Ski -
A little research shows that the 'Electro-Motive' (EM) corp was acquired by Cornell Dubilier (CDE)
in 1978. It's possible that CDE modified the manufacturing process, but I doubt it.
I've used these caps both professionally and amateur, and never found them to be particularly
failure prone. I was using them in the 60's for 400 MHz solid state amplifiers built on stripline.
We used an epoxy solvent to dissolve the brown case, resulting in a capacitor consisting of a
stack of silvered mica pieces with a copper 'wrap' around the ends, that could be soldered directly
to striplines, an early form of surface mount! Obviously the idea was zero lead inductance, and
several manufacturers later offered 'special' caps that looked just like ours for stripline use......
Actually, until a year or two ago, the first time I saw a PTO failure caused by one, I hadn't seen
any more than random, very occasional defective Dur-Micas. I've found only a couple over the last
50 years in any application aside from the PTO. The silver migration problem is exacerbated by high
voltages, and they are mostly used in resonant circuits where they see MUCH lower voltages, so that
may be part of the answer.
Still, the one in Steve's PTO is at least 40 years old, so that's not likely to be a 'manufacturing
defect'! :-)
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Fred or Ski wrote:
After working in communications for the State Police here in Michigan for 37
years, most of our equipment
was from Motorola, and we have had several problems with silver mica
capacitors made by a company that
marked there components with 'EM'. If you find a circuit that is in suspect of causing a problem,
and there is
silver mica caps with 'EM' installed, replace them. Many of our Motorola Micor radios had freq
shifting and
intermittant drive which caused erratic power output. These 'EM' caps apparently had a
contamination when
they were made and some didn't go bad for several years. I would certainly check for these parts in
the PTO's
that are operating intermittantly. I hope this helps...
Good luck and 73,
Ski-WB8YXI
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