White Ibbotson Worries

[Mikey405]

Hi all.

A couple of weeks ago I decided to have a go at a TV set which I’d had in my lounge for several years without ever having given a second glance. The set, a large White Ibbotson 4836F (So called because the picture size is 48x36) is a projection set which beams its image onto a large White-Ibbotson fold-out screen – Or at least it would do if I had the correct screen for it. The set itself doesn’t really look like a TV at all – more like a cross between a bee-hive and an amateur weather station; it stands about 3 feet tall and about 2 feet square with fluted grilles all around the bottom half, a glass-covered top with an embroidered cloth beneath and a pair of small doors covering the optics.

This not really being a standard construction, the first thing was to work out how to get to the innards of the set. It turns out that the top of the 2 main chassis sections are accessed by removing a bolt on the underside of the unit and hinging the whole back section round about the centre of its axis. The undersides of the two chassis are accessed by removing 4 screws and withdrawing a wooden panel attached the hinged section. Further access to the optical box and its many adjusters is obtained by removing 4 knurled coach-bolts on the top and lifting out the glass and embroidered base beneath.

Once access was gained to the innards, a quick survey revealed a bit of bodgery around the power supply section. One of the smoothing / reservoir caps had been bridged by a large R.S. smoothing can, which had been attached to the chassis by wrapping it round with some kind of a rubber belt and a length of string. The circuitry had been modified slightly too and there was an extra resistor wired in. Apart from that, everything else looked reasonably un-molested. There were a few old repairs to the frame circuit as well as a resistor bridging the frame-lin control but generally everything looked pretty good.

To start off with, the “extra” smoothing can was unstrung and disconnected, and the original cans given the once over. Reforming the caps for an hour-or-so using my death-trap capacitor reformer revealed that all four of them (at first glance at least) seemed perfectly good. The “extra” resistor was removed and finally power was gingerly applied.

On switching on, the large compliment of valves lit up wildly before calming down to a reasonably healthy-looking glow. Thirty seconds later and the sound of the EHT oscillator started up, followed by a most ferocious arcing noise coming from the tiny 2.5 inch Mullard MW6/2 triode tube. On examining the tube it was discovered that the glass round the insulated EHT “bowl” was cracked, and after trying to remove the EHT connector, the glass bowl came away from the rest of the tube entirely. There was no hiss or any other noise so the tube had presumably lost its vacuum, which was what was causing the nasty arcing.

The tube was then replaced with a second-hand one I had lurking in the loft and the set powered up a second time. The EHT generator started up again but this time there were no horrible noises. Unfortunately there was no picture of any description either. This set, like most projection sets, uses a “protection” circuit so that the tube isn’t damaged by the extremely bright line that would result if the line or frame sections fail. The White Ibbotson uses an EB91 double-diode to combine the outputs from the frame and line sections which in turn provides bias for the grid of the tube. If either of the scans fail then the voltage on the grid biases off the tube and no picture is displayed.

In this case, neither the line or frame circuits were working as both the resistors in the HT supply had gone open circuit. There are 3 main HT supplies in this set – One is a 350V line, another of 220V and a third at about 270V. There is a large green 6K wire wound resistor which supplies the 220V line and a similar one of 1.5K value for the 270V line. Both of these were open circuit. These funny-looking green resistors seem remarkably unreliable as every single one in the set (except one small 120 ohm one) turned out to be open circuit.

Once the two resistors had been replaced (not for the last time it turns out), the set was connected up and turned on again. This time a terribly bright and out of focus blob appeared on the screen (as viewed by looking into the lens). Adjusting the electrical focus and brightness controls brought the blob into some kind of shape, and pointing the set at the curtains in my lounge showed a wobbly raster. The Aurora was then plugged in to reveal a pretty wibbly test card with terrible vertical and horizontal linearity and very poor sync – but it was easily recognisable as a test card. The volume was turned up and some lovely loud undistorted test card music blared out the front.

After the initial jubilation, work started on the circuit in earnest. First of all a selection of capacitors in the feedback circuit of the frame stage were replaced. This wasn’t as easy as it sounds however; if ever a booby-prize were to be given out to a set-maker for the worst tag-strips and wiring practices then I think White Ibbotson may take the crown. The tag-strips themselves are incredibly weak – Apply the slightest amount of pressure to them – when trying to unravel the legs of components wrapped around them for instance – and they will quite happily snap in half. Add to this that W.I. seem to delight in trying to fit as many wires into one tag-strip contact as they possibly can then only bad things can follow. Jammed in to one of the contacts I counted no less than six components and one multi-strand wire connecting to the same point. It is a real mystery how they managed to get them in when they were dry, and how anyone is supposed to remove an individual connection from the mass is completely beyond me. In the end, and after snapping off two contacts completely, I decided that discretion probably was the better part of valour and if a tag had more than three connections jammed into it and there was no room for another component leg then I would cut off the leg of the old component, form it into a loop and feed the new component into the newly-created loop. It’s not something I particularly like doing but I felt that it was probably the best way in this particular case.

Anyway... after replacing a number of components in the frame and line circuits and the usual video-to-sync-separator coupling capacitor to cure the “cogging” on the test card, the set was powered up again. The sync was now very good; the frame lin was still not perfect but it was much much better and was adjustable with the control; the line lin was still a bit dodgy however, with the line lin control appearing to have no effect. It was noticed now that although the focus was okay, the electrical focus control was jammed at one end and only just managed to provide reasonable focus on the face of the tube. A few high-value resistors were now replaced in the frame circuit and another one of those green wire-wound resistors in series with the line linearity control was also renewed. The one component in the focus-control circuit (a 100K resistor) was measured and was okay, so the PL83 “focus amplifier” (for want of a better expression) valve was replaced. On powering the set up again, the picture was now very good. The frame was much better, although it was now cramped at the bottom, fine in the middle and cramped at the top. This had the effect of making the circle in the test card look more like a “quartic” shape (a squared-off circle). The line-lin control now worked a treat although the line-lin was still a bit odd – and the electrical focus control was still at one end.

After poking about aimlessly for a bit, wondering what could be causing the focus problem, I decided to do a few voltage checks. The voltage on the anode of the focus valve turned out to be roughly okay but the voltage on the screen grid was way too high. The focus valve is biased such that it works in a “constant current” configuration – this is so that as the focus coil around the tube heats up and its resistance changes, the current through it (and therefore, in theory, the focus) stays roughly constant, this being set by the focus control wired between the grid and cathode. After a little head-scratching and schematic-following, the peculiar voltage readings turned out to be my own stupid error. In my haste at trying to remove the “extra” components in the power supply (the extra smoothing can and extra resistor) I had inadvertently wired the 6K and 1.5K resistors round the wrong way – Thus supplying the focus G2 (as well as half the rest of the set) with far more volts than they should have had – but also depriving the IF and RF sections of the same amount.

This time, powering up the set provided a lovely sharp test card on the tube face (with the focus control roughly in the middle) and, once the picture geometry controls had been given a good twiddle, the height, width and linearities were absolutely spot on. Dragging out an old Philips projection screen and pointing the set at it however revealed that all was not well with the optics.

The only way the set could get a reasonably focused picture was to wheel it to within about 2 feet of the screen. The picture was lovely and bright and sharp, but very small. Wheeling the set back so that the picture was roughly 4 feet by 3 feet meant that it was horribly out of focus. It was at this point I phoned my pal Tone who knows about these things. Tone gave me a brief run down on what all the mechanical twiddlers on the optical box did, and so armed with this new knowledge and a copy of the rather-confusing and not-a-little-daunting Trader sheet detailing the Mullard unit, I proceeded to twiddle. It actually wasn’t as bad as I’d thought – You have to twiddle any one of the “side” knobs until you get a “strip” of the picture in focus, then you twiddle both side knobs in the same direction so that the centre of the in-focus strip passes through the centre of the picture. Now twiddle both side knobs the same amount in opposite directions until the strip of focus is either horizontal or vertical (or even diagonal) depending on which way the optical box is mounted relative to the direction of the picture – see the Trader sheet for details – and then adjust the “centre” knob until the focussed section widens out to fill the whole screen. This last bit also needs a bit of very slight (but careful and judicious) twiddling of the two side knobs as well as moving the whole TV fractionally towards or further away from the screen until the whole screen is focussed.

Having performed the focus to the best I could get it (I suspect Tone could get it even better, although it looks pretty reasonable to me), the set was reassembled and given a good polish.

And that’s it. I still have a slight issue with flyback lines on very dark scenes but that’s for another day I think (unless anyone has any ideas).

Have a brilliant Christmas everyone and thank you for making it through to the end of my rambles. I’ll post up some pictures when I can work out how to get them off my new Lumia phone and on to my PC.

Thanks all.

Kind regards.

From Mike.

[Radio Wrangler]

Epic!

David

[MonochromeMarc]

I really enjoyed your write up, nice one !

Marc.

[Tazman1966]

Another labour of love Mike and a smashing write up as usual. Nice to see another one of the sets that had been languishing untouched for absolutely ages revitalised and rejuvenated.

[McMurdo]

Very nice. I've not seen a projection set first hand. A few years ago I sold a BNIB mullard projection tube on ebay that looked a bit like that one.

[Phil G4SPZ]

Very interesting case study on a rare set, Mike, and a nice readable write-up.

[Peter.N.]

Brilliant picture for a projection set. We had a number of customers with rear projection TVs in the '50s quite complicated for the time with 400v HT and protection circuits to bias the tube off in the event of scan collapse. I think the CRT was an MW6/2.

Peter

[beery]

Hi Mikey,
Great job and great write up.
I'm amazed at how bright the picture looks even at that size.

Dodgy tagboards are a real pain and sometimes even when you are very careful with them, they just won't play the game. I've often been caught out by them
I'm glad you stuck with it.

Regards
Andy Beer

[Heatercathodeshort]

Oh boy, what a cracker!
[I have managed to get the Decca 131 into full working order. It just needs a fiddle with the mechanical focusing but the results are good. More on this after Christmas.]
This is how projection receivers should perform but at the time service guys had problems getting to grips with them and to be honest, with a busy service schedule, I can sympathise.
Well done with this one. Regards, John.

[peter_scott]

A really excellent result. So much better than the Philips 1400A that was my parents' first set. It also suffered from visible flyback but the thing that was so surprisingly bad was the lack of d.c. restoration. If White Ibbotson could do it and Decca could do it on their 100 why did Philips penny pinch?

Peter

[newlite4]

Incredible write-up Mikey, so many challenges with that set, such a blow to encounter the cracked tube, thankfully they are not rare.
Good report,
Neil

[Jac]

Thanks for the write-up Mike.

You achieved to get an excellent picture!

Jac

[brianc]

An excellent result and write-up Mike. What a super picture - far the best I have seen from a 4x3 projection set. It reminds me that I have a Philips 6027A which is also a 3x4 projection set (complete with matching screen). I will be disposing of it in the new year but seeing your results, maybe not!
Looking more closely at your first picture, the screen you have looks to be like mine - for a 6027A

[FERNSEH]

Hi Mike,
enjoyed reading your write up. That's inspired me to sort out my two projection sets, the Decca 131 which has the faulty EHT multiplier box and the Philips 600A. The latter is the really interesting one because it has the smallest screen one will ever find on a projection set.

Returning to the topic of the White-Ibbotson projection TVs. There is some useful data about these sets in the red Radio and Television servicing books.
I'll do some swotting up straight away.

DFWB.

[Mikey405]

Hi David, Marc, Taz, McMurdo, Phil, Peter, Andy, John, Peter, Neil, Jack, Brian and David.

Many thanks for all your kind comments. They're not really deserved however - it was a quite straightforward fix and there were no nasty IF faults or anything horrible to try and fix.

In answer to a few comments, the picture isn't quite as bright in the daytime as it shows on the photo and you probably wouldn't really be able to comfortably watch the set in a room with full sunlight. It is much better by far to watch it in the dark but a semi-darkened room is fine too. Luckily the room where the picture was taken only has 4 small windows and it was looking very gloomy outside - despite how it looks in the photo.

The construction of the set is really quite odd - and I can't think of any other TVs which have a glass-and-embroidery top!

As for service data, alas White Ibbotson never seemed to make two chassis exactly the same. The two varieties in the R&TVS "red" books are only superficially like the 4836F and neither of the two manuals I have for other models are exactly the same either (although one of them is near enough the same, save for a few minor modifications here and there).

I'll try and get round to posting a few more pictures of the set and its chassis shortly.

Thanks again everyone.

Kind regards.

From Mike.

[TNC]

Mike,

What an excellent write up and a fantastic restoration. I have never witnessed
a working projection set but your pictures show they can really be quite stunning!

Many thanks!

Trevor.

[Mikey405]

Thanks Trevor. You'll have to come and have a play with it at the next Solihull meetup in May.

As promised, here are some more pictures of the innards of the set and one of the cabinet itself.

Thanks all.

Kind regards.

From Mike.

[Mikey405]

And a final picture of the front with the two little doors open.

[Duke_Nukem]

Jolly good Mike ! I do have a soft spot for the old projection sets.

Guess I'm going to get nagged about a certain Etronic now, no excuses

TTFN,
Jon

[Hunts smoothing bomb]

Hello Mike,

Good writeup and interesting to read, never lost interest at all through that.
I never thought the images produced by these receivers could be so good, glad you had a spare tube that appears to be very serviceable. Whats the valve count on this electric beehive?

All the best,
Merry Christmas!