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Old 26th Jun 2022, 10:03 am   #1
Michael Maurice
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Default RFI interference on an HMV stereomaster.

I have been servicing a 1968 HMV Stereomaster record player. After servicing the amplifier I found it had this incredibly loud buzzing sound.

As I couldn't work out where it was coming from or what was causing it, I took it back to the workshop. On testing it there, it was absolutely silent, apart from a hiss and music when I turned the volume up!

It turns out that the customer has some sort of mains router extender and this is the most likely cause of the interference.

If i fit one of these is that likely to solve the problem?

I think the best place to put it is in the cabinet, bolted to the side and have the mains cable going directly to it and the output (Load) going to the amplifier.

Do you think obne would be enough or should I have two in series?
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Old 26th Jun 2022, 10:10 am   #2
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Default Re: RFI interference on an HMV stereomaster.

If the powerline adaptors are chucking out that amount of interference then there's likely to be a fault somewhere, and mains filtering is unlikely to resolve the issue. I use powerline adaptors, and while I'm not a SW listener I find there's no impact on everyday equipment use. The RFI from things like TVs and PVRs has a bigger impact.
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Old 26th Jun 2022, 11:37 am   #3
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Default Re: RFI interference on an HMV stereomaster.

I found that Powerline units commonly send out interference that is picked up by some amplifiers at a very low level and some pre-amplifers at a significantly higher level, a magnetic phono pre-amp (obviously with the most gain) being the worst. I tried some medical mains filters that reduced the interference but did not kill it completely, I chose the easier route at home and installed a cabled data network instead which is fine.
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Old 26th Jun 2022, 12:30 pm   #4
Michael Maurice
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Default Re: RFI interference on an HMV stereomaster.

The interference or 'buzz' is there irrespective of the volume setting, input setting or indeed any other setting!

Its not my house, so I can't really tell them to put in a new cable!
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Old 26th Jun 2022, 12:35 pm   #5
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Default Re: RFI interference on an HMV stereomaster.

As I said, that suggests the level of mains borne interference is *extremely* high and certainly not normal. Unfortunately SMPSUs can soldier on for years with these sorts of faults, apparently working normally. I once had a laptop charger which did this.
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Old 26th Jun 2022, 12:38 pm   #6
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Default Re: RFI interference on an HMV stereomaster.

Could this be resolved or reduced by some additional screening within the unit itself?

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Old 26th Jun 2022, 1:01 pm   #7
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Default Re: RFI interference on an HMV stereomaster.

Not sure which Stereo Master it is, I have looked at the 2032 circuit.
The interference could be “getting in” by the earth wire which that filter wouldn’t help if that’s the case.
A common mode type using a ferrite toroid may help on the mains lead and easy to try.
Long speaker leads can also be a problem but the circuit I have should be short unless external speakers are used. If they are external use toroids on those.

After saying all this the mains filter in post #1 could help, but the player uses a mains transformer and I would think( probably wrong) that it wouldn’t get through that.

Paul’s view of faulty Powerline adapter could also be correct and no amount of filtering will work.

I would try a toroid in the mains lead before anything else.

Interference problems seem to be a lot of trial and error.
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Old 26th Jun 2022, 1:03 pm   #8
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Default Re: RFI interference on an HMV stereomaster.

I don't think screening is the issue. The noise will be on the ring main, and probably on the entire house phase (and down the street too). It sounds as if it's getting through the power supply and being reproduced by the output transistors.

The source may not be powerline adaptors at all - any faulty SMPSU can cause this. It may not even be in the same premises. I once had really bad mains borne interference of unknown origin, and went down the road with a transistor radio trying to find where it was coming from. I never found the source, but you could clearly hear the interference peaking at every third house. Eventually the interference just stopped, possibly because the SMPSU responsible had finally given up the ghost.
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Old 26th Jun 2022, 1:21 pm   #9
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Default Re: RFI interference on an HMV stereomaster.

Re post #1
I should have read a little more carefully, “most likely cause”, can you prove it by switching the suspect item off and see if the interference stops.
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Old 26th Jun 2022, 6:07 pm   #10
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Default Re: RFI interference on an HMV stereomaster.

I agree with Paul's diagnosis. I had powerline extenders which caused a noise in my rebuilt stereo Mullard 5-10. I thought I'd made an error somewhere till I found that these extenders were causing it. If your customer has a similar system, you could ask them to unplug them temporarily, just to check.
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Old 26th Jun 2022, 11:18 pm   #11
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Default Re: RFI interference on an HMV stereomaster.

The buzzing may not be the direct interference, it is likely to be high frequency data bursts being demodulated by self rectification, probably in the power amp as the vol cotrol makes no difference.

Entry to hifi amps is often via common-mode pickup on speaker cables ditto for players with an extension speaker for stereo. For an all-in-one mono record player, that's unlikely.

That filter is a good first port of call. Two won't be a lot better. If it doesn't work, it goes in a mains extension as a piece of test gear for future similar problems. Furter steps would be adding large ferrite rings.

Have a read at Ian White's pages. Ian is an old friend, had a technical column in Radcom for years and is technically very competent indeed. He's put a lot of work into studying this sort of thing and testing solutions.

http://www.ifwtech.co.uk/g3sek/in-prac/

Those big clip-on cores are a tenner a pop. They are good for trying things, and then you fit a much cheaper (and slightly better) non-split version.

You might mention to your client that what he is hearing is his internet activities all being radiated strongly from his mains wiring. The encryption is not terribly good, so it's a big breach in his security. If his record player can be overloaded by this signal, then a proper receiver can receive it at far greater range.

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Old 27th Jun 2022, 10:14 am   #12
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Default Re: RFI interference on an HMV stereomaster.

I still think this is abnormal. A constant unchanging buzz is a classic symptom of a knackered SMPSU. Interference from digital communications like powerline adaptors or ADSL isn't unvarying.
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Old 28th Jun 2022, 11:24 am   #13
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Default Re: RFI interference on an HMV stereomaster.

It still needs confirmation which item is producing the interference.
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Old 24th Aug 2022, 6:05 am   #14
Michael Maurice
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Default Re: RFI interference on an HMV stereomaster.

I thought I'd bring you up to speed:

The interference was coming from one of the hub devices which plugs into the mains, proved by switching it off.

I then fitted a filter which reduced the noise considerably, I then fitted a second filter in series which completely eliminated the noise.
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Old 24th Aug 2022, 8:12 am   #15
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Default Re: RFI interference on an HMV stereomaster.

Good result.
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Old 27th Aug 2022, 9:02 pm   #16
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Default Re: RFI interference on an HMV stereomaster.

Good to know those filters actually work... was it the one you linked to that you used? good job there's plenty of room for 2 in the cabinet!

BTW Michael it looks like your domain name needs renewing
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