Author Topic: PC Engine LT video issue  (Read 1162 times)

herr-g

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PC Engine LT video issue
« on: September 09, 2008, 10:05:07 AM »
Hello,

i recently got hand over a loose defective PCE LT.
I basically works (game-sound and controls) but doesn't show any picture on the LCD. The backlight works, but nothing else up there.
I already checked and re-plugged the internal flat-cable contacts but everything is tight and well-contacted.
I'm well aware of the common TE / GT video-issue which can be repaired by replacing the small cap on the outer edge of the circuit board, but i never experienced a corresponding LT counterpart.
Since there are lots of the commonly affected caps on the LT PCB and none of them can be measured without removing it, i'd really love to hear anybody who could already solve the problem by replacing only some of them.
If not i'll start replacing any possibly defective caps within the next days and post my solution if i find one...

Best regards to the PCE community,
 herr-g

herr-g

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Re: PC Engine LT video issue
« Reply #1 on: November 27, 2022, 11:35:33 AM »
Hi,

I'm digging out my old post since my issue is still unsolved.
Meanwhile I've replaced all electrolytic capacitors. It seems that someone before me already did a partial and not so clean replacement, so I did the job again - and this time clean.
Like I use to do with all my cap replacements, I've used tantalum (bigger capacitances) and ceramic caps (smaller capacitances).

But my initial issue is still unchanged: The LT starts with clear loud sound and proper video over the extension port, but the LCD screen stays black, only with the backlight working.


I know similar issues from the GT when the -24V display voltage regulation is defective. But the LT has a slightly different and more complex voltage regulation using a 5291 PWM Converter IC.
I found that Pin 8 on the display flex connector carries -20VDC with the display disconnected (that pin on the image with the wire connected). So I believe this has to be the display supply voltage. However, with the display connected the voltage drops to roughly -7VDC.

I've checked all the traces and vias in the regulation circuit and even replaced the 5291, but without any change. All resistors and diodes / transistors within that regulation area measure normal, and the PCB has been cleaned thoroughly.

I also checked the driver PCB within the display unit. The only suspicious thing I found was three empty solder points that look like intended for a 3-pin diode or transistor (circled in red on the linked image). But I can't conclude why a missing part there would lead to such a voltage drop. as within the regulation circuit on the main PCB, all the parts measure like normal.

Unfortunately I don't have a seperate LT display lying around for a quick comparison.
Maybe someone else here has an idea or can share some experience in measuring the signals on the 5291 to narrow down the issue?
« Last Edit: November 27, 2022, 04:03:22 PM by herr-g »

Keith Courage

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Re: PC Engine LT video issue
« Reply #2 on: November 28, 2022, 11:28:36 PM »
I've worked on a few LTs with this problem and it always ended up being 1 of two things. Either old cap leakage under components near the Power port or a bad component near the power port. So if you're looking at the LT from the front, take a look at components at the rear left corner on top and below the board.

if the problem can't be solved, you can replace the screen with a composite one. I've done that before as well.

herr-g

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Re: PC Engine LT video issue
« Reply #3 on: November 29, 2022, 08:58:48 AM »
Thank you for the response.

I've checked all the components within that power regulation area quite thoroughly and cleaned it as much as I could.
All resistors, diodes and transistors measure fine. And I temporarily removed and checked what couldn't be measured in-circuit.
Today I unfortunately fried the small PNP switching transistor under the relay that switches the coil when I applied 6VDC to check the relay function. But as the relay didn't switch either, it doesn't make any difference anyway - will order a replacement transistor at some time.
Can you tell what the relay is intended for? My guess is it's for interrupting the primary power plug on the left when the LT is being powered from the opposite side (when inserted in the IFU).



Regarding the replacement LCD I actually have a lcddrv with display lying around here. For sure that would work and give a great picture. But I'd really like to keep this device stock. And as far as I remember, the replacement display is a bit smaller and would leave some free space behind the frame which would definately make it a not-so-great-alternative to me.

If someone here had an old LT display unit to give away that would definately be a huge help for finding out if my issue is caused by display or console.
« Last Edit: November 29, 2022, 12:30:00 PM by herr-g »

herr-g

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Re: PC Engine LT video issue
« Reply #4 on: November 30, 2022, 11:40:55 AM »
Today I removed the HuCard Slot and cleaned a lot of old electrolyte spill from under the plastic.
I'll give it another test tomorrow when everything has dried out properly.

Keith Courage

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Re: PC Engine LT video issue
« Reply #5 on: November 30, 2022, 04:01:57 PM »
The LCDdrv screen is way too small and wouldn't look right if you were to use it as a replacement.

I'll have to look up the exact specs but I'm pretty sure it's a 5" widescreen composite screen that I've been able to find that fits the LT perfectly as a replacement. Ends up being the exact size needed once the screen is set to 4:3 mode.

MasterofBiscuits

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Re: PC Engine LT video issue
« Reply #6 on: December 01, 2022, 06:02:33 AM »
Looking at the board, it's seems likely that corrosion/ electrolyte is the cause. I've seen it on GTs too, with the most recent one I did not showing a picture until I removed one of the transistors and fully cleaned it underneath (not the first time I've had to do it on a GT). It's very difficult to clean under a component without removing it, unless you have an ultrasonic cleaner.

I would advise carefully removing those components, fully cleaning the area and replacing with fresh solder.

And just to add, you can use 4 inch screens on the LCDDRV. Still slightly too small for the LT though https://twitter.com/iFixRetro/status/1214381258035400705

Edit: the panel is an SAT040HS54D08Y0 I believe
« Last Edit: December 01, 2022, 06:11:26 AM by MasterofBiscuits »

herr-g

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Re: PC Engine LT video issue
« Reply #7 on: December 01, 2022, 06:51:06 AM »
I just gave it a test now after everything has dried out again - unfortunately without any change.
You are right, the board had a lot of electrolyte all over it and some remains are left.
I had already removed most of the transistors for checking them out-of-circuit or swapping them vice versa with identical components. So I guess it won't make it worse to remove, clean and resolder the remaining rest. To clean it 100% I'd have do remove literally everything ...

In case of an eventual screen replacement I'd prefer the 5" composite one. Look like s...t with a too-small screen. Would be great if you could identify your recommended replacement @Keith - even if I'd hate to give up this repair ...
« Last Edit: December 01, 2022, 07:13:41 AM by herr-g »

herr-g

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Re: PC Engine LT video issue
« Reply #8 on: December 23, 2022, 08:21:44 AM »
A quick update:
I removed the HuCard Slot and practically every component in the power regulation circuit. Under the HuCard socket there was still quite a lot of "black spill" left. So it was definately a good idea I did that. Unfortunately after putting it back together not only did the display voltage not do any better after the cleaning, but also the PCE itself wouldn't start anymore and stayed on a blank screen, no matter if started via HuCard or Ext.Bus (SSDS3). Found out that the SRM2264LM SRAM was shorted (read 800 Ohms between Vcc and Vss) and the HuC6280A was dead. This must have happened when I tried to give the machine a quick test with the HuCard slot only loosely inserted, still unsoldered. Obviously the components are pretty sensitive to unreliable cartridge contacts. Well, at least more sensitive than I'd expected. I eventually transplanted the SRAM from another defective white PCE and the 6280A from a Core Grafx - now the PCE part is starting again, right as id did before my "cleaning" job.

Meanwhile I was fortunate enough to grab myself another LT which had recently been properly recapped and is actually working great. Now I finally have something for cross-testing. So I connected the display unit from my broken LT and - voila - black screen. Now I know for sure that the motherboard was never the actual issue and I can take a more detailed look in the display controller. I hope the issue is not the LCD panel itself or one of the controller ICs. In that case a replacement screen would be the only way to get the LT fully working again.

Anybody here with a spare LT screen to give away?  :D
« Last Edit: December 23, 2022, 11:49:18 AM by herr-g »

esteban

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Re: PC Engine LT video issue
« Reply #9 on: December 24, 2022, 11:18:02 PM »
ASIDE: I love that this thread represents a 14+ year epic saga for PCE LT. That is all.
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herr-g

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Re: PC Engine LT video issue
« Reply #10 on: January 16, 2023, 10:25:08 AM »
After some more detailed testing and comparing the display controller PCB with the good one from the second LT I know that the issue has to be in the driver IC  SLA7160F1S. Other than the OKI M6286 which can also be found in the Game Gear TV Tuner units I can't find any reference at all about the broken driver IC.
So unfortunately a display replacement with a 5" composite screen is my only realistic option to get it working again.

@Keith Courage can you give some details regarding your recommended replacement and how the display is best fitted inside the LTs screen housing? Could something like this be a proper option for parts?


MasterofBiscuits

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Re: PC Engine LT video issue
« Reply #11 on: January 17, 2023, 11:09:24 AM »

herr-g

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Re: PC Engine LT video issue
« Reply #12 on: January 17, 2023, 11:59:48 AM »
Wow, cool. The last time I looked the rgbdrv boards were basically unobtainable. Well, at least for gaijins.
Can you say more about panel and mounting parts and any sources to order from? At first sight the parts situation doesn't seem to have changed so far.

Some details regarding the 5" composite screen replacement would also be great, since I believe this is a far better feasible option.

MasterofBiscuits

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Re: PC Engine LT video issue
« Reply #13 on: January 17, 2023, 06:13:59 PM »
Check the Twitter link, he may be able to help you. I have never worked on an LT, but it looks like he is using the original screen frame with some modifications. I'm also not sure what LCD panel he's using with the RGBDRV. The good thing with these boards is the image size is adjustable so you can make it fit perfectly.

Yes sadly the RGBDRVs are difficult to get hold of and according to the seller, out of stock until 2024. Hopefully someone will come up with a replacement.

Necromancer

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Re: PC Engine LT video issue
« Reply #14 on: January 17, 2023, 11:28:13 PM »
Dang, biscuits, that LT is sexy as foook!
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