Iskra Question - UK Vintage Radio Repair and Restoration Discussion Forum

Nipkowdisc · 2nd May 2023, 1:19 am

Hello again

Do any of you folk remember the Iskra TV130-2?
This was a U.H.F. 625 Lines only Monochrome Portable which I think was imported to Britain from Slovenia.
I think that there were two versions where one had more Station Presets than the other.
What I want to ask is whether or not these were made with no Spot Suppression circuitry?
None of the examples that I have seen had Spot Suppression functionality but the circuitry could have been very failure prone.

Mike

Lloyd 1985 · 2nd May 2023, 10:19 am

Hi Mike,

I have one of these sets! I see after a Google search the 2 different versions, one has the power switch as part of the presets, and the other has a separate power switch. Not sure on the spot suppression, mine doesn’t show a spot on switch off, the picture just disappears as soon as the switch is operated.

Funny story about why I have one of these, when I was about 10 years old I found one dumped in a hedge, it was the first time I’d ever found anything complete, normally old TV’s were smashed up long before I got to them, so I dragged it almost home, where my dad spotted me dragging it across a field in a plastic box! He dragged me away from it and it was left in the middle of the field… on my way back from school the next day I ran to the field to see if it was still there, surprisingly it was, and it was still in one piece! Someone had nicked the box though… I managed to keep it hidden for some time, during which I tried to get it going with 9V batteries. Sadly someone found it and smashed it, but some years later I spotted one on eBay so bought it.

Regards
Lloyd

Nipkowdisc · 2nd May 2023, 12:21 pm

Thanks for that Lloyd 1985.

So your Set has working Spot Suppression.
Could there have been a batch where they accidently left out a component which caused that bit of circuitry to be disabled I wonder?
I guess your Dad knew that T.V. Picture Tubes could implode and feared that you would be injured.
I guess Battery Power would have been possible with a Battery made up from D Cells although not for very long as Sets like that usually drew at least an Amp or so.

Mike

slidertogrid · 2nd May 2023, 3:34 pm

These sets were sold widely by Freemans and Woolworths. I saw a few for repair but I can't remember the faults. Are you switching it off by the set's mains switch or cutting the power at the plug? Switch off suppression often relied on the set's own switch and won't work if the mains is unplugged. It isn't a problem I remember with them. It can also be a sign of a worn tube what's the picture like ?

Welsh Anorak · 2nd May 2023, 3:42 pm

We saw a few of these - chunky sets as you'd imagine from Eastern Europe. The top mounted buttons were a problem as was the tuner. Most other faults were pretty run of the mill - huge rectifiers that failed and were replaced by ones that were bigger in rating but smaller in size.
Spot suppression was usually pretty basic, and in many cases (though not sure about the Iskra) was simply a high value resistor across the mains switch.

Nipkowdisc · 3rd May 2023, 12:23 am

My thanks to slider to grid.

I know the type of circuit that you're thinking of.
I believe it worked by using an extra pair of contacts on the On/Off Switch to cause the Picture Tubes Control Grid to become much more negative when the set was turned off thus biasing off the Beam Current.
The picture was fine and a very bright flash in the middle of the screen was what you got when you switched off using the set's own On/Off Switch.

My thanks also to Welsh Anorak.

I can't see how a high value resistor across the mains switch could achieve Spot Suppression though.

Marconi_MPT4 · 4th May 2023, 10:18 am

Quote:

Originally Posted by Nipkowdisc

What I want to ask is whether or not these were made with no Spot Suppression circuitry?
Mike

Hi Mike,

A few thoughts. The switch off spot suppression is achieved with the help of C303 (2.2μF), R712 (3M3), and D702 (BYF404).

When the set is turned on, supply U4 (135V) charges up C303 via D702. At this point CRT g1 DC return path is via R708 (100k) and R712. D702 clamps to approx 0.6V.

Switch off, U4 supply voltage falls fairly rapidly due to R305 (5.1k), R304 (15k) and to a lesser extent R709 (15k) and IC700 discharging C712 (2.2μF). At this time D702 is eventually reverse biassed and CRT g1 potential becomes something over -100V after roughly 300ms which is then maintained for some seconds while R712 slowly discharges C303.

Possible faults might include D702 open or short circuit, C303 open circuit or very leaky.

Rich

Nipkowdisc · 4th May 2023, 12:50 pm

My thanks to Marconi MPT4.

So it did (or rather should have done) work by making the Picture Tube's Control Grid (or g.1. if you like) much more negative at switch off but by electronic rather than mechanical switching means.
I suppose that the reason all the examples of this Set that I saw didn't have the Viewing Screen Phosphor scorched in the middle was that they used Electrostatic Focusing causing the Spot to spread out as the Raster collapsed at switch off.

2nd May 2023, 1:19 am	#1
Nipkowdisc Triode Join Date: May 2010 Location: Burntwood, Staffordshire, UK. Posts: 33	Iskra Question Hello again Do any of you folk remember the Iskra TV130-2? This was a U.H.F. 625 Lines only Monochrome Portable which I think was imported to Britain from Slovenia. I think that there were two versions where one had more Station Presets than the other. What I want to ask is whether or not these were made with no Spot Suppression circuitry? None of the examples that I have seen had Spot Suppression functionality but the circuitry could have been very failure prone. Mike

2nd May 2023, 10:19 am	#2
Lloyd 1985 Nonode Join Date: Jun 2008 Location: Coningsby, Lincolnshire, UK. Posts: 2,845	Re: Iskra Question Hi Mike, I have one of these sets! I see after a Google search the 2 different versions, one has the power switch as part of the presets, and the other has a separate power switch. Not sure on the spot suppression, mine doesn’t show a spot on switch off, the picture just disappears as soon as the switch is operated. Funny story about why I have one of these, when I was about 10 years old I found one dumped in a hedge, it was the first time I’d ever found anything complete, normally old TV’s were smashed up long before I got to them, so I dragged it almost home, where my dad spotted me dragging it across a field in a plastic box! He dragged me away from it and it was left in the middle of the field… on my way back from school the next day I ran to the field to see if it was still there, surprisingly it was, and it was still in one piece! Someone had nicked the box though… I managed to keep it hidden for some time, during which I tried to get it going with 9V batteries. Sadly someone found it and smashed it, but some years later I spotted one on eBay so bought it. Regards Lloyd

2nd May 2023, 12:21 pm	#3
Nipkowdisc Triode Join Date: May 2010 Location: Burntwood, Staffordshire, UK. Posts: 33	Re: Iskra Question Thanks for that Lloyd 1985. So your Set has working Spot Suppression. Could there have been a batch where they accidently left out a component which caused that bit of circuitry to be disabled I wonder? I guess your Dad knew that T.V. Picture Tubes could implode and feared that you would be injured. I guess Battery Power would have been possible with a Battery made up from D Cells although not for very long as Sets like that usually drew at least an Amp or so. Mike

2nd May 2023, 3:34 pm	#4
slidertogrid Octode Join Date: Apr 2005 Location: Peterborough, Cambridgeshire, UK. Posts: 1,946	Re: Iskra Question These sets were sold widely by Freemans and Woolworths. I saw a few for repair but I can't remember the faults. Are you switching it off by the set's mains switch or cutting the power at the plug? Switch off suppression often relied on the set's own switch and won't work if the mains is unplugged. It isn't a problem I remember with them. It can also be a sign of a worn tube what's the picture like ?

2nd May 2023, 3:42 pm	#5
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 7,085	Re: Iskra Question We saw a few of these - chunky sets as you'd imagine from Eastern Europe. The top mounted buttons were a problem as was the tuner. Most other faults were pretty run of the mill - huge rectifiers that failed and were replaced by ones that were bigger in rating but smaller in size. Spot suppression was usually pretty basic, and in many cases (though not sure about the Iskra) was simply a high value resistor across the mains switch. __________________ Glyn www.gdelectronics.wales

3rd May 2023, 12:23 am	#6
Nipkowdisc Triode Join Date: May 2010 Location: Burntwood, Staffordshire, UK. Posts: 33	Re: Iskra Question My thanks to slider to grid. I know the type of circuit that you're thinking of. I believe it worked by using an extra pair of contacts on the On/Off Switch to cause the Picture Tubes Control Grid to become much more negative when the set was turned off thus biasing off the Beam Current. The picture was fine and a very bright flash in the middle of the screen was what you got when you switched off using the set's own On/Off Switch. My thanks also to Welsh Anorak. I can't see how a high value resistor across the mains switch could achieve Spot Suppression though.

4th May 2023, 12:50 pm	#8
Nipkowdisc Triode Join Date: May 2010 Location: Burntwood, Staffordshire, UK. Posts: 33	Re: Iskra Question My thanks to Marconi MPT4. So it did (or rather should have done) work by making the Picture Tube's Control Grid (or g.1. if you like) much more negative at switch off but by electronic rather than mechanical switching means. I suppose that the reason all the examples of this Set that I saw didn't have the Viewing Screen Phosphor scorched in the middle was that they used Electrostatic Focusing causing the Spot to spread out as the Raster collapsed at switch off.