07:21 AM

So it is really cool and safe to solder the cap to it? Was a little confused as it looked like the copper trace went "back" to this piece...got the impression that I might fry something by accidentially shorting somewhere...

Fidde_se · 01/25/2014, 08:03 AM

Yes it is completely safe to solder back a capacitor there, the pads doesn't even seem to be damaged.

wolfman · 01/25/2014, 02:46 PM

So I played around with it the whole day, and I didn´t get the screen to work. Although everything is in place so far and I although I soldered new caps into it where I had access to (left out the tiny ones in the middle and on the front of the board) the screen never went on.

I opened up the top lid and disassembled the screen block to have a look at it. Guess what? Someone had tinkered around with it and damaged a ffc. I left it the way it was and modded a 3.5" LCD screen into it, via composite Video, which works ok and is quite bright. I dont have access to brightness/contrast, but since I set it up before I built it in everything is fine so far.

Someday when I find the time I will have a look at the damaged screen, maybe I find a flexcable that would fit. Hopefully it is going to work then...

Here´s my work of today as a summary:
http://www.flickr.com/photos/65709578@N06/12138988584/#

At least something!

thesteve · 01/25/2014, 08:49 PM

T500 is a transformer, not a coil
It can fail like a coil and likely uses all 7 pins 2

wolfman · 01/26/2014, 04:23 AM

Quote from: thesteve on 01/25/2014, 08:49 PMT500 is a transformer, not a coil
It can fail like a coil and likely uses all 7 pins 2

Do you have any specs on it?

Fidde_se · 01/26/2014, 07:52 AM

You can desolder it and measure the individual linings/windings, you need a henry meter for it, you can check that the windings hasn't shorted to another one with a ohm meter though.

There is one out for every in, from the picture it looks like it's three windings, meaning 6 pins are used.
if you look under the component you would be able to see where they are connected, there should be no galvanic connection between the windings.

But I doubt this is the culprint, some of these have wires that are thicker and have more area then some of the traces meaning the traces burn of long before the winding does.

Should note that some of these I have seen do connect inside the the windings and are built that way, but it's unusual, giving false indication of a faulty component.

Usually these are hard to read from what it says on the top, even for the industry and this is most likely custom made and they are very easy for them to do when they want them in a couple thousand unlike custom ICs that costs enormous amounts of money to make.

They are usually called SMD Power Choke Coil (yes these are usually called coil even they have more then one lining/winding and should be called transformer) more like a collective name for all of them, the package is a standard one but isn't registred as one so it has no name in general, very confusing for PCB CADers.

They are used for transforming the voltage up or down and used most for switched AC adapters after the big transformer in the low voltage area (but can transform up to high voltages, but with little ampere) or in the backlight unit for screens, the later is probably what it's used for in the LT.

Fidde_se · 01/26/2014, 08:02 AM

And the pictures and links you sent me on the screen, could you post them here, and how and where you connected the cables so that others can do the same, as much info about the screen as you possibly have as it seemed to fit perfectly.

Also about anything needed to be taken out or carved in the casing, if any, to make it fit.

The cold light tube on the LT will start go out on most LTs out there on the next coming years and this will be becoming more common in doing to save them from the garbage bin.

wolfman · 01/26/2014, 09:22 AM

Sure I can do that, but it might take some time. I´ll be busy the next 1 1/2 weeks, lets see then.

What I can say for now is that I didn´t remove any portions of the motherboard, I only took out the screen completely. So apart from that the unit is rather "stocky".

BlueBMW · 01/26/2014, 11:03 AM

Since the LT has a rear expansion port like a regular PC engine it should be really easy to tap RGB or composite video for use with an LCD. Expresses are harder since you have to encode the RGB to composite when doing a screen swap.

wolfman · 01/26/2014, 12:03 PM

Quote from: BlueBMW on 01/26/2014, 11:03 AMSince the LT has a rear expansion port like a regular PC engine it should be really easy to tap RGB or composite video for use with an LCD.

Do you have recommendations on which LCDs are best for the LT? I have seen a few Adafruit/Arduino boards, but I am not sure how to get RGB with them. They all seem to only accept Composite...

I have looked at those:

possible option for TurboExpress/GT:
http://www.adafruit.com/products/911#Technical_Details

Option for LT, but without controller board (must be purchased extra):
http://www.adafruit.com/products/1596
http://www.adafruit.com/products/1591

and that would be the driver board for latter mentioned screens:
http://www.adafruit.com/products/1590

Opinions, anyone?

EDIT: The latter DO accept RGB, but I am not sure about the procedure. Would we need an additional board or could we simple feed the 3(4) signals into the board and we´re good?

BlueBMW · 01/26/2014, 12:30 PM

Turbokon has done a lot of work with screen swaps on the Express / GT... I know he's used 2.5" and 3.5" screens. Typically they've all been composite in so he's had to build an encoder circuit to go with them on expresses/GTs. I'd shoot him a message and find out which ones he uses.

wolfman · 01/26/2014, 12:57 PM

Quote from: BlueBMW on 01/26/2014, 12:30 PMTurbokon has done a lot of work with screen swaps on the Express / GT... I know he's used 2.5" and 3.5" screens. Typically they've all been composite in so he's had to build an encoder circuit to go with them on expresses/GTs. I'd shoot him a message and find out which ones he uses.

I´ve been using a cheapo car LCD I found on ebay. What puzzles me is that I don´t a fullscreen picture, although it is a 4:3 screen. Does the LCD show "native" resolution only?

What do you think?

BlueBMW · 01/26/2014, 01:01 PM

I'm not sure on that one. I know LCDs are fixed resolution displays and that sometimes you wont get a fully filled screen depending on the source. Could just be that the particular LCD you're using processes the composite signal in such a way that it doesnt fill.

wolfman · 01/26/2014, 01:26 PM

Quote from: BlueBMW on 01/26/2014, 01:01 PMI'm not sure on that one. I know LCDs are fixed resolution displays and that sometimes you wont get a fully filled screen depending on the source. Could just be that the particular LCD you're using processes the composite signal in such a way that it doesnt fill.

Thanks for the hint, I wasn´t sure about that. Sadly bigger isn´t better in that regard