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Old 12th Aug 2006, 4:58 pm   #1
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Default Marconi VRC52A Mains EHT Transformer Short?

I have a Marconi VRC52A combined radio and TV set which has been working extremely well.

A few days ago when it was powered up the picture very briefly collapsed but immediately came back again.

Two days ago there was the smell of what could be insulation breakdown etc and the set went dead.

I checked the two fuses at the back of the set and one of them had blown, I initially suspected the U52, and so, to be on the safe side, I replaced the condensers associated with that valve (I had already replaced all of the condensers in the set when restoring it, so this was a re-replacement).

I then powered the set up on radio, and it worked perfectly, however as soon as I switched to television it immediately blew a fuse and there was a slight smell of insulation breakdown.

Just to eliminate other possibilities I next powered up the set with the U52 removed as, although I knew from the radio it must be working, I wanted to be doubly sure, again as soon as I switched to television it blew a fuse. I then replaced the U52 and tried removing the U33, but again, when television was selected it blew a fuse (it seems to blow either fuse, but not both). Every time it blows a fuse it is accompanied by this slight smell.

As I have tried the set with either of the rectifiers removed this made me strongly suspect the EHT windings in the main transformer, but I would obviously like to make absolutely sure of this before sending the transformer away for a re-wind which will be expensive, as I want to make sure this is the problem and will get the set up and running again.

If anyone has any thoughts about this, or can suggest anything I may have overlooked, I would be very grateful. If you can think of another cause for this which I am missing please can you let me know, or if you feel that this is the main transformer then I would be grateful of your opinion.

Is there any chance of finding a replacement transformer for this set, if not can anyone suggest the best place to get it re-wound?

Many thanks for any advice.

Peter.
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Old 12th Aug 2006, 7:19 pm   #2
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Default Re: Marconi VRC52A Mains Transformer Short?

Hello Peter,
Oh dear this does at first thought sound like failure of the EHT transformer but this is very rare on this model because it is so well made.
Is the fuse blowing 'instant' or does ot take say 2 seconds to fail? You may have a heater wiring short in the television section. Such high currents are used in the heater circuit that a short will blow the mains fuse quickly. The radio/television switch may have broken down causing the paxolin to track and smell. Try to track down the location of the smell and listen very carefully when you apply the power with the set switched to T.V. You may hear a crackle or fizz that will give the game away. I think the eht transformer is sealed on this model so the burning smell may not be from that. Try running the set in series with a 150watt lamp or a variac if you have one. That might limit the current and give a better indication as to where the fault is located and prevent the fuse from failing. How did the picture go off? Did it get larger or just fade away? Please don't replace components until we have traced the cause! That will lead to man made faults..Hope this helps. Regards John.

Last edited by Heatercathodeshort; 12th Aug 2006 at 7:24 pm.
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Old 12th Aug 2006, 7:23 pm   #3
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Default Re: Marconi VRC52A Mains Transformer Short?

Reference suggestion of someone to re-wind the transformer, Mike Barker advertises in the BVWS Bulletin offering a coil and transformer re-wind service. As Mike is the BVWS Chairman you can be confident that your transformer will be well looked after throughout the work, and it will be in the hands of someone who understands and respects old technology (and won't see it as "just another job"). Mike's number is already available so I guess it is okay to repeat it here: 01793 536040.

As for finding an exact replacement, you could try the Vintage Wireless Museum in Dulwich, but its a bit of a long shot for finding an exact drop-in replacement. Might well have something that will do the job though.

I'll leave it to someone better qualified to advise if there are any further checks you can do prior to dispatch.
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Old 12th Aug 2006, 7:51 pm   #4
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Default Re: Marconi VRC52A Mains Transformer Short?

Hello John. Thanks for your reply and very helpful input. The fuse does blow instantly, and it can be either one of them. I will certainly give the heater wiring a check over. I did do a visual inspection underneath the main chassis and there were no obvious signs of burning - interestingly though on top of the main transformer where the EHT lead emerges there was not a covering of dust as there was on the rest of the transformer, there was what I can only really describe as a circle around the lead which was not dusty. Thanks also for the tip with the switch, I will indeed check that too, I think they were all in pretty good shape, but always good to check. When I very first checked the set after this problem arose I did catch what I think was a spark out of the corner of my eye, it came from the area of the main transformer, initially I thought this was the U52, but as the radio works I guess this is unlikely?

I have tried to see or hear a sign to give the game away as to where the fault is, but now there is nothing, it just blows the fuse instantly.

The picture went off by just fading away, there was no balloning or anything like that. I did also replace the two 0.03 EHT condensers when I worked on the set originaly with brand new ones, so I imagine they are alright.

The EHT transformer is sealed, but there are small gaps around the EHT wire entry holes, these are not black though.

I will try what you suggest with a 150 Watt lamp and see if that allows me to identify where the problem lies. We have an RCD in the house, and the chassis is earthed, but the set does not trip the RCD, so it doesn't seem as though anything is shorting to chassis.

I'll do some more investigating and let you know how I get on.

Thanks also for the tip about Mike Barker, if it does turn out to be the EHT transformer then I'll contact him.

How practical would it be to disconnect the wires which connects the EHT windings onthe main transformer and simply add a second seperate transformer for EHT?

Peter.
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Old 12th Aug 2006, 8:23 pm   #5
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Default Re: Marconi VRC52A Mains Transformer Short?

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How practical would it be to disconnect the wires which connects the EHT windings onthe main transformer and simply add a second seperate transformer for EHT?
Provided you can disconnect the faulty transformer then there's nothing to stop you providing the EHT by any other means you fancy. This could be a separate transformer (I saw this in the HMV900 displayed at the Thorn-EMI reasearch labs museum area), an old RF EHT unit or a modern EHT module such as those made by Brandenburg.

In a relatively old and rare set you might wish to carefully consider the reversability of such mods.
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Old 12th Aug 2006, 8:39 pm   #6
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Default Re: Marconi VRC52A Mains Transformer Short?

Hello again Peter,
Thanks for your reply. The short on the heater line may not appear visually [if there is one] Most of the heater wires are twisted and pass through chassis holes and the sleeving may be perished and broken down. The 150 watt lamp should allow you to monitor what is happening safely. The lamp may flicker as the short makes or breaks. Please don't replace components or fiddle with the transformer until we have proved that it is faulty. Its a LONG time since I opened one of these just to see how it was made and if I remember its full of insulating grease. The 'stain' around the A.C. feed to the rectifier may be just the rubber lead causing discolouration.
The EHT heater winding must be O.K. because removing the U33 will isolate this from the rectified A.C.feed.
The transformer can be checked by disconnecting the secondary windings in turn. Removing the U33 top cap will isolate the EHT winding. then disconnect the main heater winding, that's the first one on T5 [Marconi circuit] switch on and check after each operation. If the fuse still fails, move to the next winding and repeat followed by the last one. If the fuse still fails it looks like the EHT winding has developed a severe short of which I must admit I'm doubtful.....If the winding has broken down and a rewind is impossible, [try Rejectostats suggestion first] you will have to remove the complete EHT winding in order for the transformer to continue to be used for valve heating only. A suitable EHT transformer can then be wired into circuit. PLEASE DON'T DO THIS UNTIL WE HAVE CONFIRMED ITS FAULTY...... If the transformer is faulty I suggest you PM me for a suggestion. Check the mains input wiring to the transformer. If in doubt, disconnect the primary and feed it from the mains direct to prove the point. Regards John and good luck, watch what your doing with that mains EHT and I don't know what that grease is.....Leave it well alone for the moment or I will order confiscation of your soldering iron!.

Last edited by Heatercathodeshort; 12th Aug 2006 at 8:48 pm.
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Old 12th Aug 2006, 9:12 pm   #7
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Default Re: Marconi VRC52A Mains Transformer Short?

My suggestions may be correct but, as always, John has put the cart and the horse in the correct order. First prove that the TX is faulty beyond all reasonable doubt. Even if it means removing it from the set and firing it up, carefully, on the bench.
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Old 12th Aug 2006, 9:58 pm   #8
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Default Re: Marconi VRC52A Mains Transformer Short?

Jeffrey - Thanks for your input, that is sound advice, if it does turn out to be the transformer then I may well see what options I can take. I have a seperate mains EHT transformer in a Regentone set of mine and that has worked out very well.

John - Thanks for your further assistance, it is very much appreciated. I have not yet had chance to go through every stage that you have suggested, BUT I decided to go and simply remove the back of the set and sniff the main transformer (the things we collectors do!). The burning/insulation breakdown smell was there a little. I then powered the set up and tried to see or hear where anything was going on. The fuse blew immediately, there were no signs of any arcing, no smoke etc.

I switched off the set and immediately smelt around the main transformer. I noticed the smell was much stronger, and, espcially where the EHT AC leaves the transformer to go to the rectifier the smell was quite strong - I know there are other possibilities, but does this look as bad to you as it does to me?

The only components I have replaced so far are the electrolytics directly connected to the U52, this was bvefore I suspected the ransformer, since then I have not changed anything, and I won't until we've managed to resolve this.

From this information do you still feel I should go ahead with the 150 Watt lamp? If so then I will certainly do this to see out what I can find.

The one other thing I did do was to check the resistance of the EHT winding on the transformer from the top-cap connector of the rectifier to chassis (top can disconnected from the valve). It was approximately 300 ohms less than it was stated as being in the service data.

Your suggestion of disconnecting the windings, one at a time, is a very good one, I will try and work out which wires are for which winding and work through it this way.

Whilst the EHT winding does look possible, the only thing that puzzles me is how come the transformer can still be used for the radio without blowing a fuse as, surely, the primary must still be live?

I'll certainly not be replacing the transformer with anything until I have got further input from you, I would need advise on how to remove or disable the EHT winding if that is the problem.

Thanks again for your help, I do appreciate it. If you can let me know which approach you think is best next then I'll give it a go, either using a lamp and/or disconnecting the various secondaries.

All the best,

Peter.
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Old 13th Aug 2006, 10:16 am   #9
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Default Re: Marconi VRC52A Mains Transformer Short?

Hello again Peter,
It does appear by your description that the EHT winding has developed short circuit turns. I suppose after the best part of 60 years it can be forgiven. I would still go along with the 150watt bulb test. This will allow some voltage to pass through the set and will allow the breakdown to show itself without doing any harm or blowing fuses. Any severe overload will be passed to the lamp. The H.T. supply for the television and radio is derived from a separate transformer T6. This might be a bit confusing at first sight of the circuit. The low resistance reading of the EHT winding MAY be a tell tale but over the years I have become somewhat sceptical of published resistance readings of coils and transformers. They do tend to have a wide tolerance...
To explain the breakdown with particular regard to EHT transformers. They are wound of incredibly thin wire in layers with each layer separated by a thin wrap of paper known as an nsulating tissue. Many layers are required and the total number of turns may run into 10.000 to produce a 4.5kv A.C. supply that when rectified will produce around 5kvt for the CRT final anode. If during manufacture a single turn is allowed to 'drop' down at the end of its layer and is in near contact with a winding at lower potential, due to the large voltage build up between layers, a flash over will occur. This can take some time to show up. Another cause is expansion and contraction of the very fine wire during use of the set. This eventually causes a tiny break resulting in flash over and spectacular breakdown! A single shorting turn may pass unnoticed for a while but the increasing heat will quickly cause an avalanche effect and rapid total failure. Due to the RMS factor the insulation needs to be about twice that of the EHT voltage, hence the heavy expensive component.
It has been many years since I carried out a post mortem on a transformer from the Marconi/HMV series. You will have to remove the transformer and open up the case....Inside you will find the large EHT overwind with the lead to the U33 terminating from the top. This is not a job I would recommend you carry out without someone with experience of this standing shot gun beside you. The transformer can be easily damaged and you have to identify the windings very carefully. The winding can be literally cut off, carefully cut through PART WAY with a hacksaw and then unpicked to its start. Its not an easy job and I think this ione is filled with grease but this need not be replaced if you are removing the EHT overwind.
That's about it Peter but as you are the ONLY member of the Emiscope anode cap re fitters Society, I'm confident you will win this one.... Just make sure its faulty first as the above suggestion is the 'nothing to loose' last resort. Regards John.
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Old 13th Aug 2006, 6:02 pm   #10
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Default Re: Marconi VRC52A Mains Transformer Short?

Hello John. I do appreciate your assistance with this very much. I will certainly give the 150 watt lamp method a try and see what happens, I'll let you know what results I get.

Thanks also for explaining the reason for the EHT transformer breakdown. I am aware of the problems many encounter with mains EHT, but it is very useful to have it explained like this. I usually only run my mains EHT sets for a short period of time so as to put the mains EHT transformers under as little stress as posible, I have, for example, a Cossor 912 and a Pye D16T and I only ever run those type of sets for short periods of time so as to preserve them. How long would you tend to run mains EHT sets with their original transformers?

Thanks also for explaining how to remove the EHT winding, that does sound relatively straightforward. It's well worth stressing that you should only cut through PART WAY as well, thank you.

I agree with you about the "nothing to looe" last resort, so I'll do the final tests tomorrow and then let you know what I find and perhaps I can discuss with you the options from there? I guess the easiest option would be to fit a second transformer in the set for the EHT, I did this very successfully with my Regentonr TR20, the only problem with the Marconi is that there is not really the physical space to do this, as I mounted it next to teh chassis, so do you have any suggestions, or would you bite the bullet and pay to have the original transformer re-wound?

This set has worked incredibly well with a first class picture and so I do really want to get it up and running again.

Thanks again for the help.

All the best,

Peter.
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Old 13th Aug 2006, 7:12 pm   #11
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Default Re: Marconi VRC52A Mains Transformer Short?

Hello again Peter,
I tend to run my mains eht sets for relatively short periods but once ran the HMV 900 for around 8 hours without incident. The EHT transformer was rewound in 1975. The PYE D16T uses a very well made transformer and providing the set has been kept warm and dry I would guess that you could safely run this for a few hours. Remember when these sets were new the transmissions were only a few hours a day. The Coronation broadcast in 1953 was the first 'extended' transmission being a total transmission of around 7 hours. This resulted in huge numbers of receivers breaking down due to the strain! [I doubt if many HMV 1807's made it...] The Marconi model you have and the HMV version without the radio, particularly the 15" version were very reliable and many were still working well into the mid 60's if only as a second set.
If you can get it rewound I would go for that. Failing this a new transformer can be made up using modern laminations and will result in a smaller construction. I don't have a problem with that as the original transformer less its EHT winding will still be in service. Regards John.
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Old 14th Aug 2006, 10:48 am   #12
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Default Re: Marconi VRC52A Mains Transformer Short?

Hello John. Thanks for your reply. I thought I was right not to use the mains EHT sets for too long, it's such a shame when the transformers do break down as you lose some originality, but at least not all is lost and they can be bought back to life again. You make a good point about transmissions being relatively short when these sets were made, so it stands to reason that they were never run for long periods of time. I will look in to re-wind options and costs and new transformer costs. If you oped to mount a new transformer for the EHT and retain the original transformer for all other circuits where would you personally mount the EHT transformer in this set? Space is not so easy to find in this set as it was in my Regentone.

Thanks for the info Andy, Ed has done a transformer for my Regentone for me and he did a fantastic job, as soon as I have done the final checks I will probably contact him to see if he can make me another one, I just need to see if I can salvage the rest of the original transformer first before I buy just an EHT transformer as it may be that circumstances force me to have the original transformer re-wound.

Thanks again for the help.

Peter.
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Old 15th Aug 2006, 7:21 pm   #13
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Default Re: Marconi VRC52A Mains Transformer Short?

Hello Peter,
I have had a good look at my VC52DA chassis and as you say it is very cramped. I would suggest mounting the new transformer on a small aluminium chassis and screw it to the side of the cabinet. You will need to bond the core of the transformer or the chassis if you use one, to the main chassis with some copper braid. The EHT output can then be connected directly to the U33 anode. Regards John.
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Old 15th Aug 2006, 7:32 pm   #14
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Default Re: Marconi VRC52A Mains Transformer Short?

If you have trouble getting a suitable transformer then for rather less size and a lot less weight you could use a Brandenburg EHT module. http://www.brandenburg.co.uk/ Something like their 3590 series. Don't know about cost but I suspect £30 to £50. You would also need a small low voltage DC supply for it which you might be able to get from a heater winding. If you're lucky you might even be able to hide it under a chassis. It's also pleasantly non-lethal which could rather detract from the charm of the original arrangment Provided you do everything in a way that can be reversed you won't compromise a future restoration.
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Old 15th Aug 2006, 8:10 pm   #15
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Default Re: Marconi VRC52A Mains Transformer Short?

John - thanks for your advice, I suspect that, if I can successfully remove the EHT winding, then I will go down this avenue, it seems the easiest way around the problem. I just have to tackle the transformer first and see if I can successfully remove the EHT winding.

Jeffrey - Thanks for the advice re the Brandenberg unit. I did look in to one of these for my Regentone set with mains EHT, I was gobsmacked to find out that their cheapest unit was £330 (and they like to sell them in 3's but agreed to sell 1 on that occasion, and yes, the price was for one only). I think they make some superb power supplies, but their pricing is too expensive for me, thanks for the suggestion though, if it was a bit cheaper it would be ideal.
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Old 15th Aug 2006, 8:28 pm   #16
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Default Re: Marconi VRC52A Mains Transformer Short?

Quote:
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I was gobsmacked to find out that their cheapest unit was £330
Ouch! Some years ago (1990ish?) I spoke to a Brandenburg rep on their stand at a trade show and that's where I got the £30-£50 price tag from. Things have obviously changed.

Wonder if these guys are any cheaper:

http://www.emcohighvoltage.com/ourprods.htm

My google search was for "photomultiplier power supply"

Last edited by ppppenguin; 15th Aug 2006 at 8:33 pm. Reason: Add new supplier of HV modules
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Old 15th Aug 2006, 9:04 pm   #17
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Default Re: Marconi VRC52A Mains Transformer Short?

Well I was rather amazed myself at the price, in fact I even asked the chap to repeat it for me as I couldn't believe they were so costly.

I wonder how they make these solid state EHT supplies?

Thanks for the other link, I'll have a look at their website.

Peter.
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Old 15th Aug 2006, 9:23 pm   #18
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Default Re: Marconi VRC52A Mains Transformer Short?

I'm sure it wouldn't be too hard to design and build a simple EHT supply. The basis would be the old RF EHT units reworked for modern components. Probably a ringing choke or flyback topology with a small step up transformer and voltage multiplier. Or (I'm thinking as I'm typing) a diode split LOPT from a scrap PC monitor driven by a power FET and run well below its usual ouput. I'm sure I've seen something like this proposed as the 25kV feed for a Marx generator. Found it! http://www.electricstuff.co.uk/marxgen.htm. Look near the bottom ofthe page. Obviously whatever you did would be well throttled back compared to driving a Marx generator. In that project he suggested finding a big beefy LOPT while you would want a really compact one.
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Old 16th Aug 2006, 8:13 am   #19
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Default Re: Marconi VRC52A Mains Transformer Short?

Why not look out for an old Neon light transformer - these could be run on a lower ac input voltage, and will easily give you several thousand volts at a couple of milliamps - plenty of EHT for a tv tube, and they can be obtained as scrap from shopfitters and signmakers........

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Old 16th Aug 2006, 10:59 am   #20
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Default Re: Marconi VRC52A Mains Transformer Short?

I managed to remove the transformer from the set and just have it running on the table to-day, I wired it up with a 5 amp fuse and powered it up. It started to literally bubble inside and then hot wax literally shot out of the hole around the EHT lead, so there was a bit of a mess to clear up.

It wasn't under load, so I think this definitely shows that the transformer is the culprit sadly. At least this has enabled the tracing of the fault, now for the next step.

Thanks again to everyone for their help and advice.

Peter.
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