Capacitor reliability again. - UK Vintage Radio Repair and Restoration Discussion Forum

Station X · 15th Mar 2004, 3:32 pm

We all know that waxed paper and Hunts black capacitors are unreliable. What about other types though? In particular the metal cased TCC or similar types that look like electrolytics but aren't. In military equipment they are often stud mounted to the chassis with a single tag on the top. Also how reliable are the square or oblong shaped brown Hunts mica? capacitors?

I thought the latter were very reliable, but I'm beginning to have my doubts about some used as decouplers on an AGC line. It doesn't take much leakage through a 4M resistor to drop a lot of volts.

Graham.

Alan_Douglas · 16th Mar 2004, 12:22 am

The quality of paper used in capacitors is dependant on residual acids from the papermaking process. Some manufacturers used better paper than others, but very few paper capacitors are still good after fifty years or more.

Many capacitor checkers measure leakage in megohms, but I've found it more instructive to measure leakage directly in microamperes. I've measured dozens of 1920s - 1930s capacitors of many brands, but found only a few that leak less than 10 to 20 microamps at rated voltage. Among those were some Western Electric 1 and 2µF caps from the 1920s, potted in metal cans, and one Atwater Kent filter block, also potted in a can with a Bakelite terminal block. The key seems to be moisture penetration. No plastic or wax will keep moisture out indefinitely.

I haven't made many measurements on 1940s or later bathtub capacitors, but I do know that even a hermetic glass-to-metal seal is no guarantee of a good unit today.

Mica capacitors have been considered very reliable, but moisture gets into those too. Silvered mica caps in particular are prone to silver migration through or around the mica dielectric in the presence of moisture and applied voltage. Modern dipped silvered micas use alternating coats of phenolic and epoxy resins to retard moisture penetration, but eventually...

15th Mar 2004, 3:32 pm	#1
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Capacitor reliability again. We all know that waxed paper and Hunts black capacitors are unreliable. What about other types though? In particular the metal cased TCC or similar types that look like electrolytics but aren't. In military equipment they are often stud mounted to the chassis with a single tag on the top. Also how reliable are the square or oblong shaped brown Hunts mica? capacitors? I thought the latter were very reliable, but I'm beginning to have my doubts about some used as decouplers on an AGC line. It doesn't take much leakage through a 4M resistor to drop a lot of volts. Graham. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

16th Mar 2004, 12:22 am	#2
Alan_Douglas Retired Dormant Member Join Date: Nov 2003 Location: Cape Cod, Massachusetts Posts: 59	Re: Capacitor reliability again. The quality of paper used in capacitors is dependant on residual acids from the papermaking process. Some manufacturers used better paper than others, but very few paper capacitors are still good after fifty years or more. Many capacitor checkers measure leakage in megohms, but I've found it more instructive to measure leakage directly in microamperes. I've measured dozens of 1920s - 1930s capacitors of many brands, but found only a few that leak less than 10 to 20 microamps at rated voltage. Among those were some Western Electric 1 and 2µF caps from the 1920s, potted in metal cans, and one Atwater Kent filter block, also potted in a can with a Bakelite terminal block. The key seems to be moisture penetration. No plastic or wax will keep moisture out indefinitely. I haven't made many measurements on 1940s or later bathtub capacitors, but I do know that even a hermetic glass-to-metal seal is no guarantee of a good unit today. Mica capacitors have been considered very reliable, but moisture gets into those too. Silvered mica caps in particular are prone to silver migration through or around the mica dielectric in the presence of moisture and applied voltage. Modern dipped silvered micas use alternating coats of phenolic and epoxy resins to retard moisture penetration, but eventually...