"Harold" wrote in message
ups.com...
Richard Knoppow wrote:
"K3HVG" wrote in message
...
I had to change 100% of the micas from a BC-669D. Also,
micas in some ART-13s have also gone bad, some near
impossible to get to. I don't think many of these
components were ever even dreamed of to go this long!
On
the other hand, my SX-28, apparently in use almost
continuously since is was purchased, is still humming
(no
pun intended!!). Hope I can get away with not doing the
RF
section...? Worst B/Bs I ever saw were in a National
NC-125. They looked like they just dropped out of a bag
of
spoiled mussels.
I suspect the environment has something to do with it.
For instance, the BB's in my SP-600 were pretty awful but
I
found one in a General Radio 1001A signal generatror that
looked brand new. I replaced it on general principles but
it
tested good.
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Richard Knoppow
Los Angeles, CA, USA
I had a SP-600-JX-14 several years ago that came from a
Western
Electric monitoring station on the east coast. It came
with a SPC-10
and a speaker panel. I had been on continuously for 24/7
for many, many
years other than for retubing and check-ups. It had "ALL"
the original
black tubulars. None were split nor leaky. I used it for a
couple more
years then sold it. I have had twenty SP-600's pass
through my hands
over the years, most of those black tubulars or gray ones
were leaky
and/or split open. The enviroment in which it was in and
either in use
or sitting turned off would also make a difference.
In recent years the silver micas are now showing up bad,
probably due
to silver migration. The avc chain on the SP-600's at this
point in
time should be looked at closely. Many of them are bad.
YMMV
I have dealt with only two SP-600's. One is probably a
JX-21 but might be a JX-21(it was missing the tuning unit
cover when I got it), the other is a JX-17. Both have had
bad silver micas in various places. The JX-17 was out of
service and in storage for several years. It was re-capped
at some time in the past so I didn't have problems with the
ceramics, but I had to replace about 5 silver micas in
various places. The JX-21 had about 3 bad silver micas. I
replaced a lot of the BB's in the 21 but have not completely
recapped it except for the RF deck. I've had the 21 for some
30 years and am still workign on it on occasion. It was
basically a parts receiver when I got it. The JX-17 nearly
mint but nonetheless had problems. Its now quite stable and
works well.
BB capacitors were advertised as deluxe, high stability,
parts when new and are found in a lot of high quality
equipment. I first heard that they were trouble makers more
than 30 years ago. Silver mica caps are supposed to be among
the most stable of all but I learned long ago that they
often had a stability problem resulting in a small, rapid
variation of capacitance sometimes called scintillation. I
have no idea of what the physical change is that causes this
problem, your suggestion of migration of the silver is as
good as any. Its also obvious that dipped epoxy
encapsulation is far more stable than the old molded plastic
(Bakelite maybe) types.
BTW, the stability of the SP-600 is affected by line
voltage variation due to lack of heater regulation. I have a
Sola constant voltage line transformer that helps with this.
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Richard Knoppow
Los Angeles, CA, USA