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Old 22nd Feb 2014, 3:03 pm   #1
marsbar
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Join Date: Apr 2011
Location: Stavanger, Norway .
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Default Bang Olufsen LX2800

Still trying to fix my old TV... as you can see from pictures we still use C64 and Teletext here in Norway.

Last time I tried it: Picture showed teletext signals, to narrow. Soldered in a resistor 100 Ohm in series with 5.1V zener diode in order to lift picture up, but it got distorted in top.

Now, removed the resistor and picture looked ok! Voltage 5.3V over zener diode (under fan!) so I installed a 4.7K trimmer over diode to set it to exactly 5.1V.

Noticed a loose wire across the tube at the bottom, normally it was secured with a spring and clips, clips were broken so I tightened it up by a cable shoe and wire to the bottom as shown. This blank wire is supposed to be in direct contact with the CRT screen.

Picture now looks ok, but!


-Red lines in blue picture screen (Should not be present!)

-Slightly shadows on video signal in

-Says "Zip Bang" once in a while, picture snaps, then gets back to normal...

-TV makes a high frequency noise when on, 20kHz (My father could not hear it!)

My commodore 64 computer: (Okay, not a computer site this, but this applies to the TV modulator!)

-Gives WEAK signal out, screen has snow on 2 tested TV's

-Composite video (Direct video) is OK from C64

I have no idea how many times this TV has been repaired.
Main board has transistor, capacitors and effect resistors replaced at a TV shop ...

What can I say, great TV if I could fix it back where it was... if not an idea would be to re-use the loudspeakers! Sound is great

Any ideas how to fix it a little better?

TV seems to get better when it is warmer... just like a car!
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Last edited by marsbar; 22nd Feb 2014 at 3:15 pm.
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Old 22nd Feb 2014, 3:03 pm   #2
marsbar
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Default Re: Bang Olufsen LX2800

Here are some more pics
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Old 22nd Feb 2014, 5:07 pm   #3
Okto1984
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Default Re: Bang Olufsen LX2800

I've recently started repairing a similar, slightly older beovision from the early 80s. I got really good advice on here, so you've started well

Anyway, I knew nothing when I started and still have a lot to learn, but there's a few things I've found so far that may be useful.

Tracking down the service manual was an important step for me. There are sometimes copies to download online for free, or buy on eBay if needed. It improved my understanding a lot and identified the components. I have seen ones for your set around.

Your set also seems to have some similar components to mine (roederstein capacitors) and I tracked down the various datasheets for them online at www.datasheetarchive.com. This might be useful if you ever want to replace them. If I see correctly, some of yours have the strange pin style I found in mine called 'mounting ring', which vishay (who bought out roederstein) still make for some ranges. That took me a while to find.

As for vintage computers, if you use them often, it's worth finding a suitable cable to connect them to the set's scart socket. It gives a greatly improved picture on every TV I've tried and is kinder to your eyes.
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Old 22nd Feb 2014, 10:04 pm   #4
marsbar
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Default Re: Bang Olufsen LX2800

I will look for the factory manual! Good idea, probably top secret for service people only before WiFi!

When it comes to capacitors, I know they can dry up or short sircuit. I have no instrument to do a test...

As long as a replacement capacitor has the same or higher V, and the same or higher value of F it should work... and you install it the right way.

The fan is powered from the Audio amplifier circuit power supply, because I tapped it from a leg on the amplifier IC.

The C64 is not going to be used, as an emulator on the PC works better.
I bought it for display, but would like to fix the RF modulator. Still, it's a start, case it beautiful and scratch free.

Now, 5 years ago I repaired the TV in a local TV shop. Maybe he has spare cards and component he is going to clear out... He could probably fix it also but it would cost my shirt...
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Old 3rd Mar 2014, 3:57 pm   #5
Okto1984
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Default Re: Bang Olufsen LX2800

One problem you're having, the zip-bang and momentary loss of picture sounds similar to the problem I first had. I discovered mine was caused by the EHT cable's insulation breaking down. It could I imagine also be caused by arcing from anywhere else the high voltage is present.
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Old 9th Mar 2014, 11:36 pm   #6
marsbar
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Default Re: Bang Olufsen LX2800

Could be, my car's spark cable had the same problem if I started it cold. Cables took up moisture when stored cold, and did not work properly until warm. Could wrap some hi insulation vulk tape around the cable in it's entire length. Also, will replace ALL capacitors on the power supply board. New capacitors are 1/2 the size, started with 400V rectifier capacitor, old one probably dried out after beeing on every evening for 20 years...(and full of PCB!) New capacitors take 105 degrees c while old ones are 85 degrees c.
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Old 11th Mar 2014, 2:59 pm   #7
Welsh Anorak
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Default Re: Bang Olufsen LX2800

Hi
Just to reassure you, the capacitors used in B&O sets (and all sets after 1970 with one or two possible exceptions) don't have PCBs (carcinogenic polychlorinated biphenyls) in them.
The cables are still available from Donberg Electronics, I believe.
Glyn
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Old 16th Mar 2014, 8:54 pm   #8
Okto1984
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Default Re: Bang Olufsen LX2800

EHT cables seem to be one of Donberg's specialties, it's also where I found mine in the end. It wasn't expensive, and I feel safer with a new one than taping the old one that's failing. I don't always keep too close an eye on the TV when it's on.

As for capacitors, I've found some differences between ones with the same voltage and uf ratings. Size and leg spacing matters as they need to fit. In the case of the multi-pin versions with the mounting ring, replacing with a two pin one means the new capacitor is off center to where the original sat. Fortunately, capacitors seem to have shrunk since the 80s and I found acceptable size alternatives, but it's worth measuring before buying anything expensive.

There's also ripple current, the big ROE capacitors from the 80s that I've been looking up had high ripple current ratings that I've matched or bettered to be sure they'll work as intended.

Finally, in the audio circuits they don't all seem to sound the same. I originally put a panasonic FR on the amplifier board as one of the originals was bad - and it wasn't a good replacement. The panasonic sounded quieter and murky compared to the other channel. So, I swapped it with a nichicon FG and am very happy with the sound now, and subsequently replaced the other one too so both match. I still really like the panasonic FR in the other circuits like power supply and scan board, they have done good things there, but didn't seem to work out in the audio.

So, there's a few differences I've found so far that go beyond the voltage and uf.
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