UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Closed Thread
 
Thread Tools
Old 28th Feb 2015, 7:43 pm   #101
MonochromeMarc
Pentode
 
MonochromeMarc's Avatar
 
Join Date: Jan 2015
Location: Wakefield, West Yorkshire, UK.
Posts: 138
Default Re: Marconiphone VRC74DA

Hi Mark,

Actually those photo's could come in pretty handy when I start mine, seeing how it should be rather than how some of it is at the moment could make life a little easier.....and boy do I need all the help I can get with this one !

Good idea getting the HMV 2811 out of dock whilst it's all fresh in your mind

Marc.
MonochromeMarc is online now  
Old 28th Feb 2015, 7:59 pm   #102
Freya
Octode
 
Freya's Avatar
 
Join Date: Jan 2009
Location: Falmouth, Cornwall, UK.
Posts: 1,381
Default Re: Marconiphone VRC74DA

Glad you got the major hiccup out of the way, might be worth fitting a temporary variable resistor for the bias then when its right just measure and fit the fixed resistance.
Can we see a picture of what you do have on screen
__________________
Stephen
________________________

"It`s only an old telly"
Freya is offline  
Old 28th Feb 2015, 8:01 pm   #103
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 603
Default Re: Marconiphone VRC74DA

Well done, Mark.

Well worth the effort so far. If you don't have much luck getting your linearity correct it might be worth contacting Jon ( Duke Nukem) as he let me raid one of his 1807 chassis in order to get my set up & running. He may be able to provide you with a correct frame output transformer.

I know you've had a struggle, but sorting out a set like this is so much more rewarding and interesting. It could become pretty boring if it was just a case of replacing the waxies in order to bring a TV back to life.

SimonT.
__________________
The honesty of imperfection..........
thermionic is offline  
Old 28th Feb 2015, 8:03 pm   #104
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 4,651
Default Re: Marconiphone VRC74DA

Hi John, It certainly is a twitchy RF deck where the screening cans are involved!
I am surprised any of these ever survived, especially the console models, they really are a terrible set and a horrible design.

I note your circuit references differ from the circuit I am working from, any chance you could post the main deck portion and layout, as it would certainly help if we are using the same references.

Hi Marc, I always make a point of taking lots of photos both before and during a restoration. It is so easy to make a wiring error, having detailed photos as a reference makes life so much easier.
Good luck with your set. If you need photos of any particular area, just ask.

Mark
mark pirate is offline  
Old 28th Feb 2015, 8:06 pm   #105
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 603
Default Re: Marconiphone VRC74DA

Here's a picture on mine showing a Doris Day film.

I won't post my Test Card picture, as Johns set puts my linearity to shame!

SimonT

Oops, no , the linearity is not so bad as to put the picture sideways, sorry, trying to post from my phone!
Attached Thumbnails
Click image for larger version

Name:	image.jpg
Views:	125
Size:	63.4 KB
ID:	104847  
__________________
The honesty of imperfection..........

Last edited by thermionic; 28th Feb 2015 at 8:09 pm. Reason: Explaining my wonky picture!
thermionic is offline  
Old 28th Feb 2015, 8:10 pm   #106
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 4,651
Default Re: Marconiphone VRC74DA

Hi Simon, It would be great if Jon has the correct frame transformer going spare, I will send him a PM later.

I agree that you certainly learn more and get more satisfaction with a set like this, although it certainly has taken a lot of time to get this far and has certainly tried my patience at times!

Mark
mark pirate is offline  
Old 1st Mar 2015, 9:19 am   #107
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 6,122
Default Re: Marconiphone VRC74DA

I am working from Newnes R & TV Servicing pre 1953 models. I will sort the original EMI manual and let you have the component data later. J.
Heatercathodeshort is offline  
Old 1st Mar 2015, 9:49 am   #108
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 4,651
Default Re: Marconiphone VRC74DA

Thanks John, much appreciated.
I will resume work on the set this afternoon. I will be dragging the HMV out to borrow it's frame transformer, I also want to connect the Marconi to the CRT in the HMV to compare results.

I lashed in a new linearity pot last night, so that has helped somewhat.
I do have problems with the picture though. I know the CRT is not brilliant, but I don't believe that it is fully to blame for the current issues.

I have used the attenuator position on the set to tame the output from the Aurora, but cannot get a decent picture. The problems are that the picture is brighter toward the bottom, increasing either contrast or brightness will increase brightness from the centre of the picture with a noticeable white lines, as it is increased the lines move both up and down the screen and result in a brighter picture, but lacking in contrast and detail. any further advance on either contrast or brightness just whites out the picture.

I am sure there is still a problem on the RF deck. I will post some screen shots later today to illustrate the problem.

Mark
mark pirate is offline  
Old 1st Mar 2015, 11:29 am   #109
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 6,122
Default Re: Marconiphone VRC74DA

I am working from the omnibus edition of the EMI London models manual dated January 1954. The component numbers of the main chassis are the same.
To recap check:C36 .1uf C24 .022uf C37 .0082uf [This appears to have been adjusted during production so try a few near to this value for optimum linearity] C38 .01uf and C39 .1uf. Try reducing the value of the cathode bias resistor from 2.2k to 1k. This may help with the transfromer miss match Only a bodge but it may help the picture shape. Only problem here is that you may end up with a linear picture lacking in height... J.
Attached Thumbnails
Click image for larger version

Name:	Frame circuit..jpg
Views:	85
Size:	83.9 KB
ID:	104869   Click image for larger version

Name:	Chassis views.jpg
Views:	82
Size:	66.0 KB
ID:	104870   Click image for larger version

Name:	Cover.jpg
Views:	82
Size:	51.6 KB
ID:	104871  

Last edited by Heatercathodeshort; 1st Mar 2015 at 11:40 am.
Heatercathodeshort is offline  
Old 1st Mar 2015, 11:38 am   #110
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 6,122
Default Re: Marconiphone VRC74DA

This is a very short list of modifications and 'cures' sent to EMI dealers in 1950. I have a later list somewhere that covers a further 50 including a capacitor kit containing no less than 30 components.
A few of the mods are listed as 'try a resistor etc across RXX and see if this makes any difference'. Not an actual solving of the problem. Maybe they should have used the word 'issues'.
Happy reading.
Attached Thumbnails
Click image for larger version

Name:	1807 faults.jpg
Views:	108
Size:	52.5 KB
ID:	104872   Click image for larger version

Name:	1807 faults (1).jpg
Views:	99
Size:	77.8 KB
ID:	104873   Click image for larger version

Name:	1807 faults (2).jpg
Views:	90
Size:	94.5 KB
ID:	104874   Click image for larger version

Name:	1807 faults (3).jpg
Views:	84
Size:	106.8 KB
ID:	104875   Click image for larger version

Name:	scan0004.jpg
Views:	86
Size:	74.8 KB
ID:	104876  

Heatercathodeshort is offline  
Old 1st Mar 2015, 2:10 pm   #111
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 4,651
Default Re: Marconiphone VRC74DA

Many thanks John, It will be of great help.
Ok, I decided to fit the correct frame transformer before going any further. So I dragged out the HMV, now I have two of these nasties looking at me

Changing the TX has made no difference to the dm thing!
Swapping out the KT33C made no change either.
I will go through all the caps and resistors and report back.

The picture below is what I have at the moment.

Mark
Attached Thumbnails
Click image for larger version

Name:	Testcard.jpg
Views:	109
Size:	62.2 KB
ID:	104879  
mark pirate is offline  
Old 1st Mar 2015, 4:04 pm   #112
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 4,988
Default Re: Marconiphone VRC74DA

Hi Mark,
don't mess about checking frame timebase components in turn. Change all those capacitors marked in red by John. That action should solve the frame linearity problem. C34 (0.022mfd) is the sawtooth forming capacitor, a leak in this component will result cramping at the bottom of the picture.
C36 (0.1mfd) is the coupling capacitor will actually cause cramping at the top of the picture if it is leaky. There is a negative DC component across C34. So it follows that the KT33C will be over biased when C36 is leaky. Change it anyway. C37 (0.082mfd) and C38 (0.015mfd) must be replaced. If the correct values are not to hand you can make up the right values by mixed value capacitors in parallel. Or, 0.082mfd can be made up by two capacitors in series, 0.1mfd and 0.47mfd. Works out to be 0.084mfd, near enough I reckon.
You're on the home straight now.

DFWB.
FERNSEH is offline  
Old 1st Mar 2015, 4:36 pm   #113
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 4,651
Default Re: Marconiphone VRC74DA

I had previously replaced C34, C36, C38 and C39. Changing C40 (50uf 50v) for a 47uf 350v has now sorted the frame problems, linearity is not to far out.

The main problem is the EHT, with the focus pot fully on I am only getting 5kV
The CRT is a bit low so needs all the EHT it can muster!
If there is a way to boost it a bit, I think it would be quite watchable.
Any suggestions?

The scan coils were originally fixed in place with rubber bands, according to some of my service info
I found a better method was to use a couple of pieces of thin stiff card strips.
This has worked well and offers just enough friction to keep them in the correct place.

I spent a bit of time adjusting the focus magnet bracket until I achieved the sharpest focus possible, then centered the picture.

A little more fettling should see this set finally finished, I am a bit disappointed with the CRT, but I am too scared to give it a tickle in case I kill it

I can imagine what a dog these sets were back in their day, as John pointed out about mains voltage fluctuation. Dropping the main voltage on my variac by only 10 volts causes the picture to dim and lose horizontal lock!

Mark
Attached Thumbnails
Click image for larger version

Name:	Testcard 2.jpg
Views:	134
Size:	59.8 KB
ID:	104880   Click image for larger version

Name:	Testcard 3.jpg
Views:	127
Size:	62.5 KB
ID:	104881  
mark pirate is offline  
Old 1st Mar 2015, 4:48 pm   #114
Freya
Octode
 
Freya's Avatar
 
Join Date: Jan 2009
Location: Falmouth, Cornwall, UK.
Posts: 1,381
Default Re: Marconiphone VRC74DA

Whats the HT with the picture as it is now ?
__________________
Stephen
________________________

"It`s only an old telly"
Freya is offline  
Old 1st Mar 2015, 6:27 pm   #115
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 4,651
Default Re: Marconiphone VRC74DA

Hi stephen, HT is fine. it measures 239 and 251 at the main cap.
I tried new ones, but no difference was noted. The original is in good shape.
If I could squeeze just a little more EHT out of it, I think it would certainly help.

Mark
mark pirate is offline  
Old 1st Mar 2015, 6:31 pm   #116
Freya
Octode
 
Freya's Avatar
 
Join Date: Jan 2009
Location: Falmouth, Cornwall, UK.
Posts: 1,381
Default Re: Marconiphone VRC74DA

I had wondered if the HT was dropping, something looks odd with the contrast. Do you have the ability to test the tube ? would be worth finding out just how low it actually is.
__________________
Stephen
________________________

"It`s only an old telly"
Freya is offline  
Old 1st Mar 2015, 8:10 pm   #117
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 4,988
Default Re: Marconiphone VRC74DA

Funny thing. Some parts of the 1807 chassis are utterly reliable, the mains dropper and metal HT rectifier for example.
And they never skimped on the quality of the cabinet. Good solid EMI standard of construction, just like the pre-war sets.

DFWB.
FERNSEH is offline  
Old 1st Mar 2015, 8:57 pm   #118
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 6,122
Default Re: Marconiphone VRC74DA

The picture has the appearance of vision clipping or overload of the video amplifier. Make sure the int. limiter is set to zero. The contrast control should be about mid way for a normally contrasted picture. Check the two anode loads on the VA stage 2.2K and 6.8K and the Z77 itself. A flat tube will normally produce a correctly contrasted picture when viewed under blacked out lighting conditions but will of course be very dark. Any attempt to improve this by advancing contrast or brightness will result in the picture turning negative with a 'satin' appearance. This does not appear to be the case from your pictures but may be the case due to camera enhancement. Any attempt to boost EMI tubes can result in an O/C heater or complete loss of any emission the tube may have. J.
Heatercathodeshort is offline  
Old 1st Mar 2015, 9:25 pm   #119
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 4,651
Default Re: Marconiphone VRC74DA

I am sure that there is a problem somewhere on the RF deck, the contrast and the brightness tend to have the same effect on the picture.
The contrast alters vastly over a small amount of travel. it is near on impossible to obtain a normal picture. Scene changes when viewing a program can make the picture go from overbright to very dark.

When viewing the test card it is difficult to get either correct contrast or brightness over the whole screen. the picture focus is not to bad.

Mark
mark pirate is offline  
Old 1st Mar 2015, 9:56 pm   #120
mark pirate
Dekatron
 
mark pirate's Avatar
 
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 4,651
Default Re: Marconiphone VRC74DA

Here are some screen shots. The last photo shows a face that has lost definition.
Turning the brightness right up makes the picture go negative, but that is probably just the CRT.

Mark
Attached Thumbnails
Click image for larger version

Name:	P1120101.jpg
Views:	119
Size:	51.8 KB
ID:	104906   Click image for larger version

Name:	P1120109.jpg
Views:	109
Size:	49.1 KB
ID:	104907   Click image for larger version

Name:	P1120112.jpg
Views:	115
Size:	51.3 KB
ID:	104908  
mark pirate is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 7:59 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.