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Murphy V849U ( Includes the RBM LOPT Overwind Cure)

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PYE625
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Hi, well after a ride over to the fair city of Norwich, I am now the owner of the above set.
Here are some pics of the chassis complete with dust, but looks to be missing a metal screen from the lopt area.
Also has an RS polo on one of the dropper sections, plus a transistorised UHF tuner fitted.
Probably a re-gun tube....CME1903 "ANGLIAN" ?

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 04/09/2015 4:59 pm
Nuvistor
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Good news on the transistor UHF tuner, that made a big improvement to the performance on 625 lines.
Anglian CRT, I would agree a rebuild but if it is a good one it does not matter. Has from an earlier post I think Murphy use Mazda CRT's and in the 60's I found them much more unreliable than Mullard.

It appears to have had little work done to it, I cannot remember if that was the original thermistor location or the modified one. Flywheel sync diodes look like replacements however those were still the troublesome type but the interlace looks original. I used 1N4148 diodes with 15k to 22k resistors as per Rank modification. If you don't use the resistors the line hold will have too much lock in range and the frame interlace will be impaired.

Look forward to seeing a working set.
Frank

Frank

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Posted : 04/09/2015 6:01 pm
PYE625
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You won't be waiting long Frank.....
Re-formed the main electrolytics, cleaned it out a bit and all ready to go? dmm

Just replaced these three sweaty waxies and then first light on 405...

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 04/09/2015 7:07 pm
Nuvistor
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The tube looks good and so does the picture .

Frank

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Posted : 04/09/2015 7:27 pm
ntscuser
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Blimey that was quick! :ccf

Looking good so far :aad

I'm not familiar with that chassis but it appears to be hand wired rather than printed circuit boards?

Classic TV Theme Tunes

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Posted : 04/09/2015 7:45 pm
PYE625
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And here is 625....lets hope the LOPT holds out :aaq

The chassis is wired and no PCB'S :bba
I'm not familiar with the chassis either, and this is the first time I have had the pleasure to own one.

Obviously a bit of work to do, but not many caps to replace...think those three were the only wax types.
The dropper section needs tidying and modifying if needed, will look at that in time.

According to the seller, the set was found in a bedroom of a house recently cleared due to the death of the owner.
The set has survived pretty well I think.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 04/09/2015 7:52 pm
mark pirate
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According to the seller, the set was found in a bedroom of a house recently cleared due to the death of the owner.

It was probably working when put away, the owner probably replaced it with a colour set in the 70's
Looks like a cracking set, stored in a nice warm bedroom, fitted with a rebuilt tube AND has a good LOPT!
It certainly was a lucky find :aad

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Posted : 04/09/2015 8:56 pm
Marc
 Marc
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Hi Andrew,

Well what can one say, yet again another cracking find young Sir ! :aad
Very well done indeed :bba

Cheers,
Marc.

Marc
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RSGB call sign 2E0VTN

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Posted : 04/09/2015 8:58 pm
neil1974
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The one I acquired a while back just worked when I plugged it in :bba although it's pot luck as to whether the lopt lasts :aaj :aak .

Cheers
Neil.

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Posted : 04/09/2015 9:24 pm
PYE625
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The one I acquired a while back just worked when I plugged it in :bba although it's pot luck as to whether the lopt lasts :aaj :aak .

Cheers
Neil.

Well, after about half an hour when using 625 lines, the width does start to shrink.
So, I will apply the procedure to the overwind and see if it will dry out a bit.

I also carried out the mod for the thermistor re-position.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 05/09/2015 1:50 am
Terry
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... Anglian CRT, I would agree a rebuild but if it is a good one it does not matter. Has from an earlier post I think Murphy use Mazda CRT's and in the 60's I found them much more unreliable than Mullard ...

Our Mullard rep used to refer to that as Mazda's 'built in snuffer'!

The tubes went flat, typically when they were 15 to 18 months old, so outside the 12 month guarantee period.

When Rank took over Murphy, the latter must have had existing contracts with Thorn so RBM fitted Mullard 19" tubes and Mazda 23" tubes in both marques. The rep told us of one large dealer in the Southend area who had over 200 23" sets out on rental that needed new tubes!

On his next visit he wasn't so happy. The firm had taken the matter up with Mazda who had agreed to replace the tubes but only on condition that they signed a bulk supply agreement for Mazda to supply all their valves ...

The next thing that happened was a flurry of advertising from Mazda about their new extended two year guarantee on CRTs!

When all else fails, read the instructions

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Posted : 05/09/2015 3:00 pm
PYE625
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Hi Terry,
It sure is a re-gun and has a good bright and sharp picture.

Been having fun trying to set up the line hold though. Can set it fine on either standard, but the difference between standards is miles out. Have been through the set-up for line hold several times now to no avail, there must be a fault.

Interestingly, the so-called 625 line hold preset RV33 affects the 405 line hold just as much as it does on 625.... :aaq

Ignore all that about the line hold.....dumbo here has mixed up which control is which !!!!!!!

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 05/09/2015 4:05 pm
PYE625
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Well, after what looked a good start I'm afraid I think the LOPT is kaput.
When cold, the set works normally with roughly correct HT voltages. After a few mins, the width will start to shrink and HT drops.
It is the line stage dragging down the HT. The overwind gets a little warm to the point of being slightly soft, but not melting.
I tried leaving the overwind powered up overnight with 30VDC to try and dry it out, but no difference..... :aai

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 06/09/2015 11:22 am
sideband
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Candidate for a solid state EHT rec I think. Take the overwind off completely and use an EHT stick.

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Posted : 06/09/2015 11:43 am
Cathovisor
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Given the set's superb condition, wouldn't this be a candidate for that nice Mr. Barker to do one of his rewinds on? Ships and ha'porths of tar come to mind here...

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Posted : 06/09/2015 11:56 am
PYE625
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Here is the poor 'lil LOPT....overwind seen better days lol.
It looks to be possible to dismantle without damage, so I could try the doubler trick and keep the overwind for a rainy day.
I would like to keep the set original, so a rewind is on the cards.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 06/09/2015 12:08 pm
PYE625
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Hi, I removed the overwind and substituted a doubler which has restored normal operation on 405.

However, within the overwind, there is another smaller coil that is switched in for 625 operation.
You can see the connections in the yellow sleeve on the overwind.
I am in fact wondering if it is high voltage leakage to this winding that is the problem.

The point of the exercise is to prove beyond doubt that it was the overwind....and surprise surprise, it is.

I have removed the tar covering and will let the overwind boil steadily with some DC volts across it....whether it will be ok after some time, well, I will just have to wait and see.

The doubler in situ is not as pretty as the DY86 !!

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 06/09/2015 2:01 pm
PYE625
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Well well well.....
In the post above, I said about the small winding contained with the overwind that is switched in on 625.
It is of low resistance and is connected to ground at one end.

When I measured the continuity with a DVM between the two windings it is open circuit as you would expect.
BUT...when a voltage is applied to the overwind alone, a voltage appears at the other winding too.
For example, using 500vdc on the overwind produces about 70vdc on the other winding.

This is leakage and it increases with the temperature of the overwind.
When the overwind is cool, there is NO leakage between windings.

Now....what to do about it?
Can I pull off the overwind section and possibly improve the insulation between windings?? :aaq

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 06/09/2015 2:32 pm
PYE625
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Yes, I was able to separate the overwind from the former which has a small winding on it.
It was only possible to remove when quite warm, being heated by approx. 35 vdc across the overwind for a few mins.

First pic. is of the rather messy overwind with the paxolin former removed.
I unwound the fabric tape that is meant to insulate the small inner winding from the overwind.
I used kapton tape and then polythene tape to replace the old fabric tape, being careful to maintain a suitable thickness so the former would still fit back into the overwind doughnut.

I re-assembled the LOPT and tried it out.....SUCCESS!!
HT is normal, width is normal and the set has been on 625 for nearly two hours without any re-lapse.

So, could this be a solution to the dreaded Bush sick LOPT problem?

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 06/09/2015 4:56 pm
neil1974
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With any luck you may well have sorted it :bba . but only time will tell.

Cheers
Neil.

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Posted : 06/09/2015 5:47 pm
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