AVO Test Bridge

Its been a busy week so I've not had much time to spend in my workshop, but I did find enough time to finish off my latest project, an addition to my test equipment collection.

Whilst browsing my favourite internet auction site recently I came across this, an AVO Test Bridge.

AVO Test Bridge

I'd never seen one of these before, but a quick search on the web turned up enough info to get me interested. What better than vintage test equipment to test vintage radios?  It looked too good a deal to miss out on!

The ATB is the same size and  shape as an AVO 7, but it will test resistance, capacitance and capacitor leakage. It's basically a Wheatstone bridge, with a sensitive valve voltmeter driving a balance meter. With the desired range selected, the dial is rotated until the meter reads a minimum value; the component value is then determined by multiplying the reference for the selected range (100pF/10nF/1uF or 100R/10K/1M) by the number on the dial. An additional setting also allows an external reference resistor or capacitor to be used instead of an internal reference. The leakage tests applies a high voltage to the capacitor via a neon lamp, which flashes repeatedly if there is any leakage current.

All it needed was a new electrolytic capacitor and a couple of polyesters to replace the old wax paper types. It's simple to use, and it appears to be surprisingly accurate for something that is nearly 70 years old. (The last four digits of the serial number visible on the meter scale are 1144, indicating that it was made in November 1944)

The total cost of bits including a bit of vintage type three core mains cable came to under £5, and with a total outlay just over £30 I reckon that I got a bargain.

I went back to the seller a few days later and bought another bit of kit, an AVO allwave oscillator. I'll leave that story until another day.

Keep listening!