Marconi VT73DA Midlands Set - Request for advice.

[DoctorWho]

That's interesting John, mine definitely has the 4-wire wander plug base.

Was I correct about using a resistor around 44 ohms to connect the old heater leads together? If so, what power rating should it be?

Thanks for the help.

[beery]

Hi Peter,
I'm not sure where you got 44 ohms from.

11.5/.3 = 38.33 ohms.

The nearest E12 value would be 39 ohms, but sometimes the ranges of high power resistors that you can get are limited to popular values such as 33 or 47 ohms, so you would need to make up the value required.

For power. P=V x I. 11.5 x .3 = 3.45 watts. You will need a resistor with a power rating above this. 5 or 6 watts should be fine.

RS stock no. 485-2748 would be ok, but you have to buy 5 at a time

The best thing is to keep the heater transformer's 6V output separate from everything else except the tube heater. So it is best if you take the old series heater leads away from the tube and mount the resistor on a small tag strip on the chassis. Remember this resistor will get hot!

Hello to Heatercathode short. If you look at the pictures of Marconi sets on Jon Evans' site you will see 3/20 tubes fitted in his sets. All very strange.

Cheers
Andy

[DoctorWho]

Hello Andy,

Well I never was any good at maths!! Thanks for putting me right. I'll order 5 of the resistors you have suggested, they may well be useful in the future.

Thanks also for the tip about the transformer, I certainly shall keep it isolated from everything apart from the CRT heater.

Thanks again,

Peter.

[Duke_Nukem]

If you look at the pictures of Marconi sets on Jon Evans' site you will see 3/20 tubes fitted in his sets.

I also have another Marconiphone '73 (the plan was to make one good 'un out of two, but unfortunately that would require one of the CRT's to have something called emission ) and that too uses the 3/20, so that's 3 sets out of 3.

TTFN,
Jon

[Heatercathodeshort]

Hello!
Well I also have 3 out of 3! All my three 1807's use the Emiscope 3-18 tube with the B7B base..I have taken some shots of two of them to prove the point. The early production runs used the 4 pin 3/20 tube. The later versions used the 3/18 B7B tube. The notes are from the original service data.
The 3/20 soon became obsolete and was replaced by the 3/18. I doubt if many of the original tubes lasted more than a few years and owners were only too glad of an excuse to rid themselves of the horror rather than spend good money on tube replacement! It is interesting to read the supplementary notes issued by EMI in January 1951. The dealers were having many problems just installing these sets...The service manual is covered in 'mods' and notes!
The exact value of the resistor to replace the crt heater is not critical. Around 33 ohms will do nicely. Join together the two original leads and insulate the join. Regards, John.

[DoctorWho]

Well now, we have some very good news and, guess what, some bad news!!

I followed through the advice I've been given and, when the CRT and the transformer arrived, I fitted in the new transformer and the new CRT and, after straightening up the scanning coils and a few minor adjustments there was a cracking picture, really bright, sharp and well focussed. This shows that we now have a superb CRT which is capable of giving superb results. By adjusting the focus assembly the best possible picture was obtained.

The of course a fault developed - something tells me this will not surprise John at all! Adjusting the Line Hold simply seemed to make the picture vanish, and then, if the brightness was increased, you simply saw wavy lines, basically there was no way of getting line hold. The picture vanished and I just had a fuzzy mess which would not lock on frame or line hold at all, the sound was still fine. I checked all valves and connections in the RF strip and all was fine. After some fiddling about with the area around the slider controls the picture eventually came back and was brilliant. This was when I took the attached screen shot.

The set was then re-assembled and put in thee front room, it ran for a short while, then the picture started to go dim, bright, dim etc and then the fult returned. Now there's just a picture that cannot be line locked at all and is a fuzzy mess and which vanishes when the line hold control is adjusted.

Any ideas what may be causing this? Where should I be looking? Should i be removing the panel with the sliding controls to see what's going on behind, or is the problem likely to be elsewhere?

Any advice appreciated, and thanks again to all who helped to get the set this far. I still hope to have this set up and running properly!

Peter.

[beery]

Hi Peter,
I'm not really an expert on these sets, but you could either be looking at some line timbase fault or perhaps a low video output from the RF section, which could be due to RF instability.

If you have an isolated scope (or stick the set on an isolation transformer) you could look at the video waveform to see if it changes when the fault occurs.

I see the handle by the side, so maybe you need to wind it up again

Cheers
Andy

[DoctorWho]

Thanks for your help Andy. It wasn't until after I had taken the picture that I realised the gramophone winding handle was in shot, but, somehow, this seemed quite appropriate, I wondered if anyone would comment about it

Well I THINK the problem has now been solved, there was some quiet hissing going on as though something in the EHT circuitry may have been arcing, it took me a while to find it, but, I found a resistor near to the terminals beneath the chassis where the EHT lead connects which was hissing, arcing and hot, in fact it was black and crumbly, so I suspect this may have been breaking down.

Going by Trader Service Sheet 974 it was R36 (4.7k ohms) which was the culprit, by replacing this everything once again seems fine. The set was run out of the cabinet for a while, and then re-assembled and run again, thankfully now there are no signs of the problem.

At this moment in time the set now seems to be working extremely well, certainly the Cossor CRT gives a superb picture, nice and bright, no ion burn as it uses an ion trap, visible line structure and very watchable in full daylight.

As I replaced all of the wax condensers at the outset and any damaged resistors and potentiometers I am hopeful that the set will now run for a while without any problems. I wouldn't hesitate to use this CRT as a replacement in any similar set, it really does give first class results, the only alterations you have to make are to change the EHT connector and, of course, use the resistor to connect out the old heater leads and add a transformer for the new CRT's heater. All in all, at present, a very rewarding restoration.

Thanks again to everyone for all of the help, lets hope it runs for a while without developing any other problems, only time will tell.

Peter.

[beery]

Hi Peter,
that's great.
You've got it working in record time despite all the many nasty problems found on the way.
The pictures to look nice and bright.

I'll be interested to know how stable it is if left on for a few hours given the set's reputation.

Have a great 405 line viewing weekend!

Cheers
Andy

[DoctorWho]

Thanks Andy, so far so good anyway. I think this set has had the most problems I've ever encountered in one set, but now the picture is indeed nice and bright and stable.

The sound on this set is truly magnificent, it uses an energised speaker and has a really rich tone. The set is a joy to watch and listen to, hopefully it will remain that way for a while.

I know what you mean about the set's reputation, I'll have to see if I have the courage to leave it on for a few hours or not......... That needs some thinking about

Hope you have a good 405 weekend too.

Thanks again,

Peter.

[Heatercathodeshort]

Hello Peter.....
TE..HEE..Oh stop it John and don't be so nasty! Peter I will send you my service manual for you to copy. It might come up with some answers. This is so typical of this set I'm afraid. My manual has a lot of notes around certain components. It could be an 'out of tolerance' B36 valve. If the line oscillator is way off frequency the eht will fail or drop to a very low value. These early flyback transformers were working to ctitical levels and were 'tuned' to around line frequency [10kc's] and any deviation from this resulted in the collapse of efficiency, hence the unlocked, dim picture. The Cossor tube may have presented a larger EHt load for the old girl. A bit like Grandad shacking up with an 18 year old if you know what I mean. This may have caused a breakdown of a small component. [Stop laughing John...] Manual in post tomorrow. Regards as always, John.
Ps Just seen you have got it sorted! I'll send the manual anyway, for the next episode... J.

[Heatercathodeshort]

Hello Peter,
Its worth having a spare B36 valve to try in these sets. If they don't work they are not actually faulty! They are just not suited to this circuit. They worked fine in other sets with no problems at all. I simply cannot understand why such a simple circuit should give so much trouble.
I couple of milk bottles with the air sucked out should work with this circuit....
Thinking about it all these years on and reading the EMI notes I think most of the problems arose from the poorly designed RF/IF unit. This for the most time produced a poor video signal, unstable, ringing and poor definition due to faults and construction. This must have presented the sync seperator with a pretty poor mess to sort out. Imagine what it was like with a mains supply that varied around 15-20v and they did in those days and an off air signal that was low and mangled due to ghosting and phase shift. Oh well thank goodness for the AURORA! [Post today] Regards John. PS. 100 pages? Really?

[DoctorWho]

Thanks John, that's very kind of you. Interesting to read what you have to say about the circuit, as guess what.............

I powered the set up this morning, it was great for a few minutes, and then yesterday's fault re-appeared again, exactly the same!!!!!!!!!!!!!!!!!!!!!!

We have friends coming over to-day and so I shan't get a chance to look at it, but I hope to get a look at it tomorrow and see what's going on. I'm suspect it is in the same area as R36 which I replaced yesterday, so more investigation is needed.

I have ordered a brand new B36, so this should help.

I am determined to get this set running reasonably well if it's the last thing I ever do!!!!! The more work I do on this set the more I see why you comment about it as you do.

Any suggestions as to what I ought to be looking at around that part of the circuit, any suggestions as to what may be causing the problem? I'll see if I have a spare B36, but I don't think I do, so that will be a case of waiting for the new one to arrive.

Thanks again for your help John. I think I'll go and pull some hair out now

Peter.

[DoctorWho]

Well now, I spent some time on this set this morning and the same fault was present, I replaced some resistors I possibly suspeced and no difference, you'd turn the set on and you'd have a perfect picture for a very short time, this would then start to dissolve and the fault appear, each time you let the set cool down the same would happen again.

I then started to pay attention to the RF section, at first I wondered if a resistor there had become problematic when warm, but it didn't seem to be the case. I then physically removed valves 1-4, cleaned the pins and the holders with Servisol, put everything back together and now we have a perfect picture and sound. It's been running now for about an hour without any problems and with a superb picture!

Now, the fun bit is, we're off out for a couple of hours, it's disconnected now and the chassis is still out of the cabinet (I'm not getting caugh by putting it all back together again only to have the same fault re-appear minutes after I do so), so, when we get home, I'll power it up and see what happens.

Hopefully there may still be a good picture, do I have faith in my work? Do I have faith in this set? Goodness knows! If it is going to play ball and work properly when we get home then I'll leave it running for a couple of hours and see what happens. Here's hoping!

[Framer Dave]

Courage mon brave! Keep the faith Peter! A couple of pints and a nice lunch will make it all seem better. Remember, if the set was perfect, all you'd could do would be watch TV.

Dave

[DoctorWho]

Good point Dave. Actually, so far so good! When we got home I ran the set for over an hour out of the cabinet, not a hint of trouble anywhere. I finally re-assembled it and ran it again, still fine, so I put it away in the front room and connected it all up, currently it's running very well indeed.

One tip for others doing a set like this. I was kindly given a 6.3 volt transformer heater for the CRT, when connected up to the mains with the correct tapping though this was giving 6.8 volts, I decided hat, instead of connecting directly across the mains, I would use a tapping from the mains dropper and the third tapping down provided an output of 6.1 volts for the CRT heater, so that should result in a happy CRT hopefully.

It's good to have the set nicely up and running. Hopefully it will remain this way for a while, I can hear John laughing and waiting for the next breakdown, still, we shall see.

Currently it's first class

[DoctorWho]

I have to say that, now it is working well, it gives superb picture and sound!

Thanks for the help,

Peter.