More condenser tests

Tests were carried out by connecting each condenser to a variable HT power supply via a 1K resistor across which was a voltmeter. The supply voltage was increased and after about 5 seconds when the supply voltage registered 300V the meter reading was noted. Leakage resistance was calculated by multiplying 1K by the ratio of volts across the condenser and resistor. For example, if the voltage across the resistor was 60V the voltage across the condenser was 240V and the condenser resistance would be 1K x 240/60 = 4K If resistance measurements are made with just a multimeter you'll usually find pretty high values of resistance implying fairly low leakage. Clearly, in a normal position in a receiver, associated with the HT line, leakages will be excessive. In lower voltage applications the effects of leakage will build up slowly.

Condenser	Section	Marked value	Measured Value	Leakage @ 300V after 5 seconds	Trend see notes
10C/3399	1	100nF	171nF	75K	slowly worsening
10C/3399	2	100nF	214nF	10K	rapidly worsening
10C/3399	3	100nF	188nF	6.5K	rapidly worsening
10C/3399	1	100nF	192nF	4K	rapidly worsening
10C/3399	2	100nF	194nF	4K	rapidly worsening
10C/3399	3	100nF	202nF	4K	rapidly worsening
10C/3399	1	100nF	188nF	74K	rapidly worsening
10C/3399	2	100nF	180nF	14K	rapidly worsening
10C/3399	3	100nF	s/c	14K	rapidly worsening
10C/3401	1	500nF	805nF	1.2M	steady
New	1	100nF	123nF	none	steady
New	2	100nF	124nF	none	steady
New	3	100nF	122nF	none	steady

In most cases the voltage across the 1K resistor was rising rapidly after the measurement was taken. This is reflected by the note under the Trend column. After one of the triple condensers had been stuffed each section was tested in the same way. The voltage across the resistor measured zero mV. The short-circuit condenser appeared to fix itself on application of HT. The first two triple condensers had rubber covered leads (and dated March 1943) but the third, plus the single condenser had plastic covered leads and were made by a different manufacturer and carried no date stamp.

The accompanying pictures show how to stuff an 0.5uF condenser, type 10C/3401 with chip capacitors. Mine are each rated at 100nF and have a working voltage of 500. The tricky part is holding the chips during soldering and I've used a couple of pieces of wood held loosely in a small vise. Once a set of chips has been soldered into place you can neaten the soldering as shown. Do not attempt to bend the wires close to the chips as these are very brittle. In my example each chip measured about 125nF and it's prudent to check the capacitance during the process. I initially drilled the old condenser using a 4.5mm drill then finished with a 7/32 inch drill to accommodate a 50mm length of 5mm drinking straw. I tried closing the end in the can by melting it, or using wax, but found it was easiest to cut the end from a cotton bud and wedge this in place to prevent ingress of shreds of aluminium. Final sealing at the fixing screw is done with wax from an old condenser.

Below are pictures detailing the stuffing of a type 10C/3399 condenser. Much the same techniques as used above. Again, it's important to bend the connecting leads before soldering as the chips are very brittle. Once soldering is complete, carefully round off any solder spikes so the assembly fits inside the 5mm tube. You'll need good eyesight and a steady hand to complete the work. The hole depth was about 50mm and produced using a 4.5mm drill followed by a 7/32" drill to clear the 5mm drinking straw.

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