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Forum 141

B&O MX7000

Colour Television

Last Post by Anonymous 11 years ago

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Anonymous

(@anonymous)

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I know that this is a bit new for this forum A family member has asked me to look at this monster that went went wrong recently.

The set will only come out of standby for about 1 second before it returns to the same state. I realise that it is probably the I2C bus detecting a fault and turning the set off to protect it from damage. I thought I had read that there are a series of fault codes that can be read but I cannot find that info. Neither the service manual nor the internet are of any help.

I have checked the soldering on the scan yoke(common fault) and and the line output transistor, both are OK. Any help would be appreciated.

Posted : 09/09/2013 2:36 pm

sideband

(@sideband)

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You could try disconnecting the line output transistor and loading the power supply with a 60 watt lamp (tungsten filament type) to see if the power supply voltage comes up. If it does, the fault is probably excessive loading in the line output stage. It might not stay up due to protection circuits operating. If it's caused by excessive loading, try disconnecting the supply to field output and reconnecting the line to see if the loading is still present. Like most of these things it's a process of elimination...easier if you have a manual. If the fault proves to be the line stage itself, it could be the line output transformer but don't forget to look for bulging or otherwise suspect electrolytics first.

Rich

Posted : 09/09/2013 4:04 pm

Red_to_Black

(@red_to_black)

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As Parabola said above ^^^

If you first check for shorts and dry joints (line output transistor etc. first), and don't find anything obvious amiss, then you short out the service link and this will disable the powerfail protection and allow the set to start and may lead you to the faulty section.

It is quite clever the way the powerfail system (as described above by Parabola) works, this also checks that the panels in the set are actually connected by the way this squarewave is routed around the set via various connectors/plugs, it is also worth checking that these connectors are not dry jointed too, as this gives exactly the same symptoms.

I probably have the service manual for this set too.

Posted : 09/09/2013 6:09 pm

Anonymous

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Thanks everyone. I now understand how the powerfail works and how to go about fault finding it. I had already checked for dry joints, checked the +20V supply to the audio chips and the line output transistor for shorts. All were OK. I will do the rest(including rectifier O/C tests) tomorrow.

Parabola, the MX7000 is a minor development of the MX6000 but is close enough to allow the use of the 6000 manual for mine. I did find a copy of the 7000 manual online. Manual is a bit of a misnomer as it is little more than a circuit diagram and a few notes. Now that I know more about the powerfail it should be enough though.

I will report back when I have more to tell.

Posted : 09/09/2013 11:19 pm