Ivory Bush VHF90

[Yorkie]

Thanks you all and Alan, no as I cant find a 0.047uF suppressor cap!!

Regards

Rob

The value isn't important Rob, so long as it's a class X cap, intended for use directly across the mains between phase and neutral.

Anything from 0.1 - .047uF would be fine. (For example, the DAC90A used a 0.1uF).

Many - perhaps most sets - don't have one and it seems to make no difference, but if you fit one, it will do no harm, and might make a modest improvement. These days, Y2/X1 RFI suppression caps are mostly used to suppress RFI interference from domestic appliances such as power tools, vacuum cleaners, hair dryers and are required by EMC regulations.

My understanding of modulation hum (if it arises) is that it is caused by the mains lead to your set acting as an 'aerial’, picking up some RF energy in strong signal areas, which is modulated by the 50Hz mains frequency. (In DAC90As, any hum is most likely caused by the UL41, though I dare say a lot of restorers have needlessly replaced the twin electrolytics suspecting them as the culprit. In or out, the mains suppressor in DAC90As seems to make not one jot of difference in my experience).

Well done on sorting the set Rob!

[pwdrive]

My understanding of modulation hum (if it arises) is that it is caused by the mains lead to your set acting as an 'aerial’, picking up some RF energy in strong signal areas, which is modulated by the 50Hz mains frequency. (In DAC90As, any hum is most likely caused by the UL41, though I dare say a lot of restorers have needlessly replaced the twin electrolytics suspecting them as the culprit. In or out, the mains suppressor in DAC90As seems to make not one jot of difference in my experience).

Hello David, yes, I get that on some of my receivers where I have chopped out the mains filter capacitors and not got round to replacing them.

At my location it is worse on MW broadcasts ie: no interstation hum but hum when tuned into the station, I have also noticed that it is worse when using a receiver that's running off my Variac when the receiver in question is a 115 volt input receiver, might be compounded due to the output impedance of the Variac, it is always worst at night time here.

Lawrence.

[Bushbaby]

Ah,
I'm still running off my 3A Isolation tranformer - will this be cause of the excessive hum??

Thanks

Rob

[pwdrive]

Depends what you mean by hum, is it "modulation hum" that kind of hum is usually mains borne and usually shows up on AM broadcast band when tuned into the station, with this type of hum it's volume will increase as you advance the volume control once tuned into the station, it is usually worse on strong local stations, it is normally not present when tuning between stations, the mains filter capacitor can reduce this type of hum but the capacitor should be the X rated type for the job.
The other sources for hum are normally within the receiver, HT hum, wiring pick up hum, heater cathode hum, heater chain wiring radiation etc. When you replace the vol pot/switch pay attention to the routing of the wires as sometimes a high impedance grid circuit (the vol. control) can pick up hum from the mains switch wiring

Lawrence

[Radio Fixer]

David said:

"My understanding of modulation hum (if it arises) is that it is caused by the mains lead to your set acting as an 'aerial’, picking up some RF energy in strong signal areas, which is modulated by the 50Hz mains frequency. (In DAC90As, any hum is most likely caused by the UL41, though I dare say a lot of restorers have needlessly replaced the twin electrolytics suspecting them as the culprit. In or out, the mains suppressor in DAC90As seems to make not one jot of difference in my experience)."

My understanding of modulation hum is that it isnt the mains lead acting as an aerial and picking up etc. But rather it acting as a transmitting aerial, radiating stray RF or IF (well this would go straight thru a set unless there was an IF trap) modulated with 50 or 100 Hz . This goes around and gets picked up by the aerial to the set and comes out the speaker as HUM!!

Some say that you dont need an intermod cap as there is enough stray capacitance around the mains transformer to do the job anyway. Guess that is why just one cap on one side of the supply i/p is considered enough. The stray cap to the other side effectively decoupled this as well.

I usually fit one as I cant be botherd to test without it.

Gary

[pwdrive]

Can also be caused by Osc. radiation etc. seems to be various explanations on the web, anyone know the source of a defacto answer for the forums knowledge bank?

Lawrence.

[Mark R]

My understanding of modulation hum is that it isnt the mains lead acting as an aerial and picking up etc. But rather it acting as a transmitting aerial, radiating stray RF or IF (well this would go straight thru unless there was an IF trap) modulated with 50 or 100 Hz . This goes around and gets picked up by the aerial to the set and comes out the speaker as HUM!!

Gary

Interesting thoughts Gary, Has anyone tested this theory? Or could it be other appliances plugged in around the set on test, for example test gear or the the TV/whatever is near the radio at the time of listening?
Perhaps just the unshielded wiring of the humble abode coupled with wireless internet, wifi and TV providers?

any thoughts?

cheers Mark

PS, listening to a Hacker Mayflower II on classic FM with no hum!

[Joe]

I've no experience of isolating transformers but I can't see it IMPROVING performance. For one thing when the set is connected directly to the mains the Neutral will provide some sort of Earth.

Experiment is the best way to see. When it's got a proper plug on try it in a different room.

- Joe

[Radio Fixer]

To me the radio radiating back to itself is a lot more plausible than the mains lead picking up strong stations that some how mysteriously enter the set.

Guess its time for a serach on the web!

G

[Mark R]

I have a valve set close to the laptop in the Kitchen, along with a DAB radio, if the laptop is off there is reduced hum on the valve set, the DAB set, well that is just rubbish. So why would that be?

As an aside there are no strip lights in the kitchen.

cheers Mark