Philips 30AX set with poor convergence

[Jimmyhaflinger]

I have just found a 1983 Philips K35 "super selection" in the trash, fitted with a 30AX CRT (A56-540X) apparently, it suffered a lightning strike as the mains rectifier board had a charred spot with burnt tracks and vaporized metal

After replacing the rectifier and smps modules the set came back to life, amazingly, the "super selection" digital control module survived, but the convergence is really poor, and i mean poor, something unacceptable even on an early delta gun, it's completely off especially on the right hand side of the screen,

The CRT seems to be otherwise fine, the picture is bright and sharp with excellent greyscale, and the purity is decent too.

Maybe a partially shorted yoke?

[Michael Maurice]

Originally Posted by Jimmyhaflinger

Maybe a partially shorted yoke?

Unlikely, shorts in the yoke especially on the horizontal scanning would cause serious line picture distortion, not convergence, if on the horizontal scanning, it might stop the line O/P stage from operating.

[Heatercathodeshort]

Hello Jimmy,
The Mullard [Philips] 30AX tube gives near 100% convergence and I have never seen one adrift such as you describe. I can only think that the lightning strike has either damaged the deflection coils in some way [unlikely as Michael says] or more likely demagnetized the small disc magnets embedded in the crt bowl just underneath the coils. These are set up in the factory for correct convergence and cannot be reset. I once had a receiver that was totally magnetized after a lightning strike and I had to degauss everything before it would purify.
A massive bolt of lightning can also buckle the internal shadowmask itself but this will upset the purity somewhat. Of course the crt may also have been damaged when the set was thrown in the skip. Regards, John.

[Jimmyhaflinger]

Update:

I replaced the original yoke with another one from a weak 30AX I had, the convergence has improved a lot, now it's decent and watchable, but still not what you'd expect from a 30AX, even after tweaking the wirewound pot on the small connector board. You just can't get it right over the entire screen; the best compromise is with about 2-3 millimetres of vertical misconvergence on the blue.

This set was abandoned on the ground next to a skip, so it has unlikely to have been damaged by an impact - in fact the cabinet is in excellent conditions

The CRT is otherwise like new, the greyscale, purity and focussing are perfect, and apart from the convergence, the picture is absolutely cracking...

Maybe I just have to find 30AX yokes and swap them until I find one that works better!

[Studio263]

Has the tube been rebuilt? Look for the welding line around the neck to check. We used to buy rebuilt A66-540X tubes for B&O 8800's and these came with some rubber wedges and a set of magnetic rings that look a bit like the ones used in the Philips KT-3 (etc), only larger.

What you had to do was to get the purity right by sliding the deflection assembly back down the neck and then support the end of it with the wedges (common practice with smaller tubes). The ring magnets were then fitted over the neck up against the deflection assembly clamp and the convergence was made as good as it would get. They were quite variable and never as good as a factory original but no-one seemed to notice. Sometimes they worked better without the ring magnets so we left them off.

Even if the tube has not been rebuilt a set of the ring magnets may help you true yours up (if you can find some).

[Maarten]

On a 45AX tube, the function of the wedges (static convergence and purity) was accomplished by plastic distance pins in 4 corners of the yoke. These were factory adjusted and then melted in place. When mounting a new used yoke, you should probably file them down or cut them off and use rubber wedges instead. It is more than likely the same principle goes for 30AX tubes.

[Studio263]

There are no wedges on a 30AX when new, the coils index onto fittings bonded onto the glass. The 45AX is a bit retrograde in this respect (and a good few others besides).

[Welsh Anorak]

Hi
I tend to think there's been some magnetic disturbance due to the lightning strike - I'd thoroughly degauss the CRT then experiment with different yokes as you suggest. A convergence yoke from a later set might help with experimentation!
Glyn

[Jimmyhaflinger]

I have already degaussed the mask and rimband and the metal chassis frame with my mains degausser, but this made no difference; is it safe to attempt to degauss the gun assembly too or would this destroy the factory preset disc magnets inside the CRT?

By the way, the CRT is new (not rebuilt)

[Twinhead]

Probably strip down the CRT from the yoke and the ring magnet assembly, and do a thorough AC driven degauss on the bare neck with its electron gun assembly.

Inside the glass envelope, there are no magnets.

Victor.

[Jimmyhaflinger]

I finally got it right! It was a matter of finding the right yoke... I also learned a lot about 30AX CRTs in the process... for example:

-The factory convergence setting is stored on a magnetic ring inside the last anode of the gun, so an attempt to degauss the gun would destroy the CRT...

-The yoke from an A66-540X (26") won't work on an A56-540X (22")

-Yokes from post-1984 30AX CRTs are different and don't work well on older tubes...

-The yoke's metal clamp can get magnetized and mess up the purity.

-There are a couple of small preset magnets inside the yoke too, and I guess they were messed up by the lightning in my set's original yoke.