A Miscellany of Hums

[Joe]

I've noticed an effect on both my Ferguson radio and Wartime Civilian Receiver. This is a hum but not the usual 'mains' type.

If the sets are tuned between stations, or in the case of the WCR has the aerial pulled out, there is very little background noise. However when tuned in the station is accompanied by a hum. The stronger the station the louder the hum.

I noticed this on a DAC90a too but it seemed to stop after I moved it but I think that what's actually happened is that it is now getting a poorer signal.

The Ferguson has another hum. This was does sound like 50 cps but starts IMMEDIATELY that power is applied and I'm inclined to think it's the transformer laminations vibrating. I will now use all my will power to not add that this is what happens when somebody puts a load of great coils where the dropper should be.

That one does have a bit of 'normal' mains hum which, now I've got some spare smoothing caps, I may investigate shifting but it isn't intrusive.

As I type this I've got my AC/DC Ultra on the Light Programme with no hum.

Advise/opinions would be most welcome.

Cheers
- Joe

[ucl82]

When AC sets buzz as soon as power is applied rather than when they are warmed up it is usually a case of the transformer laminations buzzing. Sometimes it is possible to remove the transformer and boil it to get all the old wax off and then cover it in fresh wax in the hope it will reseal it properly. Very annoying if it can't be fixed though as it means a perfectly functioning transformer needs to be replaced.

[Brianc]

The hum which appears only when a station is tuned in is called Modulation Hum and is the result of RF being brought into the set via the house wiring. Any non-linearity in the set, and any valve, diode, transistor and even dry joint will be non-linear, that this mains-borne signal meets will cause modulation of the RF by the 50Hz mains so that there are now two RF signals in the set - one clean, from the aerial and the other modulated by 50Hz. When these two signals meet, there will be a resultant 50Hz "hum" produced. The caps placed across the mains input of sets are there to help prevent the RF from getting into the set, in other words, filtering the mains input. A typical cure would be to add X2 rated caps across the mains at the first possible point, probably the mains switch. Ceramic caps across the smoothing electrolytics could also help.
Hum which is present all the time will be due to inadequate HT smoothing, pickup by high impedance circuits (grids etc.) or heater-cathode leakage in valves. HT hum can be 50Hz (half wave) or 100Hz (full wave) and if the hum is 50Hz in a set with full wave rectification, then it is not due to HT smoothing.
I hope this helps a bit.

[Joe]

Thanks, Brian, it's certainly something to play around with.

I will be doing some more tidying up of the WCR as the fine wires from the coils are much longer than normal and the cotton insulation on them is past its best so I want to sleeve those. While I'm at it I can check for other bad joints.

It also makes sense regarding the Pye but I'm not ready to do battle with that so-and-so again just yet.

As for boiling the Fergy's transformer that sounds like fun even if it doesn't do any good. I remember once when I had a pal staying with me he was concerned at my apparently cooking a tin of boot polish so a what he's have made of a transformer I don't know.

- Joe