Low insulation puzzle: what's this?

[Lucien Nunes]

Damp here this morning, and the insulation resistance on a piece of workshop equipment seemed a bit low. I could influence the reading by pressing my hands against the outside of one of its controls. That, and the fact it is getting a bit stiff to operate, show that it's time for this control to be stripped down and cleaned. Metal deposits and dirt build up inside over the years and cause leakage.

The main core of the thing consists of 36 parts threaded onto a shaft. I slide them onto a long screwdriver to preserve their order and orientation as I take them off. Then as I clean each one I put it back onto the shaft.

What equipment is it part of? The maker's name will be familiar but the equipment itself probably not so much. A couple of people here will get it in one. Give the others a chance.

 

[Lucien Nunes]

Off we go...
Test insulation by selecting each electrode in turn


This is much better now the switch has been cleaned. Not getting any spurious low (<50MΩ) readings now. Hot tests next:


Just for checks I've popped a 10MΩ resistor between cathode and heater sockets and the tester does indeed show 10MΩ:


Now onto emission test. Dial down the meter range to give a good indication. 2.5mA or so, actually just off the scale on the 2.5mA range:

 

[Lucien Nunes]

So the valve is good for emission. Now back-off that standing current to zero:


And switch to mutual conductance test:


Adjust the gm dial until the meter needle is on the index mark:


And read off the mutual conductance which s 2mA/V.:

 

[Lucien Nunes]

So there it is, one happy tester and one good EF37A.

 

[Zerax]

Lucien... I think you should do ½ day sessions (a full day would blow your mind !)... "Playing with stuff with Lucien"... you could easily charge say £90 ish I reckon ! I'll put my name down. Would be popular around fathers day too maybe ?

(remember where you heard it first... only 5% commission per booking)

 

[Lucien Nunes]

Ha, one of my mentors used to do something similar, workshop sessions for you to fix your stuff under his guidance, not unlike today's local repair workshop / maker shop scene, 20 years before that became a thing.

But anyone who wants to play with electrical and electronic things can come and help with setting up museum exhibits and it doesn't cost a bean. Physical abilities permitting you are likely to get drafted in to help with lift'n'shift too though, it's not all joining-up wires and twiddling dials. Next week for example Dave and I have 15 display cabinets to collect that need to be carried down a flight of stairs. The main action takes place in the N. London / Herts / Beds areas. We have pallet racking to put up and a stage to build. Machine tools to move and woodworm to treat.

The iron is hot and the cement mixer is running. Cast iron does not levitate without assistance.