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Sobell T21

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PYE625
(@pye625)
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This set is another ebay aquisition and I will be collecting it on Saturday. Service data is in the 1956-57 volume of R&TV servicing, page 592 onwards. It has the Mullard MW 53-80 CRT.

It is quite a large screen set for the time, being 21 inch. I believe this table model was introduced in April 1956 at a price of around £90, which is very roughly the value of £2000 today.

It has been a little while since I had a repair/restoration on the go and this might well be interesting.

Watch this space for more.... 🙂

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Topic starter Posted : 13/08/2020 9:18 pm
Lloyd, Doz, Nuvistor and 1 people liked
PYE625
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The set arrives in the workshop having been collected from Dereham in Norfolk this afternoon. The set was always local to Dereham, so I would expect to find biscuits for Ch3 Tacolneston and Ch11 Mendlesham in the tuner. Hopefully, I will get to find out if I am able to establish operation. She really is a large set being approx. 22 inches square.

The first thing I decided to do was to test the CRT for emission. The set has had a replacement Solus branded CRT in the past. 

I noticed that with 6.3v heater voltage from the B&K, the heater was very dim and with no measurable emission. I was worried that the CRT might be down to air, so I then used a PSU to check the voltage for the required current of 300mA, as it would be in the series heater string with normal use. I discovered that it needed 12.6v to obtain the 300mA. The heater then glowed more like you would expect and I was delighted to see the emission climb up to a good level.

I was expecting the CRT heater voltage to be 6.3v as per the data for a Mullard MW 53-80.

IMG 5704 50
IMG 5706 50
IMG 5708 50

 

 

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Topic starter Posted : 15/08/2020 5:47 pm
Till Eulenspiegel
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It's possible the CRT has that partially short circuit heater fault that was well known in Mullard tubes. I mentioned this in my post about the RGD Deep 17 about an hour ago. When the CRT is connected to a 6.3volt transformer the heater will look abnormally bright because only one half of the M formed heater is active. In a series heater chain you'll find there is only 3 to 4volts between pin 1 and 12.

Till Eulenspiegel.

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Posted : 15/08/2020 6:20 pm
sideband
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However as stated, with 6.3V applied, the heater was very dim.....that would not have been the case with a partially shorted heater which would have been very bright. My theory is.....the tube is a regun and has been fitted with a 12.6V heater gun. 

I have come across this before. In theory the heater would probably be slightly underrun when used in the TV but as this is spread out across all heaters, the effect will be marginal.

 

Seems like you have an excellent tube albeit with a 12.6V heater.

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Posted : 15/08/2020 9:29 pm
PYE625 and Cathovisor liked
Till Eulenspiegel
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The regunner had fitted a Mazda CRM212 tetrode gun assembly?

Does the tube have the bent gun as used in all Mullard ion trap tubes? 

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Posted : 15/08/2020 9:36 pm
PYE625
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Hi, yes the CRT has a bent gun and retains the ion trap and magnetic focus assembly. I am convinced the CRT is perfectly fine and is simply a regunned type that happens to have a 12.6v heater. Current drawn by the heater is definately 300mA at 12.6 volts.

I have removed the chassis from the cabinet and also the CRT from the chassis. This has enabled me to clean away the dust and to inspect the chassis. It will be much safer and easier to work on the chassis with the CRT out of the way.

Here is the under-chassis....

IMG 5709 50

 

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Topic starter Posted : 15/08/2020 9:59 pm
PYE625
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Onto the next important item, the LOPT. Visually it appears in good condition and I will place a small current through the overwind in order to warm it slightly to aid driving off any moisture. I have re-formed the main smoothing can and it is in very good order so should be fine.

IMG 5717 50
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Topic starter Posted : 15/08/2020 10:25 pm
Till Eulenspiegel
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Leaky heater chain decoupling capacitors further up the heater chain could cause the CRT heater to be underrun. I had this problem two years ago in a Sobell T143.

Till Eulenspiegel. 

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Posted : 15/08/2020 10:28 pm
Cathovisor
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Ooh, not seen a proper telly on these pages for a while! It looks very original underneath.

Looking forward to this, Andrew...

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Posted : 15/08/2020 10:57 pm
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PYE625
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A glance at the previous under-chassis photo will reveal many Hunts mouldseal capacitors, with a sprinkling of wax types too. Many, if not all of the Hunts type have cracked and split open.

I do not especially wish to replace them all wholesale, but will seek out perhaps the most critical.... the boost HT and the mains RF bypass to name but two, with any in connection with the line drive.

Certainly enough to get the set basically operational and to establish that it is viable to proceed. So far, my gut instinct is positive.

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Topic starter Posted : 15/08/2020 11:06 pm
sideband
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Posted by: @pye625

A glance at the previous under-chassis photo will reveal many Hunts mouldseal capacitors, with a sprinkling of wax types too. Many, if not all of the Hunts type have cracked and split open.

I do not especially wish to replace them all wholesale, but will seek out perhaps the most critical.... the boost HT and the mains RF bypass to name but two, with any in connection with the line drive.

Certainly enough to get the set basically operational and to establish that it is viable to proceed. So far, my gut instinct is positive.

That is the best way. With radio's, I generally replace all the paper/moldseals in one go since they will be faulty and there are anly a few of them anyway. However with a TV there can be 30 or more so better to replace just the critical ones. Generally I would snip out the mains filter as it will either blow within a few seconds of applying mains or soon after. Then I usually change the audio coupling, line coupling, sync coupling, boost cap* and perhaps one or two decouplers directly across the HT rail, just to avoid as many explosions as possible. Then I run the set up slowly on a variac (a lamp limiter alternative if you don't have a variac). All of those paper and Moldseals will need replacement but it's good to be able to replace these as you go and see the improvements on the screen. 

*the boost cap in my Ekco 45 portable was an electrolytic rated at 500V. I didn't have a 2uF at that voltage so I took a chance and left it in situ since it measured OK......! If you ever need to clean an electrolytic can out for restuffing.......!! The explosion (which happened about 10 minutes after switching on) had to heard to be believed ? !! I ended up fitting two 4.7uF 450V caps in series with 100K 2 watt balancing resistors across them.

 

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Posted : 16/08/2020 10:07 am
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PYE625
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In fact what I might do is to introduce power to the chassis without the CRT fitted. This way, I can get an idea if the various stages are working and what needs attention if they are not. The line stage can be put out of operation by removing the screen feed to the PL36. The heater connection on the CRT base needs to be shorted to provide heater continuity, this won't matter as the mains will be at a lower level via a lamp limiter, or variac, so the rest of the valve heaters won't be overrun.

Not only that, the HT would tend to rise too much as there will be no current draw from the line stage, so mains input will have to be kept lower than normal anyway. I can test the front end and IF strips plus sound and vision stages with a signal input at the aerial. With the scan coils connected, the frame stage may be tested for basic operation too.

Later, the line stage could be brought into operation with certain precautions like removing the top cap connection to the EY86 EHT rectifier. It could be fitted and an EHT meter connected to the EHT lead, but accurate EHT readings won't be read unless I fit a high voltage capacitor to smooth the rectified EHT pulses. However, this is probably too much faffing about and it would be safer to simply re-install the CRT at this stage though.

The above tests should provide a rough idea of conditions, but of course accurate voltage readings as per the service data won't really apply until there is reasonably normal operation with the CRT installed.

It can be awkward enough working on a chassis with a 17 inch CRT fitted. This 21 inch is a beast and very bulky and heavy. It makes work on the under-chassis very difficult with the CRT installed. The chassis design is such that there is no easy way to balance the whole thing on it's side for access to the many capacitors that will need to be replaced.

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Topic starter Posted : 16/08/2020 10:21 am
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PYE625
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To give you an idea, here below is the 21 inch MW 53-80 (90 degree) CRT placed next to a 17 inch CRT. I do not know the exact type of the 17 inch, but it looks the same as a Cathodeon C17/1A (aka MW 43-69) 70 degree as fitted to my Pam 764 set. One thing though, it is NOT a Mullard MW 43-80 as this type is not as deep, being a 90 degree.

IMG 5720 50
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Topic starter Posted : 16/08/2020 10:51 am
Nuvistor
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@sideband

Yes I have had to clean up after a blown electrolytic, just a couple of times, once was a radio the other a TV. This was in the 60/70’s and the capacitors were not that old or the sets left unused.

 

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Posted : 16/08/2020 11:09 am
PYE625
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A dual section small can electrolytic containing an 8uf and 24uf rated at 350v appears reluctant to reform. It is parked usefully about 6mm from the PCL83 audio output valve. This little valve runs very hot at the best of times, so what better idea than to place an electrolytic capacitor right next to it? Hmmm.

The 24uf section provides extra smoothing/decoupling for the PCL83, so it's value may not be too important when considering a replacement. A 22uf will do. The 8uf section looks to decouple the sync seperator PFC80 triode section for the line stage, so a 10uf may do here.

 

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Topic starter Posted : 16/08/2020 12:19 pm
Nuvistor
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@pye625

Those values will be fine, the original tolerance would have been very large. Putting vulnerable components next hot items was very common.

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Posted : 16/08/2020 1:24 pm
Katie Bush
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Ah, a proper telly of the late Triassic period! - I shall look forward to following this!

If it hadn't been so far away, I actually fancied a bid on this set, but I'm glad it's gone to a good home. ?

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Posted : 16/08/2020 8:31 pm
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PYE625
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Work has certainly been carried out on the set in the distant past....aside from the replacement CRT, it looks like the frame output transformer has been changed. (The black one visible extreme left of under-chassis photo above). The one fitted is too large and overhangs the chassis slightly. The screw holes don't match either. It looks like an Ekco transformer to me. No problem though, it must have worked.

One section of the dropper resistor is open and has been replaced with an RS polo type resistor. The line drive capacitor to the PL36 grid has also been replaced with an RS type.

On the valve front, several have been replaced, namely the frame, sound and line output types. Mazda, Brimar and Tungsram are the makes that feature. The only light duty valve to have been replaced is the PCC84 in the tuner. The remainder are Mullard, including the good old EF80's that live forever.

Looking into the tuner, I was right about Ch3 and Ch11 biscuits being installed. There is also a Ch1 biscuit for London. I took the liberty to clean the tuner contacts whilst I was in there.

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Topic starter Posted : 16/08/2020 8:38 pm
PYE625
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 I cant overstate how large the cabinet for the set is.... think of the top 3/4 of a washing machine !! ?

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Topic starter Posted : 16/08/2020 8:46 pm
Katie Bush
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I thought that FOPT looked a bit odd! - Agreed, it has all the hallmarks of an EKCO part. Even the tuner looks like the ones used in late '50s EKCO sets - Cyldon, would that be?

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Posted : 16/08/2020 8:46 pm
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