A Tale of Two Sobells

[Mikey405]

Hey everyone.

I’ve been threatened with nasty things if I didn’t write up the tail of how two pals of mine came round over the weekend and fixed a pair of identical Sobell T121s (one of them my own) so here goes.

A few weeks ago I was phoned by Jon Evans (Duke_Nukem of this forum) who suggested putting our heads together to sort out a pair of Sobell T121s at the same time (one belonging to Jon and one to me) – The idea being that we could compare the sets and any faults etc. and help each other out with issues. Another mate of mine, Tony Price (also of this forum) was due to visit on the specified weekend for some TV-fixing anyway so we decided to set to it and, between the three of us, sort out the Sobells.

The Sobell T121 is a 12-inch console from 1951. It’s made entirely of Bakelite (the cabinet that is) and is really quite smart in a strange ugly kind of way (whatever that’s supposed to mean). The set looks like a bit of a cheaply-made affair but everything is reasonably standard and quite easy to work on.

The first thing we did (after both sets, half my workshop, Jon’s entire spares department, 2 spare tubes, an Invicta set with another spare tube and much more besides had been lumbered into my lounge) was to physically examine the sets. There was nothing too remarkable about either of them; they were both pretty-much intact and, save for a knob here and a back cover there, they were both complete and undamaged. Next we removed the chassis from both sets and had a good poke around with those. The tube, speaker and chassis from my set had, previously in the set’s life, been scooped out and a colour portable TV installed inside the cabinet. Luckily all the original parts had been stored and, other than a few snipped wires and a couple of wood-screws having been driven through the volume control (simply to hold a pretend volume control knob on to the colour-conversion), all parts were present and correct. Jon’s chassis was initially reluctant to come out because of a stuck knob. Pulling it this way and that did nothing other than break the pot to which the knob was attached (the contrast control). A little help from some “Plus-Gas” though was all it took to release the knob from the shaft.

Jon then set about cleaning his chassis which was extremely dirty (owing to the grease which had been used to insulate the components mounted on the line output transformer having leaked all over the place) whilst Tone and I had a go at joining up all the cut wires on my chassis. Some of the bolts / nuts / washers that held the chassis into the two sets were quite rusty so they were also removed and then lowered into a beaker of phosphoric acid to fizz gently for a while.

Next we decided to check the CRTs to see whether they had any signs of life. I didn’t have the required B12A adaptor for my trusty B&K tube tester but I did have a B7G plug and a spare B12A tube base so Tone quickly knocked up a converter and we connected up the tubes. The tube in my set measured about half-way into the “Bad” section but the tube in Jon’s set hardly read anything at all so we decided to give Jon’s tube a bit of a small buzz. One very quick and gentle blast on the “Clean” setting and the tube was reading toward the good end of “Bad”. We decided to leave it at that and see what the pictures were like before doing anything else.

Once the chassis were both clean and complete we set about reforming the smoothing caps. Jon used his fantastic 350V variable power supply with a 10K resistor whilst I used my ludicrous contrivance made up of a small isolating transformer, a 1N4009 diode, a 10K resistor, half a dozen croc-clip leads and a 13A plug. However, shortly after Jon turned his fancy power supply on, smoke started pouring out of the unit and filled the lounge with a disgusting stink. Luckily it was only the mains filter cap which had exploded, although I still can’t get rid of the nasty pong.

Next came the true measure of our individual TV fixing abilities - Jon now connected the HT power supply up to his set, diligently checking for leaky caps and other faults that might cause disaster when we turned the sets on for real. Simultaneously using readings from the multimeter and power supply, Jon took measurements at various control grids and other points in the set, working out the voltages which should be on this, the current passing through that, and generally being very clever about the other. I went off and made the tea.

Once Jon had finished showing me up, we decided to plug both the chassis into the mains. We checked to make sure all the valves were in the correct positions and then each poked a bit of wire into the tube-base socket between pins 1 and 12 to short out the heater connection before plugging the chassis into the mains.

After a reasonably long warm-up period (mostly due to the thermistor in the heater chain), and after replacing a faulty EB91 on Jon’s chassis which lit up like a candle because of a faulty filament, both sets had a bit of a crackle from the loudspeaker and a faint line whistle. Neither sets had any signs of EHT though. The sets were then both powered off again and one or two likely caps were changed before work stopped for the evening.

The next day, after everyone turned up and we’d all had breakfast – coffee (me), toast (Jon) and cigars (Tone) - we set about checking and changing various caps. Jon went about his in a very logical way whilst I blundered in and just replaced them wholesale. Jon also replaced a few resistors in salient positions whilst Tone went off to the workshop upstairs to rewire all the resistors around the edge of a new volume control switch for my set (the volume on these sets is controlled by a 6 position switch instead of a pot), and I sat around loafing for a bit.

After a fair number of leaky wax caps and nasty little hunts caps had been replaced we decided to power the chassis up again. Mine came up with a moderate line whistle but hardly any spark from either end of the EY51 and not a huge amount of life around the line output stage. Jon’s chassis burst into life with a nice big spark and plenty of EHT. Now we decided to try Jon’s chassis in the set to see what would happen.

After everything was connected up we switched on the power and, after a minute or so, were rewarded with a blank screen and some crackling from the speaker. Tone decided to make sure the ion-trap magnet was correctly aligned and so waggled and joggled it about whilst I looked out for any signs of life on the screen. We didn’t get anything at all until I accidentally spotted a flash on the screen whilst I was playing with the brightness control. More fiddling revealed a spot on the brightness control which brought up a bright diagonal out-of-focus line. Rotating the scan coils made the line horizontal but we still had frame collapse. It was a good start though. The chassis was whipped out again and Jon started work on the frame section, whilst I set to work on the EHT problem on my chassis.

After making all sorts of checks in the line output section, it was looking more and more like the lopty was duff, although the section didn’t appear to be in any real distress and there was a fairly loud whistle. On lumbering down the ‘scope from the workshop upstairs we decided to do a ring test on each of the loptys; this revealed identical waveforms which cast an enormous doubt on it being the lopty at fault. We’d already checked the negative voltage on the control grid of the PL81 and decided that it was probably okay (or at least reasonable) at about -40V but a comparison with Jon’s set showed that it should be more like -60V. Tone then had a look at the circuit and pointed out a 1M resistor (a feed to the line oscillator) which could be the culprit. The resistor was checked and found to measure 1.6M so it was replaced. The chassis was again plugged in and this time there was a lovely big spark from the anode end of the EY51 and 6.6KV at the cathode end.

My chassis was now reinstalled into the cabinet, an aerial connected and the set switched on to also reveal a blank screen. Various voltage readings were taken on the tube before it was discovered that there was no wiper on the brightness control. The control was fixed by Tone using parts from another donor pot and the chassis reinstalled. This time we actually had a picture – a half-height unsynchronised picture but it was recognisably a test card and there was nice loud sound too. The chassis was removed again, a few more components were checked and some replaced and the chassis reinstalled. Now we had a slightly taller and slightly better synchronised picture, but still a bit of work needed to get it right.

Meanwhile Jon had unfortunately discovered that the frame output transformer on his set had an open circuit primary. I went up into the loft to try and find another suitable transformer and found an old Pye FV1 chassis which we thought might yield something useful. The transformer was removed and tested and... found also to have an open circuit primary.

And that’s where we left it on Sunday evening. We will have another weekend bash in a few weeks time and I’ll finish off the tale.

I have some pictures and Jon took some as well so I’ll upload them to the Interweb in a minute and put a link up here. I’ll see if Jon can upload his pictures too.

Many thanks to Tone and Jon for all their expert brains, their sterling work and their tireless patience in the face of my total incompetence. Hopefully we’ll get both sets working properly by the next time.

And thanks everyone for reading.

Kind regards.

From Mike.

PS. Here are the few pictures I took – Jon has some more and I’ll get those uploaded as soon as Jon can send me them.
http://www.oldtechnology.net/misc/So...121_Fixing.zip

[Lloyd 1985]

Great work between you all

I quite like the styling of these sets, and would love to get my hands on one! only trouble is I don't have anywhere to put one now

You'll have both those going soon, and I can't wait to see what the pictures like on them!

Regards,
Lloyd.

[FERNSEH]

Hi Mike,
I've only got one Sobell T121 console TV. It hasn't been switched on for years. The line output transformer was replaced with one that was found in a box full of various transformers. Anyway, I fiddled around with the circuit and made it work. I remember having to replace the scan coils in order to match the replacement line OP transformer. Next, the frame output transformer had to be replaced to match the scan coils.
The circuit of this set is minimalist to the extreme. It has a strange volume control arrangement, the cathode resistor of the sound IF consists of a switched series resistor chain. It's not a very good system as the sound I F amplifier valve is an EF80 which does not have vari-mu characteristics.

This Sobell set was also marketed as a Columbia, The C501 I believe. The Sobell company did a deal with EMI, it was something to do with completing a supply contract with a major retail chain I believe. The story is well documented elsewhere.

With regards to my set, I'll take pictures of it tomorrow.

DFWB.

[Duke_Nukem]

I've emailed Mike a link to my photos but couldn't resist posting a few. As you can see my set had a bit of an issue with its diagonal hold Mike's dead posh camera will do a better job of off-screen shots.
It is a bit basic in there but seems to work and I have to say the sound quality is quite impressive thanks to quite a generous speaker.
At the last NVCF I got a Sobell T120 for a couple of quid (!) hoping it might be a donor for the Stargazer. Wrong, totally different chassis, the earlier T120 being a Plessey chassis. But the frame o/p tranny seems "about right" and is also donating a EHT cap to replace the temporary crock clip on mine.
The frame o/p is a ECL80, the triode of which is the line osc and shares the cathode. Since my line osc is a bit reluctant I suspect (hope!) the o/c frame tranny means the bias conditions in the line osc circuit are wrong, so the replacement frame tranny will hopefully improve the line stage as well.
After a couple of years of lacking enthusiasm, the weekend was just what I needed to start to get back into it. Thanks Mikey and Tone.
TTFN,
Jon

[FERNSEH]

Fired up the Sobell T121 this morning, it still works OK. This set was originally a Holme Moss receiver and was retuned to Channel B1 at the time of its restoration.
The other bakelite console set is an American Admiral 10" tube model made about 1949. Note the twelve channel tuner. The tuner unit was made by the Standard Coil Company and is very similar to the UK made Clydon type C.
In fact I used parts from a scrap Cyldon C to effect some repairs to the tuner. The CRT in the Admiral is a 10BP4, this tube uses a twin magnet ion trap magnet assembly. Although made for the 525/60 line system the set will operate satisfactorily on 625/50. Only the frame hold needs adjusting.

DFWB.

[Duke_Nukem]

The T121's cabinet seems to have been designed with another chassis in mind. The very bottom is extra thick bakelite which has a slot towards the front and three holes towards the back. Its as if the original intention would have been a two-chassis design with a PSU chassis slid into the bottom. You can see it in the pics here ... it looks like a piece of wood in the bottom of the cabinet but that's just because it was also covered in the same horrendous gunk that had also liberally dispersed itself over most of the chassis.
Anyhow, I have to say that I was quite impressed by the sound quality of this set (or rather Mikey's set as I didn't get that far).

TTFN,
Jon

[Heatercathodeshort]

What a carry on....Rooms full of television engineers, Enough tubes to keep us all going for years..Scopes hauled out of lofts..Slide rules...Rooms full of smoke from dodgy tube testers.. Sleep overs...I don't know, it was all a lot easier in my day!

Seriously, well done with those two Bakelite beasts. They had a nasty reputation but I have never understood why. A simple circuit and decent quality construction. I think most of it came from the snooty dealers who had what they considered to be blue chip agencies. [HMV 1807. That knocked their good names down a notch] Sobell gave a 2 year parts and labour guarantee on a range of models and this also upset them.

I'll be watching the progress but please jettison the beer and sandwiches in a smoke filled room. It will only end in tears. Ha! Ha! Regards, John.

[FERNSEH]

Can anyone identify the line output transformer I've fitted into my Sobell T121?
It certainly works alright. The EHT is 6.5KV at full brightness and it rises to 8KV with the tube cut-off. For the time being I'm using a silicon EHT rectifier, I'll get around to fitting the correct EY51 valve someday.

DFWB.

[audiomagpie]

Originally Posted by FERNSEH

Can anyone identify the line output transformer I've fitted into my Sobell T121?
It certainly works alright. The EHT is 6.5KV at full brightness and it rises to 8KV with the tube cut-off. For the time being I'm using a silicon EHT rectifier, I'll get around to fitting the correct EY51 valve someday.

DFWB.

A fergy 988T?

Greg

[FERNSEH]

Hi Greg,
It might be from a Ferguson 988T, it was one of a number of old line output transformers that had been under the bench for years.
The windings were configuered in a similar manner to the original Sobell part so in it went. Although the subsitution was not that straight forward.

DFWB.

[Heatercathodeshort]

Not a Ferguson 988T David. I know that LOPT very well but just cannot put a name to it. Very frustrating.
Oh by the way. When we discussed if you were going to purchase that Admiral at Harpenden all those years ago you agreed it was just a 'once seen must have' item that you did not really wish to own and declined purchase. So what happened? You must have squirreled that out of the hall while my back was turned. Shameless!
Your Sobell 121 appears to work very well so I guess the transformer must have been removed from a 12" model with a similar circuit.
I hope the 'rat pack' get those two working. I'm sure they will. Regards, John.

[Tazman1966]

Great work and a great write up too Mike. I'm sure it was a joint effort between all three of you though Mike - you're selling yourself short mate!

Now that life here at Shoukris towers is slowly returning to some semblance of normality, I'll pop up for a visit sometime.

All the best,

Tas

[Mikey405]

Thank you for the nice comments everyone and well done with your Sobell too David.

All the smoke, beer, sleep-overs, scopes in lofts and tubes falling out of sets just adds to the enjoyment John.

Thanks all. I'll report back here any progress which is being made.

Kind regards.

From Mike.

PS. Here are the other pictures that Jon took.
http://www.oldtechnology.net/misc/So..._Jons_Pics.zip

[Mikey405]

Just a quick update. Nothing much new replaced but a significant jump in picture quality by replacing one or two caps and resistors in the sync-separator and the main smoothing can. The 100uF cap was completely open-circuit.

Still stuff to do to sharpen the picture up a little bit and to get the sound a bit louder - it's very quiet just now - and a few other minor odds and ends but it's looking reasonable so far.

Thanks all.

Kind regards.

From Mike.

Pictures here:
http://www.oldtechnology.net/misc/So...1_Fixing_3.zip

[Duke_Nukem]

Having extracted a frame transformer from an old Sobell T120 plessey chassis, I thought I'd pop round to Mikey (where my Stargazer currently languishes), plug in the transformer and settle down to watch some programmes whilst sipping a hot cuppa Well, ok, at least get some vertical movement of the electron beam. Result ? Very little progress :-(

[See attached cct]
V8b (ECL80) is frame osc, V9 (also ECL80) doubles as frame o/p and line osc. P6 that looks like its in the line osc is the _frame_ lin, TM1 is line hold, P4 is the height and P3 is frame hold.

Starting with the brightness (P7), this is super critical. With pot between 0-270 deg, no screen illumination. At 271 degress a bright white line. At 271.00001 degrees the line oscillator stops (or, to be accurate, the line whistle dissapears but there is some drive)! You can usually get the line back by reducing the brightness again.

The brightness pot measures fine as does it's series resistor R2 (and, indeed, the other resistors in the CRT area i.e. R23/R19 and R25/24. The EHT visconal (C43) has been removed, C39/R51 (drive to LOP valve) replaced as has C40 and C41/R54. The line o/p valve appears to be a brimar 21A6 (if I remember right) instead of a PL81. Boost diode checked via substitution, plus there is plenty of HT volts.

Secondly, with a replacement frame tx from a Sobell T120 (a plessey chassis) I can see the frame osc running and responding to the hold control, but the anode of V9 (frame o/p) shows only a waveform at ~HT potential with around 30V p-p of _line_ frequency waveform on it which of course won't do jack in the face of frame tx / frame scan coils. It could, I guess, just be the line coils coupling into the frame coils and though the frame tx into effectively an o/c due to the frame o/p pentode being cut-off but the bias conditions on the pentode seem about right and the valve itself checked by substitution. C51 was peculiar even by usual Hunts standards (base of height control), its reading hovered between 160nF and 220nF probably based on prevailing wind direction - quite some feat since its supposed to be 20nF. Needless to say that has been replaced, as has C25 coupling cap. C26 (cathode bypass) also replaced. R35 - grid bleed for frame o/p - is a 10Meg resistor but checks out as 11Meg so I haven't changed it yet (not the easiest to get at), however the loading from a scope probe doesn't seem to have any effect at this point so I'm guessing it is for now ok.

So, anyone care to venture a possible solution ? Other than discovering the frame lin pot only works over a third of its range, and replacement of a few more components, its frustrating to be no further forwards.

TTFN,
Jon

[Duke_Nukem]

Round 3.

First things first. One cap I replaced wasn't properly soldered and could have caused lack of line drive - my excuse is I'd forgotton my specs so I couldn't actually see what the hell I was soldering, but remembered them today and soon spotted that issue. But no line whistle, no EHT.

The line osc was running fast - adjustable, but not adjustable enough. Can't help feeling it is influenced by the boost volts, which is influenced by the lop stage, which is influenced by the line drive, which is influenced by the boost .... etc !

Temporarily removing LOP valve cap and its screen grid resistor so I could take a bit more time, the drive waveform was upped by reducing one of the resistors (the set had originally 680K but I replaced it by 1M as per cct diagram and Mikey's set, so bridged the 1M with 2M2), also popped 180pF across the horizontal osc tunning cap so we could get something approaching 10KHz out of it. The line drive is warbling a bit - which varies with frame osc so perhaps the HT isn't as decoupled as it oughta.

Anyhow, reconnect up the LOP valve and - nothing.

With lots of frame-line interaction, suspicion fell on the frame coils (getting desperate by now) so the chassis was put in Mikey's set - with no improvement. Putting Mikey's chassis in my set produced a testcard, so scan coils ok and proof that CRT is OK (despite a very _very_ loose base which made me worry that a wire may have snapped by now).

Valve swapping time. Boost + HT rectifiers + Lop valve borrowed from Mikeys set, switch on - and defining warble from the LOP stage, EHT and a 2 inch raster which soon filled the screen after adjusting the height control. Thats more like it ! Straight away took the opertunity of fiddling with the ion trap.

OK, so who is the culprit ? Well, the HT was higher and we'd found with Mikey's set that anything less then very good HT meant no EHT, so Mikey had his PY82 back and I fitted another brand new one. No EHT, near silent LOP stage. D'oh, try another PY82, same. OK, put Mikey's back in. _Nothing_ !

I suggested to use the stupid piece of bakelite crap as a coffin or a large plant pot (though I beleive I inserted a few colourful metaphors in amongst that) but Mikey took a far more positive attitude and started proding around the LOP. Suddenley a burst of life. And some sparks. Though difficult to tell for sure it seems the Ey51 may have been twisted and been on the verge of shorting out the heater winding, presumably damping the LOP stage. Quite conceivable the short then came/went randomly as the chassis was manouvered. Untwisting it - and re-afixing the lead that then broke as it was untwisted (these things often seem to fracture anyway), and now we have EHT and healthy (if warbly) whistle every time the set is switched on, except the one time when there was a huge flashover from the 21A6 LOP valve which was prompltly replaced with a decidedly third-hand-looking PL81.

With a raster - of sorts - the set was treated to a proper channel 4 brummy signal. True to form, the set completely ignored it. Tone suggested some suitable places to apply a sig gen and with it turned up enough you could only just get some audio but the only screen modulation was with the sig gen connected further into IF chain. The gain control was found largely open circuit but working at the point where gain was max. Further investigation and few cap changes later found the culprit - the 1st IF transformer primary o/c :-( Tone suggested bunging a resistor across it and hope there was enough signal to couple and at least produce some kind of result. So, with the universal 10K resistor bodged in, power applied, and a faint audio signal was obtained and bits of a locked test card can be made out (clearer than the photo suggests).

The brightness control still behaves almost as an ooff-on-off switch though, and the test card - yes, it is a testcard - suggests that the horizontal osc is running fast again :-(

At which point tiredness stopped play.

TTFN,
Jon

[Mikey405]

Hi all.

Just off to bed so I'll keep it short.

Today Tas, Tone and Jon all came round and we had a go at the two Sobells (and a GEC 2041 Colour TV). I'm sure Jon will post up all the progress we made with the Sobells but below is a quick picture of the two sets side by side this evening.

A nicer version of the picture can be found here: http://www.oldtechnology.net/misc/Two_Sobells.JPG

Thanks all.

Kind regards.

From Mike.

[Hunts smoothing bomb]

Excellent work by all three of you!
Good write ups too, well done, great looking sets.

Cheers

[Heatercathodeshort]

By the look of Mikey's picture in post 17 I think a wedding is on the cards..John.

[petertheorgan]

Absolutely, they look so happy, I wonder if it might be easier to marry two television sets rather than two vintage television set collectors?