Good finds STC830.

I guess the fact that we haven't heard of the problem before indicates that most manufacturers got the physical design right. Either that or maybe listeners back in the day didn't expect too much in the audio performance department from portable battery sets.

"...too much in the audio performance department..."

In discussing battery valves RDH says that they had to perform and the oscillator in particular work down to 2/3 of the nominal battery voltage - so I reckon that the designers did pretty well!

There's always gold to be found in the RDH. Obviously the designers of the 503B didn't read that warning. And the Healing uses AWV valves so there's no excuse!

Thanks STC830. It's good to see one's observations confirmed,,,,

Now, will I modify the radio to fix it? Hmmm....

There is room to move the chassis back in the case, extend the control and tuning gang shafts and space the dial forward.

Nah.....! Too hard.

Or fit a smaller speaker higher up in the case with a new baffle. But that somewhat defeats the purpose.

Its possible that the particular valve is dodgy - maybe a loose element. Failing that maybe a mumetal or permalloy shield if you can come up with a piece.
The RDH 4th Ed. was 1953, the radio 1948 so maybe they have an excuse if the warning wasn't in earlier editions.

Or fit a smaller speaker higher up in the case with a new baffle. But that somewhat defeats the purpose.

How about a magnetic shield? Maybe layers of mu metal sheet?

The RDH 4th Ed. was 1953, the radio 1948 so maybe they have an excuse if the warning wasn't in earlier editions.

RDH editions 1 and 2 are tiny publications of 40 and 60 pages each. The third edition runs to 350 pages and I can't quickly find reference to the subject. (Quickly because that's in the 'sneezing section' my library).

The venerable 4th edition at circa 1500 pages is stretching the definition of a handbook.

I note that various mentions of the subject in RDH4 refer to the 1948 paper "Hum Reduction" by AF Dickerson, in Electronics Vol 21, No 12, p112. I haven't been able to find access to that article online.

I would be trying to see if I can get a couple of miniature valve shields. Mu metal is ideal & iron not so good (mu Metal is around 80+% NIckel).

Having said that. in desperation you could try a piece of 1.5mm sheet metal in front of them. My only concern is that it will magnetize. Ferrite may also be worth a try.

Marc

All said, I think I'd try a replacement valve first. Easiest fix if that's the problem.

Already tried replacement valves. I think I can hear a slight improvement but I doubt it would be detected on a double-blind test.
And the 2 from the Healing sound just fine in the HMV.

My understanding of magnetic shields using mu-metal is they work for AC fields. I've tried a slab of 5mm thick mild steel - actually made it worse! A tinplate shield got about the same results as swapping the valves out.

Probably my best bet would be a speaker with an M-style magnet, but that would be deeper than this one.

Actually I'm not too worried, I can't listen to most Sydney AM stations where I am at Belrose due to a localised noise source centred on about 600kHz that knocks out reception pretty much all through the house. 702 punches through it but 630 is at about 0dB S/N. Readable, but very tiring to listen to. It's nothing I have any control over, I can power the house down and it's still there.

where I am at Belrose due to a localised noise source centred on about 600kHz

AM reception in my workshop was severely affected by RFI harmonics from a poorly-earthed VFD controlling the carpark ventilation system.

Unfortunately, local sources can be many these days, but some culprits include power supplies for Optus and Telstra HFC coax and the old staple of dirty insulators on power poles which gets worse during long spells without rain.

Speaking of Optus, it might be interesting to find what escapes from their satellite dish operation at Belrose.

Those lamps with a metal base that you touch to turn on/off make lots of those humming noises across the band. You have to switch off or unplug at the powerpoint to stop it. Also my newest set-top digital box (a $28 cheapie from Big W) causes massive amounts of noise, even more when off. Again, switch off at the powerpoint is the only way to shut it up.

I'm sure these days there's any amount of noisy appliances around the neghbourhood, since the authorities don't seem to care any more.

the authorities don't seem to care any more

That's for sure. It's all "self regulated" these days, which adds up to zero regulation.

Well it's localised, just around my place, worst in the garage underneath. As I said I can power the whole place down without affecting it.

Only thing I haven't yet tried is the common-property garage door opener. That's next.

Do you have a smart meter perchance?

Neighbour with solar panels?

Meter is a long way up the building, no noise there.

No solar anywhere near.

I haven't bothered to power down the door opener so far because there's absolutely no change in the noise when it's operating (as picked up in the car).