Turbo Express Backlight problem

[mrchuck999]

Hi Guys,
1st I want to say, Thanks to everyone. The forum is an invaluable resource. 

Today I have an issue I have not seen before and could benefit from your experience.

Here are the  Details.
Unit started out with no back light. 
I did a Full cap replacement . This did not fix the backlight.
The video, audio etc are all working, its just the backlight not kicking on.

The TE backlight does not light when connected to this TE's main board (fully installed) but if I take the front half and combine it to a known working console, the light works fine.


Using  brand new batteries or a 7.5V power supply, the light would not come on. But.. If I start with the 10V power supply and unplugged it switching to batteries, the light would come on fast and every time .. plus  the batteries could keep the light running. I also noticed if the sound was turned up, I can see a "slight pulsing" in the backlight in beat to the music.      I think the voltage is too low to kick start the florescent lamp.  But adjusting -24V did not help.

I measured -23.8V at the bottom left leg of VR500. I adjusted to -24.1V with no improvement. .

Q: Is there a better place to measure -24V Is this likely to be Q900/901

Q: About Q900, Q901. What voltage do you expect to measure on Q900/901?   
I'm getting 8.5V at the pin 2 of Q900, Q901.  On a known Good TE, I measure only 7.7V.

I have a data sheet I believe to be the correct replacement for Q900/901, but i have not purchased the part yet.

http://www.nxp.com/documents/data_sheet/BF621_623.pdf

Can anyone confirm this is a proper replacement for Q900/901 ?

Thanks in advance for your input.   If you have any questions or want me to measure a point , please let me know, I'll jump right on it.

[Keith Courage]

Were you checking the voltage while the screen was plugged in? I recently had a problem where the screen was bad. The unit would show 24V when the screen wasn't hooked up but then when I plugged it in it would drop to around 15V.

[mrchuck999]

Hey thanks. Good advice. You are right. I was doing all voltage measures with the screen unplugged. I was comparing a known good unit and problem unit but both had the screen unplugged. I'll go check that and update.

[mrchuck999]

I recently had a problem where the screen was bad. The unit would show 24V when the screen wasn't hooked up but then when I plugged it in it would drop to around 15V. 

Keith, Where you measuring your -24V. .. At VR500? The leg closest to the outside-edge is -24V I also found -24 in other spots, but the closer a point is to the end of the chain might change the reading.

[Keith Courage]

I was actually measuring mine on the pin that goes straight to the screens ribbon cable. I think it's the pin 4th from the left. The steve ended up fixing this one. I had no idea that the drop in voltage meant it had a bad screen and I had no spare screens on hand at the time to test with. I thought there was something wrong on the board that was causing the drop in voltage.

[mrchuck999]

Thanks Keith. I swapped out the screen and it acted the same. The questionable screen was put on a good TE and questionable screen lit up perfect.  I did learn some from you all in this. I though the back light was all about the red plug. I see now the screen carries a signal too for the lamp. I took a screen apart a last month (different unit) and as I recall the red plug was clearly running to the lamp. But the screen only boots up when you have all 3 cables plugged in . (red-plug, screen and I think controller board too.  More later after I do some more measurements.

[Fidde_se]

Hi, you all, this comes as a surprise as I'v been working on some Complete Component Guides for a bunch of the PCEs, but been focusing on
TE/GT, and Keith and Steve were the first ones to see it (Test Drivers), anyway this is nearly a schoolbook example of when Q900,901 are bad.
I mean it's not so often this comes up technical questions about Q900,901

And don't even think about BF621/623, it's way off, first the Q900,901 (2SD1898) is NPN and BF621/623 is PNP, Collector Base voltage should be 120v and BF is 250/300, Collector Emitter Voltage should be 80v and BF is 250/300, Emitter Base Voltage should be 5v and BF is.. well 5v, lastly Collector Current DC should be 1A and BF is 50mA!!!!!!, well we should have stopped at the NPN/PNP issue... as other values can still be a little off and working but NPN/PNP can't be mixed... the correct Equivalent of today is..... bigdaddabom.... BCX56
It's common, easy to find and cheap, and a correct replacement.

Drop me a PM and I will send you some BCX56 for free.
These should be removed and/or mounted with a reflower (hotair), but it can be done without, just a regular solderiron, you just have to know
how, I will tell you later..

Of course there can be other components that have gone bad but when it's working with both Audio and Video with dead backlight it's 90%
that it's the drivers, one for each of the half of the high voltage that powers the backlight, even more funny then that I'v
been working on this right now is that I changed both Q900,901 on a GT today that fixed it.

Big parts of my guides are done but been working on different parts on different PCEs, There will be a Complete Text file with all components
, their equivalents, tons of other info, PCB drawings that has never been done in this scale before (Except NECS own secret drawings), there will be PDFs of all the Transistors, ICs, Resistor Networks and other important components and connectors, excluded (PDF wise) are standard capacitors, resistors, electrolytes.

So here is a piece of whats to come,

http://d2069331.u51.surftown.se/Temp/PC_EngineGT/
http://d2069331.u51.surftown.se/Temp/IFU-30/

The drawings of TE/GT is done, Text half assembled and PDFs not up.
The drawings of IFU-30 is incomplete, Text are done and PDFs not up.

They will be in their own Topic when fully complete.
There will also be Complete Components Guides for PCE(White), CoreGrafx I&II, CD-ROM2 and Duo-R but they will take some more time.
If some one were to donate a working or non working PCE that I don't have they will be done too.

[mrchuck999]

Hi Fidde_se,
I would be very interested in trying this out.  I PM'ed you.  let me know what you need. If you can send to the USA, let me know and I'll send you my mailing address.

-Mrchuck999

[mrchuck999]

Hi Fidde_se,
Your documentation is great! Thanks for sharing it. We really need an image showing all the traces or a schematic . Man the things we could do with a schematic.

I think there are already people working on this but I plan to develop an LED back light. This will save TE's with failed ballast or failed bulbs. I don't see them often but this week I've got one and a friend has one too. Last month I saw a TE that was dim on one side. I examined the bulb and saw the white phosphor coating was gone on end. It was minor but still its not looking good long term for these lamps.

[mrchuck999]

Can anyone verify this is a match for Q502. This is a PNP
I ran a search on the number printed on the part B1182 R H
Result- 2DB1182Q /  2DB1182Q-13 /

http://www.diodes.com/datasheets/2DB1182.pdf

[Fidde_se]

Hi yes it's a match, the R and H could have been production code (week/batch), as of mid 90s and later nearly all transistors have it/should have it, but it means ROHM, it's the manufacturer.

this line will be added:
Q502 2SB1182 SMD SC-63/TO-252/DPAK (PNP 32V 2A) Manufactured by ROHM, Equivalent 2SD1758 or 2DB1182 Manufactured by Diodes Inc

PDF here:
http://d2069331.u51.surftown.se/Temp/PC_EngineGT/2SB1182.pdf

[mrchuck999]

EDIT.. ..  The voltage  measurements are the same for both of these in the sheets

http://rohmfs.rohm.com/en/products/databook/datasheet/discrete/transistor/bipolar/2sb1182.pdf

http://www.diodes.com/datasheets/2DB1182.pdf

[Fidde_se]

Hi, yes I noticed, but it just means "up to", some components are made specifically for negative voltage like the 79XX (7905) voltage regulators vs positive 78XX (7805), but as for transistors in AC, the sinus wave gotta go threw the 0 at some point, negative voltage isn't equal to negative energy but as your edit shows you already figured that out, otherwise I would like to know what exactly -2A would mean, negative ampere that would be a new one =)

Have you fixed some BCX56 or would you like me to send you some?

[mrchuck999]

Have you fixed some BCX56 or would you like me to send you some?

I found the part locally to me and already picked them up. Thanks anyway.

[thesteve]

q900 and q901 run the backlight transformer
the transistors seem ok as they can maintain the lamp.
CCFL lamps require much higher start voltage then run voltage
any arcing or contamination will prevent lamp start