PDA

View Full Version : Turbo Express shorted?


ScoreAddict
12-24-2014, 01:26 PM
Hi!

Just got another Turbo Express to fix! Would not power on etc.

At first I thought this was another capacitator-job as the previous one. But this TE had already been re-capped. Not the greatest job but ok.

Colour markings on the new caps also indicated that they had been re-checked some time later. And it became obvious why - the fuse was gone.

Since I didn't trust the cap-job of the previous owner I removed them all, cleaned the pads (still some "fishy smell" there) and put new caps in. I have done this before and it worked just fine! Also in went another fuse (1A, 125V).

Next I put in a game and powered it on. Sound would play, but only a white display. Which started to flicker and then the TE went out again - fuse burned. And Q900 or Q901 (marked DF 0H) - not sure which - got very hot! (Touched it accidentally.) Not good!

I put in another fuse and switched it on - this time without the upper half. Fuse gone again, Q900 or Q901 very hot.

Now I'm pretty sure that I have done a proper job, so I don't think it's the caps. Since I have a second unit I tested the upper half of the Turbo Express (with the display). Works fine, screen and sound are working properly.

Any ideas where to look for the problem? Is this a short? Or could it be a voltage regulator?

Pictures required? Just ask!


Addendum:

Q900/Q901 are transistors (2SD1898) for the backlight!
Niku-Sama
12-25-2014, 05:32 AM
I was about to say it sounds like the fluorescent backlight is trying to pull too much juice probably due to age, sounds like it would be time to try your hand at an LED mod, less current and longer battery life, plus if you are able to replace that LCD wit some sort of drop in you'll get a better looking system overall
ScoreAddict
12-25-2014, 08:03 AM
I was about to say it sounds like the fluorescent backlight is trying to pull too much juice probably due to age, sounds like it would be time to try your hand at an LED mod, less current and longer battery life, plus if you are able to replace that LCD wit some sort of drop in you'll get a better looking system overall

I've just borrowed me another Turbo Express to test the upper half with. And it works just fine. Perfect sound + picture.

So it's not the old lcd/backlight-issue but something (e.g. damaged ic, short) on the lower half's pcb.
APE992
12-26-2014, 02:56 PM
Good news: nothing is shorted directly to ground.

Bad news: it sounds like something is internally damaged. I'd replace those two transistors for kicks (and because they're cheap and potentially damaged by now) and start looking at complex components just in front of them. I'm assuming both have to do with power and something is drawing too much current without drawing so much as to instantly blow the fuse. Might be the transistors, might be something else.

Disclaimer: never had a TurboExpress in front of me before. Ever.
ScoreAddict
12-27-2014, 08:33 AM
Good news: nothing is shorted directly to ground.

Yep. At least that much is obvious.


Bad news: it sounds like something is internally damaged.

Maybe the previous owner put one cap in the wrong way? Or the wrong capacity?


I'd replace those two transistors for kicks (and because they're cheap and potentially damaged by now)

That will be my next step.

I've come across one list of replacement parts where not only the capacitators are listed but also these two transistors. This may indicate that they are prone to fail (under certain circumstances).


I'm assuming both have to do with power and something is drawing too much current without drawing
so much as to instantly blow the fuse.

Interesting question! If these transistors were shorted, would they blow the fuse - but show a completely "white picture" beforehand?


Might be the transistors, might be something else.

Yep. Still no certainty.


Disclaimer: never had a TurboExpress in front of me before. Ever.

Never mind! Any help is appreciated! :)
ScoreAddict
12-30-2014, 03:04 AM
Well, I'm going to replace the two transistors. Fortunately, replacements are still readily available and very cheap.

Let's see if this fixes the issue.
ScoreAddict
01-29-2015, 08:00 PM
It's fixed! :)

The replacement 2SD189s arrived today.

Important info:

- Capacitators AND transistors (on the backlight-circuit) may both fail and have to be replaced!

- The transistors on Q900/Q901 may NOT be replaced with 2SB1260/BCX56!!! PNP- and NPN-transistors are not interchangeable!