stumped on a Turbo Express repair (pics inside)

[thesteve]

they may not be separate
the controller board switches 5V to the screen and switches the backlight supply
check your voltages at the screen connector(posted the pinout some time ago, looking for it now)

[thesteve]

http://www.pcenginefx.com/forums/index.php?topic=4869.msg186903#msg186903

[schweaty]

they may not be separate
the controller board switches 5V to the screen and switches the backlight supply
check your voltages at the screen connector(posted the pinout some time ago, looking for it now)

OK Steve, here's what i have:

1: 0.0V (of course)
2: 1.9V
3: -24.0V (bang on)
4: 0.4V
5: 3.2V
6: 3.1V
7: 3.3V
8: 5.0V  (the first reading was 4.7V but I adjusted the pot to bring it up to 5.0V)

If I can't get this fixed in the next couple of days, I might have to send it back to its owner.  I've already spent 4 days working on it without any change.  It wouldnt be fair to me or the owner to keep it much longer.

Thanks again for all the help so far.

-Dan

[thesteve]

Those numbers look ok except possibly pin4
Can you check with scope, or compare to good one?
Also check your 5v at the 470uf cap for adjustment

[schweaty]

Those numbers look ok except possibly pin4
Can you check with scope, or compare to good one?
Also check your 5v at the 470uf cap for adjustment

so i just checked my good TE voltages using the same game and same ac adaptor.  here's what I got:

1: 0.0V (duh, dan)
2: 3.53V
3: -24V (again... bang on)
4: 0.4V (same as the bad one)
5: 2.9V
6: 2.84V
7: 2.77V
8: 4.8V

so what jumped out at me was pin 2 (brightness) was nearly twice as high as the non-working TE.  Also my RGB pins are all about .5V less than the non-working TE.  that seems like a lot of variance for such low voltages.

what did you mean by check 5V at 470uF cap for adjustment?  is there a pot around there I'm not seeing or just check the voltage at the cap for at least 5V?

-Dan

[thesteve]

the screen 5V should be a bit low, so when you adjust it put meter on a non switched point
card slot, regulator, always live cap ect

[thesteve]

also your pin2 is controlled by the brightness pot

[schweaty]

i adjusted the 5V pin down to 4.8V.  Which one is the brightness pot?  As far as I can tell, the VR501 pot adjusts the 5V pin and the VR501 adjusts the 24V pin.  Neither had an effect on the brightness pin that i could see

[thesteve]

user brightness
by the vol pot

[schweaty]

another, duh-dan, moment for me.  i never really thought of the brightness wheel as a pot, but that is exactly what it is. 

no matter what that is set to (highest or lowest), pin2 only reads 1.8V.  I'm assuming its supposed to go up and down as the bright wheel is adjusted.

[thesteve]

Correct

[schweaty]

so, do you think this is the root cause of the problem (no picture on screen)?  if so, what can i do to fix it?

[thesteve]

It is the likely cause
Confirm the screen works on another board and trace pin2 voltage

[Keith Courage]

I just fixed a TE with this exact same symptom. Turns out that one of the caps on the board that I had changed had a bad trace going to one of the positive ends. Did you double check that all the caps have a voltage of some sort on their positive ends? The one that was causing my issue had no voltage at all. I just double checked each one until I found the culprit.

Not sure if this was already covered in the postings here because I just skimmed them quickly. Just thought I'd mention what I had to do for a TE I had in case anything was overlooked.

[thesteve]

well put KC
thats why i say to check the voltages on the caps
note that sometimes the voltage will be on the negative, so check across the cap, not to ground