[MarkS]

Recently been restoring my Fisher 80T preamp-tuner. The unit was functional
when I got it so no trouble shooting needed just a recap, switch and socket
cleaning, dusting, knob cleaning (previous knob disease thread). Hooked it
up to one of my power amps, and all is well, thing works great; sounds
excellent, alignment appears right on. Went on to check RIAA accuracy, not
too bad for its age. Sooo then I tried to turn it off...no dice, damn,
wouldn't shut off, a bad power switch, or so I thought. Almost destoyed the
switch fiddling with it, but it seemed to work fine- the resistance across
the power plug was inifinite when the switch was off..what the heck? Probed
the primary of the PT and well, a short to ground- double damn. "But it only
has 2 wire power cord so how in the world does.....THE POWER AMP!" Three
wire with the input plug ground lifted off chassis ground by a 2W 27 ohm
resistor. Unplugged the power amp from the 80T and it shut off- OK great.
The switch still works-phew! Bad PT? Nah! Checked the schematic and well- a
film cap from one side of the primary to ground. Thought I got all those but
no, this one was not a wax tubular but a black beauty that looked like a
resistor and it was a dead short, clipped it out, problem solved.

Dangerous it was though. I'm sure I've read why those caps exist but can't
remember. Chassis could easily been have hot and a nasty shock in waiting
like an old ac-dc radio chassis. A good reason to check for those caps and
also to check for primary to ground shorts. 'course an isolation transformer
is also a good idea when working with these old electronics.

BR's

Mark

[Brenda Ann[_2_]]

"MarkS" wrote in message
m...
Recently been restoring my Fisher 80T preamp-tuner. The unit was
functional when I got it so no trouble shooting needed just a recap,
switch and socket cleaning, dusting, knob cleaning (previous knob disease
thread). Hooked it up to one of my power amps, and all is well, thing
works great; sounds excellent, alignment appears right on. Went on to
check RIAA accuracy, not too bad for its age. Sooo then I tried to turn it
off...no dice, damn, wouldn't shut off, a bad power switch, or so I
thought. Almost destoyed the switch fiddling with it, but it seemed to
work fine- the resistance across the power plug was inifinite when the
switch was off..what the heck? Probed the primary of the PT and well, a
short to ground- double damn. "But it only has 2 wire power cord so how in
the world does.....THE POWER AMP!" Three wire with the input plug ground
lifted off chassis ground by a 2W 27 ohm resistor. Unplugged the power amp
from the 80T and it shut off- OK great. The switch still works-phew! Bad
PT? Nah! Checked the schematic and well- a film cap from one side of the
primary to ground. Thought I got all those but no, this one was not a wax
tubular but a black beauty that looked like a resistor and it was a dead
short, clipped it out, problem solved.

Dangerous it was though. I'm sure I've read why those caps exist but can't
remember. Chassis could easily been have hot and a nasty shock in waiting
like an old ac-dc radio chassis. A good reason to check for those caps and
also to check for primary to ground shorts. 'course an isolation
transformer is also a good idea when working with these old electronics.

BR's

Mark

[Brenda Ann[_2_]]

"MarkS" wrote in message
m...
Recently been restoring my Fisher 80T preamp-tuner. The unit was
functional when I got it so no trouble shooting needed just a recap,
switch and socket cleaning, dusting, knob cleaning (previous knob disease
thread). Hooked it up to one of my power amps, and all is well, thing
works great; sounds excellent, alignment appears right on. Went on to
check RIAA accuracy, not too bad for its age. Sooo then I tried to turn it
off...no dice, damn, wouldn't shut off, a bad power switch, or so I
thought. Almost destoyed the switch fiddling with it, but it seemed to
work fine- the resistance across the power plug was inifinite when the
switch was off..what the heck? Probed the primary of the PT and well, a
short to ground- double damn. "But it only has 2 wire power cord so how in
the world does.....THE POWER AMP!" Three wire with the input plug ground
lifted off chassis ground by a 2W 27 ohm resistor. Unplugged the power amp
from the 80T and it shut off- OK great. The switch still works-phew! Bad
PT? Nah! Checked the schematic and well- a film cap from one side of the
primary to ground. Thought I got all those but no, this one was not a wax
tubular but a black beauty that looked like a resistor and it was a dead
short, clipped it out, problem solved.

Dangerous it was though. I'm sure I've read why those caps exist but can't
remember. Chassis could easily been have hot and a nasty shock in waiting
like an old ac-dc radio chassis. A good reason to check for those caps and
also to check for primary to ground shorts. 'course an isolation
transformer is also a good idea when working with these old electronics.



OUCHIES...

The cap is there to ground out line noise.

Good thing you didn't plug that tuner in the opposite way... you would have
had hot AC right to the audio cables and hence to ground... you would have
at the very least blown the audio cables to kingdom come...

[Paul P[_4_]]

Dangerous it was though. I'm sure I've read why those caps exist but can't
remember. Chassis could easily been have hot and a nasty shock in waiting
like an old ac-dc radio chassis. A good reason to check for those caps and
also to check for primary to ground shorts. 'course an isolation
transformer is also a good idea when working with these old electronics.

BR's

Mark

Mark, This is an important story.

The purpose is to shunt RF noise to ground. Others can more accurately
explain the whys and wherefores.

Now for a bit of ramiling. I always replace those black beauties along with
the waxed/paper caps. There are many rationalizations to do so or not. I
prefer to replace the old breakdown prone caps (and resistors).

Those caps from line to line get a special UL rated safety cap. X type for
the line to line caps and Y type for the line to chassis (or what ever metal
you (or your accessories) can touch). There is much more detail he
http://www.justradios.com/safetytips.html.

To replace caps (and resistors) or not. I have been told it will change the
sound of the unit (radio, guitar amp, etc). I say, "Yes it will. And it
will sound pretty close to factory and the original engineered sound. BUT
if you like that "vintage" sound that you hear and want that sound on stage
(in the recording, in your bed room, etc) then that is your prerogative."
Then I add the possibilities of that vintage sound gradually decaying to
smoke or no sound at all. I periodically have this discussion with the
guitar players I run sound for.

I feel that it is perfectly acceptable if that dirty, leaky cap, out of
tolerance resistor sound or that nearly demagnetized electric guitar pickup
is what one likes and desires. Vintage sound may not be original sound.
Just be aware of that ramifications.

Thanks for reading. I rarely go into subjective detail.

Paul P.

[Phil Allison[_3_]]

"Paul P"

Mark, This is an important story.

The purpose is to shunt RF noise to ground.


** Not really.

Without the so called "death cap", the chassis of the unit will float with a
high 60Hz AC voltage and this will cause audible hum in the phono and maybe
tuner signals too.

With the cap AND the AC plug inserted the correct way around, such hum was
very much reduced.

The only SANE procedure these days is to fit a 3 core lead and plug to such
units.



..... Phil

[Richard Knoppow]

"MarkS" wrote in message
m...
Recently been restoring my Fisher 80T preamp-tuner. The
unit was functional when I got it so no trouble shooting
needed just a recap, switch and socket cleaning, dusting,
knob cleaning (previous knob disease thread). Hooked it up
to one of my power amps, and all is well, thing works
great; sounds excellent, alignment appears right on. Went
on to check RIAA accuracy, not too bad for its age. Sooo
then I tried to turn it off...no dice, damn, wouldn't shut
off, a bad power switch, or so I thought. Almost destoyed
the switch fiddling with it, but it seemed to work fine-
the resistance across the power plug was inifinite when
the switch was off..what the heck? Probed the primary of
the PT and well, a short to ground- double damn. "But it
only has 2 wire power cord so how in the world
does.....THE POWER AMP!" Three wire with the input plug
ground lifted off chassis ground by a 2W 27 ohm resistor.
Unplugged the power amp from the 80T and it shut off- OK
great. The switch still works-phew! Bad PT? Nah! Checked
the schematic and well- a film cap from one side of the
primary to ground. Thought I got all those but no, this
one was not a wax tubular but a black beauty that looked
like a resistor and it was a dead short, clipped it out,
problem solved.

Dangerous it was though. I'm sure I've read why those caps
exist but can't remember. Chassis could easily been have
hot and a nasty shock in waiting like an old ac-dc radio
chassis. A good reason to check for those caps and also to
check for primary to ground shorts. 'course an isolation
transformer is also a good idea when working with these
old electronics.

BR's

Mark

Others have said that the caps are line noise filters. I
am surprized at your finding a BB cap used for this. Its
more common for them to be either mica or ceramic caps which
are available in quite high voltages and rarely short. One
arrangement to provide some protection from a shorted cap is
to use one cap from each side of line to a junction with a
third cap which then goes to ground. That way two caps would
have to short simultaneously to ground the line. This also
doubles the voltage rating.
Black Beauty caps were made and sold to be deluxe
quality caps with an extended temperature range. They are
found in a lot of high quality equipment. At least the first
series had some problem, I don't know what happened, that
made them fail rapidly. This is illustrated by the fact that
Hammarlund began replacing BB caps with ceramics in the
SP-600 series receivers within a few years. Evidently, one
problem had to do with the seal for the oil impregnation,
one of the end leads is actually a small tube for filling
the cap. The lead at that end is fitted into the tube and
soldered on. Evidently, this arrangement was subject to
leaks and once the oil leaked out the capacitor changed
value and had increased losses. Sprague must have fixed the
problem at some point because they continued to make a
similar capacitor called the Telecap (for Television
Capacitor) for many years and these seem to have been
reliable. Perhaps they were dry type and not oil filled. I
remember being told some fifty years ago that BB caps were
poison.
BTW, I've dissected a few failed ones, not only were
they dry inside but the capacitor bodies were badly
distorted. It would be interesting to know if this
mechanical distortion was caused by the loss of oil and age
or some fault in the molding process. The windings should
have been exactly tubular. These are not strictly paper caps
but rather have a dielectric made of plastic impregnated
paper. I am not sure what purpose the oil served since in
the usual oil-filled cap it it saturates the paper
dielectric.

--
Richard Knoppow
Los Angeles
WB6KBL

[Phil Allison[_3_]]

"Richard Knoppow"


Others have said that the caps are line noise filters.


** Not me.

The action of the cap is to reduce the 60Hz voltage "floating" on the
chassis to a low value by SWAMPING the effect of stray capacitance in the
AC tranny windings and wiring.

Doing this prevents AC line noise entering audio input wiring and causing
hum modulation of AM and FM radio signals.

The same idea was used with most US made guitar amps that had only 2 core
leads.


.... Phil

[MarkS]

"Brenda Ann" wrote in message
...

"MarkS" wrote in message
m...
Recently been restoring my Fisher 80T preamp-tuner. The unit was
functional when I got it so no trouble shooting needed just a recap,
switch and socket cleaning, dusting, knob cleaning (previous knob disease
thread). Hooked it up to one of my power amps, and all is well, thing
works great; sounds excellent, alignment appears right on. Went on to
check RIAA accuracy, not too bad for its age. Sooo then I tried to turn
it off...no dice, damn, wouldn't shut off, a bad power switch, or so I
thought. Almost destoyed the switch fiddling with it, but it seemed to
work fine- the resistance across the power plug was inifinite when the
switch was off..what the heck? Probed the primary of the PT and well, a
short to ground- double damn. "But it only has 2 wire power cord so how
in the world does.....THE POWER AMP!" Three wire with the input plug
ground lifted off chassis ground by a 2W 27 ohm resistor. Unplugged the
power amp from the 80T and it shut off- OK great. The switch still
works-phew! Bad PT? Nah! Checked the schematic and well- a film cap from
one side of the primary to ground. Thought I got all those but no, this
one was not a wax tubular but a black beauty that looked like a resistor
and it was a dead short, clipped it out, problem solved.

Dangerous it was though. I'm sure I've read why those caps exist but
can't remember. Chassis could easily been have hot and a nasty shock in
waiting like an old ac-dc radio chassis. A good reason to check for those
caps and also to check for primary to ground shorts. 'course an isolation
transformer is also a good idea when working with these old electronics.



OUCHIES...

The cap is there to ground out line noise.

Good thing you didn't plug that tuner in the opposite way... you would
have had hot AC right to the audio cables and hence to ground... you would
have at the very least blown the audio cables to kingdom come...


You know its kind of lucky that it didn't happen because I plugged and
unplugged the tuner several times when I was messing with the switch. I
figure the 27 ohm resistor would have blown- probably like a gun shot. 8)

BR's

Mark

[Carter]

On 6/9/2010 1:19 AM, Richard Knoppow wrote:

I remember being told some fifty years ago that BB caps were poison.

Just curious...but when you were told "poison", do you mean literally,
as in the leaking chemical would poison you or as in 'bad news, just
don't use these things'?

Carter K8VT

[Richard Knoppow]

"Carter" wrote in message
...
On 6/9/2010 1:19 AM, Richard Knoppow wrote:

I remember being told some fifty years ago that BB caps
were poison.

Just curious...but when you were told "poison", do you
mean literally, as in the leaking chemical would poison
you or as in 'bad news, just don't use these things'?

Carter K8VT

I meant electronically. I have no idea what kind of oil
was used but it may have been a PCB, most transformer and
capacitor oil of the time was. That is one reason its so
hard to find old high-voltage oil-filled caps now.

--

--
Richard Knoppow
Los Angeles
WB6KBL

[Richard Knoppow]

"Phil Allison" wrote in message
...

"Richard Knoppow"


Others have said that the caps are line noise filters.


** Not me.

The action of the cap is to reduce the 60Hz voltage
"floating" on the chassis to a low value by SWAMPING the
effect of stray capacitance in the AC tranny windings and
wiring.

Doing this prevents AC line noise entering audio input
wiring and causing hum modulation of AM and FM radio
signals.

The same idea was used with most US made guitar amps that
had only 2 core leads.


... Phil

The capacitors I am speaking of were much too low in
value to pass any 60hz current and were found going from
each side of the line to chassis ground in many pieces of
equipment. These are typically about 0.01 uf and quite high
voltage. The reactance at 60 hz is about 260,000 ohms. They
are to by-pass high frequency current on the supply line.



--
Richard Knoppow
Los Angeles
WB6KBL

[Phil Allison[_3_]]

"Richard Knoppow"
"Phil Allison"
"Richard Knoppow"


Others have said that the caps are line noise filters.


** Not me.

The action of the cap is to reduce the 60Hz voltage "floating" on the
chassis to a low value by SWAMPING the effect of stray capacitance in
the AC tranny windings and wiring.

Doing this prevents AC line noise entering audio input wiring and causing
hum modulation of AM and FM radio signals.

The same idea was used with most US made guitar amps that had only 2 core
leads.


The capacitors I am speaking of were much too low in value to pass any
60hz current and were found going from each side of the line to chassis
ground in many pieces of equipment. These are typically about 0.01 uf and
quite high voltage. The reactance at 60 hz is about 260,000 ohms. They are
to by-pass high frequency current on the supply line.


** The AC line caps WE are speaking of some value between 0.01uF and
..05uF - used on ONE side only and of tubular film/foil or paper/foil
type.

Their working voltage rating was from 400 to 600 volts DC.

They were routinely fitted to tube power amps, pre-amps and receivers.

The stray capacitance in a power tranny is of the order of 200 - 500 pF and
typically causes the chassis to float at about half the AC supply voltage if
there is no such cap or connection to supply ground.



..... Phil

[Patrick Turner]

On Jun 9, 3:00*pm, "Phil Allison" wrote:
"Paul P"



Mark, *This is an important story.

The purpose is to shunt RF noise to ground.

** Not really.

Without the so called "death cap", the chassis of the unit will float with a
high 60Hz AC voltage and this will cause audible hum in the phono and maybe
tuner signals too.

With the cap *AND *the AC plug inserted the correct way around, such hum was
very much reduced.

The only SANE *procedure these days is to fit a 3 core lead and plug to such
units.

.... * Phil

Ah, the sound of sanity.

I thought you'd died or expired.

Patrick Turner.