Post by trombone on Jun 15, 2023 17:41:16 GMT 12
Hi everyone,
Some time ago I worked on this set. It had an open circuit field winding on its e.m. speaker. I replaced the e.m. with a p.m. speaker and put a 2.5k wirewound resistor in place of the field, which seems to be a fairly standard practice ,replaced the two 10 uFd caps with 16s and that was that.One negative of one smoothing cap goes to earth, the negative of the other smoothing cap goes to the top of the field ,or to the top of the resistor after I'd done the substitution. The radio went well and the cabinet came up quite nicely.Scroll forward a couple of years and the kitchen radio faded out so the Courier was called on to stand in for a while. Again it performed well but I realised it had an unacceptably high hum level.So back to the bench.
Bridging the two 16s with my test caps made no difference.Perhaps a valve has a heater to cathode short. Removing the valves one by one made no difference. Substituting several 2A5s for the original made no difference.
What did make a difference was replacing the 2.5k wirewound with the field of a spare e.m. speaker I had .Hum disappeared.
In this set (1936) equipped with 2.5 volt valves, the field winding is between the centre tap of the power transformer and a candohm resistor of 500 ohms to earth providing 21 volts bias for the 2A5 among other things if I am reading the circuit correctly.I am fairly certain I have done this "replace the field with a resistor and up the uFd of the caps" successfully several times before but this is ,I think, the first time there has been a hum issue.
They say you learn as you go in this game. Can anyone suggest a reason why the resistor produced the hum and why the field resolved it?
As it turns out there is room on the chassis to accommodate a spare field winding I have but I'd be interested to hear from any who can throw light on this. The circuit is on the NZVRS website as Model UU.
Some time ago I worked on this set. It had an open circuit field winding on its e.m. speaker. I replaced the e.m. with a p.m. speaker and put a 2.5k wirewound resistor in place of the field, which seems to be a fairly standard practice ,replaced the two 10 uFd caps with 16s and that was that.One negative of one smoothing cap goes to earth, the negative of the other smoothing cap goes to the top of the field ,or to the top of the resistor after I'd done the substitution. The radio went well and the cabinet came up quite nicely.Scroll forward a couple of years and the kitchen radio faded out so the Courier was called on to stand in for a while. Again it performed well but I realised it had an unacceptably high hum level.So back to the bench.
Bridging the two 16s with my test caps made no difference.Perhaps a valve has a heater to cathode short. Removing the valves one by one made no difference. Substituting several 2A5s for the original made no difference.
What did make a difference was replacing the 2.5k wirewound with the field of a spare e.m. speaker I had .Hum disappeared.
In this set (1936) equipped with 2.5 volt valves, the field winding is between the centre tap of the power transformer and a candohm resistor of 500 ohms to earth providing 21 volts bias for the 2A5 among other things if I am reading the circuit correctly.I am fairly certain I have done this "replace the field with a resistor and up the uFd of the caps" successfully several times before but this is ,I think, the first time there has been a hum issue.
They say you learn as you go in this game. Can anyone suggest a reason why the resistor produced the hum and why the field resolved it?
As it turns out there is room on the chassis to accommodate a spare field winding I have but I'd be interested to hear from any who can throw light on this. The circuit is on the NZVRS website as Model UU.
