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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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10th Aug 2020, 4:55 pm | #1 |
Triode
Join Date: May 2020
Location: Ayr, South Ayrshire, UK.
Posts: 23
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UY41 heater / Murphy U198H Advice Please
Hi,
I've restored a Murphy U198H valve radio (my first) and all was fine for a while. However on switching on one day there was total silence from the speaker. This set uses a UY41 rectifier and I am now measuring 1/2 V on its cathode rather than the schematic's 193V. I replaced the UY41 with a NOS one, but no change. This radio is transformerless with the heaters in a string. Measuring the heater voltages shows (in chain order): UY41 - 10V rather than 31V UL41 - 56.3V rather than 45V UF41 - 14.7V rather than 13V UCH42 - 17V rather than 14V UBC41 - 17.9V rather than 14V So I'm guessing that the UY41 isn't in emission, which is why there's no HT. I thought about replacing the UY41 with a 1N4007 / 100R anti-surge / 310R heater load combination, but I have a feeling that this would simply cause the over voltage on the other valves to increase. So, my basic question is what's next? I'm leaning towards the UL41 being fault, but at £22 each I'm not keen on simply replacing it without some more informed input! I'd appreciate your thoughts on this. thanks. |
10th Aug 2020, 5:23 pm | #2 |
Dekatron
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,637
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Re: UL41 heater / Murphy U198H Advice Please
You have effectively nothing on the UY cathode. What is on the anode (pin 2)?
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10th Aug 2020, 5:30 pm | #3 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
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Re: UL41 heater / Murphy U198H Advice Please
There may be additional faults yet to be discovered, and so it will make life easier for everyone if we all sing to the same hymn sheet (circuit diagram). My preference is the Trader Sheet #1375 and you can get this plus the manufacturer's service data here https://www.service-data.com/search....&search=Search or click top right of the page and search for Murphy U198H. Cheers, Jerry
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10th Aug 2020, 5:40 pm | #4 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,951
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Re: UL41 heater / Murphy U198H Advice Please
Could the UY41 have cracked a seal between a pin and hence 'gone to air' ??
If so, its cathode/filament won't get hot (because the air now in the envelope convects the heat away) so the filament will remain cool and its resistance will not rise to the design-value at which you would get the expected voltage-drop across its filament. This would also explain the higher-than-expected voltages across the filaments of the other valves. Also, with air in there rather than a vacuum, any electron-emission (and rectifying-action) the not-hot-enough cathode might muster would be seriously compromised. Look at the 'silvering' of the getter - if it shows signs of going a bit milky-white, the valve's dead. Also - if the radio uses the McMurdo tube-sockets where the individual contacts are made from a stamped brass fork, these are renowned for the brass going crystalline and the fork fracturing at the bottom of one of the tines, resulting in zero contact. |
10th Aug 2020, 6:32 pm | #5 |
Triode
Join Date: May 2020
Location: Ayr, South Ayrshire, UK.
Posts: 23
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Re: UL41 heater / Murphy U198H Advice Please
Thanks guys, I've been working from these datasheets, so I'll use them as reference. The test points are on the Murphy service sheet.
The UY41 anode voltage (pin 2) test point 48, is 220V. I've tried a second, new old stock, UY41, with exactly the same outcome, so I don't think it's that valve. I had implemented the change highlighted in the Murphy service guide for the later models where the UY41 anode to ground capacitor had it's value changed from 20nF to 50nF and was moved across the anode and cathode of the UY41. I used a 440V 47nF capacitor for this. All capacitors, except the mica ones, have been replaced and all out of tolerance resistors have been replaced. |
10th Aug 2020, 7:23 pm | #6 |
Tetrode
Join Date: Apr 2008
Location: Rhondda Cynon Taff, Wales, UK.
Posts: 87
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Re: UL41 heater / Murphy U198H Advice Please
I don't think 440V is enough across the rectifier, on negative going mains you will have the peak mains negative voltage plus the ht voltage across this capacitot
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10th Aug 2020, 7:27 pm | #7 |
Triode
Join Date: May 2020
Location: Ayr, South Ayrshire, UK.
Posts: 23
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Re: UL41 heater / Murphy U198H Advice Please
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10th Aug 2020, 8:11 pm | #8 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,783
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Re: UL41 heater / Murphy U198H Advice Please
This is a transformerless AC/DC set which half wave rectifies the mains. 220V HT is what you'd expect after losses.
My guess is that the UY41 heater-cathode insulation has failed. |
10th Aug 2020, 8:42 pm | #9 | |
Triode
Join Date: May 2020
Location: Ayr, South Ayrshire, UK.
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Re: UL41 heater / Murphy U198H Advice Please
Quote:
I also measured the resistance between the heater and anode / cathode and got open circuits. Is that a valid check or is such a short actually a very high resistance? |
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10th Aug 2020, 9:30 pm | #10 |
Moderator
Join Date: Jun 2003
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Re: UL41 heater / Murphy U198H Advice Please
In this case I'd expect this to be at least measurable, so you may well be right that this isn't the explanation.
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10th Aug 2020, 9:44 pm | #11 |
Octode
Join Date: Jan 2009
Location: Falmouth, Cornwall, UK.
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Re: UL41 heater / Murphy U198H Advice Please
You could try powering the UY41 with an external power supply if it can achieve 31 volts check it draws the correct 100mA current.
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10th Aug 2020, 9:54 pm | #12 |
Dekatron
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Location: Wigan, Greater Manchester, UK.
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Re: UL41 heater / Murphy U198H Advice Please
If the replacement UY41 is good then 10vac across the heater instead of 31vac could be a leak in the valve base between heater pins. The approximately extra 20 volts is spread across the other valve heaters.
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Frank |
11th Aug 2020, 11:03 am | #13 |
Triode
Join Date: May 2020
Location: Ayr, South Ayrshire, UK.
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Re: UL41 heater / Murphy U198H Advice Please
I liked this idea! Unfortunately there's no leak on the socket or on the valve itself.
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11th Aug 2020, 11:04 am | #14 |
Triode
Join Date: May 2020
Location: Ayr, South Ayrshire, UK.
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Re: UL41 heater / Murphy U198H Advice Please
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11th Aug 2020, 11:32 am | #15 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
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Re: UL41 heater / Murphy U198H Advice Please
Don't forget a dummy heater resistor- 330R 5W will do.
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11th Aug 2020, 11:36 am | #16 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
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Re: UL41 heater / Murphy U198H Advice Please
Broken cathode socket in rectifier valve holder?
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11th Aug 2020, 11:40 am | #17 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,611
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Re: UL41 heater / Murphy U198H Advice Please
Have you measured the voltage across the pins for both your original valve and the replacement valve? It is possible that the second valve is faulty too, but the odds of the same or similar faults happening in both of them are reasonably low I would think
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11th Aug 2020, 11:49 am | #18 |
Moderator
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Re: UY41 heater / Murphy U198H Advice Please
How about comparing the COLD resistances of the heaters of the two valves.
EDIT. I measured the heater resistance of a known good UY41 at approximately 50R.
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11th Aug 2020, 7:03 pm | #19 |
Triode
Join Date: May 2020
Location: Ayr, South Ayrshire, UK.
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Re: UY41 heater / Murphy U198H Advice Please
Thanks again for the input.
The mystery deepens... - The socket appears to be fine, no resistance at all across any of the pins and I was able to check the continuity from the top of the socket through to relevant test points on the receiver. - I used jumper leads to wire up a solid state replacement (with UY41 removed) and it worked. All heater voltages back to normal and the radio plays. - The cold resistance of the heaters on my UY41s is 50 for the old one and 45 for the new. - I ran the old UY41's heater externally and it drew 140mA at 31V As others have said, it appears unlikely that both UY41s are dead, so I wonder if there really is something up with the socket? I'm seriously tempted to simply go with a capacitive dropper and solid state rectification, but I'd really like to keep things original, and to know what's going on! EDIT: On the off chance that the moved filter cap was causing problems I disconnected it. No change. Last edited by Grotsoft; 11th Aug 2020 at 7:09 pm. |
11th Aug 2020, 7:08 pm | #20 | |
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Re: UY41 heater / Murphy U198H Advice Please
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