REPAIR GUIDE - TurboExpress/PC Engine GT: Video Fix

[farankoshan]

I wish I knew what was wrong with this little (read: ginormous) unit of mine. It's really frustrating to keep hitting dead ends, especially when all of the tricks everyone here has tried with their own TEs have worked, but just not with ours.

Plus, to send it over to any of you more experienced techs may cost more, collectively, than to just buy a brand new one..

[farankoshan]

We were able to remove the transformer from the board. Should we:

1. ...put it back and try to buy a new 4.7uF 35v cap in case it was really just a bad cap all along?

2. ...try and look for a transformer piece like this? I'm just worried about getting the voltage wrong or something..

[thesteve]

2 things....
1 was there cap juice under the transformer?
if so clean and reinstall.
2 with it installed look for any signs of light on the screen (preferably in a dark room)
if no light at all its the backlight circuit (transformer, transistors, resistors, capacitors and lamp)
the only one i have heard of having a backlight circuit failure, had a damaged trace caused by the guy replacing the caps.
repairing the trace fixed it.

[farankoshan]

I just forwarded your helpful tip to him, thesteve. Thanks so much for not giving up. I'm sure we can find what's wrong with this. I wasn't around when he was working, so he might have very well damaged that very trace. I'll ask him to be careful this time around and to see if the trace is damaged, or if he had indeed damaged it.

Will update after we reinstall and reevaluate. Thanks!

[Charlie]

It is VERY possible for the xfmr to appear bad if there is capacitor acid on the board, at or near the terminals.  The acid is conductive, the backlight power is high voltage and low current; any type of capacitor gunk in the circuit will simply short out the high voltage.  I'd make sure the entire area around the transformer and related circuitry is absolutely clean before I'd assume the xfmr is bad.

As a test, you can temporarily connect just the primary (input) side of the xfmr to the circuit, leave the secondary (output) disconnected, and then check the voltage.  That will tell you if the xfmr is good or not.

Charlie

[farankoshan]

Just got home and spoke to him:

He made a rough map of what the transformer looks like (hope this shed some light for a solution to present itself):

PRIMARY SIDE                   SECONDARY SIDE
  1 - live                                1 - dead
  2 - power supply                   2 - dead
  3 - live                                3 - dead
  4 - live
  5 - ground

And he assumes that the main power line runs between Primary 2 and Secondary 3. Is this correct?

Does the above make sense to you guys?

[thesteve]

the transformer will not have a connection from the primary to secondary.
the way it works is by running pulses of current through the primary coil, magneticly inducing voltage in the secondary.
on most you will also hav a feedback coil in the transformer, used to drive the transistors.
the transistors will short the primary coil to ground at a resonent frequency, driving the circuit.

[farankoshan]

I'm sorry, but I think we got a little lost in translation.. :p Your whole explanation went over my head. But by the sound of your answer, it's possible that I might have translated his diagram wrong as well. You see, I'm sure he knows how a transformer works, so it's probably just wording from me (ak! Sorry!)... But what do you think we ought to do at this point? And if possible... In easier English so I can explain it back to him. :s

[thesteve]

I was just explaining how the transformer works, and how the circuit works.
I have not traced out the circuit, so i cant go in to detail on what to check.
if i had a schematic, i could walk you through it.

[farankoshan]

Does a schematic of this express/gt exist?

[thesteve]

i dont know
i could do one, but charlie is better at it

[farankoshan]

I think at this point, a schematic seems to be the only way for us to really locate where the problem lies and if it can be remedied. If you or Charlie could post one up here, I'd really appreciate it.

Update: My tech friend who has been working on this unit says he's tried to shine a light onto the screen and he doesn't see any graphics moving on the LCD either. The sound & power works fine, but he doesn't see anything on the screen, even unlit. He thinks that the LCD screen doesn't just lack power from the transformer, but it doesn't even have any graphics coming out of it.

  This is getting really deflating..

[thesteve]

now that is what the cap issue has always been.

[farankoshan]

The Cap Issue... i.e. leaking out to damage other components?

[thesteve]

the cap issue is 2 fold.
the caps open up, and eat traces and feedthrough's opening circuits.
the cap juice is slightly conductive, and only likely to directly effect high voltage circuits directly.
the duo is notorious for requiring traces and feedthrough's repaired after the caps are replaced, before they work properly