Marconi VT73DA Midlands Set - Request for advice.

[beery]

Hi Peter,
I have to say that I often see these sets with 9" tubes bodged in them, so I bet the old tubes were bad.
I reckon an American 10BP4 would be a better bet as they were used a lot in the States as opposed to 9" tubes. It might be worth trying to get one shipped over.

Cheers
Andy

[FERNSEH]

Originally Posted by beery

I reackon an American 10BP4 would be a better bet as they were used alot in the States as opposed to 9" tubes. It might be worth trying to get one shipped over.

The 10BP4 is an excellent CRT and could be considered as a replacement for the Emiscope tube. However, if you ever find one make sure it is supplied with the correct ion trap magnet. a special twin magnet assembly was made for the 10BP4.
Also, the heater volts are 6.3 @ 0.3 amp. and the EHT voltage is 9000 volts.

DFWB.

[DoctorWho]

Jon - thanks for your advice, you may well be right about the CRT, I'm honestly not sure. It certainly is a weird system to achieve focus and the like!

Andy - Well if all else fauils a 9" CRT would certainly be well worth considering, at least it would get the set up and running properly again. I'll see if I can track down a 10BP4 if all else fails.

David - Thanks for your thoughts on the American CRT, it sounds like this may be the line to follow. Thanks for pointing out the need for a special ion trap magnet, I would not have known about this. Presumably I'd also need the correct base connector.

I have installed the set in the front room to-day, it is working as described before, but I'd like to get it better with a much brighter picture, only when it's been on for 15+ minutes in a darkened room can you see anything watchable.

Thanks again for the advice.

[FERNSEH]

Originally Posted by DoctorWho

I'll see if I can track down a 10BP4 if all else fails.



David - Thanks for your thoughts on the American CRT, it sounds like this may be the line to follow. Thanks for pointing out the need for a special ion trap magnet, I would not have known about this. Presumably I'd also need the correct base connector.

Hi Peter, The 10BP4 has the standard B12A base. A mains transformer will be required for the 6.3 volt 0.6amp heater.

DFWB.

[DoctorWho]

Thanks David, have you got any ideas where i could try to locate one of these tubes? I have to admit that the original tube is very por and it would be nice to see a good picture on the set.

All the best,

Peter.

[DoctorWho]

The arcing fault mentioned in an earlier post seems to have developed further, now, when powering up the set arcing can be heard as the EHT starts to come up, it manifests itself as flashing white patches in the centre of the CRT and it remains for a while until the set is fully warmed up, even when the picture is present it continues for a while, it isn't until it's fully warmed up that it stops.

Is this likely to be a further problem with the CRT, a fault in the EHT wiring, the LOPT or the rectifier valve itself? Reducing the EHT by using the Focus control does eliminate the problem, but as soon as you turn it back to full EHT it begins again until the set is fully warmed up.

Any ideas?

[Heatercathodeshort]

Hello Peter,
If that brand new EHT capacitor is a 'VISCONOL' it is probably faulty. They go down due to age rather than use. The EHT rectifier heater winding for the U35 could be breaking down to the lopt core but can be rewound by hand. It is not difficult but some peace and quiet are required... The flashing crt are classic symptoms of this. Regards, John.

[DoctorWho]

John - Thanks for your help, the new condenser is one from "Ask Jan First" in Germany, they are made currently and are extremely reliable, so hopefully that should be alright, but thank you for the warning about using old Visconol condensers. Actually, where condensers and resistors are concerned, I always use new ones, that is, made now, I don't use vintage stock, even if NOS, as they seem to so often be unreliable.

I had some time to tackle the set this morning and so checked everything over. There is a Paxolin panel beneath the chassis on to which the EHT lead terminates, together with various other components of the EHT supply and also the Line Hold circuitry (I think). It did seem that there was some arcing there. It was a bit greasy and so I gave it a good clean, this was also when I noticed a very small 0.001uf condenser going from the Line Hold control to chassis which I had missed, and so I replaced it with a new one.

It was at this point that I started to realise why other forum members have made the comments that they have about these sets!

I powered up, and, having seemingly cured the arcing fault, the Line Hold control would not lock within it's range at all, and the focus was way out. The best I could get were 3 fuzzy pictures all next to each other! I start to see just what people mean about one problem after another!

So, after studying the circuit diagram, and, basically, some guessing thrown in, I added a 100k resistor to the 330k resistor (which I had already replaced) coming from the Line Hold control. great, I could now get good Line Hold, but the focus was worse than ever!

I turned the so called 'Focus' control to maximum, so that I was getting full EHT, and loosened the bolts holding the focus magnet assembly to the top of the cabinet. Moving it too far back along the CRT did seem to cause some arcing within the neck of the CRT - I'm not sure if this is due to a worn CRT, or simply because it was not a good idea to try anyway, but, this aside, I found that by moving the whole assembly as far forward along the neck of the CRT gave reasonable results, and then, by adjusting the 3 bolts around the neck of the CRT, the picture was reasonably well centered and the line structure was visible with reasonably good focus.

The picture still needs to be viewed in a darkened room, as addressed before, this seems to be due to a worn CRT, but, at this moment in time, the set is working well without any problems that I am aware of, a nice steady picture and perfectly watchable, I will try and search out a better CRT though as previously discussed so that you can watch the set a little more easily. Now lets just see how long it lasts! Despite the poor design of some of the circuitry, I do have to say that I personally find this a very attractive set. It has a very 1940's look about it.

Thanks again to everyone for their help and assistance.

[DoctorWho]

Guess what? I ran the set yesterday evening to try and start improving the CRT, it ran well for nearly an hour and then arcing began inside, the only way to stop it was to reduce the EHT by turning the 'Focus' control down.

More investigation this morning.

John - I see your points more and more clearly............

[DoctorWho]

This set is more and more strange!

I have traced the source of the arcing, it is actually within the neck of the CRT - something tells me this is not good?

I got the set up and running this morning and had to tap the neck of the CRT several times with the handle of a screw driver, amazingly enough there is now a good picture which can be clearly viewed in a lit room!! That is a definite improvement, however, I suspect that the problem within the neck of the CRT means that the tube is very close to failing.

I'd be very grateful of any suggestions of ways to overcome this internal shorting problem, but, if there is no way, then it is definitely going to be the case of finding a new CRT.

Thanks again for the help.

Peter.

[DoctorWho]

Even that turned out to be too good to last!

Another fault has developed, theere is now uncontrolled brightness, from the service data it seems that the Grid voltage on the CRT should vary from 0-90 volts, but, now, the brightness control varies it benween 268-271, so that's way too high. Any ideas what may have caused this and what I need to do to rectify it?

I've found that by reducing the EHT voltage by turning down the Focus control the arcing inside the neck of the CRT is kept under control. With the Grid lead removed a picture is visible, just very dark of course.

If anyone can suggest a remedy for the Grid voltage issue I would be very grateful.

Thanks for the help,

Peter.

[Steve_P]

You could have cleared it, but how long for is another matter. You can run it every day and cross your fingers, or get another tube. You could be lucky.

The CRT is type 3/20. This is a triode tube, 11.5 Volt 0.3 Amp heater. EHT is at 5.5 kV. B4E Base, Made by EMI. There does not seem to be a British Equivalent, so suggestion given is the best idea.

Run it daily for a bit and see what happens!

Cheers,

Steve P

**** That message was posted as you were posting the message above. If it's not one thing its another with these! You're finding out why collectors hate them too! Anyway....

Check the links between the boards are OK. Check that the power supply is right, and check the Brightness Pot (R32) and R30. And C15.

Then, pray to God and switch on...

Cheers,

Steve P

[DoctorWho]

Steve - Thanks for the suggestions, I'll check these this evening and let you know what I find. You are so right, I am starting to see why these sets have the reputation that they do..............

[DoctorWho]

This set has just given the most spectacular fire works display I think I have ever seen!! To be honest, if I hadn't witnessed what just happened myself then I would not have believed it!

I traced through Steve's suggestions above and found that the brightness pot had indeed gone open circuit, there was no reading at all across the track or to the slider. I replaced that and then the set worked but would not line lock, adding a 50k resistor in to the Line Hold circuit cured this, I then had a pretty fair picture which could just been seen with the curtains open.

I decided that, as these issues were now resolved, and I had a pretty fair picture on the CRT, that I would leave things at that and just use the set as it is, with the EHT voltage kept down to stop the CRT arcing internally.

There were a few attempts at arcing, so I just turned down the 'Focus' control a little furhter to keep the EHT down.

It had been running on the table for a short whole when suddenly, for no reason that I am aware of, there was a massicve display of violet fireworks from the neck of the CRT. Nothing was leaning on the tube, and it wasn't under any pressure.

I turned off and examined the tube, and to my total disbelief there was a crack around the gun assembly which stretched down in to the focussing magnet assembly, you could quite literally just move the gun assembly out of the CRT!

I was totally shocked by this, I have never seen a CRT literally break before whilst not being under any stress, and the colourful display of fire works was just unbelievable.

I disconnected the CRT and checked voltages on the leads going to the CRT and everything seems to be exactly as it should be, so it is literally now only the tube which is causing the main problem.

The CRT is obviously now useless, so I would like to follow up suggestions made earlier in this thread. If anyone has any spare CRT at all, 10", 9" etc that would work in this set then please can you send me a PM? After so much work I would very much like to gety the set up and running again.

Thanks again for the help,

Peter.

[Heatercathodeshort]

Hello Peter,
Oh dear.......Well I did tell you...Uncle John knows a lot about these old dogs...You should have put a match to it at an early date...
The EMI tube is constructed of what is known as 'Hard Glass' ie PYREX, the sort your mums baking dishes were made of. My guess is that the movement of the focus magnet may have 'scored' the glass if it was in close contact with the metal assembly. This would have caused a weak point and the eventual breakdown. The EHT is not high enough to actually arc through the glass and cause a crack as the Philips G11 tubes did in the 80's due to sky high EHT under fault conditions. The problem is the odd screen size of 10". There are few UK models that use this size CRT. EMI and Cossor come to mind and even the TA10, the replacement for the earlier and pre war EMI tubes is very hard to find and even then will need some modifications. HMV 1807 and the Marconi equivalents do pop up from time to time and rough ones go quite cheap. Its a matter of waiting I'm afraid. The sparks in the neck are due to the lack of an insulating vacuum in the tube resulting in the EHT [or what there is of it on the Vt73] arcing over to the gun assembly and probably the scan coils. No harm will have come to the set during these fireworks.
Regards John.

[DoctorWho]

Well John you were well and truly spot on! It seems that you do indeed have a lot of experience with these sets. I have to say that it is a shame as they are nice looking sets cosmetically. I didn't realise these CRTs were also Pyrex, I guess that explains a lot. With your remarks about the arcing in the neck of the CRT, this was a problem all along, so I am wondering if the vacuum was already going when I got this set?

I'm very pleased that no harm will have come to the set through the events that ended the viewing yesterday. If you do happen to hear of any CRT which would work in this set I would be very grateful if you could let me know as I'd very much like to get it up and running again, even a low emission one with a dark picture would do, at least this would allow the set to be seen working.

Thanks again for the advice.

Peter.

[DoctorWho]

A friend has now found me a 10" Cossor 108K 6.3V 0.55A Triode 35mm 50deg
4-clip(7) Mag,IT CRT for use in this set.

I have placed a Wanted ad for a heater transformer for this CRT, aside from that, is there anything else I should know about how to substitute this CRT in to this set?

Does anyone know the pin connection details please? What do I do with the original heater connection leads, are they simply connected together with a resistor?

Any advice on what procedure to follow and what, if any, modifications I will need to make to the set in order to use this CRT would be appreciated.

Thank you.

Peter.

[Heatercathodeshort]

The Cossor 108K uses four wander plugs for connections. From memory the 3/20 uses the B9B all glass base. I have both tubes in my loft so if needed I will have a look. I also have the connections in an old Cossor data book. It is the first base diagram on the attachment. As suggested any six volt heater transformer will do to get it going. The 108K consumes .55amp at 6.3v. It is a very well made tube and rarely found u/s.
Regards John. PS Extra page added.

[DoctorWho]

Well, it seems that the heater was 11.5 volts and the current was 0.3 amps, so, using the formula you have given presumably this gives me 44.33 ohms. Is this right?

It is indeed a series heater chain. Am I right to assume something around 44 ohms? If so, what power/wattage rating?

Thanks for the assistance.

Peter.

[Heatercathodeshort]

Hello again Peter,
I have checked my 1807 and the tube used is the B7B based 3/16. The 3/20 does have the four wire base. All the versions I have seen use the 3/16! So many variations..Regards John.