[R-390] On the subject of replacement capacitors...

[Drew P.]

Barry done wrote:

"This capacitor is part of a "detector" that feeds the meter.  There are some precision capacitors with very specific values in the detector, but most of the rest of them are plain-Jane components.

Components directly in the bridge circuit itself need to be accurate, yes, but I'm thinking this board doesn't require all of them to be precise.

While I'm trying to isolate the bad parts and see if I can get it working again (just for the mental exercises), I plan to replace all those little electrolytics.  That's why I was asking about using a more common, less expensive part.  One wouldn't matter, but half a dozen adds up."

Aluminum electrolytic capacitors are low precision devices.  The tolerance is specified, in many instances, as -50% +100%. The capacitance value has a large dependence upon applied voltage and temperature.  In a bridge null indicator, the application you mentioned, you are looking for just that - a null.  It is beneficial to have a good sensitivity in the indicator, but you are, after all, looking for zero, and precision is not important.

I don't think that in the application,  that 4.7 uF vs 5 uF, a 6% change, would make any appreciable difference. That 6% would be swamped by tolerance and also by capacitance change incurred over temperature and voltage excursions.

If this really worries you, you could use tantalum electrolytics, at higher precision and much higher price, if available in the required voltage rating.  

Drew