Keithley 173A Electronic Flea Market find

[calexanian]

While cleaning up my stand at a recent TRW electronic swap meet I saw a very lonely looking Keithley 173A sitting in front of a bunch of 70's boat anchor unrecognizable production equipment. I had to give it a home. The guy wanted $20 for it, so I figured its worth a gamble. Fix it it, put it back in my stand next time for $35, and somebody gets a cool old meter for cheap. Plug it in and clearly something is not right. Little bit of flickering and thats about it.

Thou Shalt Check Voltages..... No biggie. Fried 7815 regulator. Don't have any, so I stick a 7812 just to get it powered up and make sure its running. No problems there. So its nagging at me. What made that regulator go bad in an otherwise normal appearing meter. I think Keithley played a little fast and loose with their supply voltages. I measured up to 38 volts +/- depending of line voltage (125 volts, max our power company says they can give you, normally I have spot on 120 here. but I run a variac up on everything now to test. I had a recent experience that now makes me include an over voltage test on everything I do) and as memory serves me that borderline in shutdown for a standard 78XX series. I recall one of our designs having issues with borderline high voltage on a 7805 and depending on manufacturer, or country of origin we had wild sporadic issues. I would think a test gear company would have been a bit more "In the numbers" lets say.

BTW it runs with the 7812, but all of the voltage measurements are way out of zero, etc etc, ordering a proper part Monday, but I know its otherwise fine. Interestingly it measures resistance bang on. Bust be a separate circuit or reference or something.