Using capacitors that have drifted...

[cerrem]

Capacitance is not always a finite number..... Sometimes capacitance is dependent on applied voltage gradient....non-linear situation...
So you have the applied DC voltage and the applied AC signal.....both have influence ....
In your case, it may be mis-labeled...

[tubedood]

Hi Juan, I only used a multimeter (Its a fluke so its a reasonably decent tool but I am not sure of how accurate the capacitance measuring of it is) when I tested those Black Beauties... the other caps I had stashed years ago (Orange Drops and CD Brown Drops) seemed to have held their values very well. The value read on all of the caps was out of circuit and as noted it is roughly twice the printed value... I can try fresh batteries and retest them. Work recalibrates the meters at intervals so it was probably checked within the past year. I don't have access to a true capacitance meter so maybe someone can check one of these caps for me. It would be a shame if all of them are pf no use.

[g1]

Agree with those above who stated that some cap. meters will mis-interpret leakage as extra capacitance.
My multi-meter has a simple cap range on it. A leaky cap will show as a higher (beyond tolerance) value capacitance. Yet if I measure the same cap on my more specialized LCR meter, it will show proper capacitance, but improper ESR or some improper parallel resistance.
Either way I know the cap is bad, but the cap range on the multi-meter could be misinterpreted as ok when it is not.

[Enzo]

I am unsure here. These shifted value caps, did you accurately measure them when they were new and now again so they show a change? Or are you only measuring them now, and they seem to be running high and are ASSUMING they have drifted high?

[tubedood]

Hi Enzo

Unfortunately I didn't check the values originally-I picked them up and into a drawer they went. They were already very old even back when I got them. I should have checked the values, had I noticed they were that far off from the printed value I probably would have left them at the store.

[Chuck H]

Admittedly, "black beauties" don't have a great rep for being on spec anyway.?. More sophisticated bench gear (like what g1 indicated) might be necessary to determine specifics. I don't know that they were even "good" caps regardless of their vintage mojo factor That is... If you didn't measure them going into the drawer then we can't know if they weren't always off value spec. There is absolutely nothing about sitting in a drawer that would cause film caps to drift. Electrolytics have a viscous electrolyte that dries out and crystallizes with age whether in use or not (actually worse "or not"). But film caps are made of stuff that will withstand dozens of human lifetimes if no electrical stresses are applied to them. I think it's probable those caps are fine, but marked incorrectly for value because they came off a run intended for the spec value. I would want to test them at near spec voltage for performance before I would even trust them at their measured value.

JM2C

[tubedood]

Cool, thanks for everyone's input it really is appreciated. If anyone can test one I will gladly donate it to be looked into to see if the rest might be useable to someone... be it in an amp or a tone control in an instrument.

Any one with the proper equipment willing to test one to see what kind of shape these might be in? I can mail it.
Dale

[SoulFetish]

I'm no "cork sniffer" either regarding caps SoulFetish hahaa . An old friend was a navy ET that helped with radar development back in the 50s I believe! He had a an electronics shop and I bought a few of them way back in the day. What he had in the bins and drawers was what was available when he stocked it. Shame they didn't age so well!

I meant no offense by it. It is a shame they may not have worked out like you hoped. I've been there.
I've scoured the earth in search of obsolete parts, nos tubes, and components long out of production. Many times I've ended up with once great sounding tubes which had gone heartbreakingly microphonic; nos newmarket germanium transistors suffering from miserable leakage and were unusable; caps and resistors no where near their manufactured values. I was just hoping to save you the trouble of hanging on to spent parts in case you were chasing ghosts.
As a technician, I depend on performance and reliability in parts. But, I still love coming across someones stock of old parts and equipment. It certainly doesn't always end in heartbreak and often there is real value. I'll have no problem using these beauties when the application calls for them. These ones tested dead on for their rated capacitance.

[Helmholtz]

Never ever saw film caps, even cheesy paper in oil ones drifting in value.
Out of tolerance? Yes, any day of the week.
And they stubbornly hold their "wrong" value forever, because they were "made" different from day 1, not that they "became" different.

But here the difference is so gross that Iīm thinking poor measurement instead.

DC leak which is equivalent to a resistor in parallel with the cap *might* explain a larger than expected *displayed* value, since "capacitance meters" typically apply a squarewave to the cap in series with a resistor, rectify AC going through and display the DC value as Capacitance (since higher DC means higher AC through so higher capacitance) BUT if a cap shows gross leak, high enough to double value with a mere 1 V RMS applied to it by the meter then that capacitor qill explode even with mere 9 or 12V DC applied.

But I donīt think thatīs the case, more likely the meter is grossly miscalibrated, out of batteries or cap is measured in circuit where other elements can affect measurement.

I have measured a number (16) of bumblebees and black beauties from the 50s with two different LCR meters (both using sine wave bursts, never saw one using square wave signals) and cross-checked on a professional HP impedance analyser. The results were identical. Leakage or additional resistors wired in parallel did not change the C results.
All of the caps measured higher than rated by 20% to 50% @1kHz and up to +100% @100Hz. Typically the higher the deviation, the higher the leakage as indicated by the Q value and the equivalent shunt resistor Rp.

[SoulFetish]

Well, Fahey’s got me on that one. I certainly can’t prove that the capacitors from the 1950s I’ve tested haven’t always been so far out of spec.
But at the same time, film caps can and do drift in value for the simple fact that they have a temperature coefficient.

[Enzo]

Temperature coefficients play out over short term, as in if it is 20 degrees now and 40 degrees later, the capacitance will change some small amount. And if we go from 40 down to 20, it will change back the other direction. But I don't think any of our film caps will change from 0.022uf to 0.047uf within the temperature range of our shops.

[J M Fahey]

I have measured a number (16) of bumblebees and black beauties from the 50s with two different LCR meters (both using sine wave bursts, never saw one using square wave signals) and cross-checked on a professional HP impedance analyser. The results were identical. Leakage or additional resistors wired in parallel did not change the C results.
All of the caps measured higher than rated by 20% to 50% @1kHz and up to +100% @100Hz. Typically the higher the deviation, the higher the leakage as indicated by the Q value and the equivalent shunt resistor Rp.

You are talking LCR meters, I am talking Capacitance reading function in standard Digital Multimeters which is whatīs expected to be found on a regular Service Tech and which I assume is used unless told otherwise:





and they use the method I mentioned: apply some AC voltage, typically a squarewave because thatīs easy/cheap to generate (we are talking less than $100 multimeters, not capacitance dedicated units or even less Lab equipment), measure how much arrives at the other end, and *convert* that to some capacitance reading.
Probably with 5% or 10% error compared to 0.1% achievable with a Lab instrument but good enough for Servicing, what we are talking about here.

The system can be cheated by *high* losses but if itīs so lossy with just 1 or 2V RMS applied (you canīt get more from a 9V battery) then itīs absolutely unusable.

One advantage of this "cheesy" system: you can use it to measure *inductance* what is not common on a Tech bench.

I have a chart somewhere , published by Electronics World or something, which made you test an inductor in the Multimeter capacitance scale , and convert the displayed "uF" value into an equivalent (same impedance because thatīs what you are actually measuring) Hy value

VERY handy and I am surprised Multimeter manufacturers donīt include that function.
It costs nothing since they already have the functional hardware built in.

FWIW even my very first multimeter back in 1969, my beloved Central 200H, suggested in the user manual how to measure capacitance: hook one end to a 6 or 12VAC transformer winding, the other to the meter on a VAC scale, and convert indicated VAC into capacitance with a handy chart.
Couldnīt read pF or uF but was quite acceptable for the typical "midrange" , say .1uF to .005uF ones, abbundant in Tube equipment way back then. (TVs, radios and amplifiers).

[Chuck H]

But I don't think any of our film caps will change from 0.022uf to 0.047uf within the temperature range of our shops.

And if they did you'd never be able to tell anyone about!

[SoulFetish]

But I don't think any of our film caps will change from 0.022uf to 0.047uf within the temperature range of our shops.

But, (...now just hear me out.). What if they did??

[Helmholtz]

You are talking LCR meters, I am talking Capacitance reading function in standard Digital Multimeters which is whatīs expected to be found on a regular Service Tech and which I assume is used unless told otherwise:





and they use the method I mentioned: apply some AC voltage, typically a squarewave because thatīs easy/cheap to generate (we are talking less than $100 multimeters, not capacitance dedicated units or even less Lab equipment), measure how much arrives at the other end, and *convert* that to some capacitance reading.
Probably with 5% or 10% error compared to 0.1% achievable with a Lab instrument but good enough for Servicing, what we are talking about here.

The system can be cheated by *high* losses but if itīs so lossy with just 1 or 2V RMS applied (you canīt get more from a 9V battery) then itīs absolutely unusable.

One advantage of this "cheesy" system: you can use it to measure *inductance* what is not common on a Tech bench.

I have a chart somewhere , published by Electronics World or something, which made you test an inductor in the Multimeter capacitance scale , and convert the displayed "uF" value into an equivalent (same impedance because thatīs what you are actually measuring) Hy value

VERY handy and I am surprised Multimeter manufacturers donīt include that function.
It costs nothing since they already have the functional hardware built in.

FWIW even my very first multimeter back in 1969, my beloved Central 200H, suggested in the user manual how to measure capacitance: hook one end to a 6 or 12VAC transformer winding, the other to the meter on a VAC scale, and convert indicated VAC into capacitance with a handy chart.
Couldnīt read pF or uF but was quite acceptable for the typical "midrange" , say .1uF to .005uF ones, abbundant in Tube equipment way back then. (TVs, radios and amplifiers).

Understood, but my main point was that capacitors of the black beauty/bumblebee type can actually show a capacitance increase of up to 100% over time. AFAIK these are PIO types. Often you can find hairline cracks in the molded housings. I assume that some interior material shows hygroscopic properties (see post #7) and absorbs moisture. Of course cap and leakage values can be expected to change with humidity and temperature.
The low voltage DCR ("insulation resistance") measured around 50MOhm. Modern film caps are rated at 10GOhm (=10000MOhms) or more.

The tone caps in the '57 to '60 Les Pauls are most probably closer to .033ĩF than to .022ĩF by now. Actually some of the caps I measured were from old Gibson guitars.

The temperature coefficient of plastic film caps is generally very small (around 1% per 10°C for Mylar/polyester caps).