REPAIR GUIDE - TurboExpress/PC Engine GT: Video Fix

[farankoshan]

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

[nat]

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]

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]

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]

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

[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