REPAIR GUIDE - TurboExpress/PC Engine GT: Video Fix

[farankoshan]

the lamp is powered by the transformer, which you said had no power.
at least we now know thats the prob.

i will soon have a spare screen, but i dont think its your issue

Hey steve! Thanks for replying.

Well, if I can have a makeshift lamp put to backlight this screen, any chance you know the specs of this lamp and how to replace it? Coz if we can replace the lamp with a new one, and it still doesn't light up, then at least we've proven beyond all doubt that it's a transformer-related issue. Rerouting power from another source is always a viable option if the transformer won't allow current. It's good news for me that this LCD still works. I just need to know how to fix the backlight.

[thesteve]

im in the process of an LCD transplant (using a 3.5" backup cam display).
the last step will be to remove the backlight power, and i will trace it out then gaining a basic schematic of it.
once the schematic is done i should be able to help you through the circuit.

[thesteve]

i may not end up tracing the circuit out.
the transformer is driven by transistors Q900 and Q901
normally when backlight circuits fail its the driver transistors.

[thesteve]

i dont have a color encoder installed yet, but i disabled the stock back-light ballast.
years back i tested my power drain on this unit and it was 600mA.
tested running the 3.5" display 97mA

[farankoshan]

So how would I go about using this info? Sorry, I got a little lost with the lingo (ballasts, transistors, etc.) What do I ask my local hardware store for?

I wonder if there would be a way to remotely power a lamp to go behind this LCD...

[thesteve]

the problem likely relates to the transformer not getting pulsed.
the transistor info should be useful to your tech guy

[farankoshan]

Ok, will see what we can do. Thanks!

[thesteve]

i have an express here showing jailbars (common for RGB modded systems).
the cause is capacitor CC505.
tracing to asses relation to a possible fix for duo mods

[Keith Courage]

I am working on a PC engine GT for the forum member tangentt right now and I am having screen issues. Already re-capped the thing, checked for bad traces off of the caps, checked the usual Q900 and Q901 transistors, and tested another screen. Basically the screen will not get bright enough. I can see a clear picture if I hold the GT at an extreme angle but if you look straight at it you can barely see anything at all. Maybe there is another bad transistor somewhere? If anyone has any guesses it would be much appreciated.

[thesteve]

id check the -24V supply, as its your drive voltage

[Keith Courage]

I checked Pin 3 for the screens ribbon cable and it is supplying 24V.

[thesteve]

it could be the 5V or contrast pin

[Keith Courage]

So I went through the pins again for the screen today just to double check things. Turns out I must have not read something correctly yesterday. The 24V pin ends up reading almost 27V when the screen is not plugged in. As soon as the screen is plugged in the voltage drops to 15V. I know that a good working express should stay very close to 24V regardless of having the screen plugged in or not.  Does that sound like there is a short somewhere to you?

[thesteve]

that sounds more like an open cap or regulation fault on the 24V line

[mrchuck999]

Hey guys,  You know the blue cap at the back light connector? Has anyone determined the value of this cap ?   I know its a long shot but I've tried almost everything else so I would like to replace it.
Mrchuck999

[macphreak4evr]

Thank you so much, I did this and the sound fix and I am back in action!!!

[RxM]

Thanks for the awesome repair guide.  I snagged a practically brand new TE off of Ebay for $34 and managed to fix it with a $1 part from Radio Shack.

It got a little hairy there for a bit though.  I thought I was being careful but still accidentally pulled the negative trace off the board when I removed the capacitor.  Thankfully the trace ripped pulled up far enough for me to follow it to another solder point.  The leg of the little transistor/MOSFET that sits just adjacent to the cap will work. 

If I had it to do over again I would definitely have tried to touch the soldering iron to the legs and gently pry them up rather than trying to pull the cap off cold.


Looks so innocent just sitting there.


Disaster strikes!  I did not notice the little trace that was ripped out of the surface of the board at first - I honestly thought it was just a strip of paint or glue or something.  The scratches you're seeing where the pad for the negative trace was are my desperate attempts to find what the pad had been attached to.  I was hoping that if I dug into the board I would find another solder point.  I realize at this point that was silly, but at the time I had no idea.


But that was exactly what I needed to find an alternate soldering point.


Here is the alternate soldering point magnified some more.

I decided to remote-mount the cap because the connections, while solid, were so fragile I didn't want to risk any more bending and twisting than I had to.  It may not be pretty but it works like a champ.

Hi Samson, i have same issue. When i went to remove the defective cap for video, my trace also lifted.  So isit ok to solder the neg lead to the mosfet transistor? Cause i dont see any other trace. Any
Help greatly appreciated. I went to do the rest of the cap replacement and now im not getting no sound or video.  Screen light up and flicker vertical. I spent countless hours trying to figure out what else could be wrong but i have no clue.

Thank you
Regards
Alex

[thesteve]

it was the 5V regulator running at 3V due to a cracked resistor R506

[fagin]

Hi,
I seem to have the same or similar problem as Keith Courage reported above.

I had previously changed the video cap a few years back - that resolved an initial dim screen issue I had. 

Recently I tried the GT and after 10 mins the screen went very dark.  Because I also had low sound, I thought I would complete a full recap.

After the full recap I still have the same screen issues:

The backlight is working - although very dim.
You have to extremely tilt the unit to see anything - this gets worse within minutes and then backlight shuts off completely.

Any help guys?

Regards,
Shaun

[thesteve]

The backlight will shut off if not used (push a button)
Also if unable to adjust brightness properly it is a 24v supply prob if not a bad screen 2

[fagin]

Thanks for the feedback.

IIRC when I measured the voltage on VR500 it was only around 10v.  I'm assuming this should be 24v.

Any pointers as to how I can perform further diagnostics and/or fix the supply problem?

(Thanks for the reply to my PM btw) - In the PM you mentioned about a possible arcing.  What should I look at to establish if this is taking place?

I've placed an order for replacement Q900 and Q901 transistors, but I'm assuming this will not fix the issue?

[thesteve]

Not sure at the pot
You should have -24V at the - pin of the 22uf cap near the pot 2

[thesteve]

The pot on the side, not the one @ the top
 2

[thesteve]

Q900,Q901 supply the backlight transformer and are all or nothing 2

[fagin]

Not sure at the pot
You should have -24V at the - pin of the 22uf cap near the pot 2

11v measured at that point.

[fagin]

Any pointers on how to resolve this low voltage issue? 

[thesteve]

First unplug the screen and recheck (keith had a shorted screen)
The video cap fixes all relate to that voltage
The pot near the side of the unit adjusts the -24V supply, the one near the top adjusts the +5V 2

[fagin]

First unplug the screen and recheck (keith had a shorted screen)

Around 11.6v with the screen unplugged.

[thesteve]

That's low
Check the large 22uf cap and the 4.7s
 2

[fagin]

By tweaking VR500 I can just about get to 14v.

Apologies, not sure what you mean by "check". 

All caps have been replaced and it's the same problem I had prior to the cap replacement.

[thesteve]

I was vague because I want you to test the caps and check their connections
Also inspect the board around them
 2

[cam]

You should fix this problem following this tutorial (in french) :

wayback.home108.com/

Best regards,

[fagin]

Thanks for that link..... where are the screens available from?

[thesteve]

If you need an original screen, anyone doing screen swaps should have a pile of good ones 2

[fagin]

It's not an OE screen I need.  Something is obviously mullered with my main board, even after looking at the areas you suggested, I didn't get anywhere.

[thesteve]

screen swaps can be done, with a composite mod

ebay sells screens, some work some dont

[coryoon]

Hi,

I have a GT with the 'screen on but no picture' issue - screen is black (but lit), sound works etc.

Is there anyone in the UK that does the Cap replacement?

Thanks.

[Rosser]

deleted post

[Moudamir]

Hello everyone,

So I follow the replacement process for the capacitors, mine was showing no video and no audio. Video only backlight sometimes shadows !!! And audio was low.
I've changed the audio circuit !!! And it's perfect. And I've changed the video one it came back for 10 sec and that's it !!! Don't know what to do !!!
I've changed total 10 capacitor out of 18 still nothing. Didn't go under the shield yet...
Any ideas ?

[Keith Courage]

Remove the shield and change the rest before looking at any other options for the problem. That way you can at least rule out the capacitors if the problem still exists.

[Moudamir]

Hey guys,

I end up changing everything all the caps, now it's worst screen stays greenish, with some shadows if you angle the TE forward.

Kind of lost !!! Any ideas ? Screen is May be dead....

[Keith Courage]

Look around for bad traces. If it was better before changing the rest then a trace or pad must have been pulled up.

[speedlolita]

Hi! Thought I'd post my issues with a PC Engine GT unit I was asked to re-cap.

After replacing all of the caps most of the issues with the unit were fixed.

However, the brightness dial isn't working as it should. That is, after the unit has been on for maybe 15 seconds. If the unit has been discharged for a while then the dial seems to work as desired. Once it settles down though the dial has little to no effect and the image just gets progressively darker.

The main video cap had leaked when I got there so I scraped away the excess junk and reflowed the transistors in that area but that didn't appear to do anything.

Aside from attempting to re-cap the video capcitors again I'm at a loss. Has anyone else experienced similar symptoms?

[thesteve]

its a power hog
battery fade will cause this

[jlued686]

My son just used these instructions as part of a job-shadow project. It worked brilliantly! Thanks for the handy guide. You made it so simple even an 8 year-old could do it!

[bovined]

I had a perfectly workinh TE but I picked it up the other day to play some Blazing Lazers and it would only power up the screen going from white then quickly to black with no audio. I tried moving the hucard a bit and it will sometimes power up to a green or orange screen and then quickly to black again. The brightness dial still works but thats it. I tried replacing the cap in question from this thread but it seems to be be behaving the exact same way afterwards. Any ideas on what is causing this issue or what the next best caps to try replacing would be?

[thesteve]

going black after white tho i havent seen it on a TE/GT, i have seen on other versions
it means the system crashed shortly after resetting the pallet registers
the cause has been bad 6280/ bad work ram/ bad trace
any one can cause this, and it could be all, but most common is bad traces

[bovined]

Just want to say big thanks to thesteve for fixing my TE! Awesome fast service and communication. A true pro!

[RetroWeirdo]

Hey y'all, I just did a recap on a TurboExpress which had stopped working (This was my 2nd time repairing one of these, 1st one came out great). This time around went a little different. When I was trying to carefully remove the portions of white glob that were covering the solder pads of the caps in the corner, there was a large piece that broke off and took a piece of the black plastic top off the copper coil pack hidden under that white goop. So I assumed this goop was acting as a insulator for the coil/glue for the 100uf 16v caps that it runs into. I attempted to use hot glue to repair this and continued the recap job.

Once I reassembled and tested the unit, I found the screen to flicker and get distorted with pixels in improper places. After double checking all of the solder points and some other trouble shooting without the back half of the shell, I was able to get the distortion to stop but the light flicker continues to occur as if possibly a slight issue with the backlight. But I wonder if this is possibly related to that coil from under the white goop. Could this occur if it is damaged or not adequately insulated?

Thanks

[Keith Courage]

Insulation of that part shouldn't effect anything. I've never seen flickering like you described. Maybe you should post some pics of the graphics issues? Could be bad ram/or damaged trace related.

Also, in the past I've accidentally uncovered the part you speak of before and have even broken off a leg/contact point before. I've always just re-soldered the wire back to where it originally went without any ill effects. Then cover up with hot glue or epoxy. So hopefully neither of the two contact points for the coil have snapped off anywhere.