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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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9th May 2017, 9:04 pm | #61 |
Dekatron
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Re: MURPHY A272C Restoration.
And HT (approx. 200 volts) on the other side of R5?
Lawrence. |
9th May 2017, 9:07 pm | #62 |
Octode
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Location: Bromyard, Herefordshire, UK.
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Re: MURPHY A272C Restoration.
Yes that's correct.
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9th May 2017, 9:29 pm | #63 |
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Re: MURPHY A272C Restoration.
So to confirm, the anode voltage is ok, the screen grid voltage is 1.5 volts, R5 is ok and C22 is ok, when the valve is unplugged the screen grid voltage rises up to the HT voltage of 200 volts?
Lawrence. |
9th May 2017, 9:31 pm | #64 |
Octode
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Re: MURPHY A272C Restoration.
Yes. That is all correct except c22 is still not connected to ground. Many thanks.
Dave. |
11th May 2017, 6:54 am | #65 |
Octode
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Re: MURPHY A272C Restoration.
I can only think it is the valve? Replace this alongside the resistors that have gone high.
Dave. |
11th May 2017, 9:27 am | #66 |
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Re: MURPHY A272C Restoration.
It's unwise to run without C22 connected as its purpose is to prevent instability.
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Phil Optimist [n]: One who is not in possession of the full facts |
11th May 2017, 1:24 pm | #67 |
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Re: MURPHY A272C Restoration.
G3 to G4 short? How is the cathode volts/current?
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25th Jun 2017, 1:27 pm | #68 |
Octode
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Re: MURPHY A272C Restoration.
Sorry it's been such a long time. I've just managed to sit in front of this set the first time since may. Life took over gor a while. Right, now where did I get to?
I remember thinking the 6c9 valve was causing the voltage issue. I will need to re read the thread i think. |
27th Jun 2017, 1:25 pm | #69 |
Octode
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Re: MURPHY A272C Restoration.
I have replaced the resistors R5 and R29 today. Not that I thought they were causing the screen grid problem on valve 2 but I'd removed them for checking and they had gone high especially R5. I think reading my previous posts and also checking c22 that it looks possible it is in fact 6C9 valve 2. Im out of ideas now. I've ordered a UCH42 new old stock valve which should be with me tomorrow or Thursday. If anyone would like to add anything they think I may have missed, please feel free.
Many thanks. Dave. |
27th Jun 2017, 2:33 pm | #70 |
Octode
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Re: MURPHY A272C Restoration.
Apologies, that should be ECH42 and not UCH42.
Im told this is a near substitute for the 6c9. Dave. |
27th Jun 2017, 3:30 pm | #71 |
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Re: MURPHY A272C Restoration.
Yes, near but not exact, so far as I can make out the 6C9 is a triode heptode and the ECH42 is a triode hexode, It's not unusual to find different screen feed regimes for the two different types, due to the lack of a suppressor grid in the latter the hexode usually requires a more stable screen voltage and is often fed from a potential divider, the heptode usually has a sliding screen voltage due to it being fed from a single series resistance.
The different screen feed regimes is basically down to the effects (or not as the case may be) of secondary emission, which, in effect can damp the IFT's primary thus reducing selectivity, the damping will vary with the AGC control voltage, more AGC voltage = more damping. Whether you will notice any difference is another matter. Lawrence. |
27th Jun 2017, 3:35 pm | #72 |
Octode
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Re: MURPHY A272C Restoration.
Lawrence,
Do you feel my diagnosis is correct? I think I've covered everything. |
27th Jun 2017, 3:47 pm | #73 |
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Re: MURPHY A272C Restoration.
Hard to say, it's been a while, is R5, C22 ok and is C22 connected in circuit.....anyways, you'll soon find out when you fit the new valve.
Lawrence. |
27th Jun 2017, 3:53 pm | #74 |
Octode
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Re: MURPHY A272C Restoration.
Yes it has been a while. Life took over for a month or too with family illness and I have a new job to start on Monday which has involved exams etc.
R5 is replaced. C22 is now in circuit and has also been replaced. |
27th Jun 2017, 4:15 pm | #75 |
Octode
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Re: MURPHY A272C Restoration.
Good news, I can confirm that valve 2, a Mazda 6C9 was indeed faulty. I was buzzing around tidying up my radio room when I thought to myself, Dave you idiot. You have another Murphy set under the bench. Indeed I had an A188 console version under the bench in the to do pile. I whipped the back off it and sure as damn it has a Mazda 6C9 in it. Swopped it into the set I'm working on and all is well. The sensitivity is tons better too. It was struggling to pick up even strong stations before and sounded shrill. It sounds much better and rich in sound now.
Dave. |
27th Jun 2017, 4:17 pm | #76 |
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Re: MURPHY A272C Restoration.
Good result.
Lawrence. |
27th Jun 2017, 4:45 pm | #77 |
Octode
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Re: MURPHY A272C Restoration.
Thanks Lawrence. I must say I was starting to doubt myself. Especially when I heard that this valve is usually very reliable. But I guess on a set of this age and it's faults I've repaired who knows how it failed. Some one could have done what I have and swopped it for a dead valve from another set. We just don't know the sets history so anything could have happened over the decades.
Dave. |
28th Jun 2017, 12:22 pm | #78 |
Octode
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Re: MURPHY A272C Restoration.
For those that are interested and have kindly followed this, even after a long break I have tested the duff 6c9 valve this morning. The valve has a short between the cathode pin 7 and screen grid and grid 4 which is pin 5. I obviously have compared this against the known good one and there is no continuity between these pins on the good one. Jus wanted to understand myself what had occurred and it wasn't educated guess work. So I've once again learned from this great set.
Dave. |
28th Jun 2017, 12:39 pm | #79 |
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Re: MURPHY A272C Restoration.
Well done Dave, and hope the new job goes well
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28th Jun 2017, 1:03 pm | #80 |
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Re: MURPHY A272C Restoration.
A rare fault but not unknown.
Be very pleased with yourself that you found the fault by solid deduction and that you were proven correct, proper pro repairer. Sam. |