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Old 23rd Jul 2017, 10:26 am   #61
1100 man
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Default Re: Nearly a Sobell 1000 DST!!

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Originally Posted by Focus Diode View Post
The boost voltage will of course have been checked and within the range specified in the manual. Personally speaking I prefer to set it towards the lower end if full width is maintained in the interest of preserving the LOPT. I was shouted down on another forum for suggesting that once but is my own opinion.
Good morning,
The range of boost adjustment, on 625 was between 750 - 790V. The book says set to between 750 and 770V 'or damage to the line output stage will result'!! On 625 the width was on the wide side and so the boost was set to 755V. On 405 the width was narrower and so the boost control needed to be set higher. What difference the 'set boost' control has on the EHT, I don't know, but like you, I would tend to think that a lower setting would be safer!!
I'll make some more detailed measurements this evening with the EHT meter and the infra-red thermometer (rather than my finger!!). It doesn't help that I can't really adjust the linearity sleeve properly which also has a big effect on the width.
All the best
Nick
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Old 23rd Jul 2017, 10:47 am   #62
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Default Re: Nearly a Sobell 1000 DST!!

The manual for the Plessey dual standard chassis as used in the Regentone 194 I have specified something similar. It also comments the width will be, "Self compensating" when the boost volts are set within the specified limits, but can be set higher or lower within the range if the width is insufficient or excessive. Like the GEC it remarks damage will result to the line output valve and transformer if set outside the range.

Sounds like all is well with your GEC if 755V allows near full width. For the width to be a bit narrower on 405 is surprising however as it was the reverse on the 2017 I once had.

Having said that on two BRC 900 related sets the width is certainly narrower on 405 compared to 625.


Cheers
Brian

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Old 23rd Jul 2017, 11:09 am   #63
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Default Re: Nearly a Sobell 1000 DST!!

The operation of a stabilised line output stage was explained in the "Recent Developments" section of the 1960/61 Radio and Television servicing book. In fact Peto-Scott introduced something similar in their 1957 range of TVs, the models TV1720 etc. Peto-Scott called the arrangement an EHT regulation circuit.
The circuit is essentially an AGC system so that width and EHT is maintained regardless of mains voltage variations, also compensates for valve aging.
Remember that the line output valve functions as a switch and the stabilising circuit controls the conduction of the valve hence the amount of energy supplied into the line output transformer primary.

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Old 23rd Jul 2017, 4:49 pm   #64
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Default Re: Nearly a Sobell 1000 DST!!

The Peto-Scott line timebase stabilising circuit. The 2KV flyback pulse present at the anode of the PL81 is reduced down to 25 to 30volts by C80 and C83 and rectified by V20 (OA71 germanium diode) and the resultant negative control bias regulates the current in the PL81. Later sets employed a diode section of a PABC80 triple-diode-triode.
Must be one of the earliest examples of a stabilised line output stage.

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Old 16th Dec 2017, 12:57 pm   #65
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Default Re: Nearly a Sobell 1000 DST!!

Thread reopened at OP's request.
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Old 16th Dec 2017, 10:42 pm   #66
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Default Re: Nearly a Sobell 1000 and 2000 DST!!

Good evening,
Well, back in the summer, I got to the point of functionality with this set but it was far from finished. It got bundled back into it's cabinet and then stood in a corner for a few months
A few weeks ago, I thought it was high time I fired it up again and watched some vintage programs in 405. So I spent a happy 1.5 hours watching some Tony Hancock (from VHS) and some BVWS films from DVD.
It became apparent after a while that there was hum on the video: A black band was moving slowly up the screen.
The sound was also grim, but I haven't set the IF's up yet so no surprise there!
Putting my head round the back, I could hear the sound of gently frying bacon! Turning all the lights out revealed a steady blue glow from the top of the primary winding to the LOPT core, so a bit of insulation breakdown there.
I switched it off and added that to the list of jobs to do.

This set uses two double can electrolytics in the power supply- one 200 + 200 mfd as reservoir and smoothing and a smaller 150 + 50 mfd to further smooth the supply to the signals stages and the frame output stage.
On my cheap Chinese capacitor tester, I measured 240, 220, 80 & 32 mfd. These testers tend to give a higher value than expected if the capacitor is leaky so I measured the leakage current at 280V DC. (they are rated at 300V)
All 4 capacitors leaked about 0.20 - 0.25 ma which I find is reasonable for old caps.
So my conclusion was that the 200/ 200 was probably perfectly OK but the 150/50 had just dried out a bit loosing capacity. It also had a bulgy end whereas the 200/ 200 was perfectly flat.
As the chassis is very clean & tidy, I am going to re-stuff the 150/50 rather than just add extra caps. Hopefully tomorrow I can open it up and remove the old innards and then get some new caps on order from RS before Christmas.

While the set is in bits, I will have to remove the LOPT again and see what I can do about the leakage. Luckily, I haven't re- dressed all the wiring loom yet so access to the transformer is not too bad.

One thing I did do back in the summer was to sort out the moulded plastic implosion screen which protrudes from the front of the cabinet. When I originally collected the set, it was lying face down in the back of a van! Consequently, there were loads of nasty scuffs and some very deep score marks which looked dreadful!
Starting with 400 grade wet & dry and progressing to 3000 grade (you can't tell which side of the paper is which!) I gradually flatted out all the damage until the entire surface was a smooth slightly matt finish.
It was then given a final polish with car body compound and an electric mop.
It looks brand new and I am very pleased with the result.
Luckily the cabinet is almost perfect and only needs a slight touch in here and there where there is minor damage.

So the next job is to remove the LOPT and see what is going on with that.

Many thanks to the moderators for re- opening this thread
All the best,
Nick
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Old 17th Dec 2017, 10:02 am   #67
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Default Re: Nearly a Sobell 1000 and 2000 DST!!

The alignment should be OK unless it has been previously fiddled. It will not drift on it's own.

Sound problems are usually due to reduction in value [eventually ending in a burn up] of the resistors associated with the EH90 AF amp 405/discriminator FM valve.
R73 18K R71 5.5K R69 180ohm. This fault will cause a very heavy drain across the HT line.

Other than this a conventional check of the output stage PCL84 and if distorted at low volume, the loudspeaker. J.
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Old 18th Dec 2017, 11:54 am   #68
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Default Re: Nearly a Sobell 1000 and 2000 DST!!

The speaker in your set is a tiddly little 5" x 3" unit and well worth replacing with a readily available 7" x 4".
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Old 18th Dec 2017, 6:41 pm   #69
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Default Re: Nearly a Sobell 1000 and 2000 DST!!

Quote:
Originally Posted by Heatercathodeshort View Post
The alignment should be OK unless it has been previously fiddled. It will not drift on it's own.

Sound problems are usually due to reduction in value [eventually ending in a burn up] of the resistors associated with the EH90 AF amp 405/discriminator FM valve.
R73 18K R71 5.5K R69 180ohm. This fault will cause a very heavy drain across the HT line.

Other than this a conventional check of the output stage PCL84 and if distorted at low volume, the loudspeaker. J.
Hi John,
Thank you for the information. When I did the original work on this set back in the summer, I checked every resistor on the PCB. I find it helpful to follow the circuit diagram and trace out what is actually on the board (or chassis) checking component values as I go. It takes a while but I find it enjoyable and also get me familiar with the circuit and layout! All the resistors, almost without exception were exactly 20% high. Strangely, several 'stock fault' resistors which I expected to be high, were spot on- rather strange!
As they were all 20% high, I didn't change any of them
I wonder why the resistors you mention would go low in value?

I've not focused my attention on the sound yet, but as Edward points out, it does have a really 'tiddly' speaker which may not be any good.
I'm surprised they fitted such a small one as the rest of the set seems very well designed and built.
Certainly, we got 20 years out of the one we had when I was a kid, although dad did have to repair it quite often towards the end!
I would be really interested if anyone has any sales literature for this model.
Many thanks
Nick
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