UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Hum pickup on inter-stage coupling transformer

Alan_G3XAQ · 29th May 2020, 6:12 pm

I've (mostly) cured the hum pickup on the audio stages of my lockdown receiver project by replacing the EI-cored mains transformer with a toroidal unit. I was quoted over £100 for a bespoke unit so I took a £16 6+6V 30VA transformer from RS and added an HT winding. I've attached some photos. The construction is described in a bit more detail in a thread on here

https://www.vintage-radio.net/forum/...d.php?t=166961

I am using another cheap 1.6VA 12+12V toroidal mains transformer to replace the broken microphone transformer. For minimum hum pickup I have to carefully orient this transformer so its core is at right angles to the core of the mains transformer.

The 5mV hum pickup referenced to the input of the audio amplifier that I had before is now reduced to about 0.1mV peak to peak, a 34dB improvement. I will try adding some tinplate shielding to see if this can be improved but even at this level the receiver will be usable.

Although it is now hard to hear I can still see the slowly decaying hum signal on a scope immediately after switching off. I am at a loss to explain it but it is no longer of any great importance.

73, Alan G3XAQ

29th May 2020, 6:12 pm	#18
Alan_G3XAQ Pentode Join Date: Apr 2018 Location: Canterbury, Kent, UK. Posts: 189	Re: Hum pickup on inter-stage coupling transformer I've (mostly) cured the hum pickup on the audio stages of my lockdown receiver project by replacing the EI-cored mains transformer with a toroidal unit. I was quoted over £100 for a bespoke unit so I took a £16 6+6V 30VA transformer from RS and added an HT winding. I've attached some photos. The construction is described in a bit more detail in a thread on here https://www.vintage-radio.net/forum/...d.php?t=166961 I am using another cheap 1.6VA 12+12V toroidal mains transformer to replace the broken microphone transformer. For minimum hum pickup I have to carefully orient this transformer so its core is at right angles to the core of the mains transformer. The 5mV hum pickup referenced to the input of the audio amplifier that I had before is now reduced to about 0.1mV peak to peak, a 34dB improvement. I will try adding some tinplate shielding to see if this can be improved but even at this level the receiver will be usable. Although it is now hard to hear I can still see the slowly decaying hum signal on a scope immediately after switching off. I am at a loss to explain it but it is no longer of any great importance. 73, Alan G3XAQ Attached Thumbnails