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]

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

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

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.