Marconi VT73DA Midlands Set - Request for advice.

[DoctorWho]

I've just been working on a Marconi VT73DA 405-line set from circa 1949/50 made for reception of the Midlands transmissions.

Some work had been done on the set prior to me owning it, some condensers had been replaced and the like and the set was giving a raster.

I just fitted a mains lead, replaced a couple of dodgy looking electrolytics and then powered up, using a Channel 4 signal of course.

The sound is absolutely perfect, but there are some picture problems which I'd like to ask for some help with please. I have been working from Trader Service Sheet 974.

The line hold control has been replaced with a pot at some point, and it is not possible to get a proper line hold on the picture, there seems to be very little adjustment available with this replacement control, most of the time you have 2 or 3 attempts at a picture next to each other, you can see the background changing with the picture, but you can't actually get a proper picture.

There is what looks like some fold over at the bottom of the picture, the top of the picture looks a bit stretched and the picture does not actually go down to the bottom of the CRT.

Any suggestions as to what to try would be appreciated. Should I re-replace the Line Hold control? If so, can I use a carbon pot, or does it have to be wire-wound? The current replacement looks like a small carbon pot, and I cannot see any value marked on it.

I have tried varying the interference limited, contrast and so forth, they alter the screen, but, as I can't get line hold, it's not really possible to go much further. The picture information, whilst present, is not terribly strong, it does not look like a hugely powerful vision signal, but that may be due to the line hold issue?

Any advice welcome? I think others here will know more than me as to what the problem is likely to be and what steps are best taken? Should any valves in the line stage, or any rectifiers being routinely replaced?

Thanks for the help,

Peter.

[Steve_P]

The line hold control is R29, which is 25K. A carbon pot should be OK.

Anyway, change C17, C18, C19, C20, R33, C21, C22, C23. The valve B36 has no equivalents, but this should be OK. The sync separator is Z68.

Change Capacitors, check/change resistors, take voltages and if you can, follow the sync trains through with a scope.

Cheers,

Steve P

[DoctorWho]

Thanks Steve, I have changed the above condensers, I now have to track down a 25K pot and replace that. i hope to do that this evening, i will then let you know how things progress.

Thanks for your help.

Peter.

[FERNSEH]

The VT73DA employs (if my memory serves me well) a superhet circuit.
The RF amplifier is a Z77/6AM6/EF91 and the frequency is the triode-hexode type X78, this is the B7G base valve that has the cathode connected internally to one of the heater pins.

DFWB.

[DoctorWho]

Thanks for your reply David, yes, that sounds right.

There is now progress, but I'm now at a stage where I am scratching my head.

The Line Hold is now working and I can lock to a picture, BUT something odd is going on.

There is a bar across the middle of the screen at the bottom half of the picture is at the top and the top half of the picture is at the bottom. The bottom part of the screen is cramped and the scan does not reach the bottom of the CRT, there is also a flicker constantly present.

The Frame Hold seems to be working, but at either extent does not rectify this problem, the picture simply rolls.

This has got me somewhat puzzled, any suggestions as to "what next" would be appreciated. How do I get the top and bottom halves of the picture back in their correct places?

Thanks for the help.

Peter.

[Tazman1966]

Hi Peter.

You sound like you're getting there!

With the frame fault it sounds to me like the FTB is running at half speed especially with the flicker that you say is present. Are you sure that you haven't got two almost complete pictures one above the other. At risk of going off topic when I was about six or seven I twiddled all the controls on the rear of my Aunty Linda's rented Bush TV161 and after frantically readjusting them all to what I thought were middle positions I was left with the symptoms that you describe on your set. It was simply that the frame hold was so far out that it had locked at half speed. Check the adjustment again from one end to the other and then the components associated with the frame oscillator section.

Good luck!

Tas

[Steve_P]

These sets were known in the trade, and for the wrong reasons!

Anyway.... C36 - C41 - Change them all. Check all the resistors too. The valves are B36 (Which I think is OK) and the Frame Output is KT33C. (This is probably OK too.)

Height control is R51 (25k), Frame Hold is R53 (25k) and R52 is Frame Linearity (22k).

Cheers,

Steve P

[DoctorWho]

Tas - Thanks for your reply. I wasn't quite sure what is causing the symptoms, but the flicker is indeed there, just as though the frame hold was badly adjusted. Sorry to seem ignorant, but what is the FTB? There definitely aren't 2 pictures, the top half of the picture is at the bottom and the bottom half is at the top, when you feed in a signal you simply get it split in 2 as described above. I have tried both Frame and Line Hold from one end of the scale to the other, this does not cure the problem. Ot looks very much like it is locked at half speed, the only odd thing is the two halves of the picture being the wrong way around.

Steve - Thanks for your further assistance. I have now changed all of the remaining wax condensers, I also replaced the resistor you suggested above (R33), it was only a little high, but I changed it so as to eliminate that problem.

I've hit a bit of a brick wall now with this strange effect of having a flickery picture split in to two. Any ideas what this is likely to be, or how to rectify it?

Thanks for the assistance.

Peter.

[FERNSEH]

Originally Posted by DoctorWho

I've hit a bit of a brick wall now with this strange effect of having a flickery picture split in to two. Any ideas what this is likely to be, or how to rectify it?

Hello Peter,
Seems as if the frame timebase is running at 25 c/s. It is possible that R41 (2,2Mohms) has gone high value. R41 is connected to the black wire of the frame blocking oscillator and the slider of the frame hold control.
It might be worth while checking C42, it is a 1 microfarad condenser and is connected to the slider of the height control.
C41 (.0022 microfarads) is the oscillator charging capacitor. It seems to be quite a low value for it's position in the circuit.

DFWB.

[Steve_P]

It's R55 in the trader sheet that Dr. Who says he is working from.

Cheers,

Steve P

[DoctorWho]

David & Steve - Thanks for the further assistance with this set. I will check all of the above areas you have highlighted and see if that identifies the problem. I'll let you know the results.

Thanks for the help.

Peter.

[Heatercathodeshort]

OOOO! This is a dreadful set...The H.T. needs to be up to scratch as a reduction of only 10-15volts will have a dramatic effect on sync and overall performance. I have never seen one of these sets perform well, even back in the early 60's when they were only 12 years old!

Don't expect a 'crisp' picture; you will have to adjust the frame and line hold controls several times during an evening's viewing. Quite normal..

The set suffers from instability due to the small decoupling capacitors used in the i.f. deck. The screen decouplers are the most 'active' in this respect.

The EHT supply from the flyback is only just enough when working well; often, to get the picture in correct focus results in a dim display.
All the resistors will have to be checked for value and some 'bodged' to get the hold controls to lock in the centre of their travel. This is quite acceptable and was issued in the many pages of modifications issued by EMI during the 1949 -1953 period. The B36 valve was blamed for many timebase faults and it was suggested that several examples be tried for 'best lock'! The truth is the valve is blameless and the circuit and components are too far out of tolerance. The B36 is very similar to the Brimar 12SN7.

This series starting with the HMV 1807 were among the first receivers to be scrapped by their unhappy owners, some after only 3 years.

My brother in law's father lived in Birmingham in 1949 when Sutton Coldfield went on the air. He bought a 2807 soon after and it never worked for more than a couple of weeks, and then rather poorly. The ultimte horror occured during the Coronation broadcast when it broke down just before the 'crowning' when he had half the street in to watch it. I don't think he ever got over it as he repeated the story almost every time I met him! That was back in 1963. There were of course a few good ones as always but they were few and far apart.
Regards John.

[DoctorWho]

Oh dear, that doesn't sound too promising. I did actually think that the picture wasn't terribly bright when properly focussed, I had put this down to an aged CRT. It's perfectly watchable (well it will be wen the picture is the right way around), but you'd need to have the room a bit darkened. I'm about to start working through the suggestions above to see if I can locate the problem.

How awful about the coronation horror John, I'd never thought about this before, but I wonder how many people had an experience like that in 1953?

Hopefully this may be a good example when repaired, I can, at least, live in hopes

[DoctorWho]

Well there is now some progress, thanks to David and Steve I followed through your suggestions and the set is now displaying a stable picture which is as it should be.

There is one remaining issue with the set, which I am not totaly sure if it can even be overcome?

When correctly adjusted for focus etc the picture is very dim, you have to draw the curtains to be able to see it, the contrast is at maximum, increasing the brightness further simply causes flyback lines to be visible.

Is this likely to be due to a low emission valve somewhere? If so, any suggestions as to which one/s would be appreciated. There seems to be more brightness available from the CRT, but, with the contrast at maximum this doesn't help too much.

I'd appreciate any further suggestions. This set certainly looks like it has the potential to be a very nice set.

Thanks again for the help,

Peter.

[DoctorWho]

Since my post earlier to-day I have been able to do some more towards making this set usable.

I've now connected in a second converter so that I can send both Channel 1 and Channel 4 all around the house, this will allow me to use any London set and also any Midlands set (something I have been unable to do before) at the same time. This of course means that I will not be collectying Midlands sets as well, but that's an entirely different topic

I have now applied this signal to the set, which is well amplified (as is the Channel 1 signal), but the problem is much the same.

You can tell that the signal is much stronger, with the contrast control at minimum on the set the signal is now coming in strongly.

The picture is locking well, it is well formed and both horizontal and vertical hold are good, but the picture is very dim and you still need to draw the curtains to be able to view it, and, even then, it is very, very dim and only just watchable. If you adjust the focus control the picture does get brighter, but at the loss of focus.

Is this likely to be due to a tired rectifier, low emission on another valve/s, or simply a tired CRT? There is plenty of brightness available on the CRT, but if turned up too far the picture detail is simply lost.

I really do like the look of this set (no accounting for taste, I know), and I would very much like to have it up and running and performing well, so any suggestions as to what I can try next would be very welcome.

Thanks again for the help,

Peter.

[Steve_P]

It sounds to me like the tube. What are the voltages on the various pins? You could try leaving it on to see if it comes up.

Cheers,

Steve P

[DoctorWho]

I did wonder that Steve. I've just turned up the focus control all of the way, and adjustd this manually with the focus magnet assembly around the neck of the CRT, that has made it a bit better, but you still need a darkened room to see a reasonable picture. The line structure is visible, there's just not a great deal of brightness, and, if you advance the brightness to far then it all goes negative and then just completely washed out.

I will try your advice of leaving it on, how long would you run it for? Somehow I have an idea that I'm highly unlikely to find a replacement CRT am I?

I haven't yet measured the voltages on the base of the CRT, but if you think that will help then I'll certainly do so.

Thanks again, any further thoughts appreciated.

Peter.

[Steve_P]

Leave it on all day today and tomorrow. Then see what it looks like. Or, if you're feeling brave. try this...

https://www.vintage-radio.net/forum/...ad.php?t=15913

Cheers,

Steve P

[DoctorWho]

I'll try running it first Steve, but, if that doesn't work, then I may attept to muster up the courage and give "boosting" a go..........

[Duke_Nukem]

There is plenty of brightness available on the CRT, but if turned up too far the picture detail is simply lost.

As there is plenty of brightness in the CRT, I'd say that the CRT sounds ok, at least certainly good enough not to risk boosting. However I've heard that the CRT's can also suffer from secondary emmision (to do with the phosphor) which can degrade the picture ; I've no experience in this area but I'd assume the degredation would be blurring on bright pictures ?

Or the set may actually be working ! Heres my thoughts on it from my website (regarding the console version):-

The focus arrangements were so stupid that you'd struggle to get a decent focussed picture even the the CRT had been perfect. This is attributable to a ""clever"" piece of circuit design whereby the focus was adjusted by varying the EHT. Bad enough to start with, but the way it was implemented also affected the line drive (i.e. picture width) - and the picture width control would also affect the focus - what a balancing act ! You choose, dim and focussed or bright and blurred !

TTFN,
Jon