Stuffing Condensers

What is about to follow is for purists who would like to restore an old radio to look like it was when first manufactured. The two problem components in a radio made during WW2 are condensers, known nowadays as capacitors, and resistors. I recently stripped out all the 0.1uF and 0.05uF condensers from an R206 receiver and carried out measurements to see how they've survived. Not one was in perfect condition because of degradation of the materials used in their manufacture. The key problem was DC resistance. In valve circuitry impedances are often extremely high and if an old condenser has a DC resistance within the range up to 5 or 10Mohms it will affect operation when used in certain circuits, for example between the anode of an LF amplifier and the grid of an output valve. Being more specific, a leaky condenser will make the output valve control grid more positive than the designer intended. Such a voltage will make the output valve draw more current than intended and if this is especially high will damage the valve and may even ruin the output transformer. Stuffing is a term used for gutting the contents of an old condenser and replacing them with a nice modern capacitor having superior characteristics and notably an extremely high DC resistance. I've recorded the steps in stuffing an old 0.1uF 350 volt working condenser to produce a new 0.1uF 500 volt working capacitor. Some time later I found a different method of doing this that works better for what are termed "radial" rather then the "axial" type shown below. Radial condensers have both leads at the same end of the case whereas axial types have a lead at each end. Click this picture to see radial condenser stuffing

Upper left: Chip capacitor, Kemet 1210 Arcshield HV 100nF Ceramic Multilayer Capacitor, 500 V dc X7R Dielectric ±10%; 3.2mm x 2.5mm x 2.1mm plus small piece of Veroboard. Above: Capacitor soldered in place with lengths of tinned copper wire to suit the intended use bent through holes in the Veroboard and soldered  On the left the circuit board inserted into the old case but not yet secured in place. Below, the circuit board carrying the chip capacitor showing the capacitance.

As I gained eperience with this topic I found that the chip capacitors are quite strong enough to dispense with the Veroboard strip and merely soldering thin insulated tinned copper wire directly to the capacitor then using a length of heatshrink sleeving to protect it, similar to the method used for R1155 types of condenser.

A few notes to accompany the pictures: The old condenser consisted of a cylinder of two long lengths of aluminium foil separated by thin paper and wound vey tightly, wrapped with an impregnated paper, and inserted into an outer cardboard tube. The ends were sealed with pitch and the whole coated with wax. The leads were secured to the ends of the aluminium via a solder cup. To remove the contents of the cardboard tube I removed the pitch by heating and scraping away then I drilled a 3mm pilot hole through the axis then using larger drills and tweezers removed the contents. The new circuit board can be secured in the old tube using the old pitch sealant or clean wax. As a precaution against contamination from particles of the residue of aluminium the circuit board can be inserted into a length of drinking straw.

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