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N Cook
Guest
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 Posted: Fri Dec 01, 2006 1:17 pm Post subject: Safety / noise immunity query |
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Just repaired a Carlsbro Hornet Combo , made in UK, 1984.
Seems as original inside otherwise.
Unusually the amp separates into 2 sections separately connected to the
wooden casing and so electrically isolated as far as any earthing.
The mains transformer is on the rear panel which has a proper earthing point
from the IEC mains inlet.
Power supply is plus and minus rails but the common is not connected to
ground at all. The power and signal lead to/from the pre-amp / reverb is 4
lines : plus , minus , common , line level signal with no earth connection.
The front panel is connected to the common , not to earth anywhere.
There is much more noise immunity from general stray pick-up if I run a wire
from the earthed back plate to the front so bridging common and ground.
If made permannent,internally, would there be a safety issue, or in fact,
leaving as is , is that safe ?
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/ |
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Guest
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| Posted: Fri Dec 01, 2006 1:18 pm Post subject: Re: Safety / noise immunity query |
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N Cook wrote:
| Quote: | Just repaired a Carlsbro Hornet Combo , made in UK, 1984.
Seems as original inside otherwise.
Unusually the amp separates into 2 sections separately connected to the
wooden casing and so electrically isolated as far as any earthing.
The mains transformer is on the rear panel which has a proper earthing point
from the IEC mains inlet.
Power supply is plus and minus rails but the common is not connected to
ground at all. The power and signal lead to/from the pre-amp / reverb is 4
lines : plus , minus , common , line level signal with no earth connection.
The front panel is connected to the common , not to earth anywhere.
There is much more noise immunity from general stray pick-up if I run a wire
from the earthed back plate to the front so bridging common and ground.
If made permannent,internally, would there be a safety issue,
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no
| Quote: | or in fact,
leaving as is , is that safe ?
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if its double inuslated, yes, if is single insulated, not entirely.
It might be floating in an attempt to avoid earth loops.
NT |
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Eeyore
Guest
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| Posted: Fri Dec 01, 2006 1:18 pm Post subject: Re: Safety / noise immunity query |
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N Cook wrote:
| Quote: | Interestingly this unit was yearly PAT tested and passing presumably earth
bond and insulation test each time.
If they had checked insulation resistance relative to the front panel , he
assuming it was earthed, then could that procedure have damaged the power
supply?
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It certainly shouldn't but I have heard of PAT testing damaging perfectlyl good
equipment since the PAT testers don't comply with the IEC standards the kit is
made to !
It's the PAT testers that are at fault. The whole idea of regular testing like
that might have made some sense 40 yrs ago but now it's very largely pointless.
It probably makes some sense wrt portable power hand tools but nothing much
else. Everything here has to mee tIEC safety regs by law anyway ( except the
'dodgy' gear - yet even that's would be likely to pass anyway ).
Graham |
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Eeyore
Guest
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| Posted: Fri Dec 01, 2006 1:18 pm Post subject: Re: Safety / noise immunity query |
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N Cook wrote:
| Quote: | Just repaired a Carlsbro Hornet Combo , made in UK, 1984.
Seems as original inside otherwise.
Unusually the amp separates into 2 sections separately connected to the
wooden casing and so electrically isolated as far as any earthing.
The mains transformer is on the rear panel which has a proper earthing point
from the IEC mains inlet.
Power supply is plus and minus rails but the common is not connected to
ground at all. The power and signal lead to/from the pre-amp / reverb is 4
lines : plus , minus , common , line level signal with no earth connection.
The front panel is connected to the common , not to earth anywhere.
There is much more noise immunity from general stray pick-up if I run a wire
from the earthed back plate to the front so bridging common and ground.
If made permannent,internally, would there be a safety issue, or in fact,
leaving as is , is that safe ?
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It would certainly be safe, yes.
Graham |
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N Cook
Guest
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| Posted: Fri Dec 01, 2006 1:18 pm Post subject: Re: Safety / noise immunity query |
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N Cook <diverse8@gazeta.pl> wrote in message
news:ekp5iv$s6v$1@inews.gazeta.pl...
| Quote: | Just repaired a Carlsbro Hornet Combo , made in UK, 1984.
Seems as original inside otherwise.
Unusually the amp separates into 2 sections separately connected to the
wooden casing and so electrically isolated as far as any earthing.
The mains transformer is on the rear panel which has a proper earthing
point
from the IEC mains inlet.
Power supply is plus and minus rails but the common is not connected to
ground at all. The power and signal lead to/from the pre-amp / reverb is 4
lines : plus , minus , common , line level signal with no earth
connection.
The front panel is connected to the common , not to earth anywhere.
There is much more noise immunity from general stray pick-up if I run a
wire
from the earthed back plate to the front so bridging common and ground.
If made permannent,internally, would there be a safety issue, or in fact,
leaving as is , is that safe ?
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/
|
Interestingly this unit was yearly PAT tested and passing presumably earth
bond and insulation test each time.
If they had checked insulation resistance relative to the front panel , he
assuming it was earthed, then could that procedure have damaged the power
supply?, not that that was the problem, it was poor soldering on output
transistors. |
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