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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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2nd Mar 2020, 7:36 pm | #1 |
Dekatron
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Sobell T121.
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. DFWB. |
2nd Mar 2020, 9:27 pm | #2 |
Heptode
Join Date: Jan 2008
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Re: Sobell T121.
A nice unusual looking set. It's amazing such a big piece of bakelite has survived!
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2nd Mar 2020, 9:59 pm | #3 |
Dekatron
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Re: Sobell T121.
The Sobell T121 keeps company with another bakelite console TV set, an Admiral 10" CRT model of 1949. Note the twelve channel tuner, made by Standard Coil Corporation it is very similar to he UK made Cyldon type C.
The American set is very heavy, weighing in at almost 100 pounds. The second attachment shows the pre-set control panel of the Sobell. DFWB. |
2nd Mar 2020, 11:00 pm | #4 | |
Hexode
Join Date: Dec 2011
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Re: Sobell T121.
Quote:
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3rd Mar 2020, 1:47 am | #5 |
Dekatron
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Re: Sobell T121.
"I was just wondering if the line scan coils have become magnetised and need de-magnetising. I had a similar problem with a Pye set several years ago and allowing a high voltage capacitor to discharge through the coil solved the problem. "
Hi Dave, I'm certain that you are correct, the line scanning coils are magnetised. Although a DC blocking capacitor doesn't appear in the circuit diagram this set has one, a 2.2 microfarad electrolytic. Tomorrow, I'll hit the scan coils with a charged up capacitor. Some time ago I did this to a Pye BV20 which had the off-set picture problem. Bang and the picture was centralised. DFWB. |
3rd Mar 2020, 10:40 am | #6 |
Pentode
Join Date: Aug 2010
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Re: Sobell T121.
There is one of these (Sobell T121) in Shropshire on eBay starting £250 or £350 BIN.
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3rd Mar 2020, 10:03 pm | #7 |
Dekatron
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Re: Sobell T121.
Whatever set the line transformer was intended for it's clear that the EHT rectifier valve did not have a 6.3 volt 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. DFWB. |
6th Mar 2020, 12:08 pm | #8 |
Dekatron
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Re: Sobell T121.
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. DFWB. |
6th Mar 2020, 4:02 pm | #9 |
Heptode
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Re: Sobell T121.
Oooooh - nice ..... very nice work [as usual] David!
(Yep - save the mustards for a rainy day ......)
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6th Mar 2020, 5:20 pm | #10 |
Dekatron
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Re: Sobell T121.
Hi Tractionist,
there's still a few still a few more polyesters to take out and replace. All low capacity types, 3300pF and 10,000pF 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 these high value capacitors. Low voltage but high capacity types. Charge up slowly and discharge fast. DFWB. |
7th Mar 2020, 9:52 am | #11 | |
Dekatron
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Re: Sobell T121.
Quote:
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7th Mar 2020, 1:38 pm | #12 |
Dekatron
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Re: Sobell T121.
That chassis looks like a reliable design David. Sobell produced some of the best receivers in the trade but were often overlooked by the snooty dealers.
David, are there any blanked off holes drilled in the side of the cabinet that may have been used to secure fancy handles? I believe that massive Bakelite cabinet could have served a dual purpose in 'funeral homes' as the Americans call them. Regards, John. |
7th Mar 2020, 2:24 pm | #13 |
Dekatron
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Re: Sobell T121.
Giving the scan coils a flash of HT didn't have any effect on the position of the picture, still got the corner shadow. So what next? Introducing a DC component into the line scanning coils could be considered.
DFWB. |
7th Mar 2020, 7:11 pm | #14 |
Hexode
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Re: Sobell T121.
Did you try reversing the capacitor? One way aids magnetism the other cancels it out!
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8th Mar 2020, 3:03 pm | #15 |
Dekatron
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Re: Sobell T121.
Unless I'm completely missing something here, surely you need to feed the scan coils with a decaying AC current for demagneisation, unless you flash the DC in such a direction that it reverses the residual magnetisation polarity to give it a shift in the other direction.
Could it be that it's due to not having the original tube type fitted - did it ever not have that corner shadowing with that particular tube? Although the actual 'shift' in one direction wouldn't likely to be anything to do with the tube. |
8th Mar 2020, 4:32 pm | #16 |
Dekatron
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Re: Sobell T121.
Davewantsone wrote: "Did you try reversing the capacitor? One way aids magnetism the other cancels it out!"
That's something I'll try tomorrow but the suggestion to demagnetise the scan coils by Techman is much better. Quote Techman: "Unless I'm completely missing something here, surely you need to feed the scan coils with a decaying AC current for demagneisation, unless you flash the DC in such a direction that it reverses the residual magnetisation polarity to give it a shift in the other direction." Remember I removed that scan coil 2.2uF DC blocking capacitor. This component wasn't included in the original circuit and was fitted in 1994 at the time I acquired and serviced the set. The original MW31-16 CRT was broken and replaced by the late production MW31-74. This tube has a cavity EHT connection instead of the 10mm cap connector. Otherwise apart from screen colour the two tubes are interchangeable. The much maligned HMV 1807 has a very similar line output stage and is like many other TV sets at time the Sobell was made. In the 1807 line scanning circuit a DC component was introduced into the line scanning coils. R60 (2.2ohms) is in series with the HT supply. DFWB. Last edited by FERNSEH; 8th Mar 2020 at 4:37 pm. |
8th Mar 2020, 4:53 pm | #17 |
Hexode
Join Date: Dec 2011
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Re: Sobell T121.
Hi This is a copy of were I got my details for the picture shift fault on my TV
Default Re: Need Help MV30(LV30) Quote: Originally Posted by Heatercathodeshort View Post Hello Dave, I was looking through some old copies of British Radio and Television May 1955 and was quite amazed to see this reference to the Pye. I have written it out as scanning often degrades the quality. PYE MV30 series. Cure for drift. These sets sometimes develop a picture drift to the left that cannot be counteracted by the shift magnets. Some sets have come in with the focus magnets hard up against the tube neck in an attempt to centre the picture. A quick cure for this trouble is to discharge a charged-up capacitor [100 or 200uf 275v wkg] across the line scan coils. If this does not move the picture back, recharge and discharge the capacitor across the coils in the reverse direction, it will move then. Repeat this until the picture will not move any further and then recentre in the normal manner. The action of this treatment is to counteract the developed residual magnetism in the scanning coils. [RVA Birkenhead] So there you go! Regards, John. Well found, John. I connected a 1N4007 to a 120uF 400v capacitor. The capacitor negative went to the blue terminal of the line scan coils. The positive via a switch went to the disconnected brown terminal of the line scan coils. I connected a variac and connected it via the diode to the capacitor. I charged the capacitor to 250V. I then disconnected the variac lead to the diode and threw the switch. Switched off and repeated. On switching on the set drift was slightly worse to the left! Did what the instructions said and repeat with connections to scan coils reversed. Repeated above 3 times. Then reconnected TV to mains supply. Hey Presto! it worked scan had gone over to right hand side of the screen with plenty of play in shift magnets to obtain a centered picture. Thanks again.[/I] Regards Dave |
9th Mar 2020, 2:17 pm | #18 |
Dekatron
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Re: Sobell T121.
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.
DFWB. |
2nd Apr 2020, 10:59 pm | #19 |
Dekatron
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Re: Sobell T121.
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 diagram of the T121 and the suggested modification to correct the picture shift problem.. 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. Resistor values between 1 to 3 ohms will be tried. DFWB. |
6th Apr 2020, 12:37 pm | #20 |
Dekatron
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Re: Sobell T121.
The first attachment shows the off-set picture without the correction resistor.
Second pic. After raising the value of the resistor to 10 ohms the picture is centralised. Third pic. 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 is still off-set. DFWB. |