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#1
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
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 |
#2
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
"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 |
#3
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
"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... |
#4
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
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. |
#5
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
"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 |
#6
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
"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 |
#7
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
"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 |
#8
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
"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 |
#9
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
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 |
#10
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
"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 |
#11
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
"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 |
#12
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
"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 |
#13
Posted to rec.antiques.radio+phono,rec.audio.tubes
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Restoration Story and a Caution
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. |
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