Hi all,

Just a quick question about faint sound still heard when the volume control is turned all the way down. I am working on an Ekco A147 and it is exhibiting this fault. I have replaced every cap in the set and checked all resistors are within 20%, I replaced the detector and output valve cathode bypass caps which were 50μF electros with new 47μF electros. All voltages are spot on.

I have had this on a couple of radios and wondered what the usual suspects are. I'm not particularly worried about it, just trying to improve my understanding of what's going on.

https://www.radiomuseum.org/r/ekco_a147a_14.html

Thanks, David.

Hi David

Most common cause of this on a non-reflex circuit is the volume pot itself.

Measure the resistance of the centre tap of the pot to ground, with the volume at minimum. Anything above zero ohms will give you some talk-through especially if there is some gain following. Or you may simply have a high resistance earth connection on the pot.

Actually removing the cathode bypass cap on the output stage will reduce the gain a little, reduce the talk-through effect and add some negative feedback which will generally improve the sound, assuming you have more gain than you need.

Does the volume pot have a switch? Sometimes the switch mechanism restricts the pot rotation.

Another cause can be rectification of stray RF amplified by the 1st audio, especially if that valve contains the detector/AGC diode. Adding a 100pf disc ceramic cap from the anode of the 1st audio stage to ground should cure that, if that's the cause.

If it is a reflex circuit, that's another story! Very careful design and lead dress can reduce but rarely completely eliminate talk-through on these radios.

Hi Ian,

It's a non-reflex circuit and the pot centre tap reads 250 Ohm resistance to ground with the volume turned to minimum so that may be it. It does have a switch and when selected to OFF it goes to about 90 Ohm. The pot is earthed OK.

I had previously checked the pot but not how you described.

I might try lifting the Cathode cap and see what difference it makes to the sound, It has plenty of gain. Interesting thought.

The radio already has a100pF cap from anode to ground in the 1st audio. I wondered why that was there.

Thanks Ian, I appreciate your help for what are probably mundane questions, I have a lot of penny drop moments reading comments, three with your comments above.

Cheers and thanks, David