How did (does) people measure line voltage on grounded chassis VTVMs? - UK Vintage Radio Repair and Restoration Discussion Forum

MrBungle · 2nd Apr 2019, 8:43 pm

I'm in the middle of refurbishing a Heathkit IM-13U VTVM. Part of this is conversion to a grounded chassis to lessen the chances of killing myself.

This begs the question, as there were commercial grounded VTVMs, of how did they measure line voltage?

Refugee · 3rd Apr 2019, 12:05 am

I have got a valve volt meter with a grounded case.
The grounding of the electronics is via two 500V capacitors connected in series.

stuarth · 3rd Apr 2019, 10:03 am

If you’re using mains powered test gear on live chassis sets, shouldn’t you be using an isolation transformer? A signal generator connected to the aerial sockets or through low value capacitors may be ok, but not anything needing a DC connection.

Stuart

MrBungle · 3rd Apr 2019, 11:56 am

That's a good point with the isolation transformer. So float the DUT and keep the meter grounded. Makes sense.

I only do secondary side stuff with this which is all earth referenced anyway (no live chassis!) using the DUT's built in isolation transformer. I have a Fluke 87V for the mains stuff. I was mostly interested in the process from a historical perspective as I worked out several ways to kill myself on that IM-13U such as the jack front being connected to the negative probe lead and the thing was floating. Changing range while measuring mains could have been fatal. I've bonded the chassis and the negative lead to earth now as it's not double insulated and the case wasn't even earthed.

dave cox · 3rd Apr 2019, 4:23 pm

They probably disconnected the earth and tried to remember to connect 'N' to the shield (after checking their insurance is up to date!). I guess Heathkit saved you the trouble of cutting the earth wire

dc

2nd Apr 2019, 8:43 pm	#1
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	How did (does) people measure line voltage on grounded chassis VTVMs? I'm in the middle of refurbishing a Heathkit IM-13U VTVM. Part of this is conversion to a grounded chassis to lessen the chances of killing myself. This begs the question, as there were commercial grounded VTVMs, of how did they measure line voltage?

3rd Apr 2019, 12:05 am	#2
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: How did (does) people measure line voltage on grounded chassis VTVMs? I have got a valve volt meter with a grounded case. The grounding of the electronics is via two 500V capacitors connected in series.

3rd Apr 2019, 10:03 am	#3
stuarth Heptode Join Date: Feb 2010 Location: Heysham, Lancashire, UK. Posts: 669	Re: How did (does) people measure line voltage on grounded chassis VTVMs? If you’re using mains powered test gear on live chassis sets, shouldn’t you be using an isolation transformer? A signal generator connected to the aerial sockets or through low value capacitors may be ok, but not anything needing a DC connection. Stuart

3rd Apr 2019, 11:56 am	#4
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: How did (does) people measure line voltage on grounded chassis VTVMs? That's a good point with the isolation transformer. So float the DUT and keep the meter grounded. Makes sense. I only do secondary side stuff with this which is all earth referenced anyway (no live chassis!) using the DUT's built in isolation transformer. I have a Fluke 87V for the mains stuff. I was mostly interested in the process from a historical perspective as I worked out several ways to kill myself on that IM-13U such as the jack front being connected to the negative probe lead and the thing was floating. Changing range while measuring mains could have been fatal. I've bonded the chassis and the negative lead to earth now as it's not double insulated and the case wasn't even earthed.

3rd Apr 2019, 4:23 pm	#5
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	Re: How did (does) people measure line voltage on grounded chassis VTVMs? They probably disconnected the earth and tried to remember to connect 'N' to the shield (after checking their insurance is up to date!). I guess Heathkit saved you the trouble of cutting the earth wire dc