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Old 10th Aug 2020, 4:55 pm   #1
Grotsoft
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Default 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.
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Old 10th Aug 2020, 5:23 pm   #2
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Default 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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Old 10th Aug 2020, 5:30 pm   #3
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Default 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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Old 10th Aug 2020, 5:40 pm   #4
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Default 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.
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Old 10th Aug 2020, 6:32 pm   #5
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Default 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.
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Old 10th Aug 2020, 7:23 pm   #6
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Default 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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Old 10th Aug 2020, 7:27 pm   #7
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Default Re: UL41 heater / Murphy U198H Advice Please

Quote:
Originally Posted by smiler411 View Post
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
I should have said 440VAC, which works out to 620V DC?
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Old 10th Aug 2020, 8:11 pm   #8
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Default 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.
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Old 10th Aug 2020, 8:42 pm   #9
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Default Re: UL41 heater / Murphy U198H Advice Please

Quote:
Originally Posted by paulsherwin View Post
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.
That was my first assumption, but a replacement UY41 didn't help.

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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Old 10th Aug 2020, 9:30 pm   #10
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Default 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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Old 10th Aug 2020, 9:44 pm   #11
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Default 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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Old 10th Aug 2020, 9:54 pm   #12
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Default 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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Old 11th Aug 2020, 11:03 am   #13
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Default Re: UL41 heater / Murphy U198H Advice Please

Quote:
Originally Posted by Nuvistor View Post
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.
I liked this idea! Unfortunately there's no leak on the socket or on the valve itself.
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Old 11th Aug 2020, 11:04 am   #14
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Default Re: UL41 heater / Murphy U198H Advice Please

Quote:
Originally Posted by Freya View Post
You could try powering the UY41 with an external power supply if it can achieve 31 volts check it draws the correct 100mA current.
Thanks, I think I'll try this, and I'll also knock up a solid state replacement to see if that makes any difference.
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Old 11th Aug 2020, 11:32 am   #15
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Default Re: UL41 heater / Murphy U198H Advice Please

Don't forget a dummy heater resistor- 330R 5W will do.
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Old 11th Aug 2020, 11:36 am   #16
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Default Re: UL41 heater / Murphy U198H Advice Please

Broken cathode socket in rectifier valve holder?
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Old 11th Aug 2020, 11:40 am   #17
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Default 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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Old 11th Aug 2020, 11:49 am   #18
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Default 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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Old 11th Aug 2020, 7:03 pm   #19
Grotsoft
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Default 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.
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Old 11th Aug 2020, 7:08 pm   #20
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Default Re: UY41 heater / Murphy U198H Advice Please

Quote:
I ran the old UY41's heater externally and it drew 140mA at 31V
What does the "new" one take? 140mA at the running voltage is far, far too high for a 100mA valve (they where specified and made to well within 5%), no wonder it is not working in the series chain. I am definitely going for both to be duff.
 
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