Hi

I am trying to repair a MX7000 that will not come out of standby.  I will not use component numbers in this description because I am working from a MX6000 service manual as there is no MX7000 service manual on the BeoWorld site.  Is the MX6000 manual sufficiently similar to the MX7000 for this kind of fault finding?  Does anyone have a MX7000 manual they could scan?  Why are the oscilloscope diagrams and parts list missing from the BeoWorld pdf files?

Firstly I discovered that the BU2508 was short circuit so I changed it and the 220uF connected to its base.  I also checked for dry joints, resoldered suspected areas and checked the BUT12 in the SMPS but this was ok.

However, after all this the problem remains; the MX7000 will not come out of standby.

The MX6000 service manual suggests that the fault that is causing this problem may be found by determining what is loading the power fail system during the 400ms after power-up.  I understand what is suggested by this, but is the service manual implying that one should disconnect the connector that is carrying the POWERFAIL signal one board at a time in order to find where the signal is being clamped?  I don't see any other way of doing it because if the signal is being clamped then surely it will not matter where in the loop it is measured because it will always read the same.  Is there a recommended sequence to doing this?

I am also curious about the 3 position switch on the video output board.  I can find no explanation as to how/when it should be used and on 4 different MX TVs (MX5500, MX4000 & 2 MX7000) it is set differently.  Can anyone explain this?

Richard

By stand-by you mean, you have the red led lluminating?

The AV9000 pcb4 (mainboard, powersupply & deflection) is the same as the MX7000.

richtoy:

  Why are the oscilloscope diagrams and parts list missing from the BeoWorld pdf files?

Richard

Hi Richard.
The user and service manuals on this site are down to Peter laboriously and selflessly scanning and posting everything he can get his hands on. They are not provided by Bang and Olufsen, just the immense devotion of one man.
Let's hear it for Peter.

Andy T.

Yes, precisely.  I also tried shorting the service mode connector to get the orange or red LED.  The LED stays red (I2C bus error ??).  I could now press TV and bypass the safety circuits but I am not confident that I could then cause more damage...

Richard 

You can only enter the service mode by shortcutting the connector on the AV board when the tv gives a picture.

Then you can enter several menu's.

But when you press TV, does the green leds on the bottom light up for just a second?

Did you replace the 220uF Cap?

As I remember you also have to search for smal C26, which comes lose. You can't see it but you have to resolder it.

Have you seen the post at the general forum? There is a MX7000 manual on this forum.

Check C102 on the PCB4. This is mentioned in the Serv manual from the AV9000. It becomes lose.

There must be a reason why the BU2508 failed.

I'm told to cut the wires from the connector on the back of the tube with the deflection coils.

Then resolder then directly. The contacts in the connector often fails. This could be one of the reasons why the chassis falls back in stand-by.

richtoy:

I changed the 220uF.  I will check and resolder C26.  Is this also on the mainboard?

Yep. PCB4. mainboard.

ipaul:

If the problem still persists, check the 2 diodes on the metal cooling plate to which also the BU2508 is mounted, these diodes are for the 8V and 15V supply and also tend to short circuit sometimes.

ipaul:

On the 8V which makes the red light come on: Yes and No, i'll try to keep it simple because it's not so hard to see in the diagram but i don't want to confuse Richard....

The TV and it's power supply can basically run in 2 modes: tv ON and tv standby (SB).

Your explanation about D36 and 35 only applies to the tv ON mode: there's an 8V line and an 8V SB coming from these respective diodes.

However, when the tv is in SB (thus with the red led on) the 8V supply does not (!!!) come from D35/36 but from the 154V line supplied by D30.

Thank you all for you help.  It was a loose C102 that cause the BU2508 to blow but there were other dry joints as well.  I found that gently pushing the large capacitors, chokes and transformers from side to side whilst observing the copper side of the PCB reveals the culprits more easily.  Joints that "look" good, even with a magnifying glass, start to show movement when the components are moved in this way.  With C102 resoldered the MX7000 came out of standby and then it was on to the next fault which was the frame power amp.  WIth this also replaced the MX7000 now fully works.

I should explain that I brought 7 broken MX/LX TVs a couple of weeks ago and hope to repair an resell them to fund the purchase and renovation of old B&O audio equipment.  Here in Holland you can pick up a broken MX7000 with BEO4 for less than euro 100.

So far I have repaired 2 x MX7000, a MX4000 and a MX3500 and I am left with a MX6000 an LX5500 and a LX2500.  I am sure that some of these TVs have been for repair but the fault could not be found so the owner buys a new set and gets what they can for the old one.  I can be sure because in some instances the TVs have not been fully reassembled and in one case a capacitor was missing!!

I started to look at the MX6000 this evening.  It turns on and the picture is complete.  However, it is out of focus, half normal height displayed in the middle of the screen and it has barrel distortion at the sides.  All power supply voltages appear ok and based on the oscillograms in my hard copy MX5500 manual the waveforms on the HOR output side (test point 49 & 50) look good.  Furthermore, the waveform on the VERT output (test point 65) is ok but the opposite side of the vertical deflection coil (test point 55) shows a very noisy sawtooth.  I also detected another strange waveform on the drain of TR35 (test point 52).  I suspected the MOSFET but after taking it out it tested ok; there was cirtainly no short.  All capacitors, chokes and transformers checked for dry joints.  A common item shared by both strange waveforms is the TDA8432 deflection processor.  Has anybody known this to fail?  Any suggestions where to look next?

This MX7000 has been acting up again lately and I would like to discuss the symptoms.  It is one of the later Beo4 model types.

The original fault (15 months ago) was repaired and at the same time all dry joints resoldered and caps replaced.  I did notice during the original repair that the set had had the two diodes and resitor that sit on the board that fits on the base of the tube replaced and that the resistor had obviously got very hot (D17, D18 & R13 on board 3 for the Horizontal centering).

The set has been used daily since the repair but then last week it again would not come out of standby.  I followed the instructions in the service manual (the LED indicated a I2C bus error so I assumed it was ok to proceed ) and when the set came out of standby R13 on board 3 very quickly began smoking and caught fire

I took the chassis out, repaired only the burn components, checked everything I could think of, reassembled and hey preso it worked

When I went back into service mode it said the last error was 8C (13IC2 deflection controller).  I reset the error indication and the set went back into service.

Today we again get the same symptoms, set won't come out of standby.  This time I just disconnected and reconnected the connector on the deflection coils.  The set still would not come out of standby but when I shorted the service connector to force it out it worked ok (no smoke ) and again the last recorded error was 8C.

So my question to the forum are:

1) Can a bad connector on the horizontal deflection coils cause a smoking R13?  It looks as if R13 partly takes the place of the horizontal deflection coil via T3 if the horizontal deflection coils become open circuit.

2) Once a fault condition occurs on an MX7000, is it latched until you go through the service mode?  My reason for asking is after playing with the plug on the deflection coils the set would still not come out of standby until I it had been in service mode.

3) Is there some other know intermittent fault that causes these symtoms?

Sorry for the long post...

Richard