While thinking about building a Kelvin Varley Divider, I came across this on ebay.
No-one had bid at the time, and it had a magnificent row of decade switches across the bottom. How can you go wrong with that! I bid and got it for $16.50. I wouldn't bother to post a teardown here unless it had something really special inside, and luckily for me, it did.

So first, some background. It was made by Sperry Piedmont for Lockheed Martin around 1960 so it is over 50 years old. The costs was $2,600 which was half the family average income in the US at the time. It has a serial number "72" - not many were made. This one was used by the Australian Air Force. Last calibrated in 2000 and so it all ties in with the notion that this is used for calibrating avionics for the Lockheed Martin Orion P-3C. Around 2000, the Australian P-3C's were upgraded to AP-3C's with all new avionics, so this instrument probably became obsolete. The Orions are one of those planes that just will not die. They are 50 years old, and right now, the US Coast Guard is upgrading theirs to last for a minimum of another 20 years.
I will not try and use this - I will be pulling it apart. It is very heavy, it needs 120V/400Hz supply and the electronic meter for measuring the potentiometer null has a maximum sensitivity of 3mV full scale. So there is nothing magical about the valve based electronics. It that part of the device, I can salvage some good switches, some nice transformers, a few wirewound resistors, a nice meter movement, some real museum-piece germanium diodes and some General Radio capacitors.
The case is brilliant, and I will be keeping that. Completely weather proof and the rubber seal in the groove on the lid is still soft after 50 years. The clips are really tight. Fantastic!
https://www.eevblog.com/forum/index.php?action=dlattach;topic=5726.0;attach=17488The inside of the lid had the circuit printed on an anodized aluminium sheet, and I have attached it below. But when I first looked at it, I was puzzled - the schematic had no decade switches on it at all!
Then I saw this down the very bottom right corner:

That must be the switches. What is a Gertsch Standard Ratio Transformer?
I went on to the internet, and discovered that they are basically exactly the same as the Tegam Ratio Transformers that IET sells today. In fact this Tegam divider probably has the same transformers that my device has, except that I have the better heavy duty switches.
http://www.tegam.com/product.asp?modelNumber=RT-60BI will go into the details of this transformer in a second post, but for now, lets just say they are amazing - unbelievable really. Gertsch made standalone dividers, but their main sales was to makers of avionics test gear for the military. So the transformers, the switches and the wiring is all 100% Gertsch and it is just put straight into my device. Being a transformer, we are looking at an AC bridge here and it just uses the 400Hz mains to power the transformer.
The reason I am saying this is there are obviously other old pieces of gear floating around with the exactly same dividers. Keep an eye out for that row of decade switches on old military test gear.
Here is the guts:

You can see the row of Gertsch switches along the top of the photo.
One of the things that really got me was look at the wiring. The wiring that Sperry has done is all with white wire! The 400V DC supply wire is the same colour as the 120V mains input wires, 6.3V filament wires and the millivolt signal wiring from the bridge. What could possibly go wrong with that?
The guy who built these obviously had one gigantic reel of white wire, and he couldn't be happier! I am glad I am not trying to fix it.
The Gertsch divider is 5 decades with 3 decades handled by one transformer, and the other two decades by a second transformer. A potentiometer provides the last decade of the output. Here is the primary transformer box:

So I have the Gertsch RT-7.
That Variac under it is one of the nicest I have ever seen or used. A fabulous light wirewound feel to it. 120V in, 0 to 134V out and even though they are using it at 400Hz, it is rated for 60Hz. 165VA. A pity it isn't 240V like our mains. I may have to sell it.

The Gertsch switches are custom built for the job. It is a 10 position switch, but they double up on every contact, so that means 20 main contacts. This is the heavy duty switch that everyone who knows these wants to see. 2 milliohm contact resistance after 40 years of use and the last 10 years doing nothing. The switches are really heavy to use - you really grab these knobs. None of this wimpy fingertip stuff.
But there is a big problem if you want to switch transformer windings. You never want to let a winding go open circuit while it is carrying current, or you will get a transient. But you also never want to short two windings together. So you cannot use a make-before-brake switch, and you cannot use a break-before-make switch. What is the solution?

The solution is to add another 18 intermediate contacts connected to the windings and to have make-before-break contacts that go from one winding to the resistor to the second winding. On the first decade the resistors are 2k2, and the other decades have 47 ohm resistors. The resistors are normally open circuit, and they switch in across the windings as you turn the switch.
I am going to strip the guts out, cut the top off the front panel so I just have the Gertsch divider switches, add some 4mm terminal sockets, and built a box to mount the panel and transformers in. I have some Bakelite sheets, so I might make the box from that for a retro look. When I build me Kelvin-Varley resistive divider, I want it to look the same so I need to work out a common style.
Back to what is special about this transformer divider. Why would you want one? I will cover that in the second post.
If you want to look at the schematic, it is below. It is basically an AC amplifier that can go from 3mv full scale to 300volts full scale. It can chop the signal to be able to do phase sensitive nulling. You can adjust the phase from -135 degrees to 135 degrees. It only works at 400Hz and has a built in 400Hz bandpass filter to reject noise and harmonics.
Richard