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Old 13th Mar 2024, 7:13 pm   #1
FERNSEH
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Default B & O MX4000 CTV dead.

Arrived in the workshop yesterday a Bang and Olufsen MX4000 colour TV receiver.
To use the term dead isn't exactly true because an AM radio set nearby is reacting is to one of the power supplies in the B & O.
No point any attempt to carry out repairs without service information.
Set was made in 1992.
Any ideas would be appreciated.

DFWB.
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Old 13th Mar 2024, 7:21 pm   #2
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Default Re: B & O MX4000 CTV dead.

Quite a complex chassis used in a number of sets using the "45AX" series of tubes. Several densely packed PCB's on a rather fragile plastic frame. Will be a bit "toasted" by now in the small cabinet space. Dry joints and expiring capacitors!

Service data not on here but can be found at

https://elektrotanya.com/bang_olufse.../download.html
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Old 13th Mar 2024, 7:24 pm   #3
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Default Re: B & O MX4000 CTV dead.

If I am not mistaken this chassis does not have an main on-off switch. Just a LED that should either glow red in standby or green when the set is "on"

Page 2-15 for power supply diagram. Need a PDF reader that can rotate, the latest Acrobat one charges for this feature!
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Last edited by bobbyball; 13th Mar 2024 at 7:34 pm. Reason: More informatiom
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Old 13th Mar 2024, 11:00 pm   #4
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Default Re: B & O MX4000 CTV dead.

Hi Robert,
many thanks for the link to the Electrotanya.com website.
I've successfully downloaded all the contents of the service manual.
It certainly is a complex receiver. Time to do reading up before any serious servicing begins.

Best regards,
David.
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Old 14th Mar 2024, 12:08 am   #5
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Default Re: B & O MX4000 CTV dead.

Do you have the remote? If not all bets are off!
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Old 14th Mar 2024, 9:28 am   #6
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Default Re: B & O MX4000 CTV dead.

Ahh the remote, yes you will need that. Beolink 1000. If I recall (all my sets are in storage), when plugged in the "standby "LED should glow red. There is a hidden button which when pressed should bring the set on - LED turns green. As Ben says you need the remote to do anything further! Can't remember where the "hidden button" is on the 4000 but I think it is next to the headphone socket.

A lot of these sets have tired tubes now, made worse by having been run at high brightness due to the "contrast screen" at the front. There is a parallel here with that KB set from the 60's, was it called a "Deep Scene" or something like....
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Old 14th Mar 2024, 10:46 am   #7
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Default Re: B & O MX4000 CTV dead.

Yes, off topic, sorry. I had a KB. Rotary UHF and the knob was illuminated red, very reliable.
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Old 14th Mar 2024, 10:54 am   #8
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Default Re: B & O MX4000 CTV dead.

Scart socket Pin 8 high "AV" works on my 4300which will get around which channel was programmed in by the user when first switched on. Otherwise, it's a screen full of noise on my set.

The sets do give a good picture and sound but, the dark filter makes the CRT work hard for its life.

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Old 14th Mar 2024, 7:45 pm   #9
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Default Re: B & O MX4000 CTV dead.

Will ask the owner of the set to bring the remote control handset to the workshop. It's pointless attempting to do any service work without it.
The attachment shows the compact chassis.

DFWB.
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Old 14th Mar 2024, 8:16 pm   #10
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Default Re: B & O MX4000 CTV dead.

Look at that for a "compact" chassis! Beware as the plastic frames and supports will have gone brittle - ask me how I know that!

Set has on-screen menus (when it is up and working!)
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Old 15th Mar 2024, 1:16 am   #11
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Default Re: B & O MX4000 CTV dead.

Check capacitors, especially if this model uses any of those plastic-encased ones, or the gold-coloured Roederstein (ROE) caps. They all had a nasty habit of failing short.
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Old 15th Mar 2024, 8:15 am   #12
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Default Re: B & O MX4000 CTV dead.

Quote:
Originally Posted by bobbyball View Post
If I am not mistaken this chassis does not have an main on-off switch. Just a LED that should either glow red in standby or green when the set is "on"

Page 2-15 for power supply diagram. Need a PDF reader that can rotate, the latest Acrobat one charges for this feature!
The free Sumatra PDF Reader will rotate L/R in 90° increments.
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Old 15th Mar 2024, 6:55 pm   #13
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Default Re: B & O MX4000 CTV dead.

These are indeed sophisticated designs.Still capable of good performance.
The 'tubes (Philips)were generally long lived,except where users insisted on overcoming the factory settings to make the picture brighter,overcoming the purpose of the contrast screen!
The screen on these and most B&O tv's is designed to reduce the glare from conventional picture's,and works well.B&O also used a monitoring circuit to auto adjust the levels to compensate for crt wear during the life of the set,again defeated by users tinkering,bah humbug.
The power supply runs continually when the set is connected to power,but at a very low level,ramping up with demand,say when switched to Tv mode.
The set can also be used in Audio only mode as an amplifier only (with power supply at lower power),as a component of B&O's "link" system concept.
Allway's repairable in my experience!
Nick

Last edited by Max Ripple; 15th Mar 2024 at 6:56 pm. Reason: typo
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Old 3rd Apr 2024, 1:01 am   #14
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Default Re: B & O MX4000 CTV dead.

The tube in my 51K5 was donated by an MX4000, I've noticed lately the tube is beginning to look a bit tired in it, it takes about half an hour for a decent picture to appear without having to crank the contrast and brightness up. after the half hour warm up picture looks good, slightly duller than it used to be but still acceptable but when its first turned on its terrible.
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Old 3rd Apr 2024, 6:21 pm   #15
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Default Re: B & O MX4000 CTV dead.

B&O may under-drive its tubes, leading to lazy cathodes. At least that seemed to be the case with their first generation wide screen models.

Regenerating the tube might be a bit too harsh a treatment, but you could try running the heater at a higher voltage 8-12V for a few minutes (in case of 12V maybe 30 seconds to a minute at most instead of a few minutes, or start out with 6, then 8, then 12 and back to 8) to burn in the cathode again (this is also done at the factory as one of the last steps of the manufacturing process, i forgot the proper word). If that doesn't help, there might be some debris in between the electrodes or the cathode is too far gone and regenerating might be the only option left.

Alternatively, there might be something wrong with the 51K5. Maybe some voltages are too low after switch on.
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Old 3rd Apr 2024, 7:13 pm   #16
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Default Re: B & O MX4000 CTV dead.

From what I remember there are a couple of electrolytic capacitors in the primary of the power supply which fail, there is also a snubber capacitor again on the primary of the power supply which goes dry jointed.
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