22nd Sep 2018, 9:46 am | #61 |
Nonode
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Re: Help needed restoring a Murphy V310
Please describe what a low emission tube looks like. I assumed they just lost brightness?
I do think l'm making progress. The contrast control is much better now as is the brightness. I have just realised I never disconnected the heater winding for the U26 when I replaced it with the stick rectifier, should I have done? Regards David
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22nd Sep 2018, 1:27 pm | #62 |
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Re: Help needed restoring a Murphy V310
Generally a low emission tube symptoms are a classic and recognized immediately by guys in the trade.
The picture is of correct shape and size but lacks overall brightness and contrast. Any attempt to increase the brightness results in the picture taking on a satin appearance with the whites inverted into a negative. The contrast control has a similar effect but can be confused in the V310 due to lack of AGC. The picture will turn negative due to excessive signal input when the contrast is advanced beyond it's correct setting. To demonstrate this I uncovered my sleeping V320. This is the 21" version of the V310 with only minor differences. The first picture shows a normal test card with good focus and contrast. The second picture shows the result when the contrast is turned well above it's correct setting. This is very similar to what you see with a low emission tube, the difference being that any attempt to obtain a bright picture with the contrast and brightness controls results in this negative display. Turning down the brightness and contrast should result in a watchable picture when viewed in subdued light i.e. a darkened room. Note that the focus remains good. As Stephen as posted, the tube may be 'soft' or in simple terms a microscopic amount of air may be present impairing the vacuum. This is usually released from the gun assembly itself but we are getting into deep water and you only need at this stage to know what is happening. The results with a soft tube are usually poor focus [with the focus control passing through the 'best' position] and flared highlights often with a surrounding halo. I presume the tube has the usual Mazda label with the data on it? Soft tubes are often encountered with second grade rebuilds. I still suspect low EHT is having a lot to do with the 'tired' appearance of the picture. Just a hunch. The CRM172 can be replaced with the Mullard MW43-69 or MW43-64 together with others. You can build a simple gentle bulb re activator with a couple of lamps and a few components. but you need to diagnose that the tube is definitely duff! John. |
22nd Sep 2018, 7:54 pm | #63 |
Nonode
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Re: Help needed restoring a Murphy V310
Thanks for that very comprehensive answer John.
I'm not sure what to do next? Regards David
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22nd Sep 2018, 8:04 pm | #64 |
Octode
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Re: Help needed restoring a Murphy V310
You could post in relevant section to see if anyone near you has crt tester / eht meter or probe, they may test for you.
I'm a bit far away or would certainly help.
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22nd Sep 2018, 8:12 pm | #65 |
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Re: Help needed restoring a Murphy V310
I am watching out for one on ebay, they do seem to turn up regularly.
What would cause low EHT? Regards David
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22nd Sep 2018, 8:35 pm | #66 |
Octode
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Re: Help needed restoring a Murphy V310
The CRT being gassy would cause excessive beam current, which in turn drags the EHT down.
But nothing is conclusive until more testing is done. In an ideal world testing the CRT is quickest and gives a good idea on its life. You could examine the tube neck and check the gettering colour to see if its milky.
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24th Sep 2018, 2:49 pm | #67 |
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Re: Help needed restoring a Murphy V310
When I replaced the U26 valve with a TV20 rectifier stick I just soldered a piece of wire to it and inserted it into the old valve base. At the time I left the heater winding in place. Today I thought I'd see what would happen if I removed the heater winding and I'm pleased to say I think the picture has improved?
The picture was also smaller and width and height had to be adjusted to fill the tube. Hopefully this is a sign that the EHT is higher? (I'm thinking opposite of the ballooning effect?). The focus is better but far from perfect. I've attached photo, sorry it's not very good but my camera phone can't seem to focus on the mirror I'm using Unfortunately another fault has shown up. Every 7 or 8 second the picture shrinks about 1/2" at the corners, the focus twitches and then it opens out again. There's no change to the line whistle. Any thoughts? Regards David
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24th Sep 2018, 5:56 pm | #68 |
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Re: Help needed restoring a Murphy V310
I think at this point it may be helpful to go over the voltages you can measure, the boost voltage, HT, heater voltage at CRT.
The latest fault could be the EHT overwind breaking down on the LOPT but that's not my area, maybe John will advise further. It must be frustrating to get so far and then be thwarted by not having the equipment needed to finally diagnose the fault yourself.
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24th Sep 2018, 8:18 pm | #69 |
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Re: Help needed restoring a Murphy V310
That's more like it! The U26 heater winding usually rots both inside and outside the transformer at the point of exit and where the wire is actually wound around the internal core. There is probably a leak to the core reducing the EHT.
Yes the higher the EHT, the smaller the picture and the reverse applies. The line output transformer may have lost a quantity of it's insulating oil. This may be the cause of your line twitching due to internal leakage with the lack of the oil insulation. The tube may be a bit low but it should still focus reasonably well. Set the dome adjustment mid way then adjust the whole assembly along the neck as listed in the service manual. You may have to tweak the ion trap to finalize. It looks like you have improved the EHT to around 12KV. It may still be too low to give the best from the tube. I have an EHT meter I can loan you. It is in a Perspex case and is quite lightweight. I can post it if you wish. Regards, John. |
24th Sep 2018, 9:26 pm | #70 | ||||
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Re: Help needed restoring a Murphy V310
Quote:
Quote:
Quote:
Quote:
When I next get time I'll recheck the voltages that I can measure Regards David
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25th Sep 2018, 9:53 am | #71 |
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Re: Help needed restoring a Murphy V310
You're welcome David. Confirm the EHT is about right and the tube can then be assessed.
The CRM172 reactivates well if the vacuum is good and it has not been done before. J. |
25th Sep 2018, 1:25 pm | #72 |
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Re: Help needed restoring a Murphy V310
I've checked some voltages with a DMM and they are much the same as they were before:-
H.T. voltage 196V (should be 205V) Boost voltage (take from C109) is a disappointing 415V (should be 480V) C.R.T. heater voltage is also disappointing at 11V (should be 12.5V) The twitchy picture seems to be worse today and I think I can faintly hear some sort of a static/fizzing noise coming from the right hand side of the chassis (looking from the back) unfortunately I can't pinpoint where it's coming from... I fear doom is around the corner . Regards David
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25th Sep 2018, 2:02 pm | #73 |
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Re: Help needed restoring a Murphy V310
You would be best looking for the corona discharge when the room is in darkness, turn the sets brightness right down too as this will increase the EHT and make it easier to find, it could be something as simple as the final anode connector arcing a bit.
All the voltages are a bit on the low side, you could try running the set on the 230 volt tap instead of the 250 volt, or as close to this figures as the sets permits. Check the voltages again one it is running and see if they are nearer to spec.
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25th Sep 2018, 3:25 pm | #74 |
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Re: Help needed restoring a Murphy V310
When you say 'you removed the heater winding' what do you mean? As it's oil-filled I assume you did not remove the turns from the LOPT?
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25th Sep 2018, 3:52 pm | #75 |
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Re: Help needed restoring a Murphy V310
Hi PJL, No I removed the heater winding from circuit (removed it from the U26 valve base). It was still attached to the EHT lead, connected to the crt and the EHT stick rectifier.
Regards David
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25th Sep 2018, 4:01 pm | #76 |
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Re: Help needed restoring a Murphy V310
I would terminate the redundant EHT rectifier heater supply with an equivalent load resistor, 5.6 ohms 2 watt.
Lawrence. |
25th Sep 2018, 6:41 pm | #77 |
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Re: Help needed restoring a Murphy V310
Sorry Lawrence, Why to that? J.
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25th Sep 2018, 6:44 pm | #78 |
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Re: Help needed restoring a Murphy V310
I would have thought that that's what the LOPTY would be expecting.
It's an interesting LOPT stage. Lawrence. |
25th Sep 2018, 7:12 pm | #79 |
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Re: Help needed restoring a Murphy V310
The winding should be disconnected completely with the positive end of the stick rectifier connected to the CRT. The base could be utilized if all other connections are removed.
I can see your logic Lawrence but it will probably damp the struggling transformer. The less load on it the better. The 'ticking' may be from the old U26 base or the plastic CRT cap. The top of the LOPT may have become leaky or the oil contaminated. As Stephen has said, it's a lights out job. J. |
25th Sep 2018, 7:17 pm | #80 |
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Re: Help needed restoring a Murphy V310
What's causing the transformer to struggle in the first place, is it partly duff?
Lawrence. |