Hello out there

I just read the great Workshop description with MX3500, LX5500...

My MX7000 is just slight different inside. I got a problem with the picture. Sometimes its only periodicly. It shows normal picture, but it has a overlay like of white horisontal lines and a lightened picture. The picture turns to flash a lot, between light and dark.

Are there anyone out there who have an idea of where i should start looking for bad soldering og bad caps?

Regards from Thomas

Is it the 'newer' type 78xx ??

I've had problems with these a few times, just replace (and re-adjust of course) the focus / G2 potmeter om the video output board (on the back of the tube).

Part is still available from dealers, i'd have to look for the number if needed.

If you do the replacement yourself also order a new 'socket' for the tube since the focuslead kind of 'snaps' in but never gets out without violence/breaking off, it's only a few euros more :).

Other possible sources of this problem are failing line output transformer and problems with the PAL generation board ... I have seen all three, but I have to admit that I would start as ipaul suggests.

Neil

Thank you very much for your reply!

I have noticed, that the color the screen lights up is slight green, and the lines are green too. Its both on television picture, and in teletext. Maybee its the green chanel in the output module?

Ipaul>> Yes the model is 7894. It seems to get off the tube quite easy, without violence, but should i change it anyway?

White lines can be caused by a defect G2/Focus potentiometer and wrong adjustment... and bad caps.

I would try first: check for bad solderings on this board, probably you will find some on the picture tube socket or the 3 bigger transistors. If you solder on this board, clean it carefully with alcohol or the new (raisin?) solder can cause sparks.

The G2 potentiometer does not like dust, moisture and smoking. So... make a sign with a cd-marker on the axle to know where the original adjustment was. Then turn it 4-5 times a little bit, this cleans the surface. Then adjust it again... you wont destroy your MX :)

Dont do Focus and G2 at the same time. The picture tube does not like it, when the picture is to bright and can magnetize. So just make small steps.

You know, that these MX have ~ 20 - 28 KiloVolt? Just a warning: even on this small board are 170-400V , so dont touch parts. Use a full plastic insulated screwdriver without metal.

Die_Bogener >>

Thank you for your input!

Im just wondering, the picture distortion is green like?? But you are right. I should try to move G2 to remove dust inside. In which area should i change caps? It will maybee be at little overkill to change all of them :)

I have found a servicemanual for the MX7000 on eserviceinfo.com. It doesnt say anything about a adjustment method to G2 or focus. Is it just to watch the screen to get the best result. (I know i should not turn it to bright to awoid damage)

Yes, im familiar with the high voltage inside. Im an engineer, but im not skilled in fault finding in televisions :) (not yet ;) )

Here you can se how the picture changes... it changes every five minutes or so.

And here you can se with the green lines:

Yes: the socket gets off the tube easily but there's a wire (for focus) stuck in that socket which, in case the G2 / focus unit is replaced needs to come out (in order to get the new wire in) and that's usually trouble.

But looking at your pics....does tapping the video output board or tube neck have any impact ??

The fact it's only green doing this gives an entirely (possible) diagnose than when it's showing white lines you see ...:

 green lines: bad solderings on output board, bad output transistor for green, tube problem......

 White lines: G2/foc unit, HT transformer.....

(amongst other options, but just to show you that there is quite a diferent approach to 'white lines' or 'green lines' so to say).

I had similar patterns with an MX3500 and it was caused by a dry joint on one of the capacitors on the video board on the back of the tube.  See this post for tips on finding them http://forum.beoworld.org/forums/post/90730.aspx

Having fixed a few of these types of B&O tv I would suggest always resoldering the usual dry joint culprits because even if they are still good now then they may not last much longer.

Richard

Thank you for your answer!

Ipaul>> Yes i also realised that it was important to describe the error were green in stead of white :) I will start out with resolder the output board again. I will change the three output transistors, and se if it helps. I have tried to condition the G2 and focus trimmer, but without success.

What about the PAL board? Unfurtunatly my service manual isnt complete. So i cant figure out the function of this.

Sorry i havnt tried to tap the board, while its running, simply cause of the high voltage :)

richtoy>> I actually read your whole post yesturday. And i think its sound as a good procedure to move the big components while looking at the solderings. But the television got service (before i owned it) and i think the most dry solders where fixed then. (i hav only found a few in the amp area)

 Regards

Yeah: because the lines are green, forget the G2 story ...:)

Instead of changing the transistors you could interchange them, i mean swap parts with the blue (or red) outputs so you can sse if the problem 'moves' (thus the lines change colour) and find the fautly part.

Another thing you can do (probably easier) is interchange cathode connections to the tube: connect blue (or red) output to the green cathode on the tube and VV.

If the lines are still green the tube is the problem, if the colour changes the electronics is the problem.

This is fairly easy to do by unsoldering one end of the resistors of the output stage connected to the tube and use 2 little wires....

ipaul>>

That sounds like a smart move, to interchange the output connections to the tube.

I just read your post a bit to late, because i already had done some soldering on the output board today. It actually showed several broken solderings at the big components. Big resistors and a capacitor. But not any related to the Green output. After resoldering. It turns out that the green lines are permanent shown on the screen now. So now it isnt a periodical defect anymore :)

It's fun, isn't ? ...fixing teles...more enjoyable than watching :)

If not a bit too much soldering somewhere maybe the 'tricky' part has died for good, which in a way makes it easier...

Probably a good time for the 'cathode swap'.