~ Testing for Capacitor Leakage in and out of circuit ~

A very common problem or if not yet a problem 'a factor' that affects the performance of old valve amplifiers is coupling capacitor leakage ~ It is often responsible for expensive valve or output transformer failure and gives many untouched old valve amplifiers a 'weather dependant' sound up to the point of failure

Capacitor Leakage can be expressed as a measure of the insulation resistance or the d.c. current a capacitor passes but either way it is only meaningful if the measurement is made at the rated voltage or at least at the operating voltage and temperature when used ~ Coupling capacitors should be rated at least to the HT supply voltage

Correspondence over the years suggests that some people are confused with coupling and decoupling using capacitors and sometimes inductors ~ Transformer coupling appears self explanatory ~ The section of QUAD 22 schematic below will be used to explain coupling and decoupling capacitor leakage in all similar valve circuits

In the examples below it is assumed that the QUAD 22 in question is powered with an incoming HT supply of 330V either from a bench supply or by a QUAD II power amplifier connected to the yellow channel ~ When using a QUAD II it may be safer to remove its EF86s to prevent probing around affecting the power amp

When testing other valve amplifiers especially integrated or power amplifiers use a suitable resistive test load and short the input and be prepared to switch off quickly ~ If you can get one a Variac will be useful because the voltages with the valves removed can be obtained steadily before damage from faulty components occurs

With integrated or power amplifiers it will be safer to remove only 1 or 2 valves at a time to prevent excessive high HT voltages at the early stages ~ Removing output valves will have the greatest effect on unregulated HT supplies but may be required if you intend to check for capacitor leakage in circuit

Schematic of QUAD22 pre-amplifier

The d.c. voltages shown in yellow above are normal with good valves and capacitors
Click here for chassis view of QUAD 22 ~ Click here or image to restore schematic

C3 is a coupling capacitor which prevents 60V appearing at the tape output socket T.O. and the grid of V1 (more information here) Coupling capacitor C13 prevents the 100V at V3A anode affecting the bias of V3B while coupling the a.c. signal between the 2 stages ~ C16 prevents 200V (actually less due to R41 and R42) reaching the amplifier output

In most valve amplifiers the valve grids sit at 0V 'ground or chassis potential' but there are exceptions one of which is the QUAD II KT66 grids which sit at about 200mV and my amplifier/buffer ~ In the circuit above the valve grids are held at 0V by R12 RV1A and R26 ~ If say C13 had an insulation resistance of 1.5MΩ then what would the voltage at the grid of V3B pin 7 be ?

Not 50V but far too high for V3Bs grid ~ Allowing for the 225V supply having a high impedance and the high 470kΩ anode load of V3A (R23) the voltage at V3B pin 7 would try to be about 43V but the grid is now forward biased and draws 10s of mA preventing V3B operating as an amplifier let alone a linear one

Even if C13s leakage resistance was a high 15MΩ the circuit would not perform correctly ~ Note C13 in later builds of the QUAD22 is a 350V rated polystyrene with insulation resistance >2000MΩ ~ its low value of 820pF gives a 20Hz high pass filter overall and you should find no need to change this capacitor or R23

C34 is a later addition input coupling capacitor to prevent d.c. reaching V3A grid ~ In the past good valve equipment ensured the high HT voltage never reached the output but transistor equipment had electrolytic output coupling capacitors and although the internal supplies were low they could via leakage pass d.c. to the high impedance input of a following valve amplifier

Coupling capacitor C16 positioned where it is would suggest that the capacitors C18 to C30 in the tone stage could be rated at only a few volts as there should be no d.c. across any of them ~ Actually as shown with the tone switch in operation there will be 160V across C18 and C19 although you may not be able to measure it

C18 and C19 are in parallel and have a combined value of 11.2nF which is in series with the 47nF of C16 ~ the total capacitance subject to the normal 200V at V3B anode is about 9nF ~ Using the formula to determine the d.c. voltage across capacitors in series we get 9nF/11.2nF x 200V = 160V which is why I use 160V polystyrene capacitors

Capacitor C8 is a de-coupling capacitor which 'couples' any a.c. including HT ripple from the screen grid of V1 to chassis ground 0V ~ C5 across the cathode bias resistor for V1 (R14) de-couples a.c. cathode current to ground which would otherwise produce an a.c. voltage across R14 that would reduce V1s gain due to 'local negative feedback'

C7 C12 and C15 de-couple any a.c. that may appear on the HT lines which includes circulating signal currents and ripple from the rectified supply ~ good de-coupling is more important as stages become more sensitive ~ Ideally decoupling capacitors should return a.c. back to 0V ground close to the cathode of the stage being 'de-coupled'

C10 couples the output of the tone control section back to V3A as negative feedback forming an active Baxandall tone control or flat a gain amplifier depending on the position of the filter switch S3 ~ It also partially de–couples the cathode resistor of V3A (R21) to ground via half of the balance control (not shown) to vary the left/right gain balance

~ Determining Capacitor Leakage in circuit ~

If a valve is removed or its heaters fail the anode coupling capacitor will be subjected to the full HT voltage and so coupling capacitors should ideally be rated for the highest likely HT voltages ~ In the circuit above with all the valves removed or before they heat up the anode coupling capacitors could have at least 330V across them

It is ~ or should be ~ safe to remove some or all valves in a well designed amplifier ~ Push Pull valve amplifiers for use by the BBC were specified to work and be stable with one output valve failed or removed ~ With an old original condition valve amp you may find that removing valves causes the coupling capacitors to fail so be careful

In the schematic above we know the 350V rated polystyrene C13 should be okay with V3 removed but what if it was not ? ~ The 225V HT would be nearer to 260V if C7 C12 and C15 were not leaky ~ which they most likely will be unless they have been recently changed for modern parts and the rest of the circuit components are good

The voltage across R26 can be used to determine C13s leakage current ~ Without V3 fitted say we measure 2V across R26 this can only be due to Leakage in C13 or across the valve bass ~ Assuming the valve base is good and our meter has 10MΩ input resistance we measure the anode voltage at V3 pin 1 as say 250V to get the voltage across C13 ?

Although our meter has a high 10MΩ input it does not measure the voltage across R26 or at V3 pin 1 accurately due to the high resistances in this part of the circuit but we do know that 1.5MΩ in parallel with the meters 10MΩ is 1.3MΩ and that this resistance has 2V across it so the leakage current is 2V/1.3MΩ ≈ 1.54µA

The leakage current also flows through R23 and produces a volt drop of 470kΩ x 1.54µA ≈ 0.723V which can almost be ignored ~ If we use the 10MΩ meter to measure the stage HT at the junction of R23 and R25 there will be a very slight loading and this voltage minus about 3V will be the voltage across C13 which can be used to determine its insulation resistance

The insulation resistance of the faulty C13 will be about 163MΩ at about 250V and if it were removed from circuit and checked with an insulation tester like the Robin KMP3050 pictured it should still measure 163MΩ unless heating the leads has changed the 'fault condition' or the capacitor is a paper in oil type and it is a misty morning in March

Any voltage across R26 is not desirable ~ Coupling capacitors should not 'couple' d.c. at all but should accurately couple a.c. between stages in a valve amplifier ~ How accurately is another subject ~ If we used a multimeter to measure the resistance directly it would not give a true reading as the measurement needs to be made close to the working voltage to be valid

Using the above technique with V3 in place we could have determined the leakage current of C13 at its operating point ≈200V but it is likely that with the grid of V3B at any positive voltage greater than about 1V or if V3B were faulty we would not get a correct result ~ removing V3 so that C13 is 'isolated' between R23 and R26 is a better test method

With V3 removed and S3 in the 'cancel' position (see full Schematic) C16 ~ which if original will be a Hunts AM108 350V rated Metallised Paper capacitor ~ is likely to be leaky and give some voltage at the output across R42 ~ It is also possible for C10 or C11 leakage current to give a voltage at the output so we need to test here with care

I stated above that C18 and C19 will have about 160V across them with S3 as shown and this is true but when we place even a 10MΩ voltmeter between the junction C16 C18 and C19 and chassis 0V we measure 0V (or a low voltage if C16 is leaky) because there is no d.c. path for the measurement current of the meter

If we try to measure a voltage across C18 or C19 the measurement problem is the same and if they are the original old leaky parts it will be difficult to determine the condition of C16 alone ~ The way to measure the leakage current and calculate the insulation resistance of C16 would be to remove one end from the junction of C18 and C19

With C16 isolated from the junction of C18 and C19 its floating end voltage can be measured to to chassis 0V ~ The voltmeter reading divided by its 10MΩ resistance is the leakage current ~ Each volt measured is 0.1µA or 100nA so our voltmeter is acting as a sensitive direct reading current meter albeit one with a high resistance

Ideally a current meter would have a low resistance but the voltmeter has the advantage that if the measured capacitor goes short the current is limited ~ In the C16 example the HT voltage with V3 removed will be about 300V and if the leakage current were as high as 10µA the meter would read 100V and you should change C16

With such a large resistance as 10MΩ charging the capacitor being tested it will take time for the charge current to stop flowing and the meter reading to stabilise ~ Often a time in seconds described by 5CR ~ 5 times the resistor value times the capacitor value ~ is long enough ~ The 47nF of C16 will need more than 24s for a valid reading

What appears to be capacitor leakage may be due to dirty valve bases and wiring ~ it may even be the body of the capacitor that is conducting due to 40 or 50 years of carbon build up from fires and traffic and it may be reduced by cleaning the bodies of some old moulded paper capacitors or the end caps of metal bodied types

Making an accurate measurement of leakage is often not required ~ it is the effect of the leakage that is important and depending on the design values it may be possible to still use new old stock capacitors that have leakage currents of µA but at what voltage and what temperature ? ~ Leakage gets worse with temperature so try to test when 'hot'

With all the QUAD 22 valves removed and the coupling capacitors good C7 C12 and C15 should have the same voltage across them which should be the incoming supply ≈330V ~ If they are old electrolytic capacitors they will have significant leakage indicated by the voltage being highest on C15 and lowest on C7 ~ The voltages across R32 R25 and R20 will indicate where and how much leakage there is

~ Measuring Leakage with a multimeter and HV PSU ~

New and new old stock (NOS) capacitors can be tested for leakage current and their insulation resistance calculated using a 10MΩ meter and high voltage bench power supply or in my case a Fluke meter and 343A voltage calibrator which can generate 1100V with 'dial up' µV precision and 25mA variable current limit

In the picture a 0.22µF Paper in Oil capacitor is connected in series with the meter across the 343A output ~ The meter input has been shunted by a 1.1MΩ resistor to give a resistance of 1MΩ so the meter reads 1µA/V and here the leakage is almost 13µA at a room temperature of 21˚C with 487V across the 500V rated capacitor ~ The 343A can be adjusted to give 500V or other voltages across the capacitor

Same setup as above testing a TCC 'Super Metalpack' 0.01µF 1000V Paper in Oil (PIO) capacitor with ceramic end caps which was hermetically sealed over 40 years ago ~ The leakage is only 2nA at 1000V at 25˚C ! and the plastic box lid on the antistatic bench is required

This type of PIO capacitor sells for a high price on ebay etc. even when pulled from old equipment and with leads grafted on and in a sorry state ~ because it is a PIO capacitor that should not damage your expensive valve amp ~ in fact after more than 50 years some of these test better than the standard PIO would have done when New

Here a 0.05µF 400V DCC type 428/S Metallised Paper capacitor again with ceramic end caps has 5nA leakage at 400V at 22˚C and the insulation resistance is 400V/5nA = 80GΩ !

The leakage will be higher for higher capacitance values and temperature but almost all hermetically sealed PIO types perform well and are safe enough for expensive vintage valve amplifiers but modern polycaps have even lower leakage and in the right design can sound better than PIO capacitors reclaimed from 1960s military equipment

In the 3 examples above the largest value capacitor is 0.22µF and I made the meter resistance 1MΩ to speed up the time to reach a stable unchanging measurement ~ Remember the 5CR time described above ~ it is now only 1.1s and because the units MΩ and µF cancel ~ multiplying the µF value by 5 gives the minimum time for a valid reading

Once isolated in the equipment or on the bench you can test the coupling capacitors C7 C12 and C15 in the QUAD 22 which are all in the same can and have a common -Ve chassis connection via the can ~ The 5CR for these 16µF capacitors will be >80s if they are in good condition and the leakage may be tens of µA or higher

When testing electrolytic capacitors and especially high value NOS (in µF not £ or $) it may be necessary to make the meter resistance 100kΩ or maybe 10kΩ but remember if you are measuring leakage at the high voltages required for valve amplifiers the lower resistor values need to be high power types ~ 500V^2/10kΩ = 25W !

If the capacitor on test goes short the meter resistor takes all the current x voltage and will get hot ~ the advantage I have using the Fluke 343A apart from the dial up voltage selection is it has an adjustable current limit and shut off so can be safely left running for a long time ~ It is often used to reform capacitors while also testing them

It is common practice to reform electrolytic capacitors using a high voltage supply and series resistor to limit the currrent ~ The resistor must be rated to allow for the capacitor shorting and the continuous current during reforming must be considered as it internally heats the capacitor causing more current to flow

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