I have several postings here about an SX-101a I am restoring. Up to
now I have done consumer radios, TVs, and an SX-71. I found the
majority of the paper caps (wax or molded) to be bad in them. By bad,
I found them to be out of tolerance (usually reading high on my old
60hz reactance bridge meter) and showing significant leakage at rated
voltage. So now I start on the SX-101a and wouldn't you know it the
first 4 paper caps I pulled check perfectly in all respects. These do
appear to be a much higher quality cap than those I have seen to date,
but paper they are. So if 4 out of 4 are good, do I need to replace
all the paper caps in this unit? Perhaps this radio never saw much
humidity, and coupling that with higher quality parts they might still
be just fine. I am tempted to reinstall the ones I just pulled to keep
the vintage look. I really don't know how to test the caps in circuit
individually unless I pull one lead, and then you are half way to
replaced anyway, so if there is high likelyhood of some bad ones out
there, I might as well replace them all. Are there certain circuit
locations more prone to fail? I have stopped further work until I get
some feedback on this.

Dan

"geojunkie" wrote in message
om...


Are the good caps C-D Black Cats? Those are mylar.

Don't ever trust those pink Tiny Chiefs or Black Beauties.

Frank Dresser

I replace caps that need to be replaced. I have found that every
electrolytic that has a small voltage rating is bad and they get changed
out without even thinking. I have found a lot of electrolytic that are
higher voltage units to be higher in value but then after they get
reformed end up reading within tolerance. I have only started looking
at leakage and so far after a good reforming they seem to be what I call
good enough to use. Of course some will not reform and they get trashed.
I have found just as many silver mica and ceramic disk caps bad as I
have found black beauty caps and the other paper caps. . Unless a cap
is in a part of the circuit where a failure would cause a major failure
like in the 75A4 where it will take out a filter I don't even think of
swapping them out unless my testing shows a problem with that part of
the circuit or the part is literally falling apart. My feeling about
the caps of old was that quality control was there major problem. I
expect that assemble of these caps was a big factor and that depending
on who did the assemble some ended up never being a problem and some
got contamination during manufact and ended up failing early in there
life.

My two cents worth,
Ron WA0KDS


geojunkie wrote:

I have several postings here about an SX-101a I am restoring. Up to
now I have done consumer radios, TVs, and an SX-71. I found the
majority of the paper caps (wax or molded) to be bad in them. By bad,
I found them to be out of tolerance (usually reading high on my old
60hz reactance bridge meter) and showing significant leakage at rated
voltage. So now I start on the SX-101a and wouldn't you know it the
first 4 paper caps I pulled check perfectly in all respects. These do
appear to be a much higher quality cap than those I have seen to date,
but paper they are. So if 4 out of 4 are good, do I need to replace
all the paper caps in this unit? Perhaps this radio never saw much
humidity, and coupling that with higher quality parts they might still
be just fine. I am tempted to reinstall the ones I just pulled to keep
the vintage look. I really don't know how to test the caps in circuit
individually unless I pull one lead, and then you are half way to
replaced anyway, so if there is high likelyhood of some bad ones out
there, I might as well replace them all. Are there certain circuit
locations more prone to fail? I have stopped further work until I get
some feedback on this.

Dan

"geojunkie" wrote in message
om...


It's easy to figure how much capacitor leakage would cause an excessive
grid voltage, a reduced screen voltage or plate voltage. It's a simple
ohm's law problem. Don't forget the power leaky bypass capacitors have
to dissapate. Cathode bypasses can get very leaky before causing
problems.

But I'd be surprised if there are any 40+ year old paper caps around
that can meet a fairly loose throw away standard now. Say 1 microamp
per microfarad at it's rated voltage.

What year was your SX 101 made? Mylar caps started showing up around
1960. Even my SW 500 (S 120) had several good quality mylar C-D Black
Cats.

Also, there was an early 60s version of the Sprague Black Beauty which
was paper-mylar rather than paper-oil. The paper-mylar cap was quite
reliable.

Frank Dresser

Frank Dresser wrote:
"geojunkie" wrote in message
om...


It's easy to figure how much capacitor leakage would cause an excessive
grid voltage, a reduced screen voltage or plate voltage. It's a simple
ohm's law problem. Don't forget the power leaky bypass capacitors have
to dissapate. Cathode bypasses can get very leaky before causing
problems.

But I'd be surprised if there are any 40+ year old paper caps around
that can meet a fairly loose throw away standard now. Say 1 microamp
per microfarad at it's rated voltage.

What year was your SX 101 made? Mylar caps started showing up around
1960. Even my SW 500 (S 120) had several good quality mylar C-D Black
Cats.

Also, there was an early 60s version of the Sprague Black Beauty which
was paper-mylar rather than paper-oil. The paper-mylar cap was quite
reliable.

Frank Dresser



Here's an excellent way to determine if paper caps should be replaced.
Count the number of caps in the radio and divide by the number of times
you want to go in and re-troubleshoot. If the quotient is equal to or
less than the original number of caps then they should be replaced.

Hope this helps,
Bill

Here's an excellent way to determine if paper caps should be replaced.
Count the number of caps in the radio and divide by the number of times
you want to go in and re-troubleshoot. If the quotient is equal to or
less than the original number of caps then they should be replaced.

Hope this helps,
Bill

Should we be getting vibes you feel strongly about this?

Avery W3AVE
Potomac, Md.

I'll concur with the other comments. I (personally) wholesale change out
caps when one or more go out and/or when I first check out a radio before
powering up (and find bad ones). Having said that, I have found both
extremes in older radios. For example, my HQ-120 was dead, blew fuses,
and had about 500 ohms plate resistance presented to the PS. All caps
were changed and the radio in now fine. In my National receivers, the
Black-Beauties split, leak oil, short, and are generally very disagreeable
components. My BC-669 transceiver (circa 1944) required all the mica caps
to be replaced, something I would not have expected, until now.
Sangamo's, included! There aren't any caps that are bullet-proof after 50
years. My SX-28 still has most of the wax/paper caps working just fine.
One of these days, though............ Finally, the sage advice to change
those parts which will precipitate other, perhaps worse, failures is
mandatory. Its just not worth the risk and to try and find some old IF
can, etc. if one smokes, after the fact. My take on the issue... 73 de
K3HVG

geojunkie wrote:

I have several postings here about an SX-101a I am restoring. Up to
now I have done consumer radios, TVs, and an SX-71. I found the
majority of the paper caps (wax or molded) to be bad in them. By bad,
I found them to be out of tolerance (usually reading high on my old
60hz reactance bridge meter) and showing significant leakage at rated
voltage. So now I start on the SX-101a and wouldn't you know it the
first 4 paper caps I pulled check perfectly in all respects. These do
appear to be a much higher quality cap than those I have seen to date,
but paper they are. So if 4 out of 4 are good, do I need to replace
all the paper caps in this unit? Perhaps this radio never saw much
humidity, and coupling that with higher quality parts they might still
be just fine. I am tempted to reinstall the ones I just pulled to keep
the vintage look. I really don't know how to test the caps in circuit
individually unless I pull one lead, and then you are half way to
replaced anyway, so if there is high likelyhood of some bad ones out
there, I might as well replace them all. Are there certain circuit
locations more prone to fail? I have stopped further work until I get
some feedback on this.

Dan

AComarow wrote:
Here's an excellent way to determine if paper caps should be replaced.
Count the number of caps in the radio and divide by the number of times
you want to go in and re-troubleshoot. If the quotient is equal to or
less than the original number of caps then they should be replaced.

Hope this helps,
Bill


Should we be getting vibes you feel strongly about this?

Avery W3AVE
Potomac, Md.

Only vibes, Ave? Maybe I wasn't clear enough!
:-)
Seriously, I question the logic of say encountering 20 caps in a 50-year
old radio and half of them have gone bad and then leaving the other
half. "Bad" is a relative term depending where it is in the circuit and
how much leakage you can tolerate but the fact is that the paper is
slowly deteriorating because of inherent acids and is only gonna get
worse with time.
So, you can replace them now or later. I enjoy replacing caps, and I
restuff the paper ones, and I really don't mind doing so. Might as well
unless you're in it for a business where your time equals money.

-Bill

"Are the good caps C-D Black Cats? Those are mylar.

Don't ever trust those pink Tiny Chiefs or Black Beauties."

This is an SX-101a that I believe is near 1962, as the knobs match
later receivers like the SX-115 and SX-117. The paper caps are pink
Tiny Chief 600v, and molded black caps that are marked only with
stripes... no lettering of any kind. I do see a "P" molded into tiny
circles on one side and a "2" in similar circles on the other side.
What brand of black caps are these? The parts list calls them both
paper caps and never mentions mylar. Sounds like you would tend to
replace them all based on this. I have been using tubular
polypropylene 630v for replacements, the yellow ones. I notice some
seem to love the orange drops. Is there a difference worth worrying
about?

I did find a 10w wirewound heater located under the RF amp, mixer and
1st osc tubes that was almost shorted and most of the components over
it are discolored. This thing is about 6" long, I think I will put a
string of maybe three new power resistors of 1/3 the value in the
metal casing and hope this distributes the heat evenly enough. This
heater is on line anytime the receiver is plugged in, as is the
oscillator filament.

geojunkie wrote:

"Are the good caps C-D Black Cats? Those are mylar.

Don't ever trust those pink Tiny Chiefs or Black Beauties."

This is an SX-101a that I believe is near 1962, as the knobs match
later receivers like the SX-115 and SX-117. The paper caps are pink
Tiny Chief 600v, and molded black caps that are marked only with
stripes... no lettering of any kind. I do see a "P" molded into tiny
circles on one side and a "2" in similar circles on the other side.
What brand of black caps are these? The parts list calls them both
paper caps and never mentions mylar.

I have seen the "2 in a circle" on most Sprague capacitors.
--
Merry Christmas!

Take care, and God bless.
Michael A. Terrell
Central Florida