Murphy Homebase Mains Hum

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sickboy
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Murphy Homebase Mains Hum

Postby sickboy » 09 Jan 2017, 19:42

I have one of these sexy Murphy homebases, and I know they suffer from mains hum - which I read somewhere was attributed to the transformer not being properly screened. The hum comes through it's own speaker when the volume is low and the squelch is at minimum. Not much of a problem really but it's annoying all the same.

I bought a cheap chinese 5A switchmode psu (http://www.ebay.co.uk/itm/222274748011? ... 1439.l2649) to put in it's place but now instead of a mains hum, I have a faint but high pitch tone coming from the speaker - again only when the volume and squelch are near minimum. I've changed the caps around the audio chip but no change. Is this an actual fault or is this just the way they are? {bnghd}
Probably doesn't help that the PSU only cost £7 including postage from China :shifty:

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Re: Murphy Homebase Mains Hum

Postby sickboy » 09 Jan 2017, 20:10

I've kinda just answered my own question.. the Murphy doesn't have a 12v input on the back of it but I connected an external power supply to it anyway and there's no hum at all. So is there anything I can do apart from adding a 12v input socket? Some kind of filter or something?

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Re: Murphy Homebase Mains Hum

Postby beamer320i » 09 Jan 2017, 20:23

Sometimes it can be a bit 'hit or miss' with switch mode PSU's - I bought five identical 8.5 amp ones from the same seller, three work fine with no issues, one creates a high pitch tone like yours and the other one creates S7 of noise!!

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Re: Murphy Homebase Mains Hum

Postby sickboy » 09 Jan 2017, 20:33

beamer320i wrote:Sometimes it can be a bit 'hit or miss' with switch mode PSU's - I bought five identical 8.5 amp ones from the same seller, three work fine with no issues, one creates a high pitch tone like yours and the other one creates S7 of noise!!


They're too good to be true really aren't they. For £7 including international shipping my expectations weren't high to be fair.

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Re: Murphy Homebase Mains Hum

Postby beamer320i » 09 Jan 2017, 20:44

I did use on of the above mentioned PSU's converting my Cobra 2000GTL, it worked beautifully in that. Since then I've experimented in various PSU's from various sellers, none have a 100% success rate.

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Re: Murphy Homebase Mains Hum

Postby dt307 » 10 Jan 2017, 00:38

check the voltage ....this way pluss side of meter to - of psu
neg side of meter to the psu case readings taken on AC not dc

had a few china sw supply that the chassis is isolated from ground and was getting ac to ground

simple fix add some disk caps between the - and earth stopped the buzzing for me
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Re: Murphy Homebase Mains Hum

Postby sickboy » 10 Jan 2017, 00:51

Cheers Chris, tried what you said and I get 0.014v

Daft idea but what if I added a 12v 5a regulator like a 78H12 to the output?

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Re: Murphy Homebase Mains Hum

Postby dt307 » 10 Jan 2017, 01:10

some of the psu make a high pitch sound and its from the psu its self
no such thing as a daft idea m8 but no i dont think that will help.
try powering from the same psu but external two or three feet away
add a smoothing cap across the output or four diodes to make a bridge rectifier to remove and ac left on it
could be just the frequency of the switching

wire it up outside i would try first
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Re: Murphy Homebase Mains Hum

Postby Retrodog » 10 Jan 2017, 08:17

The hmmmmmmmmmmmmmm is part of the rig's heritage.
Back in the day when a breaker on the side called in we used to say are you on a mmmmmmmmmmmmmmmmmmmmmurphy by any chance?
It always went down well and we had a good laugh about it.

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Re: Murphy Homebase Mains Hum

Postby sickboy » 10 Jan 2017, 09:10

dt307 wrote:add a smoothing cap across the output or four diodes to make a bridge rectifier to remove and ac left on it
could be just the frequency of the switching


I tried holding a 3300uf 50v cap across the output temporarily to see if it would have any effect on the noise but the psu just shut down then kept tripping till I removed it.
I'll try the psu outside the case today mate, cheers

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Re: Murphy Homebase Mains Hum

Postby sickboy » 10 Jan 2017, 10:20

Tried the psu about 4 or 5 feet away but there's no difference, the noise is still coming from the speaker Chris.

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Re: Murphy Homebase Mains Hum

Postby dt307 » 10 Jan 2017, 10:22

sickboy wrote:
dt307 wrote:add a smoothing cap across the output or four diodes to make a bridge rectifier to remove and ac left on it
could be just the frequency of the switching


I tried holding a 3300uf 50v cap across the output temporarily to see if it would have any effect on the noise but the psu just shut down then kept tripping till I removed it.
I'll try the psu outside the case today mate, cheers


either wrong way round or you just touched the cap on the output with the psu on?

if the psu is on then you touch the cap the cap with suck as much current as a russian hore sucks .... and trip ....no no no

connect the cap then turn it on m8 i run a cap on the same psu with no issues
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Re: Murphy Homebase Mains Hum

Postby dt307 » 10 Jan 2017, 10:26

have you try to power a different radio with it? it could be the psu is just dirty ie faulty
163dt307-chris




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Re: Murphy Homebase Mains Hum

Postby sickboy » 10 Jan 2017, 10:48

dt307 wrote:
connect the cap then turn it on m8 i run a cap on the same psu with no issues


lol ok, tried that just now and there's a noticeable improvement. So then I tried 2x 3300uf in parallel across the output and it's now brought the noise down to about 30% of what it was so we're definitely getting somewhere.

Using another rig on the same psu is fine, no noise at all so it's definitely something the Murphy is more susceptible to

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Re: Murphy Homebase Mains Hum

Postby dt307 » 10 Jan 2017, 11:36

You need to borrow my smoothing checker
If this don't work take up golf
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163dt307-chris




“I fear the day that technology will surpass our human interaction. The world will have a generation of idiots.”
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