drh4683
08-11-2010, 11:42 PM
It would have been more appropriate if I did a little more research and troublshooting before shooting my mouth off by chewing out a known reputable product with my assumption that "the capacitors must be bad" in my trans-oceanic due to a very bad hum when operated on AC. Long story short, a 1942 Zenith transoceanic bomber was playing fine on batteries yet it humed badly on AC. Naturally I suspected filter caps (wouldn't you?) but I jumped the gun and assumed the new sprague lytics I installed were at fault because the hum got worse over the last few months. It seemed to make perfect sense: Light/moderate hum after restoration, 2 years later, very bad hum.
The trouble turned out to be a leaky candohm resistor. It was leaking B+ to the chassis through it metal casing, thus screwing up the AVC, RF and IF and caused the hum. The problem gets worst with moisture absorption and the insulation material used inside the resistor was somewhat conductive, like in the megohm range, but its enough to cause all sorts of oddball problems. There is nothing to fix this other than isolating the resistor from the chassis, or install all new wire wound resistors in its place. I got this tip from another radio guy who pulled his hair out trying to fix this same problems I was having with hum and poor audio and poor sensitivity. It all was because of this dang candohm. So fortunately I spared myself the headaches of having to clip up a nice restoration job and got to find out exactly what cause is. I unmounted the candohm off the chassis as a test and the radio played perfect, every problem went away just like that and sound quality difference went from crap to excellent, reception sensitivity will now blow you away, its an outstanding dx-er with nice crisp sharp tuning. And the hum didn't just go away, I mean it doesn't exist in the slightest, which means those sprague caps I was bashing are infact flawless and perfect, like we always know them to be.
So there you have it, STILL USE THOSE SPRAGUE ATOM CAPS! THE QUALITY IS STILL THERE! Just don't listen to me and you'll be in good shape.
The trouble turned out to be a leaky candohm resistor. It was leaking B+ to the chassis through it metal casing, thus screwing up the AVC, RF and IF and caused the hum. The problem gets worst with moisture absorption and the insulation material used inside the resistor was somewhat conductive, like in the megohm range, but its enough to cause all sorts of oddball problems. There is nothing to fix this other than isolating the resistor from the chassis, or install all new wire wound resistors in its place. I got this tip from another radio guy who pulled his hair out trying to fix this same problems I was having with hum and poor audio and poor sensitivity. It all was because of this dang candohm. So fortunately I spared myself the headaches of having to clip up a nice restoration job and got to find out exactly what cause is. I unmounted the candohm off the chassis as a test and the radio played perfect, every problem went away just like that and sound quality difference went from crap to excellent, reception sensitivity will now blow you away, its an outstanding dx-er with nice crisp sharp tuning. And the hum didn't just go away, I mean it doesn't exist in the slightest, which means those sprague caps I was bashing are infact flawless and perfect, like we always know them to be.
So there you have it, STILL USE THOSE SPRAGUE ATOM CAPS! THE QUALITY IS STILL THERE! Just don't listen to me and you'll be in good shape.