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Old 7th May 2024, 7:32 pm   #1
Andy G4NPA
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Default Murphy A70 mains hum

I've managed to get my Murphy A70 working generally very well. I've recapped it realigned as per the service manual and it seems pretty sensitive on all wavebands. Voltages also check out ok.

I still have a low level mains hum on the audio though which I can't track down. Maybe I'm expecting too much from a set of this vintage but I find it a bit annoying, even with the volume control at zero.
I've tried a few things as listed.
Swapping out rectifier and audio tube
Dropping in extra smoothing caps
Disconnecting C43 - so no drive from the previous stage.
Using an alternative speaker (from an A72)
Replaced all out of spec resistors.
Nothing seems to improve matters.
Interestingly the hum on SW is marginally worse as per the service manual.

Any advice on what else would be worth trying would be very welcome
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Old 7th May 2024, 8:13 pm   #2
G6Tanuki
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Default Re: Murphy A70 mains hum

If there is hum with the volume set at zero I would be looking at something like the orientation of the output transformer with respect to the mains transformer, or the routing of heater wiring - and the grounding of the heater windings at the mains transformer.

In reality, many old radios were designed for ease of assembly, rather than HiFi, and it is normal for a bit of background hum. What constitutes excessive is a personal consideration. I have something like 20dB of hearing loss below 200Hz so don't notice hum!!!
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Old 7th May 2024, 8:42 pm   #3
cathoderay57
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Default Re: Murphy A70 mains hum

Are you sure you got C44 the right way around when you recapped, with positive connection to chassis? Jerry
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Old 7th May 2024, 9:39 pm   #4
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Default Re: Murphy A70 mains hum

A rough guide that I find helpful:
50Hz hum is from the heaters. Maybe h/c leakage or induction from the heater wiring or induction from the mains transformer or mains wiring.
100 Hz hum poor smoothing in the power supply.
Buzzy hum bad earthing/screening somewhere.

If there is a magnetising coil in the speaker as part of the HT smoothing, check tyhat any hum-cancelling coil is wire correctly if anything has been changed.
d
Not foolproof but a good place to start.
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Old 7th May 2024, 10:15 pm   #5
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Default Re: Murphy A70 mains hum

Does the hum increase when you tune to a radio station and does it reduce when you tune away from a station?

Have you increased the size of the reservoir cap?

Have you grounded the additional capacitors to any convenient chassis tags?
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Old 8th May 2024, 4:52 pm   #6
Andy G4NPA
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Default Re: Murphy A70 mains hum

Thanks everyone for the suggestions.
The hum is there even with the volume at zero and its pretty much the same with 3 different speakers - the original , a different field coil murphy speaker and a modern speaker with a resistor replacement for the field coil.
I've checked C44 and it's as per the circuit I have +ve to chassis.
The layout of the transformers etc is all original as is the heater wiring.
Linking in an extra cap in addition to C48 C49 or C43 seems to have no effect on the hum.

Pulling the the other valves other than the PEN45 audio o/p also seems to have zero effect.

I guess it could be a grounding/heater wiring issue as suggested - other than a random approach is there a recommendation for tracking that down ?
Not shure what I'm hearing is 50 or 100Hz. My spectrum analyser app on my phone shows peaks at 100 and 150hz . Not shure what to make of that !

Thanks again for the advice Andy.
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Old 8th May 2024, 6:35 pm   #7
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Default Re: Murphy A70 mains hum

Are you using a mains Earth to the chassis ?
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Old 8th May 2024, 7:36 pm   #8
Andy G4NPA
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Default Re: Murphy A70 mains hum

Yes, at the moment. I guess originaly there was no mains earth ? The set as I received it had 2 core mains cable plus someone had run a separate lead to mains earth.
Andy
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Old 9th May 2024, 9:09 am   #9
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Default Re: Murphy A70 mains hum

Is the hum present at the grid of the o/p valve on the scope,
Use the scope to determine the frequency of the hum. 50 or 100 c/s, this will guide you to the source of the hum.
Disconnect that earth wire and measure the resistance of the mains winding to chassis to ensure that there is no leakage. It should be high, in excess of 1m ohm on the Avo.
If the valve holders are paxolin you could have a leakage between grid and heater pins.
John.
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Old 9th May 2024, 12:15 pm   #10
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Default Re: Murphy A70 mains hum

Something you could try is a Pi filter, with, say, a 220 ohm resistor (5w) and 33uf between the reservoir cap and the choke. The capacitor goes between the resistor and the choke.

It will significantly reduce ripple through the choke which will cut down the magnetic coupling, and also reduce mains hum getting into the output transformer and the screen of the output valve.

I recall having seen this recommendation by Lawrence (is he still around?) in the past and it was the only thing that worked.

Regards
Gabriel

Last edited by Gabe001; 9th May 2024 at 12:24 pm.
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Old 9th May 2024, 4:33 pm   #11
Andy G4NPA
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Default Re: Murphy A70 mains hum

Thanks both for the hints. I've had a look for leakage from the primary and secondary to ground with my DVM but can't see anything - looks o/c.
I'll try the other suggestions next week when I have a bit more time.
There does seem to be a fair amount of ripple across the first filter capacitor C48 (16uf) . Also not shure why the negative sides of the filter caps don't go straight to chassis ground but go via R28/R30 plus C44. I assume this is something to do with the Gram pick up which I don't use. The previous repair done probably in the 70's the negatives of the filter caps went straight to Chassis. I've put it back as per original circuit.
Thanks, Andy
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Old 9th May 2024, 4:39 pm   #12
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Default Re: Murphy A70 mains hum

Do you have a multimeter that shows True RMS, if so what AC readings are getting at the Various Filter capacitors?
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Old 9th May 2024, 5:17 pm   #13
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Default Re: Murphy A70 mains hum

It's ripple at c49 I'd be more concerned about.

Found the original thread here. It was pertaining to a hummy Ekco.
https://www.vintage-radio.net/forum/...=177413&page=9
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Old 10th May 2024, 6:29 am   #14
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Default Re: Murphy A70 mains hum

Quote:
Originally Posted by G6Tanuki View Post
In reality, many old radios were designed for ease of assembly, rather than HiFi, and it is normal for a bit of background hum. What constitutes excessive is a personal consideration. I have something like 20dB of hearing loss below 200Hz so don't notice hum!!!
Murphy was a quality manufacture and any hum would not have been acaptable. My A70 functions without any unwanted noise other than interferences from modern poorly suppressed devices.
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Old 10th May 2024, 8:04 am   #15
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Default Re: Murphy A70 mains hum

Andy,

Remove V3 and ground G1 of V4 is the hum still present? If so, V4 could well be a faulty.

The text gives information about R28 & R30 and I have included a 'snip' from the manual about them.
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Old 10th May 2024, 11:14 pm   #16
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Default Re: Murphy A70 mains hum

One fault that you should eliminate from your enquiries is the possibility that you only have half wave rectification in the power supply.

This could be caused by on open circuit secondary HT winding, a faulty valve or valve holder.

The mains switch is on the back of the tone control so that should eliminate another possible source of hum.
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Old 10th May 2024, 11:36 pm   #17
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Default Re: Murphy A70 mains hum

The grid of V3 (HL42DD) is probably the most sensitive part of the circuit.

Has the wire/cable going to the top cap of V3 been replaced?

Is the cable shielded and grounded?

If you ground the wiper of the volume control by using a film capacitor (i.e. 0.1uF) it may tell you if the hum is introduced at the grid of V3.
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