Oh here it is. Image heavy. Sorry. One pile of old crap Heathkit GD-1U as advertised:
Don't turn it on, take it apart. Basically it's a EF92 (6AB4) based LC oscillator with a peak detecting meter attached to it. The L bit sticks out so you can couple energy to other stuff. Some criminal wiring in here, an old selenium hand grenade and a nasty nasty nasty old dual capacitor.
Unit was disassembled and front panel removed. All the nasty bits were removed. The mains cable was snipped out, the entire power supply section was removed and disposed of and all the components were tested in circuit. Fortunately all was well. The EF92 was inspected for getting poisoning and then the heater checked on a bench power supply to make sure it was still ok. Good news again. Some early victims - ick:
A new power supply section was built on a new tag strip installed in the hole the original selenium rectifier was in. I forgot to take a photo of the installed new power supply
but it was from the HT a 1n4007 series -> 22uF parallel (250v) -> 2.2k 1W series -> 22uF parallel (250v) -> out. The resistor was selected as a finger in the air estimate to give around 110 - 150V of DC for it. Came out at 141 which I was happy with. During build
Next thing the unit was scrubbed down with washing up liquid and kitchen towel and dried with a hair dryer. All crud removed. The original dial legend was knackered so I scanned it at 1200dpi, cleaned it up in gimp on Linux and printed a new one out. Trial fit:
Unit was given a quick test and reassembled. Job done. Well not quite. I identified at this point the oscillator stopped at below about 20% of the dial. A hunch pointed to the variable capacitor, confirmed by a quick continuity test. Plates were carefully bent until they didn't touch again. Damn things. Job actually done now...
So what do you use it for. Well let's check an inductance! I dug around and found a decent, stable reference capacitor, a 900pF 1% silver mica one and soldered a supposedly 5.6uH inductor across it so it is a parallel resonant LC circuit:
This was then loosely coupled to the GDO coil sticking out the of top (band A as this should fall in the 1.5-2.5MHz range ish). The flux need to cross through the main GDO coil and the inductor under test so this was the most sensible arrangement here. If you have a toroid, you can stick a couple of turns through it and then wrap them around the and of the coil a couple of times to make a coupled L circuit but I had an axial one at hand here:
You then tweak the sensitivity to about the middle and turn the frequency dial until the meter dips (hence grid dip meter). This is the LC circuit reaching resonance. Usually you'd then read the approx frequency off the dial and dig out your slide rule or nomograph and work out what the inductance is from that but (a) the dial calibration is shit and (b) it's 2019. So in a moment of anachronistic glory, lets measure it with a scope. Just wind up a few turns of hookup wire and ram it into your counter or scope and read the frequency off.
You then have L, C and F so just plug the numbers in f=1/(2*pi*sqrt(l*c)) and solve for L. Or write a program if you know how to use a calculator without an equals button
(or just do what i did and google a calculator for it)
Here we have L=?, C=900pF, F=2.276MHz ... L=5.43uH
Now it's a 5.6uH inductor so that's pretty good.
Edit: now I'm annoyed I sold my old Heathkit IB-1100 counter. That would have gone nicely with it: