05-02-2016, 02:02 AM
Quick question... I have some low level 100Hz (NZ mains is 50Hz) hum going on... not voume affected... Could using poly caps instead of electro's be responsible? I tried paralleling a 10uF cap over each of the three in turn - no appreciable difference. I also tried putting the 10uF cap across the second filter cap - again not much difference. When I put it over the reservior (first filter) cap it halved the hum. If I put a 22uF over it the hum almost disappeared although I can still hear it without straining to do so. This radio draws 90W so there's a reasonable load on the power supply.
The filter caps in the schematic call for a 12uF reservoir and an 8uF second filter... I used a 10uF reservoir and a 22uF second filter because thats what I had to hand... would this choice be partly responsible? I've never struck this before and have always found that more uF's = less hum although I am aware that the combination of the 2 smoothing caps with the field coil forms a pi-filter of sorts - my knowledge of the theory on this is limited to sweet little or nothing though.
Other than that it is playing well. All the IF's were off, but all in the same direction by approximately the same amount... maybe the previous tech working on this had a sig gen with a slightly out of whack dial... but its singing nicely.
Unfortunately it had all Philco valves in it... unfortunately because most of them were shot. I'm surprised it had any output at all... they both barely registered on my tester... so now there's a Ken-Rad and something else in there... but they work. The diodes on the 85 1st audio tube are also quite low, with the triode being borderline... The 6A7 is ok but thats the only non-Philco valve in the lineup.
I also can't get the bulb behind the shadow meter to do anything at all... but I have a service note on that somewhere so I'll play with that later...
Happy though, it goes
The filter caps in the schematic call for a 12uF reservoir and an 8uF second filter... I used a 10uF reservoir and a 22uF second filter because thats what I had to hand... would this choice be partly responsible? I've never struck this before and have always found that more uF's = less hum although I am aware that the combination of the 2 smoothing caps with the field coil forms a pi-filter of sorts - my knowledge of the theory on this is limited to sweet little or nothing though.
Other than that it is playing well. All the IF's were off, but all in the same direction by approximately the same amount... maybe the previous tech working on this had a sig gen with a slightly out of whack dial... but its singing nicely.
Unfortunately it had all Philco valves in it... unfortunately because most of them were shot. I'm surprised it had any output at all... they both barely registered on my tester... so now there's a Ken-Rad and something else in there... but they work. The diodes on the 85 1st audio tube are also quite low, with the triode being borderline... The 6A7 is ok but thats the only non-Philco valve in the lineup.
I also can't get the bulb behind the shadow meter to do anything at all... but I have a service note on that somewhere so I'll play with that later...
Happy though, it goes
There are no personal problems that can't be overcome with the liberal application of high explosives
