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Old 10-05-2021, 04:41 PM   #1
WaldoX
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Default PC Engine GT - No Video or Backlight

Hello everyone, I am obviously new here, but this seems like a good community to try and get some assistance. I have done a lot of googling already, but have not come across a thread with my exact issue.

I recently purchased a PC Engine GT from Japan. When I first received it and tested on batteries, there were no signs of life at all. Now, I wish I had left it running for a minute to see if any sound started (I have one game to test with), but I didn't. I flipped the power switch, saw no signs of life, and turned it back off. I wish I had played with the volume and brightness wheels, but I can't say that I did. I know, I did a terrible job of inspecting it's operation, but I expected to have to recap it anyway, so I think that's why I glossed over it. My bad! One of the battery terminals had quite a bit of corrosion, so I also ordered an AC Adapter and tried with that once I got it, but same results (and same crappy testing on my part).

I did a full recap last weekend, and when I first tested the system after finishing, it appeared to still be dead, which was quite soul crushing. Luckily I left it running, and after 20 or 30 seconds music started playing! Nice and loud too! It also seems like the game responded to me pressing start. Sadly though, the screen doesn't do anything at all. Not a flicker. No video, no backlight.

Seems like maybe it's not getting power? I have a multi-meter, but I'm quite new to all of this and would love a little help in knowing where to test for voltage to the screen, and any other advice you folks may have?

P.S. While most of the old caps had corrosion around their legs, none of them had spilled their contents and damaged anything around them. I got the old caps off, and cleaned up the pads and all looks good to me.

Thanks again!
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Old 10-05-2021, 06:35 PM   #2
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Default Re: PC Engine GT - No Video or Backlight

There some devices that do not start up as soon as you push the power button turn it on let it boost up and then power it down wait 30 seconds or more and do this several times and see if you get the same exact results each time if you do then it might be the way this device behaves and there might not be anything wrong with it
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Old 10-06-2021, 06:27 AM   #3
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Default Re: PC Engine GT - No Video or Backlight

there is another thread about a bad backlight in one of these.
it uses a boost convertor and a mini flourescent tube.
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Old 10-06-2021, 07:49 AM   #4
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Default Re: PC Engine GT - No Video or Backlight

Quote:
Originally Posted by stj View Post
there is another thread about a bad backlight in one of these.
it uses a boost convertor and a mini flourescent tube.
Thank you, I'll look for that thread, but I have to ask: If it was just a bad backlight, wouldn't I still be able to see a faint video image?

I get nothing at all, just a black screen, which is what makes me think maybe the screen isn't getting power at all. I'm going to try and check that with my multi-meter, but I don't exactly know where to place the leads. I should be able to figure out at least that much with more googling though. :-)
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Old 10-06-2021, 07:51 AM   #5
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Default Re: PC Engine GT - No Video or Backlight

Quote:
Originally Posted by sam_sam_sam View Post
There some devices that do not start up as soon as you push the power button turn it on let it boost up and then power it down wait 30 seconds or more and do this several times and see if you get the same exact results each time if you do then it might be the way this device behaves and there might not be anything wrong with it
I'm afraid you've misunderstood me, and that's probably my fault for rambling so long, but the screen NEVER comes on. Not a flicker, no backlight, no game. NOTHING. NOT EVER. Doesn't matter how long I wait. That certainly isn't intended behavior. :-)
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Old 10-06-2021, 04:23 PM   #6
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Default Re: PC Engine GT - No Video or Backlight

you need to run it while looking at the back of the display unit to see if the light appears to be on.
it can also be a bad display signal OR a bad contrast issue.
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Old 10-06-2021, 06:45 PM   #7
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Default Re: PC Engine GT - No Video or Backlight

Quote:
Originally Posted by WaldoX View Post
Thank you, I'll look for that thread, but I have to ask: If it was just a bad backlight, wouldn't I still be able to see a faint video image?
Depends on the LCD screen. Some will let a little light through that can then reflect back and allow you to see a very faint image. But some can't.

Quote:
Originally Posted by WaldoX View Post
I get nothing at all, just a black screen, which is what makes me think maybe the screen isn't getting power at all. I'm going to try and check that with my multi-meter, but I don't exactly know where to place the leads. I should be able to figure out at least that much with more googling though. :-)
Or perhaps help us help you by showing us what you're seeing.
That is, if possible, upload some pictures of your device - both external and internal / of the PCBs. (Please use badcaps.net attachment feature for the images, so that they don't get lost overtime to off-site hosts.) It's possible than one of us here might see something that isn't obvious. Also, it gives us a better idea of what the device looks like on the inside. I personally had never even heard of the PC Engine GT before and had to look it up (thanks Wikipedia! ) On that note, even the Wikipedia article mentions that many of these handhelds had issues with sound and display due to cheap capacitors and early LCD technology.
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Old 10-07-2021, 08:43 AM   #8
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Default Re: PC Engine GT - No Video or Backlight

Quote:
Originally Posted by momaka View Post
Depends on the LCD screen. Some will let a little light through that can then reflect back and allow you to see a very faint image. But some can't.


Or perhaps help us help you by showing us what you're seeing.
That is, if possible, upload some pictures of your device - both external and internal / of the PCBs. (Please use badcaps.net attachment feature for the images, so that they don't get lost overtime to off-site hosts.) It's possible than one of us here might see something that isn't obvious. Also, it gives us a better idea of what the device looks like on the inside. I personally had never even heard of the PC Engine GT before and had to look it up (thanks Wikipedia! ) On that note, even the Wikipedia article mentions that many of these handhelds had issues with sound and display due to cheap capacitors and early LCD technology.

Oh man.... I have to apologize. I was researching this issue for a couple of days before I decided to post for help. I was so absorbed in this problem, that I got tunnel vision and just assumed (subconsciously) that everyone at the several forums where I found related posts (including this one) knew all about this device!

I really appreciate the nice way you've brought this to my attention, and I will get some pictures uploaded just as soon as I have a chance (at work all day).

And to stj, thanks for mentioning that other thread, I think I found it, and I'll be testing some voltagles as mentioned in this post. (I think that thread was one of the ones that brought me to this forum in the first place )
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Old 10-07-2021, 09:20 PM   #9
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Default Re: PC Engine GT - No Video or Backlight

Ok, finally had a chance to tear this thing apart again. :-)

For anyone who doesn't know, but wants to, here is the device in question. It's a portable version of the PC Engine, which is the Japanese version of what we called the TurboGrafx 16 in the US. It's from 1991 and is notorious for bad capacitors, much like the Sega Game Gear.

Here's a look at the 2 PCBs and Screen. I've already replaced all of the electrolytic capacitors. There's also one cap on the other side of the button pcb, but we going to be focusing on the main board.


(click here to enlarge)


(click here to enlarge)

I did test some voltages as mentioned in a previous post:
Quote:
Originally Posted by OtherThread
Also, try for the following voltages at these Test points:
TP500 30v
TP501 -24v
TP502 5v (ish, you will see it from 4.8 to 5v depending on factory)

The 2x Variable Resistors adjust the supply rails:
VR500 LCD Voltage Adjust (-24v)
VR501 System 5v Adjust
Here's what I found:
  • TP500 30.1
  • TP501 -24.3
  • TP502 4.91
  • VR500 -23.1
  • VR501 3.1

None of those seem off by too much except for VR501, but I don't know enough about any of this to know if that's a problem. HOWEVER, I also tried to probe the power connector for the screen directly, and cannot get any sort of voltage reading out of it!

Here's a closeup.
And here's the other side.

It's the red connector I'm talking about. I assume I should get -24v from it, but I get nothing. I also don't get anything from TP901 which is next to it, but I also don't know what I should get there. Also notice what might be cap fluid lurking around? Do you think there could be damage under those grey blobs that is preventing those pins from getting power?

Any advice is appreciated! Thanks!
Attached Images
File Type: jpg main_small.jpg (110.1 KB, 6 views)
File Type: jpg trio.jpg (567.7 KB, 10 views)
File Type: jpg side1_thumb.jpg (30.7 KB, 52 views)
File Type: jpg side1.jpg (872.1 KB, 10 views)
File Type: jpg side2_thumb.jpg (34.2 KB, 54 views)
File Type: jpg side2.jpg (914.0 KB, 11 views)
File Type: jpg closeup1.jpg (484.1 KB, 6 views)
File Type: jpg closeup2.jpg (465.2 KB, 5 views)
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Old 10-08-2021, 04:36 PM   #10
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Default Re: PC Engine GT - No Video or Backlight

I did some more voltage checking. This time of Capacitors and Vias. I seem to have a problem at CC503. See pictures.

I'll be removing that cap to see what is happening underneath it. Any other advice? Thanks!
Attached Images
File Type: jpg corner_caps_front.jpg (173.4 KB, 5 views)
File Type: jpg corner_caps_back.jpg (193.1 KB, 32 views)
File Type: jpg side_caps_front.jpg (182.7 KB, 7 views)
File Type: jpg side_caps_back.jpg (185.3 KB, 9 views)
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Old 10-08-2021, 05:13 PM   #11
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Default Re: PC Engine GT - No Video or Backlight

I may have found the problem!!!! I haven't had a chance to remove that cap yet, but I realized I had taken some pictures during the recap, so I went back and found a picture of the pads for CC503. There is CLEARY corrosion on the bottom 1/3, and that seems to be exactly where it connects to the trace. I've very new to all of this, and probably thought since 2/3 of the pad looked good, all would be well. Lesson learned!



The pads looks scratched up from me trying to remove some of the corrosion with my tweazers and some deoxit. I suspect if I can't remove more of that corrosion I'll need to either scrape away some of the solder mask at the edge of the trace and connect to that? Or can I run a jumper over to the via? Thoughts?

Thanks!
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Old 10-09-2021, 06:05 PM   #12
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Default Re: PC Engine GT - No Video or Backlight

Ugh, I knew I shouldn't have been celebrating so soon, but damnit it felt good. Turn out, it was just my ignorance leading me to incorrect assumptions.

I was looking for -24V on that cap, but I don't really know what "negative voltage" is, so I didn't realize I needed to check for the voltage on the negative side of the cap. It was there all along. In fact, when I removed the cap, I found fairly pristine pads underneath; turns out I took that picture before I went back and cleaned up the pads. Here's what they actually look like:



I've looked at this board up and down a magnifying glass and I still have no idea why there's no power here:



Do you think I need to scrape away the grey goo, and is that safe?

Thanks

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File Type: jpg clean_pads.jpg (42.4 KB, 51 views)
File Type: jpg no_power.jpg (127.0 KB, 50 views)
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Old 10-09-2021, 11:50 PM   #13
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Default Re: PC Engine GT - No Video or Backlight

I'll write a more proper reply later/tomorrow or Monday, since it's early AM hours here... but just FYI, that red connector is probably for supplying power to the CCFL backlights of the LCD. IDK what voltage and current those would require with this being a handheld device, but even small CCFLs in 13-15" LCD monitors still run at 500-700V AC. So I expect the small CCFLs in this screen to still be running at relatively high voltage and probably a few mA of current or less (with total power possibly under or around a Watt.)

For this reason, don't expect to be able to measure any voltage on the red connector - not with a regular multimeter anyways. The voltage will likely be high voltage and high frequency AC. It can even damage some cheap multimeters.

And no, DON'T scrape the gray "goo" (silicone sealant). It's there to insulate the high voltage from the CCFL inverter output from jumping/arcing to anything else on the board. Only remove it if that blue cap appears to be damage or shorted... but I doubt it (it would have visible damage.)

So just as a brief answer, since the backlights are not going up, this may turn into a CCFL inverter troubleshooting. I'll take a look at the posted images and whatnot in more detail tomorrow or Monday. But in the mean time, if you want to get some ideas or just something to think about, perhaps have a quick look through this thread about troubleshooting LCD monitor inverters:
https://www.badcaps.net/forum/showthread.php?t=10419

Last edited by momaka; 10-09-2021 at 11:54 PM..
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Old 10-10-2021, 03:00 AM   #14
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Default Re: PC Engine GT - No Video or Backlight

look inside the metal can
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Old 10-11-2021, 09:28 AM   #15
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Default Re: PC Engine GT - No Video or Backlight

Quote:
Originally Posted by momaka View Post
I'll write a more proper reply later/tomorrow or Monday, since it's early AM hours here... but just FYI, that red connector is probably for supplying power to the CCFL backlights of the LCD. IDK what voltage and current those would require with this being a handheld device, but even small CCFLs in 13-15" LCD monitors still run at 500-700V AC. So I expect the small CCFLs in this screen to still be running at relatively high voltage and probably a few mA of current or less (with total power possibly under or around a Watt.)

For this reason, don't expect to be able to measure any voltage on the red connector - not with a regular multimeter anyways. The voltage will likely be high voltage and high frequency AC. It can even damage some cheap multimeters.

And no, DON'T scrape the gray "goo" (silicone sealant). It's there to insulate the high voltage from the CCFL inverter output from jumping/arcing to anything else on the board. Only remove it if that blue cap appears to be damage or shorted... but I doubt it (it would have visible damage.)

So just as a brief answer, since the backlights are not going up, this may turn into a CCFL inverter troubleshooting. I'll take a look at the posted images and whatnot in more detail tomorrow or Monday. But in the mean time, if you want to get some ideas or just something to think about, perhaps have a quick look through this thread about troubleshooting LCD monitor inverters:
https://www.badcaps.net/forum/showthread.php?t=10419

Thank you for taking the time to post all of that! It definitely reinforces the idea that I keep making mistakes because of my own ignorance! I will study this in more detail, and next time I have a chance to work on it, hopefully something here will lead me in the right direction.


Quote:
Originally Posted by stj View Post
look inside the metal can
I'll do that next time I work on it. May not be until the weekend. :-(
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Old 10-12-2021, 09:22 PM   #16
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Default Re: PC Engine GT - No Video or Backlight

OK, I looked at the board a little more carefully, but still nothing obviously wrong stands out here.

So perhaps the most logical approach here would be to start with the inverter and see why the backlights are not turning On. This is going to be a bit challenging figuring out what components belong to the inverter on this board, due to it being multi-layer... but I think Q900 and Q901 should be the main transistors used for driving the backlights inverter transformer, T900.

One test you could attempt (if possible only) is to see what voltage each of the 3 pins on these transistors read relative to ground... though if that is not possible, then remove batteries and try testing these transistors (Q900 and Q901) with your multimeter on diode test. Run these combinations of diode/forward voltage testing so that we can hopefully figure out what they are (I imagine BJTs, but could be wrong):
- Pin 1 to 2 with red (+) multimeter probe on pin 1
- Pin 1 to 3 with red (+) multimeter probe on pin 1
- Pin 1 to 2 with black (-) multimeter probe on pin 1
- Pin 1 to 3 with black (-) multimeter probe on pin 1
- Pin 2 to 3 with red (+) multimeter probe on pin 1
- Pin 2 to 3 with black (-) multimeter probe on pin 1

Report back what values you get.

You can also test other transistors (labeled with Q) this way in the area if you like. Make note in particular if you see any low readings, like less than 100 mV (or 0.1V, whichever scale your MM uses.)

Another test you can try: scape the gray silicone sealant very minimally right where you see the text "Q902" and above the "2" in particular. Scrape only a little to allow you to touch/probe the solder joint + pin under it. Once you do that, set your multimeter to 2000 Ohms scale (or if it's auto-ranging, don't worry) and measure resistance between that and the two smaller pins directly above it right under the letter "R" from the label "R900". These three pins should be the high-voltage side output of the inverter, and generally those will measure at least a few hundred Ohms. Of course, do this measurement with the power off and batteries disconnected (this is standard for all resistance measurements.)

Also, could we see another picture with more detail in the area of the upper-right corner of this picture:
https://www.badcaps.net/forum/attach...1&d=1633662298

I think only components with 9xx numbers are related to the inverter, so I don't think T500 and surrounding components have anything to do with it... but would be good to double-check anyways.
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Old 10-13-2021, 07:35 AM   #17
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Default Re: PC Engine GT - No Video or Backlight

Hello and THANK YOU for the reply! I will try my best to do these tests next time I get to work on it, but I'm honestly leaning towards just doing a modern screen mod and bypassing that old backlight power circuit. I just wish I knew how to verify that the system is generating a video signal.... any thoughts on that?

Quote:
Originally Posted by momaka View Post
Also, could we see another picture with more detail in the area of the upper-right corner of this picture:
https://www.badcaps.net/forum/attach...1&d=1633662298
This picture seems to show that region, is that good enough?


Actually, I've already ordered the screen mod kit (I'm impulsive and impatient), so I'll be trying it either way, but I could save myself some grief if I could verify the video works first. :-)
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Old 10-13-2021, 07:51 AM   #18
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Default Re: PC Engine GT - No Video or Backlight

it doesnt have a video signal, the display is fed a clocked raster from the custom Hudson chipset

maybe you can just mod the backlight to use leds
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Old 11-24-2021, 04:52 PM   #19
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Default Re: PC Engine GT - No Video or Backlight

Just wanted to update and say that my cheap chinese soldering iron has started falling apart, so I'm waiting for a new one before I attempt this screen mod.

I will update this thread though once I get around to the mod. I won't lie, this mod scares me quite a bit, it'll be some of the smallest work I've done yet. Wish me luck!
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