Marconi VT157 (THORN 406T chassis) contrast problem - UK Vintage Radio Repair and Restoration Discussion Forum

retroaudio · 14th Feb 2006, 11:22 pm

Hi,

I have a number of restored 405 line sets including the later pye washboard with the 12 channel tuner and a bush tv 24. Both have exceptional pictures comparable with any modern set.

I have two questions.

I have a Marconiphone of the late 50s. I cant remember the model but I think it is a VT 157. It is clearly a later set but I would say the picture is poorer. I t seems to have very high gain not particularly helped by an attenuator. A very bright screen with fairly extreme contrast. This means the "line " structure is very apparent i.e you can see a horizontal band structure of alternate black and white when you look at the image close up. Is this simply a feature of some monochrome sets of the period or does it point to a technical fault. there are no hold problems or linearity problems.

the other question is I seem to spend a lot of time adjusting the contrast and brightness depending on the source material supplied to the domino converter. Quite a lot of the material i have is colour such as "the saint" and various 40s and 50s films which were broadcasting fodder in the heyday of these TVs. So I feel that it is authentic to watch this stuff on old sets. I do think old monochrome source material does however reproduce better. That said Is there a logical methodical way to set up a monochrome 405 line set with the consumer controls so that further adjustment is then minimal. I dont have a test card to go by.

I look forward to your responses

Alex

ppppenguin · 15th Feb 2006, 8:31 am

Don't forget that virtually all later 405 line sets and earlier dual standard sets had mean level AGC. Also known as "always grey control" this ensured that a fadeto black meant fade to mid grey

Heatercathodeshort · 15th Feb 2006, 9:59 am

Hello,
Your Marconi VT157 is fitted with the THORN [FERGUSON] 406T chassis which is far better known. It should give a bright, clear well defined picture when operating correctly. I suspect that you may have a problem around the PCL84 video output valve or leaking AGC decoupling capacitors. This kind of fault is difficult to describe on paper but is usually easily solved with 'hands on' experience.
These sets were well received by the public and were very popular in various styles. Interlace was excellent and overall reliability good. main problems were the printed circuit board mounted dropper [URGHH!] and leaking TCC supermould capacitors mainly in the frame stage causing the picture to cramp at the bottom as the set warmed up! The MULLARD AW43-80 tubes, unlike their MAZDA equivalent the CME1702, seem to last for ever. Happy days! Regards JOHN.

14th Feb 2006, 11:22 pm	#1
retroaudio Pentode Join Date: Mar 2005 Location: Cairneyhill, Fife, UK. Posts: 135	Marconi VT157 (THORN 406T chassis) contrast problem Hi, I have a number of restored 405 line sets including the later pye washboard with the 12 channel tuner and a bush tv 24. Both have exceptional pictures comparable with any modern set. I have two questions. I have a Marconiphone of the late 50s. I cant remember the model but I think it is a VT 157. It is clearly a later set but I would say the picture is poorer. I t seems to have very high gain not particularly helped by an attenuator. A very bright screen with fairly extreme contrast. This means the "line " structure is very apparent i.e you can see a horizontal band structure of alternate black and white when you look at the image close up. Is this simply a feature of some monochrome sets of the period or does it point to a technical fault. there are no hold problems or linearity problems. the other question is I seem to spend a lot of time adjusting the contrast and brightness depending on the source material supplied to the domino converter. Quite a lot of the material i have is colour such as "the saint" and various 40s and 50s films which were broadcasting fodder in the heyday of these TVs. So I feel that it is authentic to watch this stuff on old sets. I do think old monochrome source material does however reproduce better. That said Is there a logical methodical way to set up a monochrome 405 line set with the consumer controls so that further adjustment is then minimal. I dont have a test card to go by. I look forward to your responses Alex

15th Feb 2006, 8:31 am	#2
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: marconiphone tv Don't forget that virtually all later 405 line sets and earlier dual standard sets had mean level AGC. Also known as "always grey control" this ensured that a fadeto black meant fade to mid grey

15th Feb 2006, 9:59 am	#3
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: marconiphone tv Hello, Your Marconi VT157 is fitted with the THORN [FERGUSON] 406T chassis which is far better known. It should give a bright, clear well defined picture when operating correctly. I suspect that you may have a problem around the PCL84 video output valve or leaking AGC decoupling capacitors. This kind of fault is difficult to describe on paper but is usually easily solved with 'hands on' experience. These sets were well received by the public and were very popular in various styles. Interlace was excellent and overall reliability good. main problems were the printed circuit board mounted dropper [URGHH!] and leaking TCC supermould capacitors mainly in the frame stage causing the picture to cramp at the bottom as the set warmed up! The MULLARD AW43-80 tubes, unlike their MAZDA equivalent the CME1702, seem to last for ever. Happy days! Regards JOHN.