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Murphy V849U ( Includes the RBM LOPT Overwind Cure)

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PYE625
(@pye625)
Famed V-Ratter Registered

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

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Topic starter Posted : 18/09/2015 11:00 am
Nuvistor
(@nuvistor)
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If you have a scope look at the supply lines, may see something that is not decoupled correctly.

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Posted : 18/09/2015 1:30 pm
Anonymous
(@anonymous)
Group Deactivated Account

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..

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Posted : 18/09/2015 8:12 pm
PYE625
(@pye625)
Famed V-Ratter Registered

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.

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Topic starter Posted : 18/09/2015 8:32 pm
Nuvistor
(@nuvistor)
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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

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Posted : 18/09/2015 10:54 pm
Nuvistor
(@nuvistor)
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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

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Posted : 18/09/2015 11:00 pm
PYE625
(@pye625)
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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!!

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Topic starter Posted : 18/09/2015 11:34 pm
Nuvistor
(@nuvistor)
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Surprising what we remember from many years ago, glad the information helped.

Frank

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Posted : 18/09/2015 11:37 pm
PYE625
(@pye625)
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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 :)

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Topic starter Posted : 25/09/2015 11:08 am
Nuvistor
(@nuvistor)
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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

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Posted : 25/09/2015 2:53 pm
PYE625
(@pye625)
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Hi Frank,

Thanks once again for the advice....I will check that 10k anode load resistor and replace it.

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Topic starter Posted : 25/09/2015 3:36 pm
Alastair
(@alastair)
Reputable V-Ratter Registered

R233 10K pentode amp anode-load, and R235 15K Triode C.F. cathode resistor both worth replacing--Along with the PCF80 valve. Failing/weak valve gives low contrast, which can imitate a weak CRT in this chassis......

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Posted : 25/09/2015 6:49 pm
PYE625
(@pye625)
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Hi,

With very grateful thanks to Chris aka The Teleman, I now have a LOPT screening cover and dropper resistor!!
:aad
Cover in place, just need to replace the dropper....

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Topic starter Posted : 18/11/2015 10:18 pm
Terry
(@terrykc)
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I don't think this has been mentioned before in this thread but do make sure the thermistor mod has been carried out.

If the CZ19 breaks in half because the mod hasn't been done, I doubt you'll find a replacement anywhere!

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Posted : 19/11/2015 2:29 pm
crustytv
(@crustytv)
Vrat Founder Admin

I don't think this has been mentioned before in this thread but do make sure the thermistor mod has been carried out.

Andrew already addressed this back on the 4th September viewtopic.php?f=5&t=11844#p123162

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Posted : 19/11/2015 2:34 pm
PYE625
(@pye625)
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It's a very worthwhile modification....stops the thermistor running like an electric fire !!

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Topic starter Posted : 19/11/2015 5:24 pm
Terry
(@terrykc)
Famed V-Ratter Rest in Peace

Andrew already addressed this back on the 4th September http://forum.radios-tv.co.uk/viewtopic. ... 44#p123162

Ah! I did scan the thread - twice, in fact - but missed the vital line!

It's a very worthwhile modification....stops the thermistor running like an electric fire !!

I don't know anything about the thermistor running like an electric fire - the reason for the mod is that, for some unknown reason (to me, at least) the DC component flowing through it causes it to crack neatly in the middle.

When the DC component is removed, there are no further problems.

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Posted : 19/11/2015 6:28 pm
colourstar
(@colourstar)
Honorable V-Ratter Registered

Andrew's excellent results in solving his LOPT problem have led me to taking another look at a later Rank set I have with a similar issue; i.e. width closing in and the picture gradually ballooning after only a few minutes operation. It was stored in a garage for some years before it came to me. The set is a Murphy V2015D (A640 chassis) in a very scruffy cabinet but with a really strong crt. Nothing to lose, but everything to gain in other words.

The LOPT construction looks broadly similar to Andrew's set with the 'inner core' clearly evident. Unfortunately the only psu I have to hand is a redundant one for a laptop, which is rated at 19v 6.3a. I'm not sure that would be man enough for the job of softening the overwind to enable withdrawal.

If I can successfully get the core out, I wondered about using some plumber's rubber repair tape I have in the toolbox. It's the width of 'standard' electrical masking tape, but designed for repairs to split pipes. It's non sticky, but very stretchy and, as such, self-binding, gripping itself tightly to form a seal as you wind it around.

Steve

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Posted : 30/11/2015 11:05 am
crustytv
(@crustytv)
Vrat Founder Admin

I wondered about using some plumber's rubber repair tape I have in the toolbox. It's the width of 'standard' electrical masking tape, but designed for repairs to split pipes. It's non sticky, but very stretchy and, as such, self-binding, gripping itself tightly to form a seal as you wind it around.
Steve

Steve why "spoil the ship for a hapeth of tar".

Plumbers tape is not designed for high voltage/high temperature electrical insulation application. Kapton transformer insulation tape is hardly expensive either. After Andrews success I bought myself a reel just in case one day I have to do likewise on my TV125CU

Kapton Tape
http://www.ebay.co.uk/itm/33MX-5-10-20m ... bs2W6YfGvg

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Posted : 30/11/2015 11:59 am
colourstar
(@colourstar)
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Good point Chris. I'd never heard of Kapton tape before. I'll follow the link and invest in some.

Many thanks!
Steve

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Posted : 30/11/2015 12:06 pm
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