Don't let the "mica" caps fool ya. I chased an AGC problem for two days
before I found a bad "mica" buried in an IF can. That was the straw that
broke the camels back. I then replaced ALL micas along with every bumble
bee, which, by the way, were all cracked.

--
Regards,
Gary...WZ1M
"Chuck Harris" wrote in message
...
Ancient_Hacker wrote:

Also bad news are those pea-green molded caps with the color bands.
Many Trans-Oceanics have around two dozen of those, all leaky.

I havent seen any major problem with postage-stamp or dipped

There is one variety of capacitor that looks like the silver mica
"postage stamp" type of capacitor with all of the dots, but isn't
a mica at all. It is just a paper capacitor. They are usually
dark brown in color, and fail at about the same rate as normal wax/paper
capacitors.

-Chuck

gkb wrote:
Don't let the "mica" caps fool ya.

Well, your milage may vary. I've clipped out at least a thousand old
wax caps, two hundred plus plastic tubulars, and lesse, three discrete
mica caps, and around eight of those wafer mica caps built into IF can
bases.

Mica's can go bad, they just don't seem to do so very often IMHE.

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.

"Ancient_Hacker" wrote in message
ups.com...

gkb wrote:
Don't let the "mica" caps fool ya.

Well, your milage may vary. I've clipped out at least a
thousand old
wax caps, two hundred plus plastic tubulars, and lesse,
three discrete
mica caps, and around eight of those wafer mica caps built
into IF can
bases.

Mica's can go bad, they just don't seem to do so very
often IMHE.


Well, I've replaced 4 in my JX-17. That does seem unusual
but I think they suffer from the same problem other molded
type do, namely shrinkage of the case. These do not get
leaky like the BB's but become very unstable.


--
---
Richard Knoppow
Los Angeles, CA, USA

"Steve" wrote in message
...
Richard,
I performed an electrical restoration on an SP-600 (JX-26)
last summer. The frequency accuracy was poor....WWV at
10 MHz was at 10.2 MHz (approx). What is worse IMO is the
tracking across the bands. The frequency at one end would
be high,
but at the other extreme, it'd be low.

I'm sure this can be tuned out. I just haven't tried yet.

I'm in the process of bring back another JX-26 now, so if
you have any specific questions, you can write me off
list.

Steve W6SSP
Burried in the TM 11-851 handbook (p.35 I think) and in
some Hammarlund advertising, I found a spec of 0.25% for
dial accuracy. This isn't too bad for a conventional
receiver. Also, I suspect the thing is capable of much
better adjustment than this. My experiments will have to
await getting over a back problem.


--
---
Richard Knoppow
Los Angeles, CA, USA

"Chuck Harris" wrote in
message ...
Steve wrote:
I'll second what Frank suggests about replacing ALL of
the black
beauty caps. I'm doing that now on the SP-600 I'm
restoring.
While none were a dead short, most showed some leakage,
about 100K ohms. In many circuits, that won't cause a
problem
but in some its a big deal. About 1/3 had cracked cases.

Steve

Two things make the Sprague Black Beauty caps a problem,
and both relate to their losing oil.

The cap is molded out of a black plastic, with a small
brass filler tube on the banded end. The capacitors were
filled with mineral oil through this tube, and the lead
was
placed in the hole, and crimped. They then sealed the
capacitor
by adding a little bit of solder.

So far, so good. Well, when the capacitors are installed,
very
often the banded lead is close to the lug that is being
soldered,
and it will carry the heat to the solder seal, and to the
oil in
the cap. The slight pressure increase in the cap will
softly
blow the solder out of the seal joint, and the oil will
begin to
seep out of the capacitor. Note: the oil is the
dielectric, not the
paper.

Also, most all plastics will shrink over time, and the
plastic the
Black Beauties are made from is no exception. When it
shrinks, the
leads, and the filler tube do not, and you get a small
crack, and a
leak.

If the oil is allowed to seep out, and the moisture is
allowed to
seep in, the capacitor will be compromised and become
leaky. Also,
without the oil, the WV of the capacitor is lowered,
because the oil
is the dielectric. Capacitors that are used near their WV
will often
arc over, and become shorted.... with catastrophic results
for the
rest of the circuit.

-Chuck

The BB's I found in the SP-600 are all of the dry Mylar
type. I have seen BB's with oil all over them in other
equipment. These were sold originally as highly stable, wide
temperature range, replacements for the conventional paper
type. Obviously something went wrong. The BB's I disected
had glazed looking paper and attachment ferrules at the
ends. They were not rolled up into a round tube but rather a
quite flattened one. I don't know if this is how they were
made or if its a result of the case shrinkage. In any case I
agree with those who recommend replacing them at sight with
disc ceramics.
BTW, Sangamo also sold Mylar-paper caps in Epoxy dipped
form. I wonder if these were any more reliable than the
molded ones.


--
---
Richard Knoppow
Los Angeles, CA, USA

"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.


--
---
Richard Knoppow
Los Angeles, CA, USA

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.


--
---
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

Steve wrote:
Richard,
I performed an electrical restoration on an SP-600 (JX-26)
last summer. The frequency accuracy was poor....WWV at
10 MHz was at 10.2 MHz (approx). What is worse IMO is the
tracking across the bands. The frequency at one end would be high,
but at the other extreme, it'd be low.

I'm sure this can be tuned out. I just haven't tried yet.

I'm in the process of bring back another JX-26 now, so if
you have any specific questions, you can write me off list.

Steve W6SSP

Thank you for the offer, I'm likely to take you up on it:-) At the
moment I've put off a couple of projects because of a hurt back.

Richard Knoppow
Los Angeles, CA

"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.


--
---
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.


--
---
Richard Knoppow
Los Angeles, CA, USA