Tektronix Type 130 LC Meter & S-30 Delta Standards
As far as I know, this instrument is unique, at least for the valve era. It was originally made for in-house use, then word got out, it was cleaned up for production and remained in the catalogue for 21 years. Tek people referred to it as “Elsea”.
It provides direct readings of low values of L and C and also provides a so called “guard voltage” that can be used to charge any stray capacitances due to nearby conducting materials. A great example would be measuring the inter-electrode capacitances of valves, the guard voltage is connected to all electrodes not to be measured. It can be similarly used to measure capacitances that are in-circuit. I recently used it to check a 0.5 µH deflection plate coupling choke for a Tek type 547 that I re-made after the original broke while I was disconnecting it. (The small Allen-Bradley resistor core cracked.)
The principal of operation is deceptively simple: There is a 140KHz fixed oscillator and a second 140KHz oscillator that drops in frequency when a component is added at the test terminals. The two frequencies are fed to a mixer and the difference is measured using a capacitance charge circuit that is arranged such that the voltage the capacitor charges to increases with frequency (the difference frequency) and this voltage is directly indicated on a meter. If zero reactance is present, the two oscillators dead-beat (a bit like me really) and the meter indicates zero.
The guard voltage referred to above is simply the variable oscillator output buffered by a cathode follower so that the stray surfaces are charged to the same conditions as the surfaces to be measured.
The only problem it had was oxidised pins on the variable oscillator valve that required some physical agitation together with the Deoxit treatment. After that, it worked properly and I was able to calibrate it.
Tektronix produced a switched box of standard Cs, a couple of Rs, an L and a short circuit that they termed S-30 Delta Standards, for this job:
Here is the inside. It is heavily constructed and shielded. Despite that, the connector side is bent and I chose not to attempt to straighten it:
I attached the standards box to the back of the meter using self-adhesive velcro pads, I have an abhorrence of vital accessories to equipment becoming separated. When it was made, the standards would obviously be held in a calibration lab but now that such equipment is obsolete, I make an effort to keep things united…..