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Old 16th Oct 2018, 12:10 pm   #121
Radio_Dave
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Default Re: Help needed restoring a Murphy V310

Sorry John, The 30P4 was NOS I didn't expect it to fail so quickly.

Anyhow, I've finished checking the circuit. Please refer the circuit attached, it's not the same as the Murphy Service Manual or Trader sheet.

I had mistakenly fitted C312 to the wiper of the line hold pot and not the screen grid of the 30P4.

R335 was being pushed by the Line Linearity slider against a chassis screw and it had completely worn through the resistor's body effectively making it a 500R resistor instead of 1K. I was also in the area where I had previously heard arching.

I've also borrowed a valve tester and the 30P12 and both 6/30L2's were very low emission.

Do any of the the above faults explain my collapsing picture and/or wrecked 30P4?


Regards
David
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Old 16th Oct 2018, 5:55 pm   #122
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Default Re: Help needed restoring a Murphy V310

If the 30P4 is not driven at it's control grid or oscillation stops it will overheat and the anode will glow red hot. If left switched on for an extended period the valve will self destroy but it takes a while and the 30P4 is a tough valve. It may of course just been a duff valve.

Man made faults can cause incredible confusion. This is the result of replacing too many components in one go. It is a lesson well learned for your next project. Do you still need a 30P4/MR ?
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Old 16th Oct 2018, 6:26 pm   #123
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Default Re: Help needed restoring a Murphy V310

The wiring error would have caused major problems. Have you tried it now you have corrected the faults? The 30P4 might still be usable.
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Old 16th Oct 2018, 8:04 pm   #124
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Default Re: Help needed restoring a Murphy V310

Thanks for the replies.

As soon as two new 6/30L2's arrive I'll power it up again with a new 30P4MR, 30P12 and U191 fitted as well. This time I'll keep a close eye on the 30P4.

Regards
David
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Old 16th Oct 2018, 8:13 pm   #125
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Default Re: Help needed restoring a Murphy V310

Change C306 grid coupling.
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Old 25th Oct 2018, 1:24 pm   #126
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Default Re: Help needed restoring a Murphy V310

The two valves I've been waiting for finally arrived.

Good news is that EHT is now up to 14kV, the ion trap magnet has been moved quite a bit forward onto the neck of the tube to get maximum brightness and the frame collapse seems to have stopped.

Bad news is the picture looks worse! Increasing sensitivity or contrast starts the picture twitching, especially noticeable at the top, and a sparking noise which I cannot locate. I've also gained some white horizontal lines when the brightness is turned up high (please see photo). What's causing that?

I am running out of enthusiasm now, I seem to be fighting a loosing battle but before I give up I will open up the LOPTx and see if anything is going on in there.

Regards
David
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Old 25th Oct 2018, 2:38 pm   #127
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Default Re: Help needed restoring a Murphy V310

Hi Dave,
That looks to me like an extremely low emission tube! Low contrast, very poor focus and probably the brightness control has to be at max to see anything at all?
You asked in a much earlier post what a low tube looked like, well the answer is- that!

It's not the end of the world though. Almost every set I've got came with a tube that didn't have any useful emission- no reading at all on my tester. Sometimes they recover with use but I've usually found rejuvination necessary. In almost every case this has worked amazingly well. It depends how many times this has been done in the set's life as to how well it will work.

It looks like you have a reasonable signal getting as far as the tube although without scoping it it's difficult to be sure.
The white lines are flyback lines- don't worry about those at this stage. The poor tube will tend to make these more obvious.
Check the earth connection to the black 'dag' coating on the tube. In my V410, this was sparking.

Don't give up on it yet!

All the best
Nick
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Old 29th Oct 2018, 10:54 am   #128
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Default Re: Help needed restoring a Murphy V310

I've just started removing the LOPT so I can open it up to see whats going on inside but when I removed the TV20 EHT stick I noticed it's got a rattle inside, are they supposed to rattle? Could this be the source or the mystery sparking sound and picture twitch when I advance the contrast or sensitivity controls too far?

I'll hold back from opening up the LOPT for now.

Regards
David
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Old 29th Oct 2018, 2:19 pm   #129
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Default Re: Help needed restoring a Murphy V310

EHT discharge is much more likely to happen at low brightness settings. The EHT falls as brightness increases and the tube draws more current from the LOPT.
I doubt the twitching is caused by the LOPT and I don't think your tube has enough emission to disturb the EHT as the brightness is increased!

Dismantling the LOPT would allow you to sort the U26 heater winding and then it can run with the correct valve. I don't think you will see any picture benefits though, and it's a very messy business!
I think tube testing/ rejuvinating would be my next port of call.

Cheers
Nick
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Old 29th Oct 2018, 3:45 pm   #130
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Default Re: Help needed restoring a Murphy V310

Hi Nick, The awful picture I posted earlier (post #126) was to show off those horizontal lines, which you kindly told me were flyback lines. Please see the picture below as this is what the set can produce at the moment. I didn't think it looked too bad, but I will bow to the opinions of experts .

I had set aside today to work on the LOPT, so although I'm still uncertain about the rattling EHT stick, I've opened it up! The oil seems very gritty with large build ups around the connections to the rectifier heater. there's also something else in there, is it a resistor?, that's covered in crusty coating. Although I can't see any carbon I guess this stuff still might be conducting something?

I was advised to remove the heater winding but it's right next to the overwind and I'm worried about damaging that. Not sure what to do now?


Regards
David
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Old 29th Oct 2018, 5:18 pm   #131
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Default Re: Help needed restoring a Murphy V310

Hi David,
Interesting pictures! The overwind & U26 winding are subtly different to my V410 one. You have two turns to run the heater (lucky devil!) whereas I only get one! Certainly is crusty but, like you, I can't see any black areas which are obviously burnt.

I believe the lump in series with the winding is a choke. The two turn winding only needs to generate 1.5V (or whatever the U26 heater is), but the problem comes when the U26 is connected. The heater and those two turns then assumes full EHT potential which will do it's best to arc to the transformer core.

The problem on mine was the connection rivets. I think that's where the leakage started and then that charred the paxolin.

The line stage is obviously working more than well enough to be able to light the U26 heater, so if it isn't then it must be due to leakage to the core.

My suggestion would be to carefully clean of all the crusty bits and then check very carefully for any blackening. Ensure you get continuity through the choke and winding.

I would also suggest wearing disposable gloves to minimise skin contact with the oil- we still don't know that it's not hazardous

Cheers
Nick
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Old 30th Oct 2018, 11:48 am   #132
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Default Re: Help needed restoring a Murphy V310

All the crusty oil seems to originate from that choke and it's fixings. I can't see what's wrong with it or the heater winding, all resistances check out fine, but doesn't work and drags down EHT. Because of this I've carefully removed it all and I'll keep using an TV20 stick.

I want to pass a current of 20mA through the overwind, as suggested many times on this forum. The overwind measures 136ohm so I calculate it to need 2.7vdc. Is this right, it doesn't seem like a lot compared to what I've read others have used

Regards
David
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Old 30th Oct 2018, 10:59 pm   #133
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Default Re: Help needed restoring a Murphy V310

Quote:
I want to pass a current of 20mA through the overwind, as suggested many times on this forum. The overwind measures 136ohm so I calculate it to need 2.7vdc. Is this right, it doesn't seem like a lot compared to what I've read others have used
Hi Dave,
Your calculation is correct but, alas, I don't think 20ma will do you much good!
The object of the exercise is to generate some heat in the winding to drive out the moisture. Therefore, it's power that's important, not current.

In the various LOPT's that I've done this to, I have found that about 2 Watts produces a comfortable heat in the winding.
All of my transformers, including the V410 one, have much lower overwind resistances than yours, so require quite large currents to get 2W of heat.

I did my V410 one recently and that had a resistance of 42R when cold. I passed a current of 220ma through it which caused it to get to probably 40-50 deg C. That works out at 2W.

So to get 2W dissipation in your winding of 136R, you need 121ma to pass through it. That equates to a voltage of 16.45V

You could start a bit lower than that and see what temperature it attains after half an hour. Hand warm is what you are aiming for.

Have you decided whether to order proper transformer oil ( as used by Mikey 405) or veg oil? As a short term measure, I'll probably use veg oil in mine for testing purposes.

All the best
Nick.
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Old 31st Oct 2018, 12:00 am   #134
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Default Re: Help needed restoring a Murphy V310

Thanks Nick .


Yes, I intend to use sunflower oil. I've read many old threads about these Murphy LOPT's and sunflower oil seems to be the one that's always used.

Regards
David
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Old 31st Oct 2018, 9:51 am   #135
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Default Re: Help needed restoring a Murphy V310

Hi Dave,
Looking at the pictures, your overwind is very small and must use incredibly fine wire to achieve a resistance of 136 Ohms.

It might be best to start at a much lower current, 30ma or so and see what the temperature is like after half hour. It will take much less power to get your winding warm than my much larger one.

Just a thought!

Cheers
Nick
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Old 1st Nov 2018, 1:19 pm   #136
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Default Re: Help needed restoring a Murphy V310

A new TV20 EHT stick arrived today and it definitely does not rattle. I wonder if I damaged the old one with too much heat while soldering a thick piece of wire to it. Anyhow, I'm going to make a clip for the new one so I don't have to solder on it at all.

Meanwhile the overwind is gently cooking at 30C at 85mA, which should be just under 1W. Hopefully this will be enough.

Over the weekend I'll re-pot the LOPT but before I do I wonder if should do anything to insulate that soldered joint at the business end of the overwind? When assembled it will be around a 1/4" from the inside of the metal can? Obviously it's been like that from new, unless it was painted with something that's subsequently dissolved? Any advice greatly appreciated.

Regards
David
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Old 2nd Nov 2018, 9:21 am   #137
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Default Re: Help needed restoring a Murphy V310

Hi Dave,
As I discovered, the oil acts as a very good insulator! That joint at EHT potential would certainly arc to the case in free air but is obviously designed to be fine in the oil. Considering on my transformer, the input to the overwind is only 2mm from the core, that must usually be OK when insulated by the oil.

The plan for mine, when I've managed to repair the overwind and put the transformer back together, is to suspend the whole thing in a clear container filled with cooking oil. I can then hook it back up to the chassis with temporary wires and observe any discharges within the pot. I've got a beer glass that looks to be the right diameter for the job!

I wonder why the overwinds are so different between the two transformers? My one is the same size as the whole primary winding and still only measures 42 Ohms, yet yours is tiddly and manages 136Ohm! They must both generate the same voltage.

All the best
Nick
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Old 2nd Nov 2018, 9:41 am   #138
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Default Re: Help needed restoring a Murphy V310

Given all the effort you have gone through, why use vegetable oil that will be guaranteed to break down after a maximum of two years.
Dielectric High Voltage oil that costs about eight pounds for half a litre ?
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Old 2nd Nov 2018, 10:04 am   #139
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Default Re: Help needed restoring a Murphy V310

Yes, I was concerned about the long term stability of veg oil. After all, I wouldn't use 3 year old oil to fry my bacon!
Shell 'Dialla B' transformer oil is available in small quantities from Ebay. It's what Mikey 405 used in his.

For my own project, there will now be a break before I get a chance to do any more with my transformer. I think I will order some proper oil for it in the meantime.

Cheers
Nick
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Old 2nd Nov 2018, 1:46 pm   #140
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Default Re: Help needed restoring a Murphy V310

What a dilemma! I'm up against time because I can't leave it being heated for much longer... it smells!

From what I've read in older threads sunflower oil is suggested because it mixes without reacting with the old oil. I have emptied and cleaned the can but there's still plenty of the old oil left on the transformer and I've no idea how I'd clean that away.


Regards
David
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