Air Ministry Valve Tester Housed in an wooden box Box labelled 10S/639 and the two interior sections 10S/658 and 10S/556
I knew nothing about this tester when I acquired it, though I had previously seen one similar sold on eBay for £ 41. When I took the thing apart to look inside I found that it was wired completely in black insulated wire and that someone with a large cold soldering iron had been at work. It was non functional! I tried to trace out the wiring and test individual resistors and diodes and made good the dry soldered joints. Not knowing how to operate it or how to set the switches there was little more that I could do.
After seeking help it was suggested that I contact the RAF Museum at Hendon. I was delighted to be told what it was and that for a nominal photocopying fee I could have the working instructions which included a circuit diagram part of which is shown below.
I then discovered that the fault which had eluded the amateur repairer was that the wiper of the potentiometer had scored a neat little insulated valley in the carbon track. I made a quick fix with a piece of copper foil and it now works after a fashion, but I have not compared the results from this tester with that of my AVO tester.