There are a number of tricks.? One is to put a current limited supply on the +15.? Adjust the supply to +15 volts and the current limit to roughly between 50 ma to 100 ma.? Look for the lowest voltage across each capacitor.? Suspect the ones with the lowest voltage drop.? Bear in mind that in a parallel string fed from one end, the bad one and all the ones in the string after that would have the same voltage.
Another would be to take a current trace probe (HP made one), feed the line with a pulse train, and then see where all the current is going.
The third method is the LED test.? In a dark room, look to see what capacitor lights up like a dim LED, the dull red glow may be something you could observe.? Ditto if you had an infrared camera.? If there are any filter resistors (Tektronix frequently used a 10 ohm series resistor for isolation), check for damage.
Oh, and disconnecting boards to try to isolate to a particular board isn't a bad idea.
Harvey
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On 9/7/2020 3:18 PM, Sparky wrote:
This 466 worked great for years, then suddenly I smelled something burning, and the fan stopped. All low voltages good on interface board, *except* for +15VDC. Reads about 4VDC, (and about 4 ohms to ground with power off....). If I leave power on past about 15 sec, I can smell something burning again, so I only power up long enough the check the voltages.
I'm thinking tantalum cap, but wondering a good way to isolate the offender? Assume the the +15VDC goes all over the scope, to other boards, so would be a bear to lift all tant caps one at a time. Thinking of breaking the trace somewhere near the PS source. Any other tips or likely suspects?
Thanks in advance!
