Fading picture TV24

[dominicbeesley]

Still almost there with this, but the picture gradually gets darker over the space of about an hour.

I tried monitoring the g2 and cathode voltages but these seem to stay pretty stable (g2 is affected by setting of the brightness control)

Also this set seems to quite happily display the test card for ages with no problem and only shows this problem with off-air viewing (I usually have the brightness up a fair bit more for off-air, not sure if this is Aurora related?)

I'm pretty sure its not g2 as the focus stays spot-on; or is g2 not related to focus in these (magnetically focused) sets?

The only other place I can think to look is the EHT, unfortunately I have no way of measuring this but am I right in assuming that this is probably ok (the line width would be affected if the LOPT was overheating - it doesn't seem hot just warm) or is this a likely suspect?

I'd really like to get this set to my friend before Christmas!

Any ideas for where to look gratefully accepted.

Dom

[Steve_P]

Have you changed the EY51?

Now this sounds to me like the problem that was oh so common in Dual Standard Bush/Murphy sets. I call it 'Bush Lopt Disease.'

There is a separate winding for the EHT. This gets hot and sticky and the EHT fades away in the Dual Standard Sets. On a Bush TV24, it is just getting darker. What happens if you watch a test card at the same brightness setting as the one used for watching programmes?

The insulation between the windings inside the former for the EHT are breaking down. It'll probably get worse over time. You can get away with it for now, but really you need a rewind.

Sorry to be the bearer of bad tidings of little joy, but this does happen.

Cheers,

Steve P.

[Ian - G4JQT]

If the picture expands as it gets darker, that would indicate an EHT fault. Does the picture expand/baloon off the screen into darkness when you turn the brightness right up? Is there a difference in brightness of the EY51 heater between good picture and dark picture? If so, again, possibly an EHT or EY51 fault.

Seems very odd that it's ok on the video output from the built-in test card from the Aurora. Could the fading be due to a video-signal input fault? Have you another video source you can try? Could it be a bad cable connection with the video signal going into the Aurora? Does increasing the contrast bring it back as it fades?

Ian

[dominicbeesley]

Thanks both,

The EY51 is NOS, tried earlier when I originally couldn't get a picture.

Yes, when turning up the brightness the picture balloons (only at very bright settings) does this indicate a broken LOPT?

Would it be the EHT winding that is shot?

I was hoping I wouldn't have to go down this route....is there any way of confirming this before breaking apart the LOPT?

Any information on rewinding these? I've seen Andy valves page but not sure I fully understand how it's wound.

Dom

[Steve_P]

Try swapping V8 and V10a.

As far as the tramsformer goes, contact Murphymad or Ed Dinning.

Cheers,

Steve P.

[dominicbeesley]

I've just fitted a new v8, but I'll pull v10 and give it a blast on the AVO char. meter tonight - anything's got to be worth a shot rather than pulling the LOPT apart. Though the B+ and B+Boost seem to be pretty good and stable.

I'm fairly sure its not a video problem as when cold turning the brightness up to about 1/2 way gives a bright background with visible flybacks etc but nothing when its overheated/tired

Dom

[ppppenguin]

I will always bow to Steve's extensive knowledge of mending TVs but there's one important point that may have been missed:

.....it's ok on the video output from the built-in test card from the Aurora.

If it gives a first rate test card display for long periods and if the picture doesn't balloon significantly at reasonable brightness then the LOPT should be OK. As you increase the brightness at some point you'll push the EHT over the edge and the picture will balloon but if you can get a bit too bright and it's still OK then the LOPT may well be OK. I realise that a picture with a lot of white content could push the LOP a bit harder than a test card but it seems a bit unlikely that it's broke.

Turning up the brightness for "real" pictures seems a bit odd too. The TV24 has respectable black level stability, not like later sets with AC couping and Always Grey Control.

[dominicbeesley]

A bit more fiddling tonight has revealed that it does go a bit darker on the testcard but only if you start off "too bright".

I left it on a bright test card for about two hours and the set was still useable. Will repeat the experiment with "real" pictures tomorrow.

Also the brightness and varies with focus and the focus "nice" point changes with time. I'm going to take the back off and see if the tube needs to be better held in place as I suspect something (possibly the pesky ion trap) is moving as it heats up.

Dom