Vintage caps – OK/BAD/useable and how to determine?

[lion]

Hoping for some advise.
I'm in the process of restoring an original '63 echo unit. I'm normally not a believer/convinced that certain component types/makes are responsible for any signatures sounds, but in this italian made piece it turns out that most of the coupling caps are ceramics (presumably Ingelen brand made in Austria), and for the sake of it I would like to keep as many as possible original.

The unit worked (with some issues and not perfect, but it worked) before I took it apart in pieces to refurbish everything from the ground up.

My questions is – can anyone suggest a test methode to verify the useable ceramic capacitors?

I say "useable" because I believe testings for 100% perfect caps in all aspects is quite complex and needs special equipment, but - even failing in some aspects a cap might work OK for some purposes (kind of like resistors drifted within acceptable limits, but maybe I'm missing something).

Would for instance hooking the caps up to a voltage (similar to the actual B+) and measurings for any DC leakage be any use  – and what would be acceptable values.

I've tried googling the subject but the consensus seems mostly to be to just to replace caps – and not worth the hassle trying to test them (and how), which make sense in general I believe, but in this case I'd be happy to go through the hassle to find what's useable (or not).

I hope this makes any sense, and any help or advise will be much appreciated.

lion

[kingswayguitar]

just a thought, but if you are as patient as you say, could you just use trial and error and judge the sound by ear with each cap. take a handful of new caps and test them along side the old ones, in situ?

i did something similar with a few caps on my bassman and it was surprisingly subtle. yes, the original blue caps were shot, but any replacement (even electros) was only marginally different than any other replacement. i ended up using film caps that were physically large.

i also test by ear with different types and sizes of caps in the input or output position of many of my pedals. one of my go-to jfet booster pedals has electros in both the input and output....i can almost see people's faces cringing 

[LucifersTrip]

I replace electros, but man, with ceramics, as long as they measure good within the % shown on the case, I just keep usin' em.

Though, I would like to know if a ceramic cap does actually go thru degrees of wear. Aren't they just a couple plates that just wind up shorting when you blow em out...

[Gus]

Sometime old electros still measure good. 
IMO You need to measure value and leakage at working voltage of electrolytic caps at minimum.
Be careful with what replacements you buy

[amptramp]

I added a jack to a VTVM that I had that gave about +70 volts to put through a suspect capacitor with the voltmeter probe on the other end.  Some faults have an activation voltage that shows no leakage until a certain threshold is exceeded and this was my way of checking unknown capacitors for leakage.  If you connect the cap from B+ and measure the voltage with a high-impedance voltmeter (either digital or VTVM) with the ground probe connected to ground, you should be able to see any actual or impending failures, which will show up as positive voltage.  Knowing the impedance of the meter and the applied voltage, you should be able to calculate the leakage current.  Ceramic caps should show very little leakage.

The ceramic used in caps is piezoelectric which means voltage out plus plate bending force = voltage in (with some conversion for units).  I use ceramic for high-frequency coupling and bypass where non-linearity doesn't matter and the inertia of the capacitor keeps the bending to a minimum.  At audio frequencies, there is an audible distortion caused by ceramic caps, so maybe you can improve on the "vintage" sound by using plastic film.

[Gus]

Not all ceramics caps have distortion. 
You need to specify what type ceramic cap you are posting about it.

[lion]

Thank for the replies, much appreciated.

Just to clarify – all electrolytics in the unit will be replaced, the caps in question are all non polars.
Here's what they look like.


Most of the caps in the circuit are like the two tubelar ones on the left and center and their values are 2n2, 4n7 and 10n. A few other caps (in the pF range) are like the one on the right, which I first thought to be a tantalum – but presumably are ceramic as well.
I'm basing my assumption they are ceramics according to some info I found on a vintage radio site. I guess I could sacrify one and cut it open to see wha´t's inside – if needed.

At audio frequencies, there is an audible distortion caused by ceramic caps........

Which could be good (or not so good), but the reason I want to keep them in the circuit (the tubelar ones) in case they are adding to the/any "personality" these units are said to have.

Re trial and error, comparing with other caps and testing by ear.
I'm all in for that, but as I'm about to start wiring the circuit up from scratch (chassis, valve sockets and tagstrips all stripped down for cleaning and rust removal) I'll have to get the unit up and running – and preferably with as many as the originals components as possible – before any tweaking/comparing/tests can be conducted.

If you connect the cap from B+ and measure the voltage with a high-impedance voltmeter (either digital or VTVM) with the ground probe connected to ground, you should be able to see any actual or impending failures, which will show up as positive voltage. Knowing the impedance of the meter and the applied voltage, you should be able to calculate the leakage current.  Ceramic caps should show very little leakage.

I only have a cheapo DMM with an 10M input imp to measure both values and leakage, probably not optimal - but I'll try to run some measuring tests asap.

lion

[amptramp]

Juat a note - if you are using a DMM to check leakage in a cap strung from the B+ to the voltmeter probe with the ground lead grounded, large value capacitors will show a momentary reading above zero as they charge up.  It is the final value of voltage that is actual leakage.  If you use a meter with an analog meter display, you can get a quick positive spike that settles to the leakage value.  Charging current is acceptable, continuous current has to be below the limits you set for the circuit or cap technology you are using.  For example, if you are using a cap to couple circuits that have a relatively low impedance, some leakage is acceptable from the circuit behaviour standpoint.  But if you get a lot of leakage in a capacitor that should have very little, the capacitor is deteriorating and even if the circuit can tolerate the existing value of leakage, it is going to get worse and the cap is on its way to failure.

[lion]

Ron - thanks a lot for the advise.

lion

[lion]

UPDATE

Finally got round to testing the caps last week. Strung the caps to a 370V B+ and measured free end to ground with my 10Mohm DMM.
The ceramics read between 30-300mV, except a couple which read close to or above 1V.
In contrast more of the film/foil caps from the circuit read volts of leakage.

Decide to keep the ceramics which read <300mV, and from a quick test everything seem to work fine so far.

Thanks for the helpful replies.

lion