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Paper capacitor and Hallicrafters S-40A notes
"Antonio Vernucci" wrote in message . .. Turns out to be a couple of misplaced decimal points. First of all I mis-typed, the measured value is 0.048uf, not 4.8uf. Recalculating I get: As a matter of fact a value of 4.8uF seemed real odd to me. Oh, Yikes! I did it again. The correct measured value of the capacitor is 0.048 uf, D = 0.3 I calculate: C parallel = 0.044 uf R (AC) series = 995 ohms R (AC) parallel = 12050 ohms Xc, at 1000 hz = 3315 ohms Someone please check this. Your calculations seem correct to me (assuming that by Xc you mean the reactance of Cs and not that of Cp, which is 3,617 ohm). At this point, one would still have to explain how a capacitor marked 0,02 uF can grow up to 0,044 uF, that is more than twice its value. Before formulating hypotheses (e.g. that the plates of the capacitor are closer together than originally because of loss of the wax impregnan) I would rather try to reconfirm the measurement results. Measuring the resistance of the capacitor by means of a plain digital ammeter, do you obtain a value close enough to 12 kohm? Repeating the measurement on a different scale, do you obtain similar results? My experience with lossy capacitors is that the apparent Rp varies quite a lot with the scale. Also it would be useful to repeat the test with the GR set at a diffierent frequency (should this be possible). 73 Tony I0JX There is no DC resistance, that is, its open circuit for DC but I think there is an AC resistance component in parallel with the capacitance (have to look this up). The capacitance definitely measures high as do a couple of other paper caps from the same RX. Measured on the TEK multimeter the capacitance measures even higher. I checked the TEK meter on a General Radio decade capacitor which is known to be accurate and it measures correctly. There is definitely something strange here. The hummer in the GR bridge is definitely on frequency and it shows correct values on both the GR decade box and on single precision caps. I really think something has happened to the cap internally. Also, I am not at all sure of the tolerance of these caps originally, probably quite a lot on the high side. My Xc calculation was made for the series cap value. FWIW, I measured several other old paper caps including a non-leaking Black Beauty. All were within reason of marked value, all somewhat higher but nothing like the one in the original thread. The dissipation factors were high compared to new film capacitors but a couple of them were probably still good caps. D ran from a minimum of around 0.03 to around 0.5. C values were within about 20% of marked value, all on the high side. Measured value of leaky (I mean the oil has leaked out) BB caps shows them to usually be on the low side. Probably most of the paper caps in the S-40A did not need to be replaced but I had the caps and it was not a difficult job. Its difficult to know how much, if any, this improves the performance. As mentioned two caps were thoroughly gone, one a dead short and the other completely open but most of the others were probably still servicable. It would be interesting to know what the D of these caps was when they were new. It would be interesting to know what the new film caps will be like in fifty years but I probably won't be around then (but you never know what developments there will be in medicine). And, last but not least, thanks for checking my math. -- Richard Knoppow Los Angeles WB6KBL |
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