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Old 13th Dec 2009, 3:08 pm   #1
Sam
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Default Beovision L2502/LX2500 Field Scan problem

Hi All, many moons ago I acquired a Beovision L2502 and started to get it working. In the end I failed and it was put to one side. Recently I was given a LX2500 for parts so decided to have another go.

Swopping all boards from the LX2500 into the L2502 has resulted in a working set, but it exhibits an odd problem with the field (vertical) scan. The bulk of the picture is fine, but a narrow band at the top and bottom of the picture keeps randomly changing in height. These bands expand and compress vertically, but the rest of the picture is unaffected. It happens on RF and SCART input signals.

Does anyone recognise this poor description of the problem? I can upload a picture of it, but I don't think it will provide much help. I have the B&O service book if anyone needs a scan of the pages to look at the circuits.

Thanks,

Sam
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Old 13th Dec 2009, 3:29 pm   #2
Peter.N.
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Default Re: Beovision L2502/LX2500 Field Scan problem

Sounds like dry joints possibly on the field timebase IC, try resoldering all of them, could of course be a dry joint elswhere in the field circuit but most likely the IC.

Peter
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Old 14th Dec 2009, 8:59 am   #3
Michael Maurice
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Default Re: Beovision L2502/LX2500 Field Scan problem

Check/replace all the electrolytic capacitors in the frame stage including the small ones.

I'm not sure that the timebase/power boards are interchangeable, so you might have created the problem.

The frame IC does go D/J which can cause odd faults but a sharp tap should show that up.

What was wrong with the original board?

Michael
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Old 14th Dec 2009, 8:00 pm   #4
Sam
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Default Re: Beovision L2502/LX2500 Field Scan problem

Sorry if I wasn't clear: the entire LX2500 innards were transplanted.

looks like I will just keep going through replacing caps and re-flowing joints then. I assume on the PSU and O/P board AND the oscillator board for the frame scan? Can a joint be good and then go dry?

As an aside, the original had an intermittent frame collapse for only one half of the screen (one half OK, other half collapsing). I'd tried transplanting the frame output chip, and succeeded in burning a portion of the board by shorting live and neutral on the PSU board with an oscilloscope probe. Flash, bang, and smoke!

Sam
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Old 15th Dec 2009, 12:17 am   #5
Welsh Anorak
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Default Re: Beovision L2502/LX2500 Field Scan problem

Hi
The fact that both boards have a frame problem implies there may be a fault somewhere else - possibly the original board was doing this odd thing until the IC (or whatever) gave up. It's not likely (though of course possible) both boards had a frame fault so I'd suggest looking very carefully at the scan coils and their connections, then the plugs and sockets, possibly temporarily hard-wiring them to the PCB to make sure all's well. I assume you changed all the boards but not the chassis frame - this does point to an interconnection fault IMHO, which is certainly not unknown on this model.
Good luck!
Glyn
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Old 15th Dec 2009, 8:50 am   #6
Michael Maurice
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Default Re: Beovision L2502/LX2500 Field Scan problem

The PSU, Line output and frame output board is just that, an output stage. The oscillator and drives live on another board on the other side of the set. Its worth checking for D/J on both the oscilator board and the output board.
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Old 16th Dec 2009, 1:11 pm   #7
Welsh Anorak
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Default Re: Beovision L2502/LX2500 Field Scan problem

Agreed - check the wiring to the plugs that connects the two, especially if that's the only thing left! An anomaly in the drive waveform can blow the output stage.
Glyn
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Old 22nd Dec 2009, 11:44 pm   #8
Sam
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Default Re: Beovision L2502/LX2500 Field Scan problem

Just to let everyone know, the set is now all working, installed in the Living Room, and connected up. The problem was dirty connections on the lead from the output board to the scan coils: one of the only bits of the original set left!

Thanks for all the help.

Sam
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