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Old 7th Oct 2018, 4:56 pm   #21
FERNSEH
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Default Re: Murphy V410 TV

Fernseh wrote: "and also a curious disturbance on the picture which takes the form of random horizontal lines. The latter is the result of line timebase pulses entering the IF amplifier and causing bursts of oscillation. The effect is most apparent when the contrast control is turned down to minimum."
Further investigation of the interference on the picture proves that fault is not caused by line pulses entering the vision IF amplifier but is of a random nature.
The decoupling capacitor C17 is known to be a troublesome component and in fact today it failed causing the IF amplifier to oscillate. C17 is a critical component and is not the usual low inductance ceramic type and is one of those capacitors that resembles a resistor. Replacing C17 with a 1000pF polyester capacitor solved the oscillation fault but the disturbance is still apparent on the picture but does not appear on the video waveform displayed on the oscilloscope. C19 is a ceramic capacitor.

DFWB.
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Old 11th Oct 2018, 9:58 am   #22
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Default Re: Murphy V410 TV

Results from yesterdays work on the V410. The frame linearity fault turned out be a leaky capacitor C92. This capacitor is part of the frame linearity circuit and is hidden behind the centre of chassis preset adjustments panel.

So that's the frame timebase sorted out but there is still that vision fault to investigate.

DFWB.
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Old 11th Oct 2018, 5:38 pm   #23
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Default Re: Murphy V410 TV

Never seen that one David. I wonder where it is being introduced? J.
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Old 11th Oct 2018, 8:38 pm   #24
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Default Re: Murphy V410 TV

Hi David,
Yes, that's much better! Just the picture disturbance to sort now.

Presumably, if you fed in a signal with the video content set to black level, any disturbance would be easier to see on the 'scope? TC'C' looks a mess on a 'scope so I usually use a greyscale pattern for fault finding.

I guess you could also disconnect the detector. At least that would confirm whether the disturbance is getting in via the video amp or is coming from the IF stages.

I'm sure you don't need me to suggest fault finding techniques for you!

I know I said I wouldn't, but I might just have to get mine on the bench later and have a play with it:- The radio will just have to wait (again)!

Do you know any history about your set? Why does it look like it's never been used? It should be covered in nicotine, spiders and filth- very strange!

Cheers
Nick

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Old 11th Oct 2018, 9:19 pm   #25
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Default Re: Murphy V410 TV

1100 Man wrote: "Presumably, if you fed in a signal with the video content set to black level, any disturbance would be easier to see on the 'scope? TC'C' looks a mess on a 'scope so I usually use a greyscale pattern for fault finding."
Hi Nick, The interference is apparent when the aerial is disconnected. The 'scope was connected across the video demodulator load resistor and disturbance appears as spikes with an amplitude of about 3 volts.
The vision interference fault has been solved. The cause of the problem originated in the sound IF amplifier. A decoupling capacitor C62 was found to be open circuit. It was the same type that was well known for causing problems in the vision IF amplifier. The offending capacitors were made by Erie and have the appearance of a resistor.

The picture is now excellent.

DFWB.
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Old 11th Oct 2018, 9:54 pm   #26
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Default Re: Murphy V410 TV

That's more like it!

DFWB.
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Old 11th Oct 2018, 11:34 pm   #27
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Default Re: Murphy V410 TV

looking good, bright enough as a daily set?
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Old 12th Oct 2018, 10:06 am   #28
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Hi Kevin,
It certainly is. The V410 is the best model in the Murphy "sound mirror" series.
However, the set which is the subject of this topic does still have one problem, after two hours the picture starts to balloon when the brightness control is turned up or on bright picture scenes.
The line output transformer can becomes too hot to touch. Needs an oil change?

DFWB.
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Old 12th Oct 2018, 10:17 am   #29
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Default Re: Murphy V410 TV

Quote:
Originally Posted by FERNSEH View Post
The vision interference fault has been solved. The cause of the problem originated in the sound IF amplifier. A decoupling capacitor C62 was found to be open circuit.
DFWB.
Very impressive David! These are extremely difficult faults to locate.

Jac
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Old 12th Oct 2018, 11:15 am   #30
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Default Re: Murphy V410 TV

Good morning Jac,
It certainly was a difficult fault condition to solve.
As a result of that decoupling capacitor C62 having gone OC the sound IF amplifier had become a squegging oscillator and the burst of oscillations were fed back to the vision IF amplifier.

It's possible that this was a well known fault in Murphy TV receivers and might even been considered fifty years ago common a fault, an easy fix.

DFWB.
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Old 12th Oct 2018, 11:17 am   #31
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Default Re: Murphy V410 TV

Looking excellent as these sets can perform.
Now what highly technical way did you diagnose the fault or did you use the old trick of a small screwdriver and a finger on each IF valve control grid?
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Old 12th Oct 2018, 11:22 am   #32
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Default Re: Murphy V410 TV

Quote:
Originally Posted by FERNSEH View Post

It's possible that this was a well known fault in Murphy TV receivers and might even been considered fifty years ago common a fault, an easy fix.

DFWB.
Those capacitors were a common fault back in the day, an instability probe was the weapon of choice where I worked back then.

Lawrence.
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Old 12th Oct 2018, 11:27 am   #33
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Hi Frank,
Nothing very technical, although the oscilloscope was used to monitor interference spikes. The only effective method to trace these faults is of course the screwdriver applied across transformer windings as well as a capacitor connected between the IF valve grids and ground.

DFWB.
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Old 13th Oct 2018, 11:15 am   #34
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Default Re: Murphy V410 TV

Looking back through your 410 postings David I should have made it clear in post 10 that the sound decoupler when O/C caused all manner of odd picture faults.

It was well known at the time but that was over 50 odd years ago!

Oh, in your post 4, I have just noticed some sort of matting on the floor of your workshop.
Can you tell me the colour code please as it is so badly worn that I cannot read it.

It appears to have been woven with shoddy or recycled felt hats of various colours and ages. Is it not a health hazard?

Just curious, John.
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Old 13th Oct 2018, 12:01 pm   #35
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Default Re: Murphy V410 TV

Does the rogue capacitor look like these?
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Old 13th Oct 2018, 1:35 pm   #36
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Default Re: Murphy V410 TV

Hi Dazzelevision,
that's exactly the type of capacitor that failed in the Murphy V410.

All the other decoupling capacitors are low inductance ceramic types, so I'd guess the Murphy designers chose the Erie capacitors because these did have a certain amount of inductance in in order to increase the gain of the vision and sound IF amplifiers. We really need sight of the Murphy service manual to read the circuit description and why such specific components were used.

The performance of the V410 series is impressive, all helped by the special attention paid to the design of the IF amplifiers. Also further helped by the first use in a TV set the then new in 1958 Mazda 30L15 cascode RF amplifier valve.

DFWB.
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Old 13th Oct 2018, 4:52 pm   #37
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Quote:
Originally Posted by FERNSEH View Post
Hi Dazzelevision,
that's exactly the type of capacitor that failed in the Murphy V410.

All the other decoupling capacitors are low inductance ceramic types, so I'd guess the Murphy designers chose the Erie capacitors.........

DFWB.
I’m pretty sure these were made by Dubilier and not Erie, as some have “DCC” marked on them, which is short for Dubilier Condenser Company.
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Old 16th Oct 2018, 11:04 pm   #38
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Default Re: Murphy V410 TV

Meanwhile, my own V410 has seen some action! Thanks to David for keeping the thread warm and inspiring me to do some work on my one!

Rather than embark on a full restoration, I decided to do a minimum amount of work just to assess whether the LOPT was in good shape or not.
Firstly, I removed the complete chassis from the CRT to make access to components easier. This is very easy- just undo 4 bolts, remove the tube base and what has to be the biggest EHT connector ever, and lift the entire chassis complete with scan coils off the back of the tube. Thanks Fernseh for the tip

The inside face of the chassis was covered in a coating of what looked like used diesel engine oil, it would not have looked out of place in my Montego! Very black. Very dirty. Obviously oil from the LOPT, the top of which was also very oily.
I didn't need the crt at this stage, so wired a valve with 300ma heater across it's pins. That way I could operate the chassis with no tube and be able to get to both sides of it with ease.

First port of call was the mains filter cap. There was no need to remove this as it had been mis- wired from new! Both sides of it went to chassis as it had been connected to the wrong terminal on the mains switch! Probably made it more reliable that way!
Removing a valve so that nothing operated, I gradually increased the mains voltage while monitoring the HT current. Over an hour or so, I increased it to the point of getting an HT of 280V (with no load don't forget). Nothing got hot, and the voltage at all the decoupling electrolytics measured the same as the HT, so at least they were not too leaky.
So far so good and time to complete the heater circuit and see what would happen.

After a short time the gentle purr of the frame output transformer could be heard and then the familiar line whistle which gradually increased in intensity. At least I can still hear 405 line whistle,: sadly 625 is a distant memory!

A very healthy 1/4" long arc could be drawn from the top of the EHT rectifier, but the heater was not alight. The boost volts were 380V so not too bad but not high enough. The HT was also a bit low at 170V.
The screen volts were very low on the 30P4 due to the resistors doubling in value and the grid was more positive than it should be. Replacing those and the coupling cap improved matters but not much. I was now happy after measuring the line oscillator voltages, that the LOPT was being driven as it should be.

I tried another U26 EHT rectifier and this time there was a glow from the heater. Then the whole valve started to glow bright blue and the heater went out. Hmm! Very pretty but probably not a good sign!
I tried another one and this time there was a firework display within the valve and it also lit up bright blue. Again very pretty but definitely not right!
Time to do a bit of reading on the forum about Murphy LOPT's.

It would seem this is a common problem- the U26 heater winding leaks to the core of the LOPT within the transformer can.
The obvious way to confirm this seemed to be to disconnect the top cap of the U26, that way there would be no high voltage on the heater winding. Sure enough, the heater lit up and the HT and boost also increased. So I think that confirms the diagnosis.

Whilst it would seem that this can be overcome with a TV18/20 stick rectifier, where's the fun in that?! Opening up the LOPT seems much more fun as there is endless scope for getting absolutely filthy!! I'm also intrigued as to what's in the can! Plus I do like a bit of LOPT surgery and I don't doubt it could do with an oil and filter change!
I believe I read that John (HCS) cuts the can about an inch down from the top, but any advice would be welcome as I don't want to cut into the windings!

So I think open heart surgery on the LOPT will be the first thing before I embark on sorting the rest of the chassis.
All the best
Nick
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Old 16th Oct 2018, 11:43 pm   #39
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Default Re: Murphy V410 TV

The first picture shows the incorrectly fitted mains filter cap. The right hand side goes to chassis and the left hand side goes to, well, chassis via the black wire!

2nd & 3rd picture shows the distorted mask. This has the consistency of firm putty. I have seen references to masks being destroyed by oil leaking from the transformer and there certainly has been leakage from this one!
4th picture is of a very black & oily LOPT can.

5th picture shows the frame stage & output transformer bristling with capacitors. The output transformer is festooned with components and connection tags some of which have nothing to do with the frame stage! There are 4 wax caps and 4 resistors stuck to it!
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Old 23rd Oct 2018, 11:57 pm   #40
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Default Re: Murphy V410 TV

Having, hopefully, repaired the damaged heater winding within the LOPT can, I wanted to test it before putting it all back together and filling it with oil.
I therefore connected it with some longer leads and supported it in a manner that would enable the set to be powered up.

It all seems to work fine but the problem I've got is a bright uncontrollable raster. Despite extensive testing & experimenting, whatever I do, the voltage on the cathode is exactly the same as the voltage on the grid. The brightness control (connected to the grid) causes both the grid and the cathode voltage to vary and they are always the same!

The other strange thing is that reducing the brightness towards minimum does not change the screen brightness one bit but it does increase the EHT to a point where the uninsulated connection of the U26 heater winding flashes over to the core of the LOPT.

So my conclusion is that there is a grid- cathode short. I assume this is possible although I guess heater - cathode shorts are more common?

Am I correct in assuming a H-C short would reduce the cathode voltage to zero as the heater is grounded?

I don't have my tube tester here which would test for inter- electrode shorts. Is there any other way I can test for a grid- cathode leak?

Any help appreciated on this one as it's not something I've experienced before!

Many thanks
Nick
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