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NEC TG-16/TE/TurboDuo => TG/PCE Repair/Mod Discussion => Topic started by: maded2 on October 05, 2022, 11:25:41 AM

Title: PC Engine GT - showing only 3.1V on the 5V (Q502) output
Post by: maded2 on October 05, 2022, 11:25:41 AM
hi all,
    Trying to fix my friend's PC Engine GT (recapped few years ago).   It has stopped working, the display backlight is on, no game is running.  So I looking at it for him.  I power up using main adaptor (7.2V) input.  I see 30V on TP500, -24V on TP501,  7.6V on TP503.  But when I checked the 5V output (Q502) I only get 3.1V.  I tried changing VR501 with no effect.

   I then look for possible shorts and found C900 appears to be shorted.  But I read on another post here saying it should fine.

So any expert here can give me any hints to what to look for? or possible component to tests.   With only 3.1V, I can't see the CPU will boot.

thanks,
eddie
Title: Re: PC Engine GT - showing only 3.1V on the 5V (Q502) output
Post by: Keith Courage on October 08, 2022, 06:26:11 AM
The transistor at 900 can actually be removed as it just controls the backlight. Meaning, if the transistor was bad then you should still hear game music.

What does the screen look like? Is it solid white?
Title: Re: PC Engine GT - showing only 3.1V on the 5V (Q502) output
Post by: maded2 on October 08, 2022, 09:15:25 PM
Quote from: Keith Courage on October 08, 2022, 06:26:11 AM
The transistor at 900 can actually be removed as it just controls the backlight. Meaning, if the transistor was bad then you should still hear game music.

What does the screen look like? Is it solid white?

hi, yes just solid white screen.  So not getting full 5V, I don't think the CPU is running.  Any idea what to check related to the 5V?



thanks.
eddie
Title: Re: PC Engine GT - showing only 3.1V on the 5V (Q502) output
Post by: Keith Courage on October 09, 2022, 06:56:19 AM
What voltage is the main Voltage regulator putting out? I can't remember the part number on it but it should be at 7V.

(https://i.postimg.cc/jdtddghm/759px-NEC-Turbo-Express-Motherboard-Top-2x.jpg)
Title: Re: PC Engine GT - showing only 3.1V on the 5V (Q502) output
Post by: maded2 on October 09, 2022, 07:29:16 AM
I read Q502 regulator is B1182, I read 3.1V when power on.  I believe it normally read 5V

thanks,
eddie
Title: Re: PC Engine GT - showing only 3.1V on the 5V (Q502) output
Post by: MasterofBiscuits on October 10, 2022, 03:40:57 PM
Quote from: maded2 on October 09, 2022, 07:29:16 AM
I read Q502 regulator is B1182, I read 3.1V when power on.  I believe it normally read 5V

thanks,
eddie

Yes the large pad should be 5v as that is the output. It sounds like you may have a short in the circuit ahead of the regulator. The backlight circuit is powered from here so it could be that.

One thing you can try is desoldering the BLH jumper at the top right of the board (next to the green cube component), then test the voltage again. This will disconnect the backlight circuit and may show you if that's the problem. Also check VR501, IIRC this pot that can control the output voltage.
Title: Re: PC Engine GT - showing only 3.1V on the 5V (Q502) output
Post by: Keith Courage on October 11, 2022, 11:04:45 PM
That's correct big pad is 5v on the regulator. My mistake. One of the legs of the voltage regulator is almost 7V. Same as Power switch.