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.

 

[timhoward]

Bad picture, best I could find online.
Take it you have a vintage AVO panel tester?

 

[Lucien Nunes]

Got it. Anyone not with us, keep watching.

 

[Lucien Nunes]

VCM IV

 

[DPG]

Did I get it in post #21 then?

 

[timhoward]

Did I get it in post #21 then?

I'd say so!

 

[Lucien Nunes]

OK yes joint win, didn't see your edit, when I looked at that post it only mentioned the resistance decade box.

 

[DPG]

Thanks flip. Made my brain hurt!

 

[DPG]

OK yes joint win, didn't see your edit, when I looked at that post it only mentioned the resistance decade box.

Yes, it occurred to me afterwards. Luckily well within the edit window.

 

[DPG]

My next guess was going to be some sort of capacitance or inductance measuring bridge.

 

[Lucien Nunes]

Back in an hour or so...

 

[timhoward]

I wouldn't have got it without the valve specification clue:

KT66
026 540 310 A100V, S60V, MAV6 HTR6
It's been many years, anode, screen, ma/v, heater I think?

 

[DPG]

I wouldn't have got it without the valve specification clue:

KT66
026 540 310 A100V, S60V, MAV6 HTR6
It's been many years, anode, screen, ma/v, heater I think?

Didn't cotton on to that! Nice one

 

[Lucien Nunes]

Those settings are not for the VCM though. The 2-panel tester is a simple zero-bias tester that is not well suited to later high-gain power bottles and has to test them at low voltages. The VCM operates the valve under realistic operating conditions and can be used to plot curves. A:250, G2:250, G1: -15V. Then you are looking for Ia=65mA and gm=6.3mA/V.

Those still not with us, the rotary switch assembly I was servicing, called a 'roller switch', allows each 'test lead' of the instrument circuit to be connected to any pin of the valveholder (=socket). There were thousands of different types of valves on dozens of different types of bases, with the same pin being used for different functions on different valves. I..e for a KT66, in addition to dialling in the anode voltage as 250V, you also have to dial in that the anode is on pin 3. and the same for all the other electrodes.

Different testers had different ways of doing this but the AVO roller switch is generally considered one of the best and slickest. The settings are given in the AVO data book but if you know the valve pinout you can work them out on the fly.

For a reminder of the KT66, one of Britain's best-loved audio output bottles, see:

KT66 @ The Valve Museum

www.r-type.org

or download the dataheet here:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwi0odDxm6HxAhXWA2MBHQGlAwwQFjAAegQIBRAF&url=https%3A%2F%2Ffrank.pocnet.net%2Fsheets%2F084%2Fk%2FKT66_GEC.pdf&usg=AOvVaw2vfZVC_ScvGOenDNbR6XBD

 

[SJD]

I wouldn't have got it without the valve specification clue: KT66

At that point, I was wondering was this some newer Kewtech MFT model I'd not heard of . . . . .

 

[telectrix]

Those settings are not for the VCM though. The 2-panel tester is a simple zero-bias tester that is not well suited to later high-gain power bottles and has to test them at low voltages. The VCM operates the valve under realistic operating conditions and can be used to plot curves. A:250, G2:250, G1: -15V. Then you are looking for Ia=65mA and gm=6.3mA/V.

Those still not with us, the rotary switch assembly I was servicing, called a 'roller switch', allows each 'test lead' of the instrument circuit to be connected to any pin of the valveholder (=socket). There were thousands of different types of valves on dozens of different types of bases, with the same pin being used for different functions on different valves. I..e for a KT66, in addition to dialling in the anode voltage as 250V, you also have to dial in that the anode is on pin 3. and the same for all the other electrodes.

Different testers had different ways of doing this but the AVO roller switch is generally considered one of the best and slickest. The settings are given in the AVO data book but if you know the valve pinout you can work them out on the fly.

For a reminder of the KT66, one of Britain's best-loved audio output bottles, see:

KT66 @ The Valve Museum

www.r-type.org

or download the dataheet here:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwi0odDxm6HxAhXWA2MBHQGlAwwQFjAAegQIBRAF&url=https%3A%2F%2Ffrank.pocnet.net%2Fsheets%2F084%2Fk%2FKT66_GEC.pdf&usg=AOvVaw2vfZVC_ScvGOenDNbR6XBD

I'm convinced now. yous was born on Mars and come here to confuse us all into abject surrender.

 

[Lucien Nunes]

It's only what the chap behind the counter at the telly shop would have been doing 60 years ago. I typed '40 years' and had to correct it. These new-fangled transistors aren't so new any more.

 

[static zap]

My electronics are running hot ,
(they need to be for Thermionic emission - Insert Diode here)
Them were the days 90V batteries !

 

[brianmoooore]

Them were the days 90V batteries !

Over the last few weeks I've been using my new JCB toy to lay new drains for a couple of my properties.
Dug up hundreds of cells from these batteries in one particular place.

 

[Lucien Nunes]

Roller switch back in place and wired up

 

[Lucien Nunes]

Wiring checked and ready to use. I'm going to test an EF37A because that KT66 is known to be duff.
Pinout is similar (it's fairly standard for octal-based pentodes and tetrodes) except the g1 is on the top cap and pin 5 is g3 which is brought out separately from the cathode. 026 510 310 connects g3 and the cathode (pin 8) externally. Vh=6.3, Va=250, Vg2=100, Vg1=-2V. Looking for 3mA and 1.8mA/V.

 

[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.