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Old 18th Aug 2017, 9:56 pm   #1
TV_Madness
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Default Sobell SC270 Restoration

Hi All

This is a set that I have had in my possession for many years, however it has been in storage for the last 4 or so. A couple of weeks ago, a little voice in the back of my head said "you really ought to do something about that old Sobell Matt...". So that's what I have set forth on doing.

This is a Sobell SC270 console and is similar to the later SC370 which uses the same chassis as the T193 (and a few other Sobell / McMichael sets). The SC270 uses a very 50s valve line up, with PL81 / PY81 in the Line OP stage, together with the ubiquitous EY51 EHT rectifier (must be one of the last sets to use this). It also uses a great many of those moulded black Hunts caps, all of which I have changed on sight. Many of these were in such a bad way that they were simply falling apart. Although a few tested close to their stated values, I felt it best that all should be changed given their age. I was thinking that maybe at a later date, I would try and cut open the old caps and stuff new ones inside to make the set look a bit more authentic with the back off. Not a priority for the moment however.

I set forth on changing out any of the caps that I missed the first time around when I had a go at this set, however it had developed an intermittent frame lock / frame collapse fault since. The lock issue was simply the coupling between the sync separator and the pulse shaper triode being OC (C91), whilst the frame collapse was traced down to an intermittent OC in the anode feed resistor to the first half of the frame oscillator multivibrator (R74). there is still a slight frame linearity issue to sort out too (most likely caused by a couple of low value caps in the frame OP stage that are quite tricky to get to and are yet to be replaced).

This set has always had a contrast issue since I took over its ownership some 10 years or so back, and even after this mass component overhaul, I still haven't sorted this out fully. I am sure this is not CRT related either. Every now and again, the contrast will restore itself to a good, bright and clear viewable picture, however it intermittently reverts back to a state where only the very brightest highlights are visible and all other grey tones are lost entirely. I'm not sure if this is some sort of AGC issue or something. The only reason why I say so is that when it is in this state, the contrast control is totally inoperative.

I have attached a couple of shots of the progress so far. Although the picture looks as though the CRT is indeed duff, I just had the brightness turned right down to stop it from overwhelming the camera. In reality, it is a very bright and sharp tube.

Cheers

Matt
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Old 18th Aug 2017, 10:01 pm   #2
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Default Re: Sobell SC270 Restoration

...And the heap of baddies that came out of it!
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Old 18th Aug 2017, 11:05 pm   #3
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Default Re: Sobell SC270 Restoration

That's a great start.


I hope poor puss wasn't in the set when it was turned on!


Cheers. SimonT
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Old 18th Aug 2017, 11:31 pm   #4
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Default Re: Sobell SC270 Restoration

Hi Matt,
That looks a very clean, tidy chassis- almost brand new, although you do have a cat in there which you might want to remove before next power up!!
I've just dug out the circuit and they certainly didn't spare any expense on the valve line up- 18 plus the crt! And VHF radio as well. I imagine it should be capable of excellent results.
Does it use the double sided pcb's like the later GEC/ Sobell sets? I must say I've been very impressed with them on the GEC 2000 I've been doing. Is the Line o/p stage on a pcb or hand wired?
The circuit mentions resistor packs- sounds intriguing- what form do they take?
Interesting about the contrast problem. It does look like the contrast is in the AGC circuit and affects the overall gain. Presumably, to get only the highlights and a dark picture, there must be too much gain or not enough brightness.
When the fault occurs, what happens when you increase the brightness?
Keep up the good work,
All the best
Nick
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Old 19th Aug 2017, 6:05 am   #5
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Default Re: Sobell SC270 Restoration

That's an excellent restoration job, well done.
As for the contrast problem another possibility is the tuner but tapping on the knob usually shows this up. It would do no harm to try new tuner and IF valves. Gently tapping in situ will reveal which one is responsible, if any.

They could also be a dry joint in the coaxial socket.

Is the problem heat sensitive or does it appear randomly?

Brian
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Old 19th Aug 2017, 1:01 pm   #6
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Default Re: Sobell SC270 Restoration

You can see a resistor pack on the timebase board top right in the photo, it's between the valves running vertical. At least that resistor pack is just normal resistors on a PCB, later ones had resistors and capacitors encapsulated in a cement.
Either replace that type of pack or hack out the cement around the faulty component.
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Old 19th Aug 2017, 6:54 pm   #7
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Default Re: Sobell SC270 Restoration

It looks like a nice set. What sort of state is the printed circuit in? Some of the early ones were a little bit fragile, especially in the hands of repairers more used to tag strips, mechanically-sound joints and solder almost as an afterthought.

I see your friend approves, too
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Old 21st Aug 2017, 8:45 pm   #8
TV_Madness
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Default Re: Sobell SC270 Restoration

Nick - It does indeed use double sided PCBs. They appear to be of quite decent quality and aren't in too bad a shape. The line output stage is mounted on the timebase PCB. The lower end of the PCB disappears into the line stage screening box and the PL/PY81 are mounted on the very bottom of the board.

Brian - I haven't tried swapping out IF / Tuner valves just yet, however this is a very good suggestion. I have plenty on offer, so will give this a go. I have tapped just about every corner of this set and try as I may, I can't get the fault to come back with any sort of "percussive persuasion". It does however seem that the warmer the thing gets, the less reluctant it is to go away. There was one occasion where the thing sat there for a good 3 or so minutes displaying a cracking picture, and then as if somebody threw a switch, lights out again. I have pulled the tuner apart and given everything a good clean-up inside. The same fault is present across all channels, so it would appear as though it isn't mechanical / gunk related.

Varying the brightness control certainly brightens the picture up. It doesn't lose any definition either, so this must be some sort of gain issue in the vision stages.

As a side note, the sound is absolutely fantastic and there are no complaints to be had here.

P.S. - The cat is safe (I always leave any set that I'm working on unplugged if I'm out of the room for this very reason!). I'd turned my back for two seconds to get some tools and when I got back, there she was. Little monster
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