Archive for September 7th, 2018

Tektronix AM503 Current Probe Amplifier: Failed Electrolytic Capacitor

Posted by Ed in Electronics Workbench on 2018-09-07

The same Tektronix AM503 current probe amp with the slightly disconnected JFET developed a nasty 120 Hz hum, which can mean only one thing:

Tek AM503 - corroded capacitor

Tek AM503 – corroded capacitor

Dunno how I missed that while I had the cover off, but there it is. For future reference it’s C165, 47 µF 50 V, with an 8709 date code.

This being a PCB made back in the old days, likely with self-adhesive tape strips, the component pads inside ground and power pours lack thermal isolation decorations.

The solder joints on the bottom side looks better than these, honest:

Tek AM503 - replaced capacitor

Tek AM503 – replaced capacitor

The new cap is 56 µF 50 F, which seems Close Enough.

The other caps have reasonable ESRs, but I must lay in a stock of high-voltage ‘lytics, as the “new” one is definitely old enough to know better, too.

Now that I check, the amp still produces a very low amplitude 2 MHz sine wave on its 1 mA/div and 2 mA/div settings, suggesting Something Is Not Right™ in the front end, but it works well enough to let me defer that fix until later.

 

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