I'm rebuilding an old Heathkit IT-28 Capacitor/Resistor checker. It uses a Wheatstone bridge design with a Magic Eye tube for null indication. It is a fairly simple design with a rectifier tube and a multi-element tube for the only active element (it contains two rectifiers and a triode), and a Magic Eye tube.
This is a Magic Eye tube, a null is indicated when the dark portion is at a minimum:
Measurement range is from 10pF to 1000uF, or compared against an external standard capacitor. Resistance from 5 ohms to 50M ohms, or compared against an external standard. Test signal is 10Vac at 60Hz, or an external sine wave generator can be used, frequency up to 10kHz.
It can also measure inductors but only with an external reference, ratio up to 25:1.
Leakage can be tested at selected voltages from 3 to 600Vdc.
The manual is pretty thorough, describing the theory of operation, build manual, and schematics.http://tubularelectronics.com/Heath_Manual_Collection/Heath_Manuals_IT-NE/IT-28/IT-28.pdf
The biggest issue with these is that Heathkit provided some paper capacitors that do not age well, and poor assembly/soldering by whomever put it together. This particular Heathkit appears to be put together well, and it looks like some of the capacitors were upgraded at assembly time.
However, it blew the 1/2A slow blow fuse when powered up. So I suspect the aluminum electrolytic capacitors in the power supply have gone bad. It has two 40uF 350V capacitors in series to get the voltage rating, and giving a total capacitance of 20uF. This is subject to problems when the capacitors are slightly different values due to tolerances. With older leaky caps, it will tend to even out the voltage. If I were to replace them with modern 47uF electrolytics, I'd add 1M to 10M resistors in parallel to ensure equalized voltages.
Instead, I think I'll use a 20uF 1kVdc 5% film capacitor, it seems to be an economical alternative at $7.40 in single quantities from Digikey:http://www.digikey.com/product-detail/en/vishay-bc-components/MKP1848S62010JY5F/BC2777-ND/3053110
Or from Mouser a 20uF 900Vdc 10% film capacitor for $4.55 each:http://www.mouser.com/ProductDetail/AVX/FE37M6C0206KB/?qs=sGAEpiMZZMv1cc3ydrPrF9Ta4vR0G8Q2ljgetnD1K%2f8%3d
Amazon 20uF 900V 10% film capacitor for $8.76 plus $7.00 shipping:https://www.amazon.com/Film-Capacitors-900volts-20uF-6mohms/dp/B00MEKTSMA/ref=sr_1_5?ie=UTF8&qid=1484163349&sr=8-5&keywords=20uf+film+900v
I had some other things to order from Mouser, so I ordered two from them. It is $7.99 for Priority Mail USPS, or ground UPS or Fed Ex shipping.
The 6AX4 rectifier tube is looking possibly not 100%, based on the internal silvering starting to go black around the edges. I have some 1N4007 1kV 1A diodes I can repair it with, while leaving the 6AX4 in place just for looks. The 6BN8 and the 6E5 Magic Eye tube don't have any obvious problems, I'll know more once I can fire it up.
Before I do this, I'll be disconnecting the transformer from the circuit by pulling the 6AX4, putting a fuse back in, and bringing it up slowly on a variac (variable transformer) while monitoring current and voltage.
A page about restoring this instrument:http://www.californiahistoricalradio.com/restorations/heathkit-it-28-restoration/
Restoration will happen at OlyMEGA during "Tuesdays With Steve", members only. I'm happy to field questions while I'm working.https://www.meetup.com/OlyMEGA/events/236813827/