26-11-2012, 06:21 PM
(This post was last modified: 26-11-2012, 06:29 PM by 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
"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
