This Sobell set is needing some attention. As the first attachment shows the picture is shifted to the left and no amount of adjustment will centralise the raster. Possibly a DC component in the scanning coils?
The line output transformer is not the original and is some unidentified component that was found in the spares box. Same goes for the scanning coils and frame output transformer. Originally a Holme Moss receiver it has been retuned to channel B1.
The replacement Mullard MW31-74 CRT displays a bright picture with an EHT of 8KV. Late production tube, made in 1966. A few aspects of the design are not nice, the volume control for example which is a switched resistor chain in the cathode circuit of the IF amplifier. Also, the scant regard to safety.
A similar chassis was fitted in a table model and also a Columbia branded set which was the result of some business deal between EMI and Michael Sobell's Radio and Allied Industries concern. The has Emitron valves which made by Mullard. It's possible the original CRT was also an Emitron.
The story of the shares exchange transaction can be read in the Setmakers book.
Till Eulenspiegel.
Very nice!!! I'd love one of these, but have no where to put it currently! There was one for £450 at last years NVCF, but I couldn't bring myself to part with that much, which I suppose was a good thing, if it had been a bit cheaper (or should I say, if the price had fallen into what I had in my wallet at the time!) I'd have bought it, and then be wondering where it was going to live! The one on eBay currently is very tempting, but the space issue is still ever present...
As for the shifted picture, could the scan coils have become magnetized? Remember that Philips 385U that I was working on way back when, and I had messed up the scan coils with a big magnet, and ended up having to buy a colour CRT degausser to sort it out. Got to be worth a try if you have a degassing coil knocking about.
Regards,
Lloyd
Hi Lloyd,
Magnetised scan coils? That's definitely one possibility. Also, I must check to see if the DC blocking capacitor exists in this set. Will report my findings about the off-set picture fault tomorrow.
The original line transformer had bifilar windings in series with the top end of the heater chain, the Pye FV1 series has something similar. The PY80 booster diode has limited heater to cathode voltage ratings so the bifilar windings introduce a balancing voltage to compensate for this. The transformer currently fitted hasn't the windings but it would seem I've got away without such things.
Till Eulenspiegel.
Whatever set the line transformer was intended for it's clear that the EHT rectifier valve did not have a 6.3volt heater. The winding has only five turns so it's likely the EHT rectifier was a Mazda U25 or the like.
Two neat brackets have been found for the repositioned line output transformer.
Till Eulenspiegel.
That looks so familiar, and yet I can't place it, though it reminds me of a TX in a very early Ultra TV set that my brother once had. It was a roundie, and it was badged as a Bermuda model, BBC only, but I somehow doubt my own memory.
Hi Marion, whatever make of set the transformer was taken from it follows the design of a line output stage which was the typical circuit arrangement in the late forties and fifties. My take of this line output stage is the line output valve functions an amplifier rather than as a switch which was normal practise in line output stages in later sets. In the Sobell T121 "Stargazer" the PL81 receives it grid bias from a 270ohm cathode resistor. Not grid current biasing which became the norm later on.
Till Eulenspiegel.
The Sobell T121 was restored to working order in 1994. Lots of Mullard polyester capacitors were used, excellent components but I believe these should be employed in other resto projects.
So it's out with the polyesters and in with restuffed Hunts, TCC and Dubilier capacitors. The under chassis looks more authentic now.
How about these high capacity low voltage capacitors to give the scan coils a jolt?
Till Eulenspiegel.
Ooh, David, are you going all 'audiophile' on us? That's the favoured haunt of the much vaunted Mullard "mustards" these days, isn't it?
Now then, it's looking more like original stock under the floor now, and even I have to admit, it's a big improvement over what it was, visually, that is. Will you be changing those last few mustards as well?
Hi Marion,
the Audiophools are not getting those salvaged polyester capacitors. Instead, I'll use them in Philips radios and TVs where they should be. The recent restoration of the Stella ST1007 TV set required that all the leaky black tar capacitors had to be replaced and it was only right a set made by Philips should have Philips replacement parts fitted.
Still a few polyesters to take out and replace. Meanwhile, all this work resulted in no improvement to the off-set picture fault. It's time to give those scan coils a blast from one of those high value capacitors.
Till Eulenspiegel.
A simple modification as shown in the second attachment will centralise the picture and remove the corner shadow. The value of the resistor Rs will be something like 2.2 to 3.3 ohms.
The additional resistor is effectively in series with the scanning coils and introduces a DC component into the waveform.
The HMV employs the resistor R60 to compensate for some of the line output valve anode current flowing through the scanning coils.
Till Eulenspiegel.
@till I like the thinking! Such a simple idea, and quite logical when you think about!
Put the Ever-Ready transistor TV aside and turned my attention to the Sobell T121 and it's off-set picture fault.
Refer to my previous post showing the circuit diagrams of the T121 and the HMV 1807 and Marconi VT53A.
Found a very convenient wire link between the HT smoothing capacitor C57 and the tag strip which serves as the start point for the HT line. Remove the wire link. Remove the HT feed wire to the line OP transformer. Solder a wire from the vacated tag on the transformer direct to C57. From C57 take another wire to the new tag strip fitted close to the frame output transformer. A wire is soldered to the other tag and goes direct to the main HT line. The picture shift resistor value will be determined by trial and error, or A.O.T. Resistor values between 1 to 3.3 ohms will be tried.
We'll know tomorrow if the modification works.
Till Eulenspiegel.
From my isolation workshop. The first attachment shows the off-set picture without the correction resistor.
After raising the value of the resistor to 10 ohms the picture is centralised.
The left side castellations appear compressed, critical adjustment of the line hold control flips the picture to the right.
After short circuiting the correction resistor the scanning coils were removed from the neck of the tube and then refitted back to front. The picture still appeared off-set.
Till Eulenspiegel.
