Ferguson 606T Very odd effect.

[Heatercathodeshort]

This is not a trick picture but exactly the sight that met my eyes on opening my collection outbuilding. The mains isolator had just been turned on but none of the connected receivers had time to fully warm up before this was noticed.
I have never seen a fault condition such as this, the only exception being a receiver with it's EHT derived from a mains transformer where the normal HT supply is disconnected or has broken down.
This is a Ferguson 606T from 1960, the EHT supply of course is derived from the line flyback.
So what is going on? ...................John.

[ms660]

Anything to do with the switch off spot suppression circuit?

Lawrence.

[Heatercathodeshort]

Wow!That would need some suppression! John.

[ms660]

I was meaning from the point of view that the CRT might have retained a charge, I assume it's not a permanent effect, just at early warming up after switch on.

Lawrence.

[Lloyd 1985]

When was it last powered up before this photo? Certainly an interesting effect! I’ve got a set here that when it’s been off for a while but then mains reconnected it shows a blob for a couple of seconds as the tube warms, but normally only if it had recently been powered up, as in within a minute or so. I can’t remember which set it was now!

The real question is; will it do it again?

Regards
Lloyd

[FERNSEH]

Been looking at the circuit diagram of the Ferguson 606T, it can be found in the 1960/61 Radio and Television servicing book, pages 327 thru to 334.
Doesn't appear to be any switch-off spot suppression in this set.
From the same book, on page 28, a method of spot suppression. The bottom end of the brightness control is returned to the un-switched side of mains neutral, see attachment. On switching the set off the HT supply doesn't decay instantaneously. Because the chassis return of the brightness control has been broken the grid volts rise to near HT potential causing the beam current of the CRT to rise which in turn discharges the EHT capacitor.
This was the method of spot suppression used by Pye. I know this works because my V600A has it.
Another method used for spot suppression was to insert a voltage dependent resistor in the earthy side of the brightness control.

DFWB.

[Heatercathodeshort]

I can see what you mean Laurence. The main isolator had not been switched on for over a week so any charge in the CRT would have been discharged rapidly. The blob was very bright and startled me for a few seconds.
It faded after about ten minutes and the strange symptom may repeat itself. When present the brightness control had no effect on the blob.

The answer is a very strange one, quite simple in fact. I'll give you a day or two before I reveal the faulty item. John.

[Freya]

Has the set been fully restored previously ?, I suspect a capacitor is at fault.

[high_vacuum_house]

Good evening,
The thing I find strange in the picture is that the blob appears to be almost at the top of the screen. As the timebases are valve based, they probably would not be operational so quickly therefore there would be no beam deflection and the spot would be directly in the centre of the screen. There must be a strong and constant strength (I.e not generated by an AC current source which would elongate the blob in one direction) magnetic field present to deflect the spot.

Christopher Capener

[peter_scott]

It looks as if the CRT has been given enough volts for Jac's test.

Peter

[PJL]

Whilst isolated, the building mains had 'charged up' somehow?

[Graham G3ZVT]

Someone shining an ultra-violet laser on the screen?

[Lloyd 1985]

Reflection of the moon!

[ms660]

I knew it was a beam fault.

Lawrence.

[davyrocket2]

It has the look of a light bulb reflecting on the screen.

[Heatercathodeshort]

OK I have had my fun and well done to Rambo who managed to get close to the cause.
The pictures show the outside weather boarding to my museum building and an internal picture taken of the sun beam entering through a knot hole in the timber.

The knot must have dropped out in the heat of last summer. The end wall faces south.

It looks very convincing at first sight of it but as can be seen in one of the pictures, the defined blob can also be seen reflected in other receivers on display. The knot hole can be seen on the external picture.

Thanks for all your very clever suggestions and I hope the wind up has kept you amused. Regards, John.

[Lloyd 1985]

Haha! Brilliant! I’d have never thought of that one

Regards
Lloyd

[mark2collection]

It's a shame the knot's that high John.

Camera Obscura? Could still work I suppose(?)

Mark

[peter_scott]

Hmm! Knot what I thought then.

Peter

[Graham G3ZVT]

Originally Posted by Heatercathodeshort

OK I have had my fun and well done to Rambo who managed to get close to the cause.

I'd call it "Thinking outside the box"(!)

There was a launderette opposite my bedroom in my parents house, and a perfect upside-down image of the illuminated sign was projected on the far wall, by means of a chink in the curtains.