An observation.

[bob orbell]

Hi all, a few weeks ago I changed all the umbilical plugs and sockets for five pin instead of the supplied four pin, Richard did this first a long time ago, this now gives a solid earth connection at both Pre and Phono, I tried with and without the 100 ohm lift resistors in both PSU's and found without was indeed the quietest, Phono has gained best out of this and now with full volume there is barely a sound, now, I am wondering, and please don't bite me, in the standard form with four pin connectors and the 100 ohm earth lift there is NO SOLID earth connection to Phono or Pre, if a fault developed in say Pre III and the 250-275 volt HT line makes contact with the chasse, would it blow the fuse ? and if one touches the Pre's volume control, could it be game over , or am I wrong. Bob

[John Caswell]

It will depend whether you are earthed or not. As yourself why birds that sit in the power lines don't get frazzled, it's because they do not have path between themselves and earth.
If you are insulated from earth then you can touch the 240V power line with impunity.
I am not suggesting anyone tries this but it is a fact, unipotential and all that.
I use 5 pin plugs on my pre/psu/phono 3 so that they are all at common earth.
I also have a very heavy earth strap between my Pre3 and both amplifiers (Thank you Richard) Power is supplied by a 6mm TWE spur from the dis board with a dedicated 10mm earth feed thru the dis board to the company earth on the incoming cable.
As far as the HT shorting to chassis, I would think that the fuse fuse would blow first or possibly the rectifier diodes failing. As far as touching the VC in fault conditions like this bear in mind the HT -Ve is at earth/chassis potential so unless your house earthing is at fault and you are earthed at the same time nothing is likely to happen. I would also think that reflex action would throw you off the connection.

John

[bob orbell]

Thank you for the reply John, I am going to do some tests later with a now not used PSU board and a spare transformer, I do like your last sentence so if you don't hear from me again you will all know the answer. Bob

[Richard]

Originally Posted by bob orbell

Thank you for the reply John, I am going to do some tests later with a now not used PSU board and a spare transformer, I do like your last sentence so if you don't hear from me again you will all know the answer. Bob

Go carefully Bob! I had a quick google and can't see mention of it but I was told many years ago that one reason for going AC mains was that it caused you to let go of it whereas DC mains (around til the 50's) caused you to hang on to it...

[bob orbell]

Thanks Richard, I am not going to be the conductor, I will try introducing the Pos HT to chassis first with an earth leakage device on the incoming AC, then a fuse etc, it matters not if anything goes pop or bang, but I do have my doubts. Bob

[bob orbell]

Test results, PSU III populated board, transformer connected, 250 mA fuse, no connection to Pre, only Phono III, Variac on incoming mains, first test DC voltage at 180 v shorted to Phono chassis, voltage now 0 at Phono, still AC present at transformer sec. voltage now lifted to 260 v at Phono, shorted to chassis, no bang 0 volts at Phono, no voltage on chassis, R6 getting very warm, transformer sec down 20 volts, second test, earth leakage trip connected and results the same as above, so it seams that the resistors are - will carry the load if a problem occurs. Bob

[A Stuart]

Just a detail regarding post #5.
I would not think (open to correction) that an earth leakage device would trip on account of anything that comes out of the other side of a transformer.

Secondly, I remember being told in school physics (before RCDs were the norm, though Google tells me it had already been patented in 1965 by Charles Dalziel) that the fuse protects the wiring from causing fires, not you from dangerous shock.

And, totally off topic, I was reminded by the mention of sticking to DC, of a recent fire safety talk pointing out what is blantantly obvious if you think about it - To tell if it is hot the other side of a door by touching the handle, use the back of your hand so the reflex is to snatch it away, not spasm and grab onto it.

[John Caswell]

Richard wrote
"I was told many years ago that one reason for going AC mains was that it caused you to let go of it whereas DC mains (around til the 50's) caused you to hang on to it... True. Back "when I were a lad doing TV servicing in the days of live chassis" there was a lot of DC mains in W London. So two of us would go on a service call, one to do the servicing and the other one to watch out in case the service engineer got himself across the mains, so he could then be dragged off in a hurry. Nasty stuff DC mains. The thing also to remember is that until the fuse blows Battersea power station is on the other end, by then of course you could be dead

John

[bikerhifinut]

Indeed.
Now here's a thing, If anyone wants to bond their PSU and Phono or Pre chassis earths together then there should be (if you built the kit to the destructions) a nice hefty earth post on each unit. So save yourself the effort and if it worries you that much get a length of wire and connect the earth posts together. it'll do exactly the same job.

A.

[bob orbell]

Interesting you mention that Andy, I did exactly that before going five pin and it buzzed very slightly removing the pick up arm ground connection removed the buzz but vinyl reply was orid the five pin sorted it, then I also removed the 100 ohm earth lift resistor from each PSU and now total silence from CD and just a very gentle hum from Phono at full volume, so you are correct when you said we don't always agree, stay safe all. Bob