REPAIR GUIDE - TurboExpress/PC Engine GT: Video Fix

[RetroWeirdo]

I've seen something similar happen when the 1 10uf cap pads get ruined but not exactly the same. Also, are you by chance using a generic ac adapter? Does the same thing happen on batteries? I wonder if this is the sign of an actual bad screen.

The burn in on the screen is completely normal.

I will recheck the 10uf again. No I first tried this unit reassembled on batteries and noticed the problem, since then I have been trouble shooting it partially disassembled with power from a 1st party NEC TE AC adapter. That would kinda suck if the screen has a legit problem, Though it might be nice to upgrade to a newer LCD.

[thesteve]

Looks like something isn't getting the correct voltage. Make sure there are not any broken traces to any of the caps you installed.

I'm getting 7.8v from an AC adapter.
When unit powers on it drops to 3.8v
Checked some the caps, most read about 3.5-3.7v
Measured on pin 3 of the display ribbon cable only get -2.5v

Anything else I should check and where does the 24v start ?

Thanks

if your supply is dropping below 6V it wont work

[hard_gb]

Please, the images in the post are gone. I need them to make the modification. Thank you
 
 

It's long been known that by replacing all the capacitors in the audio circuit on the Express PCB you are able to cure all sound issues, but video issues have been a bit of a grey area. What has been unknown (until now) is just which component is directly linked to the common video problem these units are beginning to have. Let's give a round of applause to Sinistron, the real hero in all this, for donating a TurboExpress with bad video for me to butcher on the operating table. It was a very long and tedious process, as I ended up replacing almost everything on the board by the time I was done and I had to play test after every component which meant quite a bit of assembly and disassembly. The culprit turned out to be the smallest, most innocuous capacitor on the entire board and the only reason it wasn't the very last one I did was because there was one cap I didn't have a proper uF/voltage match for. At the very least all this paid off and finally put to rest just what causes the no video issue. Real quick: the capacitor in question is in the power circuit, specifically, the line that supplies the mains to the LCD. This cap fails, dries out, bleeds, etc causing insufficient voltage supplied to run the LCD.

So let's get down to it.... I took photos documenting the entire process, but I'll only post the relevant ones here.


Circled in green is the audio circuit. We already know what these caps are for, so we ignored them for this procedure.
Circled in red is the culprit for the video issue. You can barely see it, can't you?


Since I didn't know where the problem was specifically, I removed the shield covering the CPU, VDC, etc. The caps circled in blue are the ones that are exposed by removing the shield. It is not necessary to replace these to fix the video, but Sinistron got a freebie on this one. I included this pic so folks who might want to replace EVERYTHING can see just what's under the shield.


Circled in red is the replacement cap I installed in place of the bad on. The cap is a 4.7uF 35-volt electrolytic. This is THE culprit of the backlight-but-no-picture video issue. Replace it and your video works again.


Just a final look at the board after I was done with it. I had to run wires to remote-mount a few of the replacement caps due to not enough room on the board, etc. FYI, there is plenty of open space down behind where the batteries sit where you can remote-mount caps if you need to.

There you have it. It's a simple fix, really, and luckily it's probably the most easily accessible cap on the entire board.

[MasterofBiscuits]

I believe the cap being referred to below is CC501 which is at the top of the board. This info is outdated though, you need to replace all the caps as they will leak and corrode the PCB.

Please, the images in the post are gone. I need them to make the modification. Thank you
 
 

It's long been known that by replacing all the capacitors in the audio circuit on the Express PCB you are able to cure all sound issues, but video issues have been a bit of a grey area. What has been unknown (until now) is just which component is directly linked to the common video problem these units are beginning to have. Let's give a round of applause to Sinistron, the real hero in all this, for donating a TurboExpress with bad video for me to butcher on the operating table. It was a very long and tedious process, as I ended up replacing almost everything on the board by the time I was done and I had to play test after every component which meant quite a bit of assembly and disassembly. The culprit turned out to be the smallest, most innocuous capacitor on the entire board and the only reason it wasn't the very last one I did was because there was one cap I didn't have a proper uF/voltage match for. At the very least all this paid off and finally put to rest just what causes the no video issue. Real quick: the capacitor in question is in the power circuit, specifically, the line that supplies the mains to the LCD. This cap fails, dries out, bleeds, etc causing insufficient voltage supplied to run the LCD.

So let's get down to it.... I took photos documenting the entire process, but I'll only post the relevant ones here.


Circled in green is the audio circuit. We already know what these caps are for, so we ignored them for this procedure.
Circled in red is the culprit for the video issue. You can barely see it, can't you?


Since I didn't know where the problem was specifically, I removed the shield covering the CPU, VDC, etc. The caps circled in blue are the ones that are exposed by removing the shield. It is not necessary to replace these to fix the video, but Sinistron got a freebie on this one. I included this pic so folks who might want to replace EVERYTHING can see just what's under the shield.


Circled in red is the replacement cap I installed in place of the bad on. The cap is a 4.7uF 35-volt electrolytic. This is THE culprit of the backlight-but-no-picture video issue. Replace it and your video works again.


Just a final look at the board after I was done with it. I had to run wires to remote-mount a few of the replacement caps due to not enough room on the board, etc. FYI, there is plenty of open space down behind where the batteries sit where you can remote-mount caps if you need to.

There you have it. It's a simple fix, really, and luckily it's probably the most easily accessible cap on the entire board.

[Cassius]

Hi, I’m currently working on a Pc engine gt that I find in my spare parts drawer, the voltages were all over the place and it had suffered heavy cap leakage, a recap hadn’t moved me any father until I noticed that d502 was missing off the board as 2 out of its 3 pads were dissolved, I’ve ordered the 1ss184 as recommended and  I think I’ve identified the points where I can bypass it but can anyone confirm where the points are ? Thanks in advance.