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Paper capacitor and Hallicrafters S-40A notes
While the high C value may not seem to be a problem I suspect it may
indicate some deformation of the capacitor elements or some other problem. I suspect the the high C reading is not due to a real capacity increase, but rather to the fact that the meter shows a C higher than real when the measured capacitor is very lossy, i.e. it has a fairly low resistance in parallel. 73 Tony I0JX |
Paper capacitor and Hallicrafters S-40A notes (resistors too?)
"Richard Knoppow" wrote in message I am in the process of restoring a Hallicrafters S-40A and have posted a bit about this already. First of all, I replaced all the paper caps. This is generally recommended when rebuilding old electronic equipment. Most of these were the original wax filled types. I measured these for capacitance and dissipation factor. D is the inverse of Q and is a way of measuring ESR but has a constant value. The result was: One cap a dead short. One cap open The rest had C much higher than the marked value and rather high D, anywhere from about 0.2 to about 0.5, I think one was even higher. There were a couple of molded plastic caps, one a blue colored Cornell-Dubilier the other an Astron. These were probably replacements. Both were close to the marked C value with D of around 0.1, probably still good. The paper caps were made by Industrial Condenser Co, of Chicago and Sprague. For comparison, new film caps measure right on the nose for C and have D too low to measure on this bridge. The measurements were made on a General Radio Type 650A, an oldie but goodie, which has been calibrated with precision resistors and caps. The capacitance measurements are made at 1.0Khz. If I get curious enough I may check the RF impedance of a couple of these caps on a Q meter but I suspect they are not very good. While the high C value may not seem to be a problem I suspect it may indicate some deformation of the capacitor elements or some other problem. These were not remotely precision caps when made but the values are so far off the marked values that I suspect a change in the internals rather than manufacturing tollerance. So, the advice to shotgun paper caps seems to be right on. Another note, probably should be a separate post. I mentioned before that the bandspread cap went the opposite direction of any other I've seen in a Hallicrafters RX. I got a couple of responses that others also had similar S-40's. I found a couple of other oddities in my receiver (for instance someone had connected the output of the BFO directly to the detecto rather than by means of the wire "gimmic" cap, it won't work this way). I am now pretty much convinced that the band spread cap was modified to make it "set" at 100 rather than the usual zero used in H receivers. I was able to make it go in the right direction by crossing the dial drive cable but I don't like makeshifts. Also, despite the nearly symmetrical construction of the band spread cap I suspect the minimum capacitance is slightly different when its running in the correct direction. That would affect the "normal" dial calibration. The RX calibrates OK but I am still going to have a go at removing the stop peg. I think I will have to drill it out but will try something less "invasive" first. BTW, I used to consider Hallicrafters stuff junk but after rebuilding this set and an S-20R have changed my mind. Actually both are well made and well designed. Hallicrafters definitely built their equipment to fit certain price ranges, mostly economy. I think they did quite well in delivering good performance value at the prices they aimed at. One proof is that the stuff is still working after 60 or 70 or more years. The above really applies to the components too. While paper caps have a bad reputation keep in mind that most of the ones we deal with are very old and have worked well for a long time. Plastic film was not available when these guys were made and other types of caps, mostly tin-can oil-filled types, were very expensive in comparison to the common wax filled types. I am not sure they lasted any longer. One exception is the oil-filled molded plastic Sprague Black Beauty cap. These were made and sold as extra-high quality, long life caps but Sangamo evidenty had manufacturing problems, one of which was the way the oil filling tube was sealed. These began failing within a couple of years of manufacture where the "less reliable" wax filled paper caps went along for many decades. Anyone ever bother to check resistor values? During overhaul and repair of lots of tube era marine electronics, many composition (and films sometimes) resistors had changed value considerably. Especially those used in voltage dropping circuits. High value resistors ( half megohm or more) seemed to be pretty wild too. Of course checking frequently meant disconnecting from associated circuitry to check is a real pain in the you know what, but the end result, (including the condenser replacements) resulted in amazing results! Anybody find similar resistor drift? Old Chief Lynn |
Paper capacitor and Hallicrafters S-40A notes (resistors too?)
"Lynn" wrote in message ... "Richard Knoppow" wrote in message I am in the process of restoring a Hallicrafters S-40A and have posted a bit about this already. First of all, I replaced all the paper caps. This is generally recommended when rebuilding old electronic equipment. Most of these were the original wax filled types. I measured these for capacitance and dissipation factor. D is the inverse of Q and is a way of measuring ESR but has a constant value. The result was: One cap a dead short. One cap open The rest had C much higher than the marked value and rather high D, anywhere from about 0.2 to about 0.5, I think one was even higher. There were a couple of molded plastic caps, one a blue colored Cornell-Dubilier the other an Astron. These were probably replacements. Both were close to the marked C value with D of around 0.1, probably still good. The paper caps were made by Industrial Condenser Co, of Chicago and Sprague. For comparison, new film caps measure right on the nose for C and have D too low to measure on this bridge. The measurements were made on a General Radio Type 650A, an oldie but goodie, which has been calibrated with precision resistors and caps. The capacitance measurements are made at 1.0Khz. If I get curious enough I may check the RF impedance of a couple of these caps on a Q meter but I suspect they are not very good. While the high C value may not seem to be a problem I suspect it may indicate some deformation of the capacitor elements or some other problem. These were not remotely precision caps when made but the values are so far off the marked values that I suspect a change in the internals rather than manufacturing tollerance. So, the advice to shotgun paper caps seems to be right on. Another note, probably should be a separate post. I mentioned before that the bandspread cap went the opposite direction of any other I've seen in a Hallicrafters RX. I got a couple of responses that others also had similar S-40's. I found a couple of other oddities in my receiver (for instance someone had connected the output of the BFO directly to the detecto rather than by means of the wire "gimmic" cap, it won't work this way). I am now pretty much convinced that the band spread cap was modified to make it "set" at 100 rather than the usual zero used in H receivers. I was able to make it go in the right direction by crossing the dial drive cable but I don't like makeshifts. Also, despite the nearly symmetrical construction of the band spread cap I suspect the minimum capacitance is slightly different when its running in the correct direction. That would affect the "normal" dial calibration. The RX calibrates OK but I am still going to have a go at removing the stop peg. I think I will have to drill it out but will try something less "invasive" first. BTW, I used to consider Hallicrafters stuff junk but after rebuilding this set and an S-20R have changed my mind. Actually both are well made and well designed. Hallicrafters definitely built their equipment to fit certain price ranges, mostly economy. I think they did quite well in delivering good performance value at the prices they aimed at. One proof is that the stuff is still working after 60 or 70 or more years. The above really applies to the components too. While paper caps have a bad reputation keep in mind that most of the ones we deal with are very old and have worked well for a long time. Plastic film was not available when these guys were made and other types of caps, mostly tin-can oil-filled types, were very expensive in comparison to the common wax filled types. I am not sure they lasted any longer. One exception is the oil-filled molded plastic Sprague Black Beauty cap. These were made and sold as extra-high quality, long life caps but Sangamo evidenty had manufacturing problems, one of which was the way the oil filling tube was sealed. These began failing within a couple of years of manufacture where the "less reliable" wax filled paper caps went along for many decades. Anyone ever bother to check resistor values? During overhaul and repair of lots of tube era marine electronics, many composition (and films sometimes) resistors had changed value considerably. Especially those used in voltage dropping circuits. High value resistors ( half megohm or more) seemed to be pretty wild too. Of course checking frequently meant disconnecting from associated circuitry to check is a real pain in the you know what, but the end result, (including the condenser replacements) resulted in amazing results! Anybody find similar resistor drift? Old Chief Lynn I do check resistor values. Its typical for carbon composition resistors (aka mud resistors) to go up in value when heated and with age. For the most part these were not precision resistors although some were sold as 5% resistors. Most of those found in equipment built before about 1946 were +/- 20%. After that 10% tollerance was more usual. They can be quite far off but seldom fail catastrophically. Also, composition resistors are noisy and were always noisy even when new. IMO Ohmite were the best of a bad lot. Older carbon film resistors often had staked end caps. The caps can develop poor contact or even corrosion at which point the resistor goes open. Many early metal film resistors used a similar method of attaching leads. If done right end capped units can be quite reliable but many were not. Another method of attaching leads was used primarily on precision resistors and high reliability parts. This was the use of conductive epoxy resin to glue the ends of the leads to holes in the ceramic core. When made correctly the break stength of the connection is greater than the wire lead. Metal film resistors can be made to have a very low or even zero temperature co-efficient and, in general, are the quietest of resistors. Both deposited carbon and metal resistors can go open if sufficiently overloaded. Military parts are tested for a seven times overload in manufacture. The bad ones sound like Chinese fireworks going off. -- -- Richard Knoppow Los Angeles WB6KBL |
Paper capacitor and Hallicrafters S-40A notes
"Antonio Vernucci" wrote in message .. . While the high C value may not seem to be a problem I suspect it may indicate some deformation of the capacitor elements or some other problem. I suspect the the high C reading is not due to a real capacity increase, but rather to the fact that the meter shows a C higher than real when the measured capacitor is very lossy, i.e. it has a fairly low resistance in parallel. 73 Tony I0JX That might happen using a capacitance meter than does not measure the series and parallel resistance. The GR bridge does, which is not to say that you might not be right. It will measure capacitors that my Tektronix multi-meter will not measure, or, rather, will not give a stable indication on. Whatever is the case, these are bad guys indeed. My main reason for mentioning this was to confirm the wisdom replacing paper caps where they are encountered. -- -- Richard Knoppow Los Angeles WB6KBL |
Paper capacitor and Hallicrafters S-40A notes (resistors too?)
On Mar 13, 12:33*am, "Lynn" wrote:
* * *Anyone ever bother to check resistor values? During overhaul and repair of lots of tube era marine electronics, many composition (and films sometimes) resistors had changed value considerably. Especially those used in voltage dropping circuits. High value resistors ( half megohm or more) seemed to be pretty wild too. * * *Of course checking frequently meant disconnecting from associated circuitry to check is a real pain in the you know what, but the end result, (including the condenser replacements) resulted in amazing results! * * *Anybody find similar resistor drift? I do. My general rule is to replace or bridge resistors if they deviate 15 percent or more from their marked value. However, if their value isn't all that critical (e.g. a large value resistor in series with a screen grid), I'll leave it alone. I prefer to replace rather than bridge; however, if I don't have a suitable replacement I'll bridge if the resistor has drifted up in value, which is often the case. Dave Drumheller, K3WQ |
Paper capacitor and Hallicrafters S-40A notes (resistors too?)
I have found, over the years, that carbon comp resistors go high. In some
restorations, especially on radios that you just want to function, I leave the resistors alone, where they have increased in value moderately. My reasoning is that since the line voltage is higher now, a reduction in voltage caused by the higher value resistors, will somewhat compensate for the increased voltage. This would especially apply to screen and cathode resistors. However, in critical work, it may pay to pay attention to the resistors. My Collins receiver uses a bridge circuit for the S-meter and the zero point kept changing. At first I thought it was a leaky capacitor. I checked, then finally replaced them. No improvement. I then carefully checked the tubes. No improvement. Finally, I replaced the carbon comp resistors with stable ones and the meter drift was cured. 73, Colin K7FM |
Paper capacitor and Hallicrafters S-40A notes (resistors too?)
Lynn wrote:
Anyone ever bother to check resistor values? During overhaul and repair of lots of tube era marine electronics, many composition (and films sometimes) resistors had changed value considerably. Especially those used in voltage dropping circuits. High value resistors ( half megohm or more) seemed to be pretty wild too. Of course checking frequently meant disconnecting from associated circuitry to check is a real pain in the you know what, but the end result, (including the condenser replacements) resulted in amazing results! Anybody find similar resistor drift? My general method is to check them all in-circuit with a DVM. The thing is, resistors tend to fail by rising in value, rather than falling. And the errors due to in-circuit testing all reduce the measured value (since there is stuff shunted around them). So, if the measured value is lower than it should be, I go on to the next resistor. But if the measured value is more than 10% or so higher than the value on the resistor, I'll replace it. Sometimes I miss resistors this way, since if a much lower value thing is across it, the test is useless. But it doesn't take more than a few minutes it and catches a lot of them. Things like plate resistors often are worries, because they are generally pretty high values, they tend to get hot, and they are in a part of the circuit where if they get noisy it'll be a problem. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
Paper capacitor and Hallicrafters S-40A notes
That might happen using a capacitance meter than does
not measure the series and parallel resistance. The GR bridge does, which is not to say that you might not be right. Dick, what I meant to say isd the following. |
Paper capacitor and Hallicrafters S-40A notes
That might happen using a capacitance meter than does
not measure the series and parallel resistance. The GR bridge does, which is not to say that you might not be right. Dick, what I meant to say is the following: At 1kHz, a 500 pF capacitor with a 500,000 ohm resistance in parallel is equivalent to the series of of 702 pF capacitor and 144,200 ohm resistior (using the well known parallel-to-seriel translation formulas). So, if your meter has the ability to separately measure the series resistance and capacitance, it should correctly indicate 702 pF, i.e. a value higher than that marked on the capacitor. 73 Tony I0JX |
Paper capacitor and Hallicrafters S-40A notes
Re my previous message, I have downloaded a GR bridge manual from BAMA. It
reports that: - the bridge measures the series capacitance - if D is low, the series capacitance almost coincides with the parallel capacitance - but if D is high, they differ significantly. A chart is provided to convert series capacitance into parallel capacitance. This confirms that, if D is high and if the loss is caused by a parallel resistance (as it actually is), one must convert the measured capacitance value using the chart. 73 Tony I0JX |
Paper capacitor and Hallicrafters S-40A notes
"Antonio Vernucci" wrote in message . .. Re my previous message, I have downloaded a GR bridge manual from BAMA. It reports that: - the bridge measures the series capacitance - if D is low, the series capacitance almost coincides with the parallel capacitance - but if D is high, they differ significantly. A chart is provided to convert series capacitance into parallel capacitance. This confirms that, if D is high and if the loss is caused by a parallel resistance (as it actually is), one must convert the measured capacitance value using the chart. 73 Tony I0JX I think you are looking at a manual for a later model bridge. My 650A manual has the formulas but not charts. I remeasured a bad cap and calculated the parallel capacitance, series resistance, and parallel resistance. This is a paper cap rated at 0.02 uf. The values I got a Cs = 4.8 uf D = 0.3 Cp = 4.3 uf Rs = 994 ohms Rp = 12 kohms Not a very good cap. New plastic film caps measure very close to the marked value and have a D which is below the residual of the bridge (essentially zero) While there is an error from the rather high D it is not significant in terms of this measurement, that is, the value of the cap measures nearly three times its marked value. I have not dissected one of these but suspect the winding is distorted. That would also affect the voltage rating. What I mean is that the plates of the capacitor are closer together than originally, probably because of loss of the wax impregnant. I found other caps in this RX which had high values so this one is not unique. I have not measured the caps at RF but I seems like an interesting project and a practical use for my Boonton Q-Meter:-) BTW, I think my math is OK but maybe not. -- -- Richard Knoppow Los Angeles WB6KBL |
Paper capacitor and Hallicrafters S-40A notes
I think you are looking at a manual for a later model bridge. My 650A
manual has the formulas but not charts. Yes. However, in the 650A manual there is a similar statement. At page 3, item 9, they say that the bridge measures the series capacitance, and they also give the formula for calculating the parallel capacitance (that is what we need, as the leaky capacitors have a resistence in parallel). I remeasured a bad cap and calculated the parallel capacitance, series resistance, and parallel resistance. This is a paper cap rated at 0.02 uf. The values I got a Cs = 4.8 uf D = 0.3 Cp = 4.3 uf Rs = 994 ohms Rp = 12 kohms Not a very good cap. Your Cp/Cs ratio corresponds to that calculated using the formula at page 3. However the other figures do not tie up with what my spreadsheet gives at 1kHz, that is: - for measured Cs= 4.8uF and D=0.3 (that is a reactance / resistance ratio = 3.33), then Rs should be about 10 ohm, rather than 994 ohm Moreover: - the series of 4.8uF and 994 ohm would corresponds to Cp= 5,335 pF and Rp= 995 ohm - the parallel of 4.3uF of 12 kohm would corresponds to Cs= 4.3uF and Rs= 0.1 ohm I get values close enough to yours only if I set a frequency close to 10 Hz, not 1 kHz (unless I did something wrong). Anyway, you may measure the parallel resistance of your capacitor with an ohmeter, and check that you really read a value as low a 12 kohm. New plastic film caps measure very close to the marked value and have a D which is below the residual of the bridge (essentially zero) While there is an error from the rather high D it is not significant in terms of this measurement, that is, the value of the cap measures nearly three times its marked value. why just three times? I would say that the ratio between 4.3uF and 0.02uF is more than 200 I have not dissected one of these but suspect the winding is distorted. That would also affect the voltage rating. What I mean is that the plates of the capacitor are closer together than originally, probably because of loss of the wax impregnant. I found other caps in this RX which had high values so this one is not unique. I have not measured the caps at RF but I seems like an interesting project and a practical use for my Boonton Q-Meter:-) BTW, I think my math is OK but maybe not. 73 Tony I0JX - Rome, Italy |
Paper capacitor and Hallicrafters S-40A notes
"Antonio Vernucci" wrote in message . .. I think you are looking at a manual for a later model bridge. My 650A manual has the formulas but not charts. Yes. However, in the 650A manual there is a similar statement. At page 3, item 9, they say that the bridge measures the series capacitance, and they also give the formula for calculating the parallel capacitance (that is what we need, as the leaky capacitors have a resistence in parallel). I remeasured a bad cap and calculated the parallel capacitance, series resistance, and parallel resistance. This is a paper cap rated at 0.02 uf. The values I got a Cs = 4.8 uf D = 0.3 Cp = 4.3 uf Rs = 994 ohms Rp = 12 kohms Not a very good cap. Your Cp/Cs ratio corresponds to that calculated using the formula at page 3. However the other figures do not tie up with what my spreadsheet gives at 1kHz, that is: - for measured Cs= 4.8uF and D=0.3 (that is a reactance / resistance ratio = 3.33), then Rs should be about 10 ohm, rather than 994 ohm Moreover: - the series of 4.8uF and 994 ohm would corresponds to Cp= 5,335 pF and Rp= 995 ohm - the parallel of 4.3uF of 12 kohm would corresponds to Cs= 4.3uF and Rs= 0.1 ohm I get values close enough to yours only if I set a frequency close to 10 Hz, not 1 kHz (unless I did something wrong). Anyway, you may measure the parallel resistance of your capacitor with an ohmeter, and check that you really read a value as low a 12 kohm. New plastic film caps measure very close to the marked value and have a D which is below the residual of the bridge (essentially zero) While there is an error from the rather high D it is not significant in terms of this measurement, that is, the value of the cap measures nearly three times its marked value. why just three times? I would say that the ratio between 4.3uF and 0.02uF is more than 200 I have not dissected one of these but suspect the winding is distorted. That would also affect the voltage rating. What I mean is that the plates of the capacitor are closer together than originally, probably because of loss of the wax impregnant. I found other caps in this RX which had high values so this one is not unique. I have not measured the caps at RF but I seems like an interesting project and a practical use for my Boonton Q-Meter:-) BTW, I think my math is OK but maybe not. 73 Tony I0JX - Rome, Italy I will recalculate, I may have misplaced a decimal point. -- Richard Knoppow Los Angeles WB6KBL |
Paper capacitor and Hallicrafters S-40A notes
"Antonio Vernucci" wrote in message . .. I think you are looking at a manual for a later model bridge. My 650A manual has the formulas but not charts. Yes. However, in the 650A manual there is a similar statement. At page 3, item 9, they say that the bridge measures the series capacitance, and they also give the formula for calculating the parallel capacitance (that is what we need, as the leaky capacitors have a resistence in parallel). I remeasured a bad cap and calculated the parallel capacitance, series resistance, and parallel resistance. This is a paper cap rated at 0.02 uf. The values I got a Cs = 4.8 uf D = 0.3 Cp = 4.3 uf Rs = 994 ohms Rp = 12 kohms Not a very good cap. Your Cp/Cs ratio corresponds to that calculated using the formula at page 3. However the other figures do not tie up with what my spreadsheet gives at 1kHz, that is: - for measured Cs= 4.8uF and D=0.3 (that is a reactance / resistance ratio = 3.33), then Rs should be about 10 ohm, rather than 994 ohm Moreover: - the series of 4.8uF and 994 ohm would corresponds to Cp= 5,335 pF and Rp= 995 ohm - the parallel of 4.3uF of 12 kohm would corresponds to Cs= 4.3uF and Rs= 0.1 ohm I get values close enough to yours only if I set a frequency close to 10 Hz, not 1 kHz (unless I did something wrong). Anyway, you may measure the parallel resistance of your capacitor with an ohmeter, and check that you really read a value as low a 12 kohm. New plastic film caps measure very close to the marked value and have a D which is below the residual of the bridge (essentially zero) While there is an error from the rather high D it is not significant in terms of this measurement, that is, the value of the cap measures nearly three times its marked value. why just three times? I would say that the ratio between 4.3uF and 0.02uF is more than 200 I have not dissected one of these but suspect the winding is distorted. That would also affect the voltage rating. What I mean is that the plates of the capacitor are closer together than originally, probably because of loss of the wax impregnant. I found other caps in this RX which had high values so this one is not unique. I have not measured the caps at RF but I seems like an interesting project and a practical use for my Boonton Q-Meter:-) BTW, I think my math is OK but maybe not. 73 Tony I0JX - Rome, Italy 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: C parallel = 0.044 uf R series = 99.5 ohms R parallel = 1205 ohms -- Richard Knoppow Los Angeles WB6KBL |
Paper capacitor and Hallicrafters S-40A notes
"Richard Knoppow" wrote in message m... "Antonio Vernucci" wrote in message . .. I think you are looking at a manual for a later model bridge. My 650A manual has the formulas but not charts. Yes. However, in the 650A manual there is a similar statement. At page 3, item 9, they say that the bridge measures the series capacitance, and they also give the formula for calculating the parallel capacitance (that is what we need, as the leaky capacitors have a resistence in parallel). I remeasured a bad cap and calculated the parallel capacitance, series resistance, and parallel resistance. This is a paper cap rated at 0.02 uf. The values I got a Cs = 4.8 uf D = 0.3 Cp = 4.3 uf Rs = 994 ohms Rp = 12 kohms Not a very good cap. Your Cp/Cs ratio corresponds to that calculated using the formula at page 3. However the other figures do not tie up with what my spreadsheet gives at 1kHz, that is: - for measured Cs= 4.8uF and D=0.3 (that is a reactance / resistance ratio = 3.33), then Rs should be about 10 ohm, rather than 994 ohm Moreover: - the series of 4.8uF and 994 ohm would corresponds to Cp= 5,335 pF and Rp= 995 ohm - the parallel of 4.3uF of 12 kohm would corresponds to Cs= 4.3uF and Rs= 0.1 ohm I get values close enough to yours only if I set a frequency close to 10 Hz, not 1 kHz (unless I did something wrong). Anyway, you may measure the parallel resistance of your capacitor with an ohmeter, and check that you really read a value as low a 12 kohm. New plastic film caps measure very close to the marked value and have a D which is below the residual of the bridge (essentially zero) While there is an error from the rather high D it is not significant in terms of this measurement, that is, the value of the cap measures nearly three times its marked value. why just three times? I would say that the ratio between 4.3uF and 0.02uF is more than 200 I have not dissected one of these but suspect the winding is distorted. That would also affect the voltage rating. What I mean is that the plates of the capacitor are closer together than originally, probably because of loss of the wax impregnant. I found other caps in this RX which had high values so this one is not unique. I have not measured the caps at RF but I seems like an interesting project and a practical use for my Boonton Q-Meter:-) BTW, I think my math is OK but maybe not. 73 Tony I0JX - Rome, Italy 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: C parallel = 0.044 uf R series = 99.5 ohms R parallel = 1205 ohms -- Richard Knoppow Los Angeles WB6KBL 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. Formulas a Cp = Cs / 1+D^2 Rs = D/wC where w = 2*pi*f Rp = (1+D^2)/D^2)*Rs All measurements and calculations for f = 1000 hz -- Richard Knoppow Los Angeles WB6KBL |
Paper capacitor and Hallicrafters S-40A notes
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 |
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 |
Paper capacitor and Hallicrafters S-40A notes
Richard Knoppow wrote:
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). I suspect they'll be hanging in there. Some, actually many, of the film caps from the 1950s are still reliable and I can only guess that modern ones will be better still...with the exceptions of some that may turn out to have come from crummy manufacturers. |
Paper capacitor and Hallicrafters S-40A notes
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). Well, in that case I believe that the calculations we did cannot be strictly valid. Anyway, it is an interesting issue. By the way I also have an HT-40 and found no need for changing capacitors, as the leaky capacitors are placed where they make no harm (e.g. screen grid bypass). 73 Tony I0JX Rome, Italy |
Paper capacitor and Hallicrafters S-40A notes
Antonio Vernucci wrote:
By the way I also have an HT-40 and found no need for changing capacitors, as the leaky capacitors are placed where they make no harm (e.g. screen grid bypass). 73 Tony I0JX Rome, Italy I always consider 'leaky' to mean that they are somewhere along the road to 'short' and just haven't arrived yet. Leakage, particularly on HT circuits, will ultimately lead to heating and then is just a matter of time. I figure we all have a certain mental tolerance of how much we can live with in a certain part of the circuit but the other factor is 'time'. Same applies to drifted carbon resistors. -Bill |
Paper capacitor and Hallicrafters S-40A notes
I always consider 'leaky' to mean that they are somewhere along the road to
'short' and just haven't arrived yet. Leakage, particularly on HT circuits, will ultimately lead to heating and then is just a matter of time. You are certainly right in saying that changing the leaky capacitors would be the correct way to go from the technical viewpoint. However I adopt a different phylosophy. I use the HT-40 (or other boatanchors radios) for maybe a few hours per year in total, just for fun. For normal operations, I use recent equipment. So, the risk of failure caused by the HT-40 leaky capacitors is moderate. On the other hand, if I would replace the leaky capacitors, I would have the certainty (and not just the risk) of spoiling the originality of the radio. So, in the end I prefer to leave the radio as it is, as long as it works acceptably. 73 Tony I0JX |
Paper capacitor and Hallicrafters S-40A notes
So, the risk of failure caused by the HT-40 leaky capacitors is moderate.
On the other hand, if I would replace the leaky capacitors, I would have the certainty (and not just the risk) of spoiling the originality of the radio. I am of the opinion the originality of the radio has been compromised when the caps (and other parts) go bad and do not perform up to original engineering intent. Like a Fender Stratocaster or Les Paul with an old leaky bubble bee cap that sounds nice, mellow, fuzzy...(insert description). It did not sound that way from the factory either. This old bubble bee is a whole new sound. Pleasing and desirable, perhaps, but not original. And there is the engineering part of me that says if it is not operating with in spec it is broke. But this is a whole other philosophical discussion. Just my opinion, Paul P. |
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