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Old 6th Jun 2021, 2:46 pm   #1
ChristianFletcher
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Default Murphy Type V350

I’m just starting my second television restoration on a Murphy Type 350. I haven’t changed out Any components as yet. The set had the selenium rectifier replaced with a silicon diode by the previous own and I have reformed the electrolytic and brought the set up on a variac. The set had slightly high HT due probably due to the use of the silicon diode. I have just over 10kv to the picture tube anode.

The picture is static and does have very solid horizontal and vertical hold. Has very poor volume when fed from the standard converter signal source.

So with a test card input I see only the top half the screen with the bottom lines finally compressed into a white line. When starting to search for information on this type of thing it’s probably important to get the terminology and description of the problem accurately described. So my first question is this what is called a collapsed frame.

So I’m thinking that this problem is caused by lack of drive to the frame scan coils so where should I start to diagnose the frame collapse from. I am thinking I should probably check I have the 500v boost voltage. Is it safe to measure the boost voltage without cremating the volt meter. What is the effect of having a low boost voltage.

My next checks would probably be to check the voltage around frame amplifier and frame output valve. I am wondering why the frame is collapsed in a none linearly fashion in my mind it should be equally collapsed about a central horizontal line ?

Note on the picture they are upside down the lower part of the screen is black

Thanks for any advice regards Chris
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Last edited by ChristianFletcher; 6th Jun 2021 at 3:04 pm.
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Old 6th Jun 2021, 4:00 pm   #2
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Default Re: Murphy Type V350

Looking at the circuit diagram more closely I think my theory of operation regards the boost is wrong in my previous post, With the boosted voltage being recovered from the scan coil rather than driving them. We live and learn hopefully

So does flyback from both the line and frame coils contribute to the boost voltage ?
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Last edited by ChristianFletcher; 6th Jun 2021 at 4:14 pm.
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Old 6th Jun 2021, 5:22 pm   #3
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Default Re: Murphy Type V350

These perform very well. It is stuffed with wax and other types of capacitors that leak especially after 63 years! Replacing these will sort most of the problems. Start with the line output stage, follow on to the frame timebase to get the raster correct, checking as you go then you can move on to video, I.F. and sound.

The Mazda CME141 tube is very reliable and the same goes for the LOPT which in the V350 is an open type, no nasty oil to deal with.
Regards, John.
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Old 6th Jun 2021, 5:38 pm   #4
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Default Re: Murphy Type V350

That is a very typical frame fault.. a 'partial' frame collapse. As HKS says, it will undoubtedly be caused by leaky wax capacitors in the frame output and oscillator stage. These will all need changing but while you are learning, change one at a time so you can see the effect.

No doubt the poor volume will be caused by the same problem. All the leaky waxies will eventually need to be changed. Don't leave the set running for too long since other components (like output transformers and frame output transformers) can fail while all the leaky waxies are still in place and these items are far more difficult to source for the sake of a few cheap capacitors.

It's quite safe to measure the boost HT with a standard meter. Use the 750 or 1000V range and make sure you are using good, well-insulated test prods.
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Old 6th Jun 2021, 5:42 pm   #5
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Default Re: Murphy Type V350

Hello John

I was hoping to try and diagnose the problem with the frame collapse rather than just bulk changing the capacitors. I agree the waxy capacitors will be bad and need changing but the line output looks ok I have pretty much full 10Kv and the width is correct so I want to diagnose what causes of the frame collapse. I could just start by change out the capacitors in the frame stage. Or will the line stage effect the frame height.
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Old 6th Jun 2021, 5:43 pm   #6
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Default Re: Murphy Type V350

Thanks Sideband that does sound like a better plan more in line with what I am thinking. But what is actually missing to make the frame collapse in this way and why would the oscillator effect the vertical height.
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Old 6th Jun 2021, 6:31 pm   #7
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Default Re: Murphy Type V350

The field bottom cramp may be caused by a number of possibilities.
Low emission output valve, incorrect voltages, incorrect biassing, leaky capacitors in no particular order.. But change all the relevant wax caps in the field stage, one at a time as suggested as a first port of call.
The oscillator may cause the symptoms if the waveform it is generating is distorted due to faulty operating conditions.
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Old 7th Jun 2021, 12:04 pm   #8
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Default Re: Murphy Type V350

Quote:
Originally Posted by ChristianFletcher View Post
But what is actually missing to make the frame collapse in this way
Half the drive waveform! If you could examine the frame output waveform on a scope, you would probably only see half of the sawtooth with a very squished bottom (or top) half.


Quote:
Originally Posted by ChristianFletcher View Post
and why would the oscillator effect the vertical height.
Lack of drive/poor waveform....There are a number of shaping capacitors and critical time-constants going on in a frame stage and very often more than one thing can cause similar problems. There is also the cathode bypass capacitor which can cause part of the problem.
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Old 7th Jun 2021, 5:15 pm   #9
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Default Re: Murphy Type V350

Great comments and information thanks everyone

Regards Chris
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Old 7th Jun 2021, 8:36 pm   #10
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Default Re: Murphy Type V350

There's a Murphy V350 upstairs above the shop. I might have introduced it as topic over two years ago.
Simply took a chance and plugged it into the mains, no bangs or flashes.
Just gave time to allow it warm up. The set works surprisingly well considering no components have been replaced. CRT is good.
Not so good is the condition of the cabinet, it has completely disintegrated, no chance of repairing it. A new cabinet will have to be made, a winter project perhaps.

DFWB.
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Old 23rd Jun 2021, 5:35 pm   #11
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Default Re: Murphy Type V350

I have been doing work on the Murphy and as can be seen in the picture the frame is severely collapsed. I have changed out a few of the very sad looking capacitor and the electrolytic in the linearity control. My question is could the frame collapse be a mechanical issue or perhaps a faulty frame coil.

Looking at the scope waveform these are all slightly low but I wouldn’t have thought that would produce such a distorted frame. Adjusting the frame height and linearity do increase the frame height vertical but the bottom line where the frame is collapsed into doesn’t move at all it’s stuck halfway up the screen and doesn’t respond to adjustment.

What’s the most logical way to diagnose the frame collapse. I am think I should really be looking at the final drive to the frame coils to see if the magnitude is correct. Some of the scope pictures look very linear and not like capacitor charging curves

Thanks Chris

Picture 3 Anode V7
Picture 4 Grid V10
Picture 5 Anode V10
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Old 23rd Jun 2021, 6:29 pm   #12
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Default Re: Murphy Type V350

I think I have partly answered my own question regarding mechanical tube setup fault or frame winding. Changing out C91 in the frame linearity circuit has made a big improvement. Although the picture is still horribly distorted. Makes me think this is going to be a long slog of changing out capacitors.
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Old 23rd Jun 2021, 7:26 pm   #13
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Default Re: Murphy Type V350

Hey Chris.

Some good progress. You’ll need to change almost all the caps in the frame stage in order to obtain a good linear scan. Experience has shown that even a very small amount of leakage will upset the correct operation here. Change one at a time and see how it effects the scan.

Your results so far are very promising.


Cheers.


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Old 23rd Jun 2021, 9:30 pm   #14
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Default Re: Murphy Type V350

That is coming along nicely, the frame looks fairly linear now. Your distortion looks like ‘Cogging’, caused by a leaking capacitor feeding the grid of the sync separator.

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Old 23rd Jun 2021, 10:10 pm   #15
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Default Re: Murphy Type V350

Thanks

Loyd can you explain the effect that you describe as Cogging as never heard this term before

Thanks regards Chris
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Old 24th Jun 2021, 9:17 am   #16
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Default Re: Murphy Type V350

It’s the horizontal displacement you can see on the picture, it follows light/dark areas. From what I’ve read about it it’s caused by picture information entering the sync separator, causing it to trigger too early or too late. I’m sure others could explain it better than me!

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