Re: Never assume anything!
It's a real puzzle when a piece of kit from a reputable manufacturer turns out to have been wrongly wired half a century ago.
I had a recent example with a Bruel & Kjaer dual condenser microphone power supply with versatile switching to route either or both microphones to the two outputs.
The audio worked fine, but the two little neon lamps showing the active input(s) made no sense. I assumed it was just a design oddity. It was only when I got hold of a second identical unit that DID make sense, with the neon adjacent to the active input(s) lighting up, that I realised my first unit was actually defective. Couldn't now live with it - had to rectify the problem!
Hampered by not having the circuit diagram (in the hen's teeth category), I removed the relevant reed relays - they checked out fine. Transistors - all good.
Finally, having as a last resort traced out the very tidy cable-form connections, I noticed a minor wiring colour difference between the two units. Sure enough, the defective unit had two wires swapped over in the cable-form. Reversing the two finally put the indicators right. It spoiled the immaculate look of the cable-form though - clear evidence that it had been wired wrongly all those years ago.
A satisfying puzzle to solve!
Martin
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