Recently I received a bad AM503 current probe amplifier from a buddy. He had a couple of good AM503s and did not need the bad one so he generously passed it on.
Initial evaluation showed that the output signal had a very large AC component and the overload LED was lit.
The immediate suspicion was that a dipped tantalum had shorted and caused some issue. But none of the three in the device was shorted.
So the supply rails were examined. Sure enough, the problem was obvious – the ripple on the negative 16 volt rail was 7 volts. Ouch.
A visual inspection of the PCB showed what had happened.
The 47uF electrolytic on the -16V output, C165, lost its negative lead right off the case:
I surmised this might be due to excessive ripple causing the capacitor to overheat. My buddy calculated that the capacitor should be rated for at least 0.5A ripple current and have low ESR in order to handle the transients that could be caused when the probe is measuring very large signals.
After the capacitor was replaced the AM503 worked properly.
But then I happened to look inside one of the good AM503s, and found the same problem with the same capacitor. In this case, the lead was still attached (by a thread, it seems -- it fell off when the capacitor was desoldered.)
Here is the second AM503, with C165 leaking badly just above the large blue electrolytic.
Interesting to see the same failure on two separate units, one with 1981 date codes and the other with 1991 date codes. Also, it is interesting that the corresponding capacitor on the +16 volt rail, C155, looked perfectly intact on both units.
It may be just a fluke that two units had problems with exactly the same part. But I thought it worth passing along in case other owners want to examine their AM503s.