I have just aquired an nice condition MX7000 with what appears to be a wierd fault.

The picture has something wrong with colour, it is mainly black and white but some colour is present but obviously incorrect.

I hooked up a DVD with some test images (Y/CVBS video output) and the PAL colour bars appear but in black and white.

If I switch the video output of the DVD to RBG then the PAL colour bars appear in colour but in the wrong order (teletext also has incorrect colours)

The PAL colour bars should be:

W Y C G M R Bl Bk

but I see:

W M Y R C Bl G Bk

I suspect the PAL decoder board and have checked the ESR of all elcos, all good.

I now suspect either the TDA 4510 colour decoder or the TDA 4661 64us delay line.

Is there any other check aybody can suggest to further pinpoint the fault (am I even in the right area?)

Richard

Hello Richard,

I have had a couple of MX and LX televisions present the no colour fault on composite input / RF off air reception. Sometimes the colour will lock after the set has warmed up for 20mins, with the other the fault was permanent. Using an ESR tester, all capacitors on the colour decode boards test fine, and within spec.

What has worked for me is performing readjustment of the colour subcarrier frequency (see the service manual). It seems that some sets are prone to drift after 15 - 20 years, and this adjustment shifts to be on the threshold of the operating point for colour lock.

I am not sure about the incorrect colour bars / wrong menu colours, but you should try this adjustment first, and see if it restores colour lock before looking to any other faults.

Best regards,

Colin.

Thanks for your reply Colin.  By "performing readjustment of the colour subcarrier frequency" I presume you mean the PLL ref osc i.e 12C48?

Today I did some more investigation.  First of all the colour bars appeared in colour today from the outset with the output set to either RGB or composite and the TV is fitted with the PAL/SECAM varient of board 12.  All tests made today were done with the DVD switched to composite output.

The test DVD I have also has chapters of solid colours to check purity and these are also appear swapped (Red instead of Green, Green instead of Blue and Blue instead of Red).

With input of PAL colour bars I also used a scope on all recommended test points on board 12 and they all look like the example oscillograms in the service manual.

The symptoms I see suggest a 120 degree phase error of the colour.  Which component failure other that the TDA 4650 colour decoder could cause such symptoms?

Hello Richard,

The PLL ref oscillator is the adjustment I referred to.

If you have the same wrong colour fault regardless of whether the source is composite or RGB then you can rule out the colour decoder, as running RGB into the set bypasses the Pal decoder. (double check that you really have a fully wired scart, or you could be seeing composite in both instances)

If the fault exists with an RGB input then the RGB colour signals are somehow mixed up in the set! Has it been serviced prior to you picking it up? Are all the boards correct for the set (especially the tube neck board). Also check the interboard wiring is correct with no swapped plugs. Check the tube neck board for any modifications (wire bridges, cut tracks)

If the fault is only from the colour decoder board, carefully check all soldered connections especially the ICs which run warm, with a magnifying glass and rework if necessary. If the PAL switch is working in the wrong phase, then red and green will be transposed, but not blue, so this seems unlikely.

Also, you mention that the teletext colours are wrong. Teletext is internally generated as RGB, so this also points to a problem external to the colour decoder.

Well tonight I traced all 3 solid colour signals all the way through to the tube neck.  They are all as expected but give the wrong colour[:'(]  When the set came on today however there was significant colour bleed (the other primary colour) at the corners of the picture.  It was almost as if the convergence was out.  I switched back to normal TV and the picture was fine (from a convergence point of view) but then by chance I noticed a difference at switch on to other MXs we have, ther was no deguass buzz.  A few checks later and D8 (the one that drives the triac) was open circuit.  I changed this switched on and the picture was perfect

I can only assume that months of use with no deguass (previous owners stated he had watched in this state for some time) had caused the electron guns to hit the wrong phosphore dots.

Anybody else ever seen this kind of effect?

Richard.

Not so bad that you would expect wrong colour decoding, but otherwise I've seen some awful cases of discolorization. Dunno how universal this is, but unlike B&O, many TV sets sold here only do the degaussing when you turn them on with the mechanical mains switch, not when coming out of standby during normal operation.

I believe it also depends on the orientation of the set compared to the Earth's magnetic field!

Well done! I would never have  have guessed a gross purity error could give completely solid wrong colours. Every degauss fault  I have seen the colour was all over the place, rainbow fashion.