It is currently Tue Dec 12, 2017 4:57 pm

Murphy V849U ( Includes the RBM LOPT Overwind Cure)

 
Posts: 3712
Joined: Sun Feb 12, 2012 7:43 pm
Location: North Hykeham, Lincolnshire and Ilford, Essex (but not for much longer ...!)

Re: Murphy V849U

Post by Terrykc » Sun Sep 06, 2015 7:40 pm

PYE625 wrote:On a slightly different note, this set has a transistorised UHF tuner fitted.
I would guess it would have had a valve UHF tuner originally ?

Yes. In its original form it was released as the VHF only TV125. The TV125U with (valved) UHF tuner didn't start appearing until the opening of BBC2 was approaching.

The difference in price was 8 guineas, the same as the difference in price between the TV125 and the TV125U. As the sets were still under guarantee it saved any arguments to retrofit the UHF for the price of the tuner alone.

Interestingly, though, the TV125 was only fitted with a shorting link on the plug and, presumably, your tuner is a later Bush one - because of the mechanical arrangements, including the push button unit, needed to fit the cabinet ...

 
Posts: 715
Joined: Tue Jan 27, 2015 2:02 pm
Location: Chatteris, Cambs

Re: Murphy V849U

Post by PYE625 » Mon Sep 07, 2015 9:27 am

Thanks Terry,
On the back cover, it has the last couple of digits missing so I can't tell if it's V849, or V849U.
On the VHF tuner there is a date of 4 DEC 1964 stamped.

So far the LOPT is fighting fit....been on for long periods now and perfect operation. :)
All voltages spot on and bags of width available.
Had to change the PCL85 due to the bottom creeping up.
Nice to have the mustard caps, no 'orrible wax ones. :bba

PS....anyone have a scrap chassis I could pinch the line stage screening cover from? :bba :bba

 
Posts: 2665
Joined: Sun Sep 01, 2013 6:19 pm
Location: Behind the sofa

Re: Murphy V849U

Post by Cathovisor » Mon Sep 07, 2015 10:41 am

I seem to recall that the TV125 had a rather bad press regarding system switch reliability; think I saw an article in Service Skill about it...

 
Posts: 3712
Joined: Sun Feb 12, 2012 7:43 pm
Location: North Hykeham, Lincolnshire and Ilford, Essex (but not for much longer ...!)

Re: Murphy V849U

Post by Terrykc » Mon Sep 07, 2015 6:12 pm

PYE625 wrote:... On the VHF tuner there is a date of 4 DEC 1964 stamped ...

That probably makes it too early for the U version - Crystal Palace had only just closed down (4 days earlier!) the UHF transmitters so that they could be retuned to Channel 33 for the BBC2 launch.

Remember also that the BBC had announced that the new channel would be aimed at minority interests and interest nosedived overnight - I encountered a queue of customers in the shop the following morning all wanting to cancel their upgrade orders - so the original owner probably didn't bother for a couple of years.

It wasn't until the BBC created a demand with Match of the Day that UHF conversions really took off and this owner benefitted from a transistorised tuner rather than its much lower gain valved predecessor.

 
Posts: 715
Joined: Tue Jan 27, 2015 2:02 pm
Location: Chatteris, Cambs

Re: Murphy V849U

Post by PYE625 » Mon Sep 07, 2015 7:07 pm

Thanks for the interesting info Terry. :aad
I would imagine the set had always been in Norwich, and therefore Tacloneston the local transmitter (BBC), or Mendlesham (ITV) a little further out.
The resistors on the UHF tuner look more modern than the rest of the set, so it may well have been fitted quite a bit later.
The tuning buttons are all set in the high end of UHF, where Tacloneston is (or was for analogue).
UHF BBC2 started on 9th Sept 1967 from Tacloneston on channel 55, BBC1 and ITV early October 1970.

 
Posts: 3712
Joined: Sun Feb 12, 2012 7:43 pm
Location: North Hykeham, Lincolnshire and Ilford, Essex (but not for much longer ...!)

Re: Murphy V849U

Post by Terrykc » Wed Sep 09, 2015 12:06 pm

When BBC2 started in April 1964, all UHF tuners were valved.

Transistorised UHF tuners started to appear in sets about a year or so later and, within a short time, the valved type were history.

From your date, Andrew, Tacolneston was over three years behind Crystal Palace, so well into the transistor area for conversions.

Meanwhile, we still had valved UHF tuners on our shelves, left over from when so many customers cancelled their upgrades ...

 
Posts: 715
Joined: Tue Jan 27, 2015 2:02 pm
Location: Chatteris, Cambs

Re: Murphy V849U

Post by PYE625 » Fri Sep 11, 2015 11:38 am

Hi, well it looks like the LOPT is happy now and so am I.
Been several days and no hint of any problems. :)

 
Posts: 715
Joined: Tue Jan 27, 2015 2:02 pm
Location: Chatteris, Cambs

Re: Murphy V849U

Post by PYE625 » Fri Sep 11, 2015 9:52 pm

Replacement of the "CenTerCel" metal rectifier MR23a and b with a couple of IN4148's cured a problem of too much contrast.
(AGC delay).

The same goes for MR31/32 but this time for poor line lock, I used OA91's here.
I think either type will work ok, but only had a couple of the OA type left.

 
Posts: 715
Joined: Tue Jan 27, 2015 2:02 pm
Location: Chatteris, Cambs

Re: Murphy V849U

Post by PYE625 » Thu Sep 17, 2015 6:33 pm

Hi all,
This is a niggly wiggly kind of fault that has always been there.
It is only just noticeable on 625, but worse on 405.
Basically, it is small wiggles in the verticals that seem to move down the picture.

First photo is taken with the set on tune and you can just make out the wiggles.
Second photo is with the 405 tuning deliberately mis-tuned to a position where sound on vision would normally occur as this seems to exaggerate the fault.
The focus of the camera is slightly out so as not to see the line structure.
The effect is also worse with a weak signal, see last photo.

Bearing in mind that the LOPT screening can is missing, could the line stage be radiating RF?

Any ideas? :aab
Attachments
rsz_003.jpg
rsz_001.jpg
rsz_005.jpg

 
Posts: 489
Joined: Tue Mar 24, 2015 12:01 am
Location: Wigan

Re: Murphy V849U

Post by nuvistor » Thu Sep 17, 2015 7:29 pm

Looks like 50c/s ripple, if the smoothing is ok, check any connections to chassis for high resistance and then heater cathode leakage in the valves.
It does not appear to put a dark bar on the video so I would be checking Sync and timebase circuits first, any hum in the sound?

Frank

 
Posts: 3712
Joined: Sun Feb 12, 2012 7:43 pm
Location: North Hykeham, Lincolnshire and Ilford, Essex (but not for much longer ...!)

Re: Murphy V849U

Post by Terrykc » Thu Sep 17, 2015 7:46 pm

As posted earlier:

viewtopic.php?f=10&t=11888#p123637

perhaps a simple screen composed of cooking foil glued onto cardboard could prove/disprove whether the missing screen is instrumental in causing this problem ...?

 
Posts: 489
Joined: Tue Mar 24, 2015 12:01 am
Location: Wigan

Re: Murphy V849U

Post by nuvistor » Thu Sep 17, 2015 8:32 pm

A test screen is a very good idea, I don't remember having the can removed causing problems, it was very often removed during repair but I always refitted them and it will be easy to check if it as any effect.
Also worth checking lead dressing.

 
Posts: 3712
Joined: Sun Feb 12, 2012 7:43 pm
Location: North Hykeham, Lincolnshire and Ilford, Essex (but not for much longer ...!)

Re: Murphy V849U

Post by Terrykc » Thu Sep 17, 2015 9:26 pm

PYE625 wrote:... Bearing in mind that the LOPT screening can is missing, could the line stage be radiating RF?

The fact that the problem is exacerbated by reducing the signal level does suggest that RF radiation is indeed the cause ...

 
Posts: 847
Joined: Wed Nov 16, 2011 8:11 pm
Location: Cheltenham

Re: Murphy V849U

Post by Doz » Thu Sep 17, 2015 10:38 pm

nuvistor wrote:Looks like 50c/s ripple, if the smoothing is ok, check any connections to chassis for high resistance and then heater cathode leakage in the valves.
It does not appear to put a dark bar on the video so I would be checking Sync and timebase circuits first, any hum in the sound?

Frank


Looks a bit more HF than 50Hz

 
Posts: 489
Joined: Tue Mar 24, 2015 12:01 am
Location: Wigan

Re: Murphy V849U

Post by nuvistor » Fri Sep 18, 2015 6:58 am

Doz wrote:
Looks a bit more HF than 50Hz


I thought the pattern was in a band a few inches high moving either up or down the screen. I probably misunderstood.
Frank

 
Posts: 715
Joined: Tue Jan 27, 2015 2:02 pm
Location: Chatteris, Cambs

Re: Murphy V849U

Post by PYE625 » Fri Sep 18, 2015 9:00 am

Thanks for all the input fellas',
It does seem to be HF, there is no hum on sound or anything resembling a 50 or 100 Hz ripple.
I tried a temporary screen made up of foil on cardboard carefully placed in a similar fashion to the line stage screening cover, but no difference whatsoever.
One by one I substituted all valves to no avail. I even tried touching a 1000pf cap across each decoupling cap in the IF stage to try and see any difference, but none.
All cable looms/leads are undisturbed and moving any around where possible proved fruitless.
The fault, as I said at first, is not only confined to 405. It is only just noticeable on 625 but hard to photograph.
It is confined to this set only, I did try my Ekco TC435 and was ok.

Got to admit I'm flummoxed lol. :aab

 
Posts: 489
Joined: Tue Mar 24, 2015 12:01 am
Location: Wigan

Re: Murphy V849U

Post by nuvistor » Fri Sep 18, 2015 11:30 am

If you have a scope look at the supply lines, may see something that is not decoupled correctly.

 
Posts: 311
Joined: Sat Feb 09, 2013 6:08 pm
Location: due South of Selby, near the Pepperpots, house smells of two stroke.

Re: Murphy V849U

Post by Wolfie » Fri Sep 18, 2015 6:12 pm

Could it be the TV interfering with your video source?

Reminds me very much of some of the strange problems experience by putting a TV on top of a VCR..

 
Posts: 715
Joined: Tue Jan 27, 2015 2:02 pm
Location: Chatteris, Cambs

Re: Murphy V849U

Post by PYE625 » Fri Sep 18, 2015 6:32 pm

Hi,
Well I scoped all supply rails and nothing odd.

Wolfie, you are right and it does look very similar to that kind of interference, but I tried moving the set and even a substitute Ekco set all to no avail.
It has got me well and truly stumped.

One thing I have not mentioned so far is that I have painstakingly performed a re-alignment of the IF stages as per the service data.
I went through the whole procedure twice and I have to say the results are fine on both standards and 405 is better with the sound/vision tuning in correct positions now (definition was previously poor when tuned for best sound).
Needless to say, the wiggles were there before and remain exactly the same, so it must be a fault of some kind.

 
Posts: 489
Joined: Tue Mar 24, 2015 12:01 am
Location: Wigan

Re: Murphy V849U

Post by nuvistor » Fri Sep 18, 2015 8:54 pm

This set as flywheel sync for the line timebase, the flywheel effect should smooth out those line by line timing variations which show up very well on that last photo with the noisy signal.

Is your scope dual trace, if so trigger one channel on the sync pulse and the other on the oscillator output to the PL36 grid.. If there is a timing change between the two, ie sync solid but osc varies line by line, check the osc and flywheel feedback.
It may look worse on 405 due to the line sweep is approx 100micro seconds and 625 being approx 64 micro seconds.

These are just thoughts on we're to look next, maybe completely irrevalant but if you have no other ideas it is worth a look.

From your early post I may have understood incorrect, is this a band of jagged verticals a few inches high and this band moves up or down the picture?
Frank

 
Posts: 489
Joined: Tue Mar 24, 2015 12:01 am
Location: Wigan

Re: Murphy V849U

Post by nuvistor » Fri Sep 18, 2015 9:00 pm

Noticed you have changed the flywheel sync diodes, I am not sure if OA91 diodes are suitable, I always used silicon ,such as 1N4148 but the service bulletin from Rank advised that 15k to 22k resistors were fitted in series with these diodes. I found that the line lock was to large and the time constant altered if the resistors were not fitted.
They advised these resistors for the interlace diodes if silicon ones replaced the originals.
Frank

 
Posts: 715
Joined: Tue Jan 27, 2015 2:02 pm
Location: Chatteris, Cambs

Re: Murphy V849U

Post by PYE625 » Fri Sep 18, 2015 9:34 pm

Frank !! You are a TOP MAN :aad :aad

Fitting IN4148's with 18k resistors in series has solved the problem 100% !!

Right at the beginning I had very poor line lock and replaced the block with the OA91's.

I had almost given up, many thanks for the advice which I would never have known.

Look at the below pic's now....even with a very weak signal, the wiggles are banished forever!!
Attachments
rsz_003.jpg
rsz_11002.jpg
Last edited by PYE625 on Fri Sep 18, 2015 9:43 pm, edited 1 time in total.

 
Posts: 489
Joined: Tue Mar 24, 2015 12:01 am
Location: Wigan

Re: Murphy V849U

Post by nuvistor » Fri Sep 18, 2015 9:37 pm

Surprising what we remember from many years ago, glad the information helped.

Frank

 
Posts: 715
Joined: Tue Jan 27, 2015 2:02 pm
Location: Chatteris, Cambs

Re: Murphy V849U

Post by PYE625 » Fri Sep 25, 2015 9:08 am

Hi, just a little update on the LOPT repair....

It's now been a few weeks and all is well. (Please see my previous post's on the subject).
I think I can safely say that in this case, it was the fabric tape between the high voltage over-wind and the smaller earthy winding on the same former that was the problem. Clearly the insulation decreased dramatically with temperature. Whether this is indeed due to moisture ingress into the fabric tape, or a general breakdown in its insulation properties, I don't know.
But replacing the old fabric tape with polythene tape solved the issue completely.

Whether this would apply to any other Bush LOPT's with similar symptoms is an unknown.
That is for anyone reading this to find out..... :qq1 :qq1
Well worth a try, and it would be very interesting to see :)

 
Posts: 489
Joined: Tue Mar 24, 2015 12:01 am
Location: Wigan

Re: Murphy V849U

Post by nuvistor » Fri Sep 25, 2015 12:53 pm

A stock fault on theses sets is the 10k ( how did I remember that value?)video anode load resistor, Rank fitted a 1watt component but is was not up to the job. I cannot remember whether it went O/C or low in value.
They recommended fitting it "standing off" the valve holder, the original was buried underneath other components.

I don't think the resistor in your set is too far off the correct value but worth checking.

You could well be correct about the reason for some faulty LOPTX's, good find.
Frank

PreviousNext

Return to Black & White Television



Who is online

Users browsing this forum: No registered users and 4 guests