REPAIR GUIDE - TurboExpress/PC Engine GT: Video Fix

thesteve · 05/30/2011, 04:02 PM

found the cause of the top left part of the screen too bright.
the 100uF 16V cap by the fuse was bad

farankoshan · 08/30/2011, 01:02 AM

Right now, my friend and I (more him actually) have checked the board and the 4.7uF 35v contacts are all right. And testing it with a multimeter shows proper current to and from it, through the board. The thing is, now he's trying to nail down a problem with the LCD. He's tested the power circuit that routes power to the LCD. (forgive my caveman terms) It's one of the cables you stick into the red port on the board. He keeps saying that the contacts at 4.7uF 35v are all fine, but the multimeter shows no power going through that red "teethy" port, hence the LCD isn't getting the power it needs. We turn on the TE and the audio works fine, even the start buttons (with the help of sound cues) get us past the start screen, but still no video coming out of that LCD. So he may be right.

Any ideas? Are we on the right track?

thesteve · 08/30/2011, 04:17 AM

the lcd has a florescent backlight.
is the backlight working?

farankoshan · 08/30/2011, 05:55 AM

Quote from: thesteve on 08/30/2011, 04:17 AMthe lcd has a florescent backlight.
is the backlight working?

How can you check that?

Right now, we're pretty stumped on our problem. Still not getting any successful video out of the TurboExpress. Will be taking photos of both sides of the boards & will attempt to translate (he's not very fluent in English) what my fix-it friend needs to know from this forum. I hope you guys will be able to help.

He's been able to test & is almost positively sure that both the 4.7uF 35v cap itself and the traces on the board around that cap work out just fine, so it's not that problem.

Off the bat though, he was wondering if anyone here has a diagram of the board and the leads that go around it. He's tried to isolate where the problem is, and it seems that the "red teethy port" I mentioned earlier is not giving power to the screen. Does this make sense?

farankoshan · 08/30/2011, 06:29 AM

Ok, still no video and I've included 4 pictures for reference.

img2354w.jpg
Board (cap side)

img2355tj.jpg
Board (back)

img2356p.jpg
My best closeup of the 4.7uF 35v cap, in case any of you spot something we don't.

closeupofproblem.jpg

Closeup of our problem spot
(Everything that's within the yellow border is our problem, front and back. help..

)

Ok, of what I have been able to glean from my fix-it friend, is that if you look at the last photo, the "red teethy port" is not giving any power to the screen. He's tested the light blue cap that it's connected to on the other side, and he says that the current is just fine, so it should be working.
Also, I've highlighted (in a pink rectangle) 5 studs that he wanted me to point out to you guys. I'm not sure why, but he seems to say that there's a problem he can't isolate around these 5 studs. Please meet me halfway in trying to understand what he's saying. He's pretty brilliant, if you ask me, he just doesn't think in English, so let's all try to see how we can make it easier for me to translate what you guys say to him.

Ok, from what I was able to gather from the "dead end" that he's hit, he's asking 2 questions:

1. (See the last photo, pink arrow) What is feeding this port it's power? Where is this port supposed to draw its power from? I think he needs to know the route and source so he can isolate where the problem is.

2. What's the voltage requirement of this red port? He figures if he knows that much, and he can't properly route power to it, he can find a way to cheat and get power there another way. Now whether this is a good idea or not, I hope you all understand that we've tried to test every cap the whole day and nothing is helping.

Any help would be MOST appreciated.

thesteve · 08/30/2011, 12:14 PM

that port provides high voltage AC to a flourescent lamp.
the purple hilighted studs appear to be the transformer/balast for the lamp.
the two 100uf 16V caps next to the transformer provide the power for 2 transistors that drive the transformer.

farankoshan · 08/30/2011, 02:41 PM

big help! thanks thesteve! whats the voltage requirement for that port?

thesteve · 08/30/2011, 05:05 PM

not sure on voltage, likely 600V+ with no load, dropping to around 30V once lamp starts.
the only way to test it is with a lamp.

Charlie · 08/30/2011, 05:27 PM

He is checking for AC, right? Not DC? And the value can be anywhere between 100V and 300V; typically 225V.

Charlie

farankoshan · 08/30/2011, 09:58 PM

Thanks guys! I forward the info to him and...well, we'll see what he can whip up. Anything you guys have been able to gather from the photos? Or if you need specific photos of my board?

farankoshan · 08/31/2011, 03:33 AM

Ok, with your help & some more tinkering, he was able to locate the source of the problem!
When we first opened up the TurboExpress, we noticed a massive amount of leakage all over the board. It was really wet. We thought to simply clean it up and hoped none of the leaks affected any of the other components. While everything still seems alright, he's finally deduced that the transformer for the lamp (the one we've been talking about above) seems to have shorted out. All the traces and components leading up to it, and after it seems fine, but that transformer is what's dead.

Does anyone know where I can source this from? I have a feeling that if we can replace this one piece, my Turbo Express will be back up and running.

Any ideas? Anyone here have the part?

BlueBMW · 08/31/2011, 07:30 AM

Is that transformer just for the flourescent tube for back light? Worst case maybe you could rig up an LED array as a replacement backlight.

thesteve · 08/31/2011, 12:12 PM

yes its just for the backlight.
yes an LED array would work.
id be surprised if its a bad transformer

farankoshan · 08/31/2011, 12:42 PM

I'm really stumped at this point.
But if the backlight lamp was the only problem, wouldn't I be able to see graphics on the screen? Kind of like how the non-backlit Game Boy Color used to look like?

BlueBMW · 08/31/2011, 01:24 PM

Its very hard to see without a backlight because there isn't much behind the screen to reflect light back at you. A flashlight right on it may work but it still wont be easy to see.

farankoshan · 08/31/2011, 01:25 PM

I wish I knew what was wrong with this little (read: ginormous) unit of mine. It's really frustrating to keep hitting dead ends, especially when all of the tricks everyone here has tried with their own TEs have worked, but just not with ours.

Plus, to send it over to any of you more experienced techs may cost more, collectively, than to just buy a brand new one..

farankoshan · 08/31/2011, 09:49 PM

We were able to remove the transformer from the board. Should we:

1. ...put it back and try to buy a new 4.7uF 35v cap in case it was really just a bad cap all along?

2. ...try and look for a transformer piece like this? I'm just worried about getting the voltage wrong or something..

thesteve · 08/31/2011, 09:58 PM

2 things....
1 was there cap juice under the transformer?
if so clean and reinstall.
2 with it installed look for any signs of light on the screen (preferably in a dark room)
if no light at all its the backlight circuit (transformer, transistors, resistors, capacitors and lamp)
the only one i have heard of having a backlight circuit failure, had a damaged trace caused by the guy replacing the caps.
repairing the trace fixed it.

farankoshan · 08/31/2011, 11:11 PM

I just forwarded your helpful tip to him, thesteve. Thanks so much for not giving up. I'm sure we can find what's wrong with this. I wasn't around when he was working, so he might have very well damaged that very trace. I'll ask him to be careful this time around and to see if the trace is damaged, or if he had indeed damaged it.

Will update after we reinstall and reevaluate. Thanks!

Charlie · 09/01/2011, 04:47 AM

It is VERY possible for the xfmr to appear bad if there is capacitor acid on the board, at or near the terminals. The acid is conductive, the backlight power is high voltage and low current; any type of capacitor gunk in the circuit will simply short out the high voltage. I'd make sure the entire area around the transformer and related circuitry is absolutely clean before I'd assume the xfmr is bad.

As a test, you can temporarily connect just the primary (input) side of the xfmr to the circuit, leave the secondary (output) disconnected, and then check the voltage. That will tell you if the xfmr is good or not.

Charlie

farankoshan · 09/01/2011, 08:13 AM

Just got home and spoke to him:

He made a rough map of what the transformer looks like (hope this shed some light for a solution to present itself):

PRIMARY SIDE SECONDARY SIDE
  1 - live 1 - dead
  2 - power supply 2 - dead
  3 - live 3 - dead
  4 - live
  5 - ground

And he assumes that the main power line runs between Primary 2 and Secondary 3. Is this correct?

Does the above make sense to you guys?

thesteve · 09/01/2011, 12:13 PM

the transformer will not have a connection from the primary to secondary.
the way it works is by running pulses of current through the primary coil, magneticly inducing voltage in the secondary.
on most you will also hav a feedback coil in the transformer, used to drive the transistors.
the transistors will short the primary coil to ground at a resonent frequency, driving the circuit.

farankoshan · 09/01/2011, 02:50 PM

I'm sorry, but I think we got a little lost in translation.. :p Your whole explanation went over my head. But by the sound of your answer, it's possible that I might have translated his diagram wrong as well. You see, I'm sure he knows how a transformer works, so it's probably just wording from me (ak! Sorry!)... But what do you think we ought to do at this point? And if possible... In easier English so I can explain it back to him. :s

thesteve · 09/01/2011, 05:05 PM

I was just explaining how the transformer works, and how the circuit works.
I have not traced out the circuit, so i cant go in to detail on what to check.
if i had a schematic, i could walk you through it.

farankoshan · 09/01/2011, 11:17 PM

Does a schematic of this express/gt exist?

thesteve · 09/02/2011, 12:03 AM

i dont know
i could do one, but charlie is better at it

farankoshan · 09/02/2011, 06:44 AM

I think at this point, a schematic seems to be the only way for us to really locate where the problem lies and if it can be remedied. If you or Charlie could post one up here, I'd really appreciate it.

Update: My tech friend who has been working on this unit says he's tried to shine a light onto the screen and he doesn't see any graphics moving on the LCD either. The sound & power works fine, but he doesn't see anything on the screen, even unlit. He thinks that the LCD screen doesn't just lack power from the transformer, but it doesn't even have any graphics coming out of it.

This is getting really deflating..

thesteve · 09/02/2011, 12:14 PM

now that is what the cap issue has always been.

farankoshan · 09/02/2011, 03:56 PM

The Cap Issue... i.e. leaking out to damage other components?

thesteve · 09/02/2011, 05:10 PM

the cap issue is 2 fold.
the caps open up, and eat traces and feedthrough's opening circuits.
the cap juice is slightly conductive, and only likely to directly effect high voltage circuits directly.
the duo is notorious for requiring traces and feedthrough's repaired after the caps are replaced, before they work properly

nat · 09/02/2011, 09:41 PM

Without going back and reading 5 pages, let me ask.... Did you actually replace the video capacitor? If not, why not? It can look "just fine" and not be.

thesteve · 09/02/2011, 09:59 PM

yes its a full cap change. (just looked at the pics)
take a close look at C900 (its a ceramic cap on the back of the board, by the transformer)

farankoshan · 09/04/2011, 02:16 PM

So everything looks fine, doesn't it? This is why it's so damn frustrating.. you guys think it might really be a bad transformer or a bad screen? Or worse, both...?

thesteve · 09/04/2011, 02:43 PM

id suspect you either had no pic or no backlight, not both

farankoshan · 09/04/2011, 02:54 PM

I think it's backlight. As my tech-friend was able to deduce from his tests, the transformer may very well have been shorted out. I think it's safe to say that this little TurboExpress is officially...blind.

I can use this as a big music player I guess...

thesteve · 09/04/2011, 03:08 PM

do you have it with you?
join us in chat sometime

farankoshan · 09/05/2011, 04:49 PM

What do you mean?

Quote from: thesteve on 09/04/2011, 03:08 PMdo you have it with you?
join us in chat sometime

When and what time? Time zone?

thesteve · 09/05/2011, 04:54 PM

i am usa west coast, others are east coast, asia and other places.
you will find knowledgeable people on just about any time you look.

Charlie · 09/07/2011, 06:19 PM

Just out of curiosity, why have you narrowed the problem to the transformer? What did you do to prove it is not the circuit that "runs" the transformer?

Charlie

chop5 · 09/08/2011, 01:23 AM

farankoshan you can send the motherboard and slot only in a small necklace box or something to save on shipping. just pack it good in a static free bag. one of us can assemble it with one of our units and be able to service it. just an idea.

thesteve · 09/08/2011, 04:39 AM

well said chop.
im sure it can be fixed by one of us, i just was trying to save some shipping costs by trying to walk him through it.

farankoshan · 09/08/2011, 02:55 PM

This is a pretty good idea actually.

I will weigh my options and study packing methods.

You guys have been really helpful so far, and I appreciate it!

farankoshan · 09/13/2011, 02:50 PM

Quote from: farankoshan on 09/01/2011, 08:13 AMJust got home and spoke to him:

He made a rough map of what the transformer looks like (hope this shed some light for a solution to present itself):

PRIMARY SIDE SECONDARY SIDE
1 - live 1 - dead
2 - power supply 2 - dead
3 - live 3 - dead
4 - live
5 - ground

And he assumes that the main power line runs between Primary 2 and Secondary 3. Is this correct?

Does the above make sense to you guys?

Does this guy's video relate to the above?

In the middle of the video, he shows a diagram that seems to look like my problem area. Does this make any sense to you guys? You think that this diagram will help me try again to fix Video on my TGExpress?

thesteve · 09/13/2011, 05:13 PM

nope
he is showing just the cap fixes

nat · 09/13/2011, 11:19 PM

Nice how the guy in that video doesn't credit any of the original sources for the fixes.

farankoshan · 09/14/2011, 05:41 AM

He does for a quick bit, but not the ORIGINAL ones... Oh well...

nat · 09/14/2011, 07:58 PM

Yeah I heard him mention some guy who I'd never heard of, "Morse One" or something, not that "Morse One" had any part in discovering these fixes either.

Oh, well is right...

farankoshan · 09/20/2011, 12:54 AM

I just wanted to reawaken the discussion on a video fix:

This thread does really well explaining capacitor switches to help backlit-but-no-video problems, but what about if both video or backlight are out?

farankoshan · 09/23/2011, 12:56 AM

Ok, so as I have been posting earlier on this thread, my TurboExpress isn't giving any video, just audio.

So today, I got myself a bright LED torch and shined it onto the screen and it appears there is video actually being generated & there is movement and sprite work on that screen! So it appears that the lamp is the problem. Any suggestions as to how to get this fixed, or specs so I can go to a hardware shop and get a replacement lamp?

thesteve · 09/23/2011, 05:56 AM

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