EMI Push About Coils

[peter scott]

Many years ago I removed the push about coil from my HMV901 because it appeared to distort the edge focus and I didn't really need it to get my picture centred.

More recently I decided to try it again and once again decided to remove it but removing it didn't get me back to the edge focus I had previously and my scan now had some trapezoidal distortion. Checking the push about coil I found that its iron core had some retained magnetism and I wondered whether the CRT cage and the iron core of the scan coils also had retained magnetism. Just today I tried using a degaussing coil around the CRT area and have been able to largely remove the scan distortion and also improved the edge focus.

If you have one of these early EMI sets fitted with a push about coil I would highly recommend removing it and degaussing the CRT cage and scan coils. You my need to make other arrangements to hold the scan coils up close to the cone of the CRT.

Peter

[ppppenguin]

Interesting. Always thought of degauusing with colour CRTs. TBH, I've never got round to investigating the scan coil setup on my 702. One day I'll take out the CRT, put the EHT unit back together with Ed's rewound transformer (it's a horrible bodge at the moment but it give an accurate 5kV) and generally give the set some TLC.

[Murphyv310]

Hi.
I'm wondering if there is residual magnetism from switching off the large transformers of these sets? Most likely not but the Ekco TSC30 has similar issues.

[peter scott]

Hi Trevor,

There may be some stray fields from transformers but I think they are designed to contain most of their fields. The push about coil is actually designed to spread its field and it is as close as it could be to the scan coils. I did wonder about a stray field from the speaker magnet (permanent magnet in the 702/901) but my compass hardly moved at the back of the magnet.

I think the Ekco TSC30A has a permanent magnet focus. What material is the core in the Ekco scan coils?

Peter

[Murphyv310]

Hi Peter.
No the Ekco has a focus coil, although the tube is a bit low the coil distorts the picture and makes it appear to rotate, edge focus is poor. Initially it actually was worse but replacing both EHT smoothing capacitors made a big improvement, its still far from correct though.
The interesting things about this set is it has a date stamp on the chassis of Sept 1939 but the set wasn't built till 1946 so somehow Ekco were able to retain metalwork without it being used for the war effort, plus of course it was chopped up into a table model some many years ago. Our cabinet Guru David Boynes rebuilt the set as it should be, beautiful job and I'm very proud to own a set that has strong prewar connections.

[Terry]

The interesting things about this set is it has a date stamp on the chassis of Sept 1939 ...

Presumably the first of September 1939, as there would have been no point in continuing to produce the metalwork after the Television Service closed down for the duration ...

[Murphyv310]

Hi Terry.
I wonder though how quick the factories stopped TV production after the announcement. I always remember my Dad telling me two things, he was an aeronautical design draughtsman and civilian work carried on for some weeks after the speech so quite possible in the radio industry. Also it was felt initially the war would be short lived.

[Terry]

The 1st September was Friday, so the end of the working week. Management would have had all weekend to decide what to do or not to do before the following Monday. There was, presumably, still a demand for the work your dad was involved with but not for TV sets without programming.

As for a short lived war, this sounds more like the WWI "We'll all be home for Christmas" belief. Considering the military might that Germany had demonstrated and the speed with which they had occupied Poland, plus the fact that the events of what had happened just over 20 years earlier were still firmly etched in many peoples' minds, it sounds unlikely to me, Trevor.

[Murphyv310]

Possibly Terry, I doubt we will ever know. So if these chassis were only stamped out for one day I'd say they are very rare. I'm still surprised it survived till 1946, the extra base for the sound interference limiter can be quite clearly seen as a later addition.

[Geordie McBoyne]

Link to the UKVRRR: https://www.vintage-radio.net/forum/show...p?t=104480

There was/is a problem with the focus coil. I do remember that the picture was off-set and also tilted when the scan coils were positioned horizontally. Turning the scan coils about 15 degrees positioned the picture correctly in the CRT mask.

Geordie McBoyne.