Safety / noise immunity query

[N Cook]

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 ?

 

[N Cook]

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 ?

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.

 

[Eeyore]

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 ?

It would certainly be safe, yes.


Graham

 

[Eeyore]

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?

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

 

[[email protected]]

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,
no

or in fact,
leaving as is , is that safe ?

if its double inuslated, yes, if is single insulated, not entirely.

It might be floating in an attempt to avoid earth loops.


NT

 

[N Cook]

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

In the hundreds , maybe > thousand pieces of kit I've PAT tested/
3 failed over the years, one had a corroded earth point, one had internal
leakage in mains transformer and another had a 2 Meg ohm resistor mains live
to earth as part of its design (old radio). Dozens of times all sorts of
potential nasties inside (UK) mains plugs, usually too much stripped back
conductors or totally wrong fuses, interestingly never one with aluminium
foil wrapped around a blown fuse.

 

[Arfa Daily]

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 ?

See my reply further down on original thread for this repair, regarding
value of burnt out resistor.

Arfa