I am just started looking at a RCA Victor 28x and found it full of many different kinds of trashy capacitors, many of which are paper and difficult, if not impossible to read. There is one such capacitor which is almost readable and is attached to a tune tank circuit. Why would anyone put a paper capacitor in a tank circuit is beyond me. From the attached file, figure 1 shows the paper capacitor in general and figure two is via a magnify glass. The best I can read is 111-0.305. The schematic (see figure 3) states it is a 33 which I assume is 33uF as all of the others are in uF. According to the bill of materials, that capacitor should be a 33 mmfd or 34nf. I cut one end and measure the capacitance and found it to be 34 pf. I am sure it leaks like crazy and is not very accurate.
I need to establish the real value for this capacitor. Is it really uF, nF, or pF? I do not see why one would use anything but ceramic capacitors for that part.
Here is a link to the pictures referenced above:
https://1drv.ms/b/s!Ap6T6enyw9BPgat6Ppsi-G1S3C64ag
33µF would be too high for that part of the circuit, 33pf sounds right.
Edit: Also, 33µF was not a common value back then.
As to my experience, when there is no value identifier by a capacitor, or resistor in a schematic, it is usually the smallest usual multiplier – i.e. pF by capacitors, ohms by resistors. Your reading of 34 pF therefore does not seem very off to me. The leakage of that capacitor is a different topic however. From what I can see in the picture, I would say the cap has more of an oval shape rather then cylidrical, thus it might not necesarily be a paper cap, but it might be mica.
I would like to add, that a ceramic cap in an oscillator circuit would be the worst possible choice because of its temperature dependence.
Hope you find this answer useful and I am not talking nonsense. 🙂
There is no way that C4 is 33µF. It would be way too big and as KE4EST has mentioned earlier, 33µF (non-polarized) I doubt it existed in ordinary consumer electronics at that time.
I agree that the 33pF is likely to be the proper size/value for it. And if the bill of materials say mmfd well, the answer is the following: in the old days instead of clearly marking pF it was marked as –> mmf = MMF = Mmfd = MMFD = micro-microfarad or picofarad = 1 millionth of a microfarad or 1E-12 Farad. As a side note MCL has actually mentioned this in one of his past videos but for the life of me I cannot remember which one it was or, where (YouTube or Patreon)
mmfd = micro micro farad = pico farad. Pico farads were not used in earlier days, so micro micro was used as the description. So your 34 pF is accurate. This is assuming the bill of materials is correct.
I know this is not helpful but as I have degrees in mechanical engineering, chemistry and biology with an M.S.T. but am a new bee to electronics I have found that when it comes to schematics, charts, list of materials, graphs in many disciplines there is a serious lack of standardization when it comes to information. Often there is no Keys to tell anyone new what is being represented, lack of figures in key locations, multiple systems, jargon, etc. That makes it hard to figure out what is going on unless one has years of experience in that discipline like Mr. C. I would like to see schematics that are labeled completely showing ALL data with a Key for all the acronyms and all values clearly marked at all points so there is no question of what should be going on at any point on a schematic, chart, list, graph or what ever is being represented. But often what I find is poorly done and mostly because those who did the work are experts in their fields that they assume things or take for granted that others know what they are illustrating. Kind of like Albert Einstein was a great physicist but a lousy teacher. Hopefully soon that will be corrected with advancements in AI where computers and automation will fix that problem. I only say this because it is a problem in almost all fields of engineering and science.
@ovi4 Here is a quick cap question, Why are the filter caps in most receivers Huge in size? I see that when I look at replacements same value they are much smaller. I am working on a Sansui 3300 now and the filter cap is 3300uf 100V, 3″ in height, 2″ in dia. The replacements same value uf and volts are almost half that size. Is there something I should know? I watched all of Mr. C’s videos and don’t remember him addressing dimension size of filter caps when replacing them. I compared all specs on both sights Mr. C gave me. Digikey and Mouser.
@john-barron Regarding the physical size between old and new there is nothing to worry about. The reason for that is that in the old days, the technology and materials used (for the inner guts of a cap) were very different. With today’s tendency for miniaturization (hence the SMD technology) the major advantage is, that the smaller you go, the less quantity of materials needed = which costs less. The dielectric used in today’s caps is also generally thinner therefore needs smaller space again, all to do with the advancements in technology. In other words, today’s technology is all about cost-effectiveness and miniaturization with less “emphasis” on sturdiness and durability. If you know what I mean.
@ovi4 Yep – modern dielectrics like mylar (polyester), polypropylene, polystyrene, Teflon, are all much superior insulators/dielectrics to the oil-impregnated paper that was originally used, so can be made much thinner for the same voltage rating. The smaller size reduces the surface area for noise to couple into, so was one of the reasons why the foil-end bands disappeared.