January 2016
I’m trying to resurrect an old Halicrafter’s communications receiver from at least the ‘50s. I’m planning to replace the electrolytic capacitors in the power supply with capacitors I salvaged from a more recent TV set.
However, I’ve read that electrolytic capacitors — once formed at a certain voltage — can take months, if not years, to reform at a new voltage. Until then, the capacitor value can be significantly off from what’s on the label.
Can anyone shed some light on this, and any thoughts on whether I’ll risk damaging the receiver if I use the caps formed at the higher voltages found in the TV circuit?
Please log in to post an answer.
The thin aluminum oxide dielectric (energy-storing) layer in electrolytic capacitors is formed on the specially-treated anode metal, and the electrolyte contacts the outer can (on early parts) or the cathode foil (on later “dry” parts). When radios have been unpowered for a very long time, electrolytic capacitors tend to lose their dielectric layer and their voltage rating, but not usually much capacitance. They may destroy themselves and other parts when re-powered, unless slow-start techniques are used to renew their dielectric.
When restarting long-idle sets, it’s best to use a variable-voltage (Variac) transformer to increase voltage slowly over several days. Note that Variacs are usually not isolated. An alternative method is to power up with an incandescent lamp socket in series with the AC line, starting with a 60-watt bulb and increasing the wattage gradually. Turn the set off occasionally and check for excessive heating.
I have repaired many radios using higher voltage TV capacitors, and never known one to take more than a week, unless it was defective. If the TV parts aren’t ancient, they may still be sufficiently formed for your new (lower) voltage, but for safety, follow the above procedures.
There is a strong shock hazard posed by AC/DC sets. Many early versions of these sets had all B(-) connections grounded to the chassis, including one side of the switched line cord, no matter which way it’s plugged in. Missing or wrong size screws or knobs or rotting rubber mounting grommets could make outer metal cabinets lethal. Some otherwise-well-built later radios, such as the Hallicrafters SX-41, had a live chassis. Most later sets used an isolated internal ground system to minimize the hazard. It’s safest to work on AC-DC sets using an isolation transformer, especially if you’ll be connecting any AC-powered test gear. You can make your own, using back-to-back filament or power transformers of appropriate power. Good luck with your repair!
Let me start with a rule of thumb, electrolytic caps drop some value if formed to higher voltage (within working range). This is because of thickening of barrier layer, BUT this can vary, that is why most are rated plus or minus 20%. They generally only need to be big enough (power filter, or bypass). Don’t expect much value drift moving to lower voltage. Other concerns are, internal resistance (can screw with bypass performance in audio stages, and heats cap at high duty cycle).
