Safety / noise immunity query

Dec 01, 2006 6 Replies

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N Cook 19 years ago

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

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N Cook 19 years ago

point

connection.

wire

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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Eeyore 19 years ago

It would certainly be safe, yes.

Graham

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Eeyore 19 years ago

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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meow2222 19 years ago

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

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

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N Cook 19 years ago

earth

power

good

kit is

pointless.

much

the

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.

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Arfa Daily 19 years ago

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

Arfa

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