[Hallicrafters] postage stamp mica caps

[Roy Morgan]

At 03:23 PM 6/23/02 -0400, JM/CO wrote:
...I once made a similar "find", but upon electrical
>inspection, discovered that they were ALL bad. Use a megger. This is the
>only way to check these at their rated voltage.

Well... if you don't have a megger, there is an alternative.  Here is my 
explanation on how to go about this with more common equipment.


Capacitor leakage testing

1) Find a B+ supply that will deliver a voltage as high or higher than the 
capacitors rated working value.  A variable supply is nice but not 
necessary.  Current capacity is not important - a few milliamperes is fine.

2) Get a VTVM or a DVM with high input impedance (10 megohms is  common).

3) Set the voltmeter to measure volts on a range above the supply 
voltage.  Connect the common terminal of the supply to the common terminal 
of the voltmeter.

4) With the supply off for safety, connect the capacitor from the high side 
of the supply to the high side of the voltmeter.

5) Turn on the supply.

6) Observe the meter.

The meters input resistance causes it to operate as very sensitive 
microammeter.  A reading of 10 volts across 10 megohms indicates a current 
of one microampere.  One volt one tenth microampere, or 100 picoamperes.

Example: with a supply voltage of 350 volts and voltmeter indication of 50 
volts, the capacitor is conducting a current of  50 microamperes.  The 
capactor has an impressed voltage of 300 volts (350 minus 50).  You can 
figure the capacitor leakage resistance by Ohm's law, or by 
proportions.  Note, it has 6 times the voltage as the voltmeter, so it has 
6 times the resistance, or 60 megohms.

In my experience, it is common for old paper capacitors to measure one 
quarter to three quarters of the supply voltage in this setup.  It is also 
common for modern film capacitors to measure less than a few tenths of a volt.

Consider the case of an old paper .01 uF capacitor feeding the audio output 
tube in a receiver. The preceding stage operates at a plate voltage of 200 
volts.  The old paper capacitor leaks about 100 microamperes.  The output 
tube grid resistor is 100Kohms.  The voltage developed across the grid 
resistor from the leakage is 10 volts.  This 10 volts reduces the 
grid-cathode bias on the audio out put tube from -minus 14 volts to minus 4 
volts.  In the case of a 6V6, or 6AQ5, that will increase the standing 
plate current from a normal 25 or 30 ma to about 80 or 100 ma.  The audio 
will sound terrible and the tube will last only a few hours instead of a 
few thousand hours.  Leakage in the blocking cap at the audio pre-amp stage 
is even more damaging to the sound since the stage operates at lower bias 
levels.

Consider the case of a screen bypass capacitor in a receiver IF stage.  The 
B+ supply is 220 volts, normal screen current is 5 ma, screen resistor is 
22K, and screen voltage is about 110 volts.  The tube operates with normal 
gain.  Now, if the screen bypass cap leaks 3 millamps, the screen voltage 
will go down to something like 60 volts.  The tube will operate a lower 
gain, will not respond in the same way to AGC voltage, and will be more 
subject to overload on strong signals.  If many IF and RF stages are having 
similar screen bypass leakage problems, your radio will be quite dead.  I 
have a number of as-yet un-re-capped  receivers like this.

You can measure screen bypass and grid coupling capacitors in circuit by 
pulling out one or more tubes and measuring voltages on either side of the 
cap.  Take into account the voltmeter input resistance and any resistance 
to ground on the non-B+ side of the cap, such as the grid resistor.  You 
can do this withOUT removing any modules form the chassis in the 
R-390.  Count your tube pin numbers in the right direction when working 
from the top of the chassis.

Note:  Many older radios were measured with 1000 ohms-per-volt meters and 
the reported normal tube voltages reflect this.  Most affected are screen 
voltage and  voltages in high resistance circuits.  Notable examples are 
the TV-7 tube tester and most pre-war receivers.  Your TV-7 will not be 
calibrated right if you do it with a modern 10-megohm input resistance 
meter.  Just add a resistor in parallel with the meter appropriate to the 
scale you are using.

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan at nist.gov --