UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - AC Millivoltmeter

6SN7WGTB · 21st Oct 2021, 5:48 pm

Just acquired a rather nice Kikusui dual channel AC MV.

It (a) a 240V-labelled model and (b) has the original mains cable which is TWIN core.

So, there is no protective earth.

The chassis is a steel/aluminium and plenty exposed.

The two channel inputs are UHF-type sockets with an additional ground terminal.

The two pre-amp outputs are 4mm banana binding posts, red and black.

Each channel 'ground' (the UHF shroud, the gnd terminal and the black output post) is actually connected to the chassis via what measures as 47Ω. (so channel to channel ground measures 94Ω).

There were at least THREE versions of this AC voltmeter, with very similar numbers (a 1831, an 1831A and a 1831B model). I quote from the manuals:

(A model) This had a switch on the front which EITHER floated/disconnected the channel grounds from the chassis OR "connected to the case ground with respective resistors of which resistances are sufficiently low as compared with the input resistance". (it's 1MΩ input impedance)
(B model) No switch. But "the outer conductor and GND terminals are electrically connected to the meter panel and chassis"

So I have a no switch model with the A model via resistor grounding mode.

I've spoken to Kikusui and they have no manual for my model (the A and B are on their website).

Can anyone explain (a) why no PE, and (b) why I shouldn't earth the chassis when I rewire it?

Yes, I could of course create an A model by adding a switch but my most pressing issue is the PE.

21st Oct 2021, 5:48 pm	#1
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 444	AC Millivoltmeter - earthing question Just acquired a rather nice Kikusui dual channel AC MV. It (a) a 240V-labelled model and (b) has the original mains cable which is TWIN core. So, there is no protective earth. The chassis is a steel/aluminium and plenty exposed. The two channel inputs are UHF-type sockets with an additional ground terminal. The two pre-amp outputs are 4mm banana binding posts, red and black. Each channel 'ground' (the UHF shroud, the gnd terminal and the black output post) is actually connected to the chassis via what measures as 47Ω. (so channel to channel ground measures 94Ω). There were at least THREE versions of this AC voltmeter, with very similar numbers (a 1831, an 1831A and a 1831B model). I quote from the manuals: (A model) This had a switch on the front which EITHER floated/disconnected the channel grounds from the chassis OR "connected to the case ground with respective resistors of which resistances are sufficiently low as compared with the input resistance". (it's 1MΩ input impedance) (B model) No switch. But "the outer conductor and GND terminals are electrically connected to the meter panel and chassis" So I have a no switch model with the A model via resistor grounding mode. I've spoken to Kikusui and they have no manual for my model (the A and B are on their website). Can anyone explain (a) why no PE, and (b) why I shouldn't earth the chassis when I rewire it? Yes, I could of course create an A model by adding a switch but my most pressing issue is the PE.